Magnets around the House:Headphones Refrigerator magnets Computer speakers Telephone receiversPhone ringers Microwave tubesSeal around refrigerator doorFloppy disk recording and reading headAudio tape recording and playback headVideo tape recording and playback headCredit card magnetic strip TV deflection coilComputer monitor Computer hard drive Power supply transformers

Magnets in your Car:Starter motorA/C clutchInterior fan motorElectric door locksWindshield wiper motorElectric window motorSide-view mirror adjuster motorCD/tape player motor and playbackEngine speed sensorsAlternatorStarter relayWindshield washer pump motor

Uses of magnets

• Magnets were first put to use in navigation because they always point north and south.

What is Magnetism?What is Magnetism?

Magnetism is the force of attraction or repulsion of magnetic material due to the arrangement of its electrons.

lab

• Magnets stick to some things but not to others.

• Find different things to test with your magnet.• What is your conclusion?

Magnets

– not all objects are affected by the force of magnetism• ex. wood, glass, paper, plastic

– common metals affected by magnetism are iron, nickel, and cobalt

Poles of a magnet alwaysCome in pairs!

Magnetic Poles– poles – two ends of a magnet. every magnet

has two poles: north (N) pole and south (S) pole

N S

N S N S

Break one bar magnet in half, and you have two smaller bar magnets!

Properties of Magnets

• if a north pole and a south pole are brought together, they will attract each other

– opposites attract

Properties of Magnets

• if the north pole of one magnet is brought near the north pole of another magnet, they will repel each other

• if two south poles are brought together, they will repel each other

• Ferromagnetic materials and magnetisation

• Use 2 paper clips. Put the paper clips close together and observe what happens.

What happens to the paper clips?• Are the paper clips magnetic?• Now take a permanent bar magnet and rub it

once along 1 of the paper clips.• Remove the magnet and put the paper clip

which was touched by the magnet close to the other paper clip and observe what happens.

• A magnetic field is a region in space where a magnet or object made of magnetic material will experience a non-contact force.

Investigation : Field around a Bar Magnet• Take a bar magnet and place it on a flat

surface. Place a sheet of white paper over the bar magnet and sprinkle some iron filings onto the paper.

• Give the paper a shake to evenly distribute the iron filings.

• In your workbