7.Magnetic Effect of electric Current.doc(VII)

Magnetic Effect of Electric Current

Explanation/ Reason/ Laws/ Rules/ Devices/ Practical – electric Bell and electromagnet.

In 1820 Hans Christian Oersted during his experiment found that when an electric current flows in a wire it moves a compass needle and this effect lasts as long as the current flows through the wire. This experiment established the relation between electricity and magnetism.

If we place a compass near to a electric current carrying wire we can observe a deflection in compass needle. The needle of compass gets deflected by a magnetic field produced by current carrying wire. This effect which produced by the flow of electric current is called “Magnetic Effect” of electric current.

In this unit we will learn about Magnetism, Magnet, Magnetic effect of electric current and its applications.

Magnetism:

The magnetism is the property possessed by certain bodies of attracting or repelling other bodies of magnetic substances.

Magnet:

A magnet is an object or a device that gives off an external magnetic field. Basically, it applies a force over a distance on other magnets, electrical currents, beams of charge, circuits, or magnetic materials.The basic atomic structure of a Magnet seems to align most of the molecules in the same direction. Since many atoms have a magnetic moment (tiny magnetic field), all of the moments can add up to create a magnet. Scientists use the word hysteresis to describe the way the atoms stay aligned.

There are two types of magnets

Natural Magnet:

The magnet found in nature are called “natural magnets”

Artificial Magnet

The magnets which are made artificially are called Artificial Magnet.

On the basis of life of Magnetism the magnets can be classified into two categories

Permanent Magnet

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The magnet which retains their magnetism for a long time is called permanent magnet. The strongest and best permanent magnets are made of alloys like Almico ( Aluminium, Nickel, Cobalt and Iron); Permalloy (Cobalt, Nickel and Iron or Nickel and Iron); Vicalloy (Cobalt, Iron and Vanadium).

Temporary Magnet

The magnets which retain their magnetism for a short time are called temporary magnet. All electromagnets are temporary electromagnet because they show magnetism till the flow of electric current when the flow of electric current stops the magnetism ends in coil.

Magnetic Lines

It is possible to see this force through a simple experiment:

Bar Magnet Experiment

Put a Bar Magnet under a sheet of glass and sprinkle Iron Filings on the glass. The "lines of force" from the Magnet show up clearly as the Filings form a pattern. Notice that the attractive forces are greatest at the two "ends" of the Magnet, where the majority of Filings gather. We call these "ends" "poles."

The density of the pattern represents the strength of the field, which is the magnitude of the force exerted upon a magnetic material placed at the point in the field. These lines are called "lines of magnetic flux."

Magnetic flux: The total number of lines of force around a magnet is called magnetic flux.

Types of MagnetsThere are many different types of magnets. Permanent magnets never lose their magnetism. There are materials in the world that are called ferromagnetic. Those materials are able to create and hold a specific alignment of their atoms. Since many atoms have a magnetic moment (tiny magnetic field), all of the moments can add up to create a magnet. Scientists use the word hysteresis to describe the way the atoms stay aligned.

Most of the magnets you see around you are man-made. Since they weren't originally magnetic, they lose their magnetic characteristics over time. Dropping them, for

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example, weakens their magnetism; as does heating them, or hammering on them, etc.

There are also air-core magnets. Air-core magnets are created by current flowing through a wire. That current produces the magnetic field. You create an air-core magnet by wrapping miles of wire around in a doughnut shape (toroid). When you send current through the wire, a magnetic field is created inside of the doughnut. Scientists sometimes use air-core magnets to study fusion reactions.

Electromagnets are different because they have a ferromagnetic material (usually iron or steel) located inside of

the coils of wire. The core isn't air; it is something that aids in producing magnetic effects, so electromagnets are typically stronger than a comparable air-core magnet. Air-core and electromagnets can be turned on and off. They both depend on currents of electricity to give them magnetic characteristics. Not only can they be turned on and off, but they can also be made much stronger than ordinary magnets. You might see an electromagnet at work in a junkyard lifting old cars off the ground

Magnetic Field

A magnetic field is defined as a region in which a magnetic force is present. In a

magnetic field, the magnetic dipole (two equal and oppositely charged or magnetized

poles separated by a distance) experiences a turning force, which tends to align it

parallel to the direction of the field. The concept of a magnetic field can be understood

with the help of the following activity:

● Place a piece of cardboard over a magnet

● Sprinkle some iron filings onto the cardboard

● Tap the cardboard gently and draw what you see

● The iron filings show the magnetic field of the magnet

Magnetic Effect:

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The effect which applies a force over magnetic materials, beam of charges, electrical current or on other magnet is called Magnetic effect.

Magnetic Lines of Force Due To Current in a Straight Wire

The direction of the magnetic field due to a current may be studied by drawing the magnetic lines of force. A vertical

wire AB is passed through a horizontal cardboard PQRS. Ion filings are sprinkled on the cardboard. Current is passed

through it by connecting a battery to it. Iron filings spread evenly on the cardboard. When a compass needle is placed

on the cardboard, the direction of the needle will show the direction of the magnetic field. The point on the cardboard

where the north pole of the needle is siturated is marked. The needle is shifted a little so that its south pole takes the

same position where the north pole was situated previously. The position of the north pole is marked. If the current is

strong the lines will be circular. The arrows on the circular lines show the direction of the magnetic field.

Magnetic Field Lines Due to Straight Wire

If the direction of the current is reversed, the lines will still be circular, but the directions of the lines will be reversed,

which can be verified using the compass needle.

End

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The direction of the magnetic field around a current carrying conductor can be explained by a simple rule known as

Maxwell's right hand grip rule. If we hold the current carrying wire in our right hand in such a way that the

thumb is stretched along the direction of the current, then the curled fingers give the direction of the

magnetic field produced by the current.

Maxwell's Right Hand Grip Rule

Magnetic Field due to a Solenoid

When a long wire is coiled in the shape of a spring so that the turns are closely spaced and insulated from each other

it forms a solenoid. Generally, a wire is coiled over a non-conducting hollow cylindrical tube. An iron rod is

often inserted inside the hollow tube. This rod is called the core.


The free ends of the solenoid are connected to a battery to pass current through the solenoid. This produces a

magnetic field. The magnetic field inside the coil is almost constant in magnitude and direction. The current

carrying solenoid produces magnetic field similar to that of a bar magnet. One end of the solenoid becomes

the north pole and the other end becomes a south pole.

The magnitude of the field depends on the following factors. The magnetic field is directly proportional to:

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● the amount of current passing through the solenoid

● the number of turns of the solenoid. It also depends on the core material.

Since the magnetic field formed by the solenoid is temporary it is used to make electromagnets. Electromagnets are

used in electric bells, cranes, etc.

Electromagnetic Induction

The process of producing electricity by magnetic field is called electromagnetic induction.

Electric current can also be induced through a wire loop, by moving it near a fixed magnet. So a current is induced

either by moving a magnet near the loop or by moving the loop near a magnet. It is the relative motion

between the two which is important. It does not matter which of the two is moved. Thus the electromagnetic

induction takes place because of the relative motion between a magnet and a coil. The induced current

exists as long as there is a relative motion between the coil and the magnet.

When the magnet is moved faster, then the amount of current induced is found to be higher. Normally moving the

magnets in a linear fashion is difficult. Hence a different arrangement is used.

The figure given below shows a wire loop, a section AB of which lies in a magnetic field. A galvanometer is connected

to the loop.


The wire is directed along south-north direction and the magnetic field is from west to east. When the loop is pulled

up such that the wire AB moves upwards in the field, a current is induced in the loop as shown in the figure.

The direction of the current will be from A to B, i.e., from south to north. If the loop is pushed down

vertically, the direction of the current in the wire will be from B to A.

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Fleming's Right Hand Rule

The direction of the current in a wire moving perpendicular to itself and to a magnetic field may be found by Fleming's

right hand rule. If the thumb, forefinger and middle finger of the right hand are stretched in a mutually

perpendicular direction, in such a way that the forefinger directs towards the magnetic field, the thumb

shows the motion of the wire, then the middle finger shows the direction of the induced current.

So the phenomenon electromagnetic induction paved us the way to generate current without the electrochemical cells. It formed the principle underlying the working of dynamos.

● An electric current produces a magnetic field

● A magnetic field exists in the region surrounding a magnet, in which the force of the magnet can be detected

● Field lines are used to represent a magnetic field

● The magnetic field lines of a straight current are circular with centres on the wire carrying the current

● The magnetic field inside a current carrying solenoid is uniform and parallel to the axis. It behaves like a bar

magnet

● An electromagnet consists of a soft iron core wrapped with an insulated copper wire

● When a current carrying wire is placed in a magnetic field, a force acts on the wire. The direction of force is

given by Fleming's left-hand rule. This is the basis of electric motor

● An electric motor is a device that converts electrical energy into mechanical energy

● The phenomenon by which an emf or current is induced in a conductor due to change in the magnetic field

near the conductor is known as electromagnetic induction

● The direction of the induced current is given by Fleming's right-hand rule. This forms the basis of the electric

generator

● An electric generator is a device that converts mechanical energy into electrical energy

● In our houses we receive AC electric power of 220 V with a frequency of 50 Hz. There are two wires - the live

wire and neutral wire. The potential difference between the two wires is 220 V

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● Earthing and electric fuse are the two commonly used safety measures in electrical circuits. It prevents

electric shock

● Electric fuse is a safety device used for protecting the circuits due to overloading and short-circuiting

● Rules

●

Rules for Determining the Direction of Magnetic Field

The direction of magnetic field around a current carrying conductor can be determined by using one of the laws given

here.

