Electricity

All of us agree the importance of electricity in our daily lives.

But what is electricity?

Electric Charge and Electrical Forces:

Electrons have a negative electrical charge.

Protons have a positive electrical charge.

These charges interact to create an electrical force.

Like charges produce repulsive forces – so they repel each other (e.g. electron and electron or proton and proton repel each other).

Unlike charges produce attractive forces – so they attract each other (e.g. electron and proton attract each other).

Electrostatic Charge:

Electrons move from atom to atom to create ions.positively charge ions result from the loss of electrons

and are called cations.Negatively charge ions result from the gain of electrons

and are called anions.

The charge on an ion is called an electrostatic charge.

An object becomes electrostatically charged byFriction,which transfers electrons between two objects

in contact,Contact with a charged body which results in the

transfer of electrons,Induction which produces a charge redistribution of

electrons in a material.

Arbitrary numbers of protons (+) and electrons (-) on a comb and in hair (A) before and (B) after combing.

Combing transfers electrons from the hair to the comb by friction, resulting in a negative charge on the comb and a positive charge on the hair.

Chargingby

Induction

Electrical Conductors and Insulators:

Electrical conductors are materials that allows free movement of electrons inside Metals are good conductors of electricity. Silver

is the best electrical conductor.

Electrical nonconductors (insulators) are materials that do not allow movement of electrons easily.Examples are wood, rubber etc.

Semiconductors are materials whose conductivity lies in between those of conductors and insulators. Examples are silicon, arsenic, germanium.

Measuring Electrical Charges:

The fundamental charge is the electrical charge on an electron and has a magnitude of 1.6021892 X 10-19 C

The electrical charge (q) is a discrete quantity and it is always measured as

q = n ewhere e is the fundamental charge.

Conservation of charge is a fundamental principle which states that charge can neither be created or destroyed but can only move from one atom to another.

Coulomb’s law: Electrical force is directly proportional to the

product of the electrical charges and inversely proportional to the square of the distance. This is known as Coulomb’s law.Mathematically,

where, F is the electrical force, k is a constant and has the value of 9.00 x 109

Newtonmeters2/coulomb2 (9.00 x 10 9 Nm2/C2), q1 represents the electrical charge of object 1 and q2

represents the electrical charge of object 2, and d is the distance between the two objects. Electrical force is a VECTOR quantity and is directed

along the line of action

2

21

d

qqkF

Force Fields:

The configuration of space around an object is changed by the presence of an electrical charge.

The electrical charge produces a force field, called as electrical field

Coulomb’s Law: | F | = k | Q qo | / r2

Rearranged: | F | = | qo [k Q/r2] |

Gives us:F = qo E

where the electric field E is:

| E | = | k Q / r2 |

A map of the electrical field can be created by bringing a positive test charge into an electrical field.

When brought near a negative charge the test charge is attracted to the unlike charge and when brought near a positive charge the test charge is repelled.

You can draw vector arrows to indicate the direction of the electrical field.

This is represented by drawing lines of force or electrical field lines,These lines are closer together when the field is

stronger and farther apart when it is weaker.

A positive test charge is used by convention to identify the properties of an electric field. The vector arrow points in the direction of the force that the test charge would experience.

Electric Lines of force diagram for (A) a negative charge and (B) a positive charge when the charges have the same magnitude as the test charge.

Electrical Potential:An electrical charge has an electrical field that

surrounds it.In order to move a second charge through this field

work must be done.Bringing a like charge particle into this field will

require work since like charges repel each other and bringing an opposite charged particle into the field will require work to keep the charges separated.In both of these cases the electrical potential is

changed.

The potential difference (PD) that is created by doing 1.00 joule of work in moving 1.00 coulomb of charge is defined as 1.00 volt.A volt is a measure of the potential difference

between two points,electric potential = work done,

charge Or, PD=W QThe voltage of an electrical charge is the energy

transfer per coulomb.The energy transfer can be measured by the work

that is done to move the charge or by the work that the charge can do because of the position of the field.