Right Hand Thumb Rule

Imagine the conductor to be held in your right hand with the fingers curled around it. If the thumb points in the

direction of the current, then the curled fingers show the direction of the magnetic field

Maxwell's Cork- Screw Rule

Imagine a right-handed corkscrew being rotated along the wire in the direction of the current. The direction of rotation

of the thumb gives the direction of the magnetic lines of force.

Ampere's Swimming Rule

Ampere's swimming rule states that "if a man swims along the wire carrying current such that his face is always

towards the magnetic needle with current entering his feet and leaving his head then the North Pole of the

magnetic needle is always deflected towards his left hand".

Clock Rule

According to the clock rule "When an observer, looking at the face of the coil, finds the current to be flowing in the

anti-clockwise direction, then the face of the coil will behave like the North Pole. While if the current is in the

clockwise direction, the face of the coil will behave like South Pole.

Fleming's Rule

Fleming's rules help us to predict the movement of a current carrying conductor placed in a magnetic field and the

direction of the induced current.

Fleming's Left Hand Rule

Extend the thumb, forefinger, and the middle finger of the left hand in such a way that all the three are mutually

perpendicular to each another. If the forefinger points in the direction of the magnetic field and the middle

finger in the direction of the current, then, the thumb points in the direction of the force exerted on the

conductor.

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Stretch the forefinger, the middle finger and the thumb of the right hand, such that they are mutually perpendicular to

each other. If forefinger indicates the direction of the magnetic field, the thumb indicates the direction of

motion of the conductor, then, middle finger indicates the direction of induced current in the conductor.

Major progress in understanding magnetism came after the relationship between electricity and magnetism was established by Hans Christian Oersted in 1820. He found that an electric current moves a compass needle and this effect lasts as long as the current flows through the wire. It is then possible to produce magnetism without any magnetic substance at all.

A coil of wire could produce a magnetic field exactly like the field around a permanent magnet.

Magnetic Field

A magnetic field is defined as a region in which a magnetic force is present. In a magnetic field, the magnetic dipole (two equal and oppositely charged or magnetized poles separated by a distance) experiences a turning force, which tends to align it parallel to the direction of the field. The concept of a magnetic field can be understood with the help of the following activity:

Place a piece of cardboard over a magnet

Sprinkle some iron filings onto the cardboard

Tap the cardboard gently and draw what you see

The iron filings show the magnetic field of the magnet

Magnetic Lines of Force Due To Current in a Straight Wire

The direction of the magnetic field due to a current may be studied by drawing the magnetic lines of force. A vertical wire AB is passed through a horizontal cardboard PQRS. Ion filings are sprinkled on the cardboard. Current is passed through it by connecting a battery to it. Iron filings spread evenly on the cardboard. When a compass needle is placed on the cardboard, the direction of the needle will show the direction of the magnetic field. The point on the cardboard where the north pole of the needle is siturated is marked. The needle is shifted a little so that its south pole takes the same position where the north pole was situated previously. The position of the north pole is marked. If the current is strong the lines will be circular. The arrows on the circular lines show the direction of the magnetic field.

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Magnetic Field Lines Due to Straight Wire

If the direction of the current is reversed, the lines will still be circular, but the directions of the lines will be reversed, which can be verified using the compass needle.


The direction of the magnetic field around a current carrying conductor can be explained by a simple rule known as Maxwell's right hand grip rule. If we hold the current carrying wire in our right hand in such a way that the thumb is stretched along the direction of the current, then the curled fingers give the direction of the magnetic field produced by the current.



When a long wire is coiled in the shape of a spring so that the turns are closely spaced and insulated from each other it forms a solenoid. Generally, a wire is coiled over a non-conducting hollow cylindrical tube. An iron rod is often inserted inside the hollow tube. This rod is called

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the core.


The free ends of the solenoid are connected to a battery to pass current through the solenoid. This produces a magnetic field. The magnetic field inside the coil is almost constant in magnitude and direction. The current carrying solenoid produces magnetic field similar to that of a bar magnet. One end of the solenoid becomes the north pole and the other end becomes a south pole.

The magnitude of the field depends on the following factors. The magnetic field is directly proportional to:

the amount of current passing through the solenoid

the number of turns of the solenoid. It also depends on the core material.

Since the magnetic field formed by the solenoid is temporary it is used to make electromagnets. Electromagnets are used in electric bells, cranes, etc.


The process of producing electricity by magnetic field is called electromagnetic induction.

Electric current can also be induced through a wire loop, by moving it near a fixed magnet. So a current is induced either by moving a magnet near the loop or by moving the loop near a magnet. It is the relative motion between the two which is important. It does not matter which of the two is moved. Thus the electromagnetic induction takes place because of the relative motion between a magnet and a coil. The induced current exists as long as there is a relative motion between the coil and the magnet.

When the magnet is moved faster, then the amount of current induced is found to be higher. Normally moving the magnets in a linear fashion is difficult. Hence a different arrangement is

4


used.

The figure given below shows a wire loop, a section AB of which lies in a magnetic field. A galvanometer is connected to the loop.


The wire is directed along south-north direction and the magnetic field is from west to east. When the loop is pulled up such that the wire AB moves upwards in the field, a current is induced in the loop as shown in the figure. The direction of the current will be from A to B, i.e., from south to north. If the loop is pushed down vertically, the direction of the current in the wire will be from B to A.


The direction of the current in a wire moving perpendicular to itself and to a magnetic field may be found by Fleming's right hand rule. If the thumb, forefinger and middle finger of the right hand are stretched in a mutually perpendicular direction, in such a way that the forefinger directs towards the magnetic field, the thumb shows the motion of the wire, then the middle finger shows the direction of the induced current.

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So the phenomenon electromagnetic induction paved us the way to generate current without the electrochemical cells. It formed the principle underlying the working of dynamos.

An electric motor is a device which converts electrical energy into mechanical energy.

It works on the principle that when an electric current is passed through a conductor placed normally in a magnetic field a force acts on the conductor as a result of which the conductor begins to move. The direction of the force is obtained with the help of Fleming's left hand rule.

Construction

The figure below shows the construction of an electric motor. The main parts of an electric motor are:

D.C. Motor

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the armature coil ABCD mounted on an axle

the commutator that is a split ring divided in two parts S1 and S2

a pair of brushes B1 and B2

a horse - shoe electromagnet

The coil ABCD is wound round a soft iron and is placed in between the pole pieces of a powerful horse - shoe magnet. The coil is free to rotate about its axis. The ends of the coil A and D are connected to split parts of the ring S1 and S2 respectively. Two brushes B1 and B2, made of carbon or copper, touch the split rings S1 and S2 respectively. A dc source is connected across the brushes B1 and B2. When the coil rotates, the split rings rotate but the brushes do not move.

A wheel can be mounted on the axle placed along the axis of the coil so as to drive the desired parts of the machine such as electric fan, washing machine etc. where the motor is used.

Working

The plane of the coil is horizontal and the split ring S1 touches the brush B1 while the split ring S2 touches the brush B2. The brush B1 is connected to the anode of the d.c. battery while the brush B2 is connected to the cathode. The current flows in the coil in the direction ABCD. The arms BC and DA being parallel to the magnetic field experience no force.

According to Fleming's left hand rule force 'F' acting on the arm AB, is inward and perpendicular to the plane of the coil and the force on the arm CD is in just in the opposite direction. The forces on the arms AB and CD being equal and opposite form an anticlockwise couple, due to which the coil begins to rotate. It rotates in such a way that the arm AB goes in and the arm CD comes out.

When the coil reaches the vertical position, the couple becomes zero since the forces on the arms now become collinear. But due to the inertia of motion, the coil does not stop in this position. As the coil passes from the vertical the split ring S1 comes in contact with the brush B2, while the split ring S2 comes in contact with the brush B1. Now the current flows through the coil in the direction DCBA and the forces acting on the arms DC and AB of the coil again form an anticlockwise, couple due to which the coil remains rotating in the same direction. Thus, whenever the coil comes in the vertical position, the direction of the current through the coil reverses and the coil continues to rotate in the same direction.

The deflecting couple on the coil is maximum when the plane of the coil is parallel to the direction of the magnetic field and the deflecting couple is minimum when the plane of the coil is perpendicular to the magnet field.

The speed of rotation of the coil depends on the deflecting couple acting on the coil. Hence the

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speed of rotation of the coil can be increased by,

increasing the number of turns of the coil

increasing the strength of the current

increasing the area of the coil

increasing the strength of the magnetic field.

The electric generator is a machine for producing electric current. The electric generator or dynamo converts mechanical energy into electrical energy.

DC Generator

Principle

The generator is an application of electromagnetic induction. It works on the principle that when a wire is moved in a magnetic field, then the current is induced in the coil. A rectangular coil is made to rotate rapidly in the magnetic field between the poles of a horse shoe type magnet. When the coil rotates, it cuts the lines of magnetic force, due to which a current is produced in the generator coil. This current can be used to run the various electrical appliances.

Construction

A simple D.C. generator consists of a rectangular coil ABCD which can be rotated rapidly between the poles N and S of a strong horse-shoe type magnet M. The generator coil is made of a large number of turns of insulated copper wire. The two ends of the coil are connected to the two

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copper half rings (or split rings) R1 and R2 of a commutator. There are two carbon brushes B1 and B2 which press lightly against the two half rings. When the coil is rotated, the two half rings R1 and R2 touch the two carbon brushes B1 and B2 one by one. So the current produced in the rotating coil can be tapped out through the commutator half rings and into the carbon brushes. From the carbon brushes B1 and B2 we can supply current into various electrical appliances like radio, television, electric bulb etc.