ELECTRIC CURRENT:

Electric current means the flow of charges which is analogous to water flow

It is the charge that flows, and the current is defined as the flow of the charge.

An electrical circuit contains some device that acts as a source of energy as it gives charges a higher potential against an electrical field.

The charges do work as they flow through the circuit to a lower potential.

The charges flow through connecting wires to make a continuous path.

A switch is a means of interrupting or completing the circuit.

The source of the electrical potential is the voltage source.

A simple electric circuit has a voltage source (such as a generator or battery) that maintains the electrical potential, some device (such as a lamp or motor ) where work is done by the potential, and continuous pathways for the current to flow.

Voltage is a measure of the potential difference between two places in a circuit.Voltage is measured in joules/coloumb.

The rate at which an electrical current (I) flows is the charge (q) that moves through a cross section of a conductor in a give unit of time (t),

I = q/t.

the units of current are coulombs/second.A coulomb/second is an ampere (amp).

What is the nature of the electric current carried by these conducting lines? It is an electric field that moves at near the speed of light. The field causes a net motion of electrons that constitutes a flow of charge, a current.

(A) A metal conductor without a current has immovable positive ions surrounded by a swarm of randomly moving electrons.

(B) An electric field causes the electrons to shift positions, creating a separation charge as the electrons move with a zigzag motion from collisions with stationary positive ions and other electrons.

Electrical Resistance:

Electrical resistance is the resistance to movement of electrons being accelerated with an energy loss. Materials have the property of reducing a current and

that is electrical resistance (R).Resistance is a ratio between the potential difference (V)

between two points and the resulting current (I).R = V/I

The ratio of volts/amp is called an ohm ().The relationship between voltage, current, and

resistance is:V =I R

This is known as Ohms Law.

The magnitude of the electrical resistance of a conductor depends on four variables: The length of the conductor. The cross-sectional area of the conductor. The material the conductor is made of. The temperature of the conductor.

Resistors in SeriesResistors can be connected in series; that is, the

current flows through them one after another. The circuit here shows three resistors connected in series, and the direction of current is indicated by the arrow.

Note that since there is only one path for the current to travel, the current through each of the resistors is the same.

I1= I2 = I3

Also, the voltage drops across the resistors must add up to the total voltage supplied by the battery:

V total = V1+V2+V3

R equivalent = R1 + R2 + R3

Resistors in Parallel Resistors can be connected such that they branch out

from a single point (known as a node), and join up again somewhere else in the circuit. This is known as a parallel connection. Each of the three resistors in the figure below is another path for current to travel between points A and B.

At A the potential must be the same for each resistor. Similarly, at B the potential must also be the same for each resistor.

So, between points A and B, the potential difference is the same. That is, each of the three resistors in the parallel circuit must have the same voltage.

V1 =V2 = V3Also, the current splits as it travels from A to B.

So, the sum of the currents through the three branches is the same as the current at A and at B (where the currents from the branch reunite).

I = I1 +I2 + I3

Electrical Power and Electrical Work:

All electrical circuits have three parts in common.A voltage source.An electrical deviceConducting wires.

The work done (W) by a voltage source is equal to the work done by the electrical field in an electrical device,

Work = Power x Time.The electrical potential is measured in

joules/coulomb and a quantity of charge is measured in coulombs, so the electrical work is measure in joules.

A joule/second is a unit of power called the watt.Power = current x potential

Or, P = I V

This meter measures the amount of electric work done in the circuits, usually over a time period of a month. The work is measured in kWhr.

Magnetism

All of us are familiar with magnets. In a magnet we have magnetic poles – the north and the south pole.

A North seeking pole is called the North Pole.A South seeking pole is called the South Pole.

Like magnetic poles repel and unlike magnetic poles attract.