Working

Let us suppose that the generator coil ABCD is initially in the horizontal position. As the coil rotates in the anticlockwise direction between the pole N and S of the magnet the side AB of the coil moves down cutting the magnetic lines of force near the N-pole of the magnet and side DC moves up, cutting the lines of force near the S-pole of the magnet. Due to this, induced current is produced in the sides AB and DC of the coil. On applying Fleming's right hand rule to the sides AB and DC of the coil we find that the currents in them are in the directions B to A and D to C respectively. Thus the induced currents in the two sides of the coil are in the same direction and we get an effective induced current in the direction BADC. Due to this the brush B1 becomes the positive pole and brush B2 becomes the negative pole of the generator.

After half revolution, the sides AB and DC of the coil will interchange their positions. The side AB will come on the right hand side and starts moving up whereas side DC will come on the left hand side and start moving down. But when sides of the coil interchange their positions, then the two commutator half rings R1 and R2 automatically change their contacts from one carbon brush to the other. Due to this change, the current keeps flowing in the same direction. Thus a DC generator supplies a current only in one direction.

If the current flows always in the same direction, it is called 'direct current'. Direction current is represented as DC or dc. The current derived from a cell or a battery is direct current - since it is unidirectional. The positive and negative terminals are fixed. If the current changes direction after equal intervals of time, it is called alternating current. Alternating current can be written as AC or ac. Most of the power stations generate alternating current. The following are the circuit elements representing dc and ac.

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The difference can be learnt by drawing voltage-time graph.

The DC sources are described only in turns of the steady voltage and AC sources are described with maximum voltage and the frequency with which the voltage varies.

Magnetic Field Basics Magnetic fields are different from electric fields. Although both types of fields are

interconnected, they do different things. The idea of magnetic field lines and magnetic fields was first examined by Michael Faraday and later by James Clerk Maxwell. Both of these English scientists made great discoveries in the field of electromagnetism.

Magnetic fields are areas where an object exhibits a magnetic influence. The fields affect neighboring objects along things called magnetic field lines. A magnetic object can attract or push away another magnetic object. You also need to remember that magnetic forces are NOT related to gravity. The amount of gravity is based on an object's mass, while magnetic strength is based on the material that the object is made of.

If you place an object in a magnetic field, it will be affected, and the effect will happen along field lines. Many classroom experiments watch small pieces of iron (Fe) line up around magnets along the field lines. Magnetic poles are the points where the magnetic field lines begin and end. Field lines converge or come together at the poles. You have probably heard of the poles of the Earth. Those poles are places where our planets field lines come together. We call those poles north and south because that's where they're located on Earth. All magnetic objects have field lines and poles. It can be as small as an atom or as large as a star.

Attracted and RepulsedYou know about charged particles. There are positive and negative charges. You also

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know that positive charges are attracted to negative charges. A French scientist named Andre-Marie Ampere studied the relationship between electricity and magnetism. He discovered that magnetic fields are produced by moving charges (current). And moving charges are affected by magnets. Stationary charges, on the other hand, do not

produce magnetic fields, and are not affected by magnets. Two wires, with current flowing, when placed next to each other, may attract or repel like two magnets. It all has to do with moving charges.

Earth's Magnetic Field

Magnets are simple examples of natural magnetic fields. But guess what? The Earth has a huge magnetic field. Because the core of our planet is filled with molten iron (Fe), there is a large field that protects the Earth from space radiation and particles such as the solar wind. When you look at tiny magnets, they are working in a similar way. The magnet has a field around it.

As noted earlier, current in wires produces a magnetic effect. You can increase the strength of that magnetic field by increasing the current through the wire. We can use this principle to make artificial, adjustable magnets called electromagnets, by making coils of wire, and then passing current through the coils. Flowing Electrons

Electric current is very similar to a flowing river. The river flows from one spot to another and the speed it moves is the speed of the current. The size of the current flow is related more to the size of the river than it is to the speed of the river. A river carries more water each second than a stream, even if both flow at the same speed. With electricity, current is a measure of the amount of charge transferred over a period of time. Current is a flow of electrons, or individual negative

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charges. When charge flows, it carries energy that can be used to do work. Scientists measure current with units called amperes.

Current and HeatOne of the results of current is the heating of the conductor. When an electric stove

heats up, it's because of the flow of current. The electrons have a mass (however small), and when they move through the conductor, there are collisions that produce heat. The more electrons bumping into the atoms of the conductor, the more heat is created, so higher current generally means greater heat.

Scientists used to think that the flow of current always heated up the object, but with modern superconductors, that is not always true, or at least not as true as with normal materials. Superconducting materials seem to have less interaction between atoms and current, so the moving charges lose much less energy.

Spaces Between AtomsEverything that is matter can conduct electricity, but not everything does it well.

Scientists use the terms conductors, insulators, and semi-conductors. The labels are used to describe how easily energy is transferred through the object by moving charge. The spaces between the atoms, as well as the type of atoms, determines whether an object a good conductor or a good insulator (poor conductor).

Usable Current

There are two main kinds of electric current, direct current (DC) and alternating current (AC). They are easy to remember. Direct current is a flow of charge always in one direction. Alternating current is a flow of charge back and forth, changing its direction many times in one second. Batteries produce DC current, while the outlets in our homes use AC current.

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Be very careful if you work with electricity. NEVER touch the plugs in your house. That

electricity is very powerful and it can hurt you… badly. Electricity from batteries can also injure you. We have burned ourselves when working with batteries and electromagnets, so we know what can happen. To be safe, go get an adult to help you with any experiments.

Resisting CurrentThe collisions between electrons and atoms in a conductor cause resistance to the flow

of charge. We measure that resistance in order to determine the effect that it will have on current. Scientists measure resistance in ohms (rhymes with homes). There is a magical little formula used to figure out the resistance in an electrical system. That formula is called Ohm's Law, V=IR.

Measuring ResistanceThe symbol "V" is used to represent something called the potential difference.

Potential difference is the amount of work done in moving a charge between two points, divided by the size of the charge. That's kind of complicated, though. You can think of potential as electrical height. High potential (near positive charge) is kind of like being on top of a hill. Low potential (near negative charge) is kind of like being in a valley. So potential difference indicates the difference in electrical height between two points. The greater that difference, the more likely it is that charge will move. The potential difference is measured in volts, and potential is commonly referred to as voltage. "I" is the symbol for current and "R" is the symbol for the resistance of the system. Current is measured in amperes and resistance is measured in ohms.

How can you think of resistance? Have you ever gone to a baseball game? Between innings, we like going to get some food. There are always people between the counter and us. Resistance to current is similar to you trying to make your way through the crowds to get your hot dog. You have to weave your way through the people to reach your goal. The more people in your way, the more resistance. If everyone is in their seats it is super-easy to get your food. There would be very little resistance.

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Let's go back to that equation and look at it in terms of resistance. When you move the values around you get

R=V/I. In English that means the resistance of a system is based on voltage and current. Not all conductors follow Ohm's law.

Resistance is also based on the resistivity of a material. The resistivity of a material changes because of chemical makeup or the temperature. Copper is a better conductor than wood so copper would have lower resistivity. That resistivity combines with (1) the distance and (2) the space that charges have to move in (thin vs. thick wires) to affect the "R" value. Greater length results in more resistance, and thick wires result in less. When people connect speakers,

they usually use wires that are as short and thick as possible.

Knocking Electrons AroundIn metals, electrons carry the charges of the current as it flows.

What stops the electrons? What offers the resistance to that current? Nothing allows a perfect flow of current, not even superconductors. In metal, there are tiny flaws. You can't see them because they are on a molecular level. Those

imperfections cause the electrons to collide with the metal atoms. When they hit the metal, the electrons lose energy. Where does that energy go? It is usually turned into heat. You can watch a hot plate heat up, or maybe a stove top. They heat up because of the collisions between electrons and the metal. Imperfections mean collisions; collisions mean heat.

Faraday BasicsFaraday's law of induction is one of the important concepts of electricity. It looks at the

way changing magnetic fields can cause current to flow in wires. Basically, it is a formula/concept that describes how potential difference (voltage difference) is created and how much is created. It's a huge concept to understand that the changing of a magnetic field can create voltage.

Faraday's WorkMichael Faraday was an English physicist working in the early 1800's. He worked with

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another scientist named Sir Humphrey Davy. Faraday's big discovery happened in 1831 when he found that when you change a magnetic field, you can create an electric current. He did a lot of other work with electricity such as making generators and experimenting with electrochemistry and electrolysis.

Faraday's experiments started with magnetic fields that stayed the same. That setup did not induce current. It was only when he started to change the magnetic fields that the current and voltage were induced (created). He discovered that the changes in the magnetic field and the size of the field were related to the amount of current created. Scientists also use the term magnetic flux. Magnetic flux is a value that is the strength of the magnetic field multiplied by the surface area of the device.

Faraday's LawYou're going to have to review your Greek letters when you memorize the real formula.

Here are the basics...

E=dB/dt

"E" is the value of voltage induced (the old name for voltage was "ElectroMotive Force", or EMF. That's the "E" in the equation). The change in time for the experiment is "dt". Time is measured in seconds. Last is "dB" which stands for the change in magnetic flux. The magnetic flux is the field lines of the magnetic field. The flux is equal to BA, where B is the magnetic field strength, and A is the area. This formula is a bit harder than those you may have seen before.

In English: the amount of voltage created is equal to the change in magnetic flux divided by the change in time. The bigger the change you have in the magnetic field, the

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greater amount of voltage. Coulomb Basics

Coulomb's Law is one of the basic ideas of electricity in physics. The law looks at the forces created between two charged objects. As distance increases, the forces and electric fields decrease. This simple idea was converted into a relatively simple formula. The force between the objects can be positive or negative depending on whether the objects are attracted to each other or repelled.

Think about a few concepts before you continue reading. Some charges are attracted to each other. Positive and negative charges like to move towards each other. Similar charges such as two positive or two negative push away from each other. You also need to understand that forces between objects become stronger as they move together and weaker as they move apart. You could yell at someone from far away, and they would barely hear you. If you

yelled the same amount when you were together, it would be more powerful and loud.