Every magnet has ends, or poles, about which the magnetic properties seem to be concentrated. As this photo shows, more iron filings are attracted to the poles, revealing their location.

Magnetic Fields:

A magnet that is moved in space near a second magnet experiences a magnetic field.

A magnetic field can be represented by field lines.

The strength of the magnetic field is greater where the lines are closer together and weaker where they are farther apart.

These lines are a map of the magnetic field around a bar magnet. The needle of a magnetic compass will follow the lines, with the north end showing the direction of the field.

The Source of Magnetic Fields:

Permanent Magnets:Moving electrons produce magnetic fields.

In most materials these magnetic fields cancel one another and neutralize the overall magnetic effect.

In other materials such as iron, cobalt, and nickel, the atoms behave as tiny magnets because of certain orientations of the electrons inside the atom.

These atoms are grouped in a tiny region called the magnetic domain.

Our Earth is a big magnet.

The Earth’s magnetic field is thought to originate with moving charges.

The core is probably composed of iron and nickel, which flows as the Earth rotates, creating electrical currents that result in the Earth’s magnetic field.

The earth's magnetic field. Note that the magnetic north pole and the geographic North Pole are not in the same place. Note also that the magnetic north pole acts as if the south pole of a huge bar magnet were inside the earth. You know that it must be a magnetic south pole since the north end of a magnetic compass is attracted to it and opposite poles attract.

A bar magnet cut into halves always makes new, complete magnets with both a north and a south pole. The poles always come in pairs. You can not separate a pair into single poles.

Electric Currents

and

Magnetism

Oersted discovered that a compass needle below a wire (A) pointed north when there was not a current, (B) moved at right angles when a current flowed one way, and (C) moved at right angles in the opposite direction when the current was reversed.

(A) In a piece of iron, the magnetic domains have random arrangement that cancels any overall magnetic effect (not magnetic). (B) When an external magnetic field is applied to the iron, the magnetic domains are realigned, and those parallel to the field grow in size at the expense of the other domains, and the iron becomes magnetized.

A magnetic compass shows the presence and direction of the magnetic field around a straight length of current-carrying wire.

When a current is run through a cylindrical coil of wire, a solenoid, it produces a magnetic field like the magnetic field of a bar magnet. The solenoid is known as electromagnet.

Applications of Electromagnets:

Electric Meters:

The strength of the magnetic field produced by an electromagnet is proportional to the electric current in the electromagnet.

A galvanometer measures electrical current by measuring the magnetic field.

A galvanometer can measure current, potential difference, and resistance.

A galvanometer measures the direction and relative strength of an electric current from the magnetic field it produces. A coil of wire wrapped around an iron core becomes an electromagnet that rotates in the field of a permanent magnet. The rotation moves pointer on a scale.

Electric Motors:An electrical motor is an electromagnetic device

that converts electrical energy into mechanical energy.

A motor has two working parts - a stationary magnet called a field magnet and a cylindrical, movable electromagnet called an armature.

The armature is on an axle and rotates in the magnetic field of the field magnet.

The axle is used to do work.

Electromagnetic Induction

Induced Current:If a loop of wire is moved in a magnetic field a

voltage is induced in the wire.The voltage is called an induced voltage and the

resulting current is called an induced current.The induction is called electromagnetic

induction.

A current is induced in a coil of wire moved through a magnetic field. The direction of the current depends on the direction of motion.

The magnitude of the induced voltage is proportional to:

The number of wire loops cutting across the magnetic field lines.

The strength of the magnetic field.

The rate at which magnetic field lines are cut by the wire.

Applications:DC and AC Generators,Transformers (step-up and step-down).

Electric currentscreate

magnetic fields.

Charges movingin a magnetic field

experience anelectromagnetic force.

A changing magnetic fieldcreates an electric field.

Induced electric currentsact so as to oppose the

motion that caused them.

FAITH ACADEMY, NEW DELHI-5. YEAR: 2008-09