Coulomb's WorkCharles Augustin de Coulomb was a French scientist working in

the late 1700's. A little earlier, a British scientist named Henry Cavendish came up with similar ideas. Coulomb received most of the credit for the work on electric forces because Cavendish did not publish all of his work. The

world never knew about Cavendish's work until decades after he died.

Coulomb's LawBut you're here to learn about the law. When you have two charged particles, an

electric force is created. If you have larger charges, the forces will be larger. If you use those two ideas, and add the fact that charges can attract and repel each other you will understand Coulomb's Law. It's a formula that measures the electrical forces between two objects.

F=kq1q2/r2

"F" is the resulting force between the two charges. The distance between the two charges is "r." The "r" actually stands for "radius of separation" but you just

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need to know it is a distance. The "q1" and "q2" are values for the amount of charge in each of the particles. Scientists use Coulombs as units to measure charge. The constant of

the equation is "k." As you learn more physics, you will see that this formula is very similar to a formula from Newton's work with gravity.

What is a Magnet? A magnet is an object or a device that gives off an external magnetic field. Basically, it

applies a force over a distance on other magnets, electrical currents, beams of charge, circuits, or magnetic materials. Magnetism can even be caused by electrical currents.

While you might think of metal magnets such as the ones you use in class, there are many different types of magnetic materials. Iron (Fe) is an easy material to use. Other elements such as neodymium (Nd) and samarium (Sm) are also used in magnets. Neodymium magnets are some of the strongest on Earth.

Different Types of MagnetsThere are many different types of magnets. Permanent magnets never lose their

magnetism. There are materials in the world that are called ferromagnetic. Those materials are able to create and hold a specific alignment of their atoms. Since many atoms have a magnetic moment (tiny magnetic field), all of the moments can add up to create a magnet. Scientists use the word hysteresis to describe the way the atoms stay aligned.

Most of the magnets you see around you are man-made. Since they weren't originally magnetic, they lose their magnetic characteristics over time. Dropping them, for example, weakens their magnetism; as does heating them, or hammering on them, etc.

There are also air-core magnets. Air-core magnets are created by current flowing through a wire. That current produces the magnetic field. You create an air-core magnet by wrapping miles of wire around in a doughnut shape (toroid). When you send current through the wire, a magnetic field is created inside of the doughnut. Scientists sometimes use air-core magnets to study fusion reactions.

Electromagnets are different because they have a ferromagnetic material

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(usually iron or steel) located inside of the coils of wire. The core isn't air, it is something that aids in producing magnetic effects, so electromagnets are typically stronger than a

comparable air-core magnet. Air-core and electromagnets can be turned on and off. They both depend on currents of electricity to give them magnetic characteristics. Not only can they be turned on and off, but they can also be made much stronger than ordinary magnets. You might see an electromagnet at work in a junkyard lifting old cars off the ground

A Direct CurrentThere are two main types of current in our world. One is direct current (DC) which is a

constant stream of charges in one direction. The other is alternating current (AC) that is a stream of charges that reverses direction. Let's look at DC power which was refined by Thomas Edison in the 1800s.

Moving in One DirectionThe current in DC circuits is moving in a constant direction. The amount of current can

change, but it will always flow from one point to another. Before we move on, we need to explain that physicists, as well as electricians, refer to something called conventional current.

Do you remember that we talked about physicists agreeing to always use positive charges to determine how electric field lines would be drawn? Following through on that agreement, they also agreed to explain charge flow in terms of positive charges rather than electrons. So although electrons would flow from negative to positive, by convention (agreement), physicists refer to conventional current as a flow from high potential/voltage (positive) to low potential/voltage (negative). Reminding you that potential is like electrical height, this means that conventional current flows "downhill", which makes sense.

Electrons move from areas where there are excess of negative charges to areas where there are a deficiency (or positive charge). Electrons move from "-" to "+", but conventional current is considered to move in the other direction. When you set up a circuit, conventional current is considered to move from the "+" to the "-" side.

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The idea about using positive charges in forming explanations comes from Benjamin Franklin. In Franklin's day, we didn't know about protons and electrons. Franklin believed that something moved through electrical wires, and he called these things "charge". He assumed there was only one kind of charge, and he logically assumed that charge would flow from a spot that had an excess

(extra), to a spot that had a deficiency (too few). He called the spot with an excess "positive" and the spot with a deficiency "negative". So, for Franklin, charge flowed from positive to negative. We simply honor his

achievements by continuing with this idea.

MagnetismThe generation of electric power depends on Magnetism or the principles of Magnets. Most of us have seen a Magnet's ability to attract certain metals, (i.e. Iron). Any material that can attract metals is called a "magnet." The attractive ability of these materials is called "magnetic force." Certain specimens of Iron Ore possess this attracting property when they are taken from the earth. One name for this material is "magnetite" or "lodestones."

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MagnetsThe basic atomic structure of a Magnet seems to align most of the molecules in the same direction. It is possible to see this force through a simple experiment:

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Bar Magnet Experiment

Put a Bar Magnet under a sheet of glass and sprinkle Iron Filings on the glass. The "lines of force" from the Magnet show up clearly as the Filings form a pattern. Notice that the attractive forces are greatest at the two "ends" of the Magnet, where the majority of Filings gather. We call these "ends" "poles."

The density of the pattern represents the strength of the field, which is the magnitude of the force exerted upon a magnetic material placed at the point in the field. These lines are called "lines of magnetic flux."

Magnet PoleExperime

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If we suspend a Magnet by a string from its center so that it is free to turn, it will turn until the Axis lines up with its Poles. The Pole which points north is called the "north pole" and the other is called the "south pole." These are usually designated by an N and S marked on the Magnets.

Laws of AttractionExperiment

Let's add another Magnet to our experiment, and we shall notice another key property of Magnets.

The "like" Poles will repel one another; while the "unlike" Poles will attract one another. This is a very important principle since the generation of electric power depends on these Laws of Attraction.

Almost all commercially available Magnets are artificial. They were manufactured to be Magnets by using other Magnets to create the correct molecular alignment.

There are two types of Magnets: "temporary" and "permanent." Temporary Magnets are those that will hold their Magnetism only as long as the magnetizing force is maintained. These are usually found inside Motors.

Permanent Magnets are those that will hold their Magnetism after the

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magnetizing force has been removed and will continue to be Magnets for as long as they are not disturbed by being jarred or heated.

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Electromagnetic FieldsThe flow of electricity through a Conductor produces both an

electric and magnetic field around the Conductor. Collectively, these two fields are referred to as an "electromagnetic field" (EMF). The strength of the Electric Field is measured in volts per meter and varies with the amount of the source voltage. The higher the source voltage, the higher the strength of the field. Electric Field strength decreases rapidly with distance from the "source."

Electric Field

Electric Fields are produced both naturally and by any Conductor carrying electricity. The strength of the earth's natural Electric Field varies, but on average is about one-thousandth of a volt per meter. Electric Field strength typically varies from 10-to-150 volts per meter under Electric Distribution Lines and 5-to-100 volts per meter inside homes and workplaces.

The strength of a Magnetic Field is typically measured in units of "gauss" or "milligauss" and varies with the amount of current moving through a Conductor. Lines or devices requiring high levels of current flow produce stronger Magnetic Fields than those with low current flow. For example, the measure of a Magnetic Field directly under a high-voltage Transmission Line is somewhere between 20-to-650 milligauss.

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The Magnetic Field measured underneath a lower-power Distribution Line is .5-to-30 milligauss.

Magnetic Fields produced by Electrical Circuits drop off rapidly with distance from the "source." The Magnetic Field produced by a Microwave at 1 foot is 70-to-100 milligauss while at five feet away, the Magnetic Field strength drops to five milligauss.

"Shielding" (walls, houses, trees, other vegetation, soil, and other large dense objects) blocks Electric Fields. Magnetic Fields, on the other hand, pass easily through most objects and are only blocked by structures containing large amounts of Iron or Iron-alloy metals.

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Electromagnets

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Electromagnets play an essential role in the operation of Generators, Motors, Transformers, and Relays. Wrapping an insulated Conductor Wire around an Iron object (i.e. large nail), and then passing an electrical current through the Wire, construct Electromagnets. The strength of the Electromagnet depends on the number of "wraps", the size of the Wire, and the amount of current flowing through the Wire.

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Magnetic Induction PrinciplesMichael Faraday discovered in 1831 that if a "coil" of

Copper Wire is rotated in a Magnetic Field in such a way as to cut across the "lines of magnetic force," an electric charge is created or induced in the Wires. This is the basic principle by which practically all our present day electric current is generated.

Generators use "magnetic induction" to produce electrical energy. Moving Wires through a Magnetic Field generates electrical current. The Wire "loop" inside the Generator is mechanically driven by some source of rotary motion. The source of power for the rotation might be fossil fuels, falling water, or nuclear energy. As the Wire loop spins inside the Magnetic Field, an electric current is produced in the Wire. This current becomes the basis for commercially available electrical energy.

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Which of the following is NOT a property of a Magnetic Field?

A Strength

B Weight

C Direction

D Poles of Polarity

The highest level of an Electromagnetic Field (EMF) is produced by?

A Toaster

B Air Conditioner

C Low-Voltage Distribution Line

D High-Voltage Transmission Line

Which of the following does NOT have an effect on the strength of a Magnetic Field from an Electromagnet?

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A Number of Wire "wraps" or Coils

B Size of Wire used for "wraps" or Coils

C Amount of Current flowing through "wraps" or Coils

D Type of Insulation material used on the "wraps" or Coils

Which of the following describes the principle behind the operation of a modern day Electrical Generator?

A Magnetic Induction

B Friction Resistance

C Chemical Action

D Static Action

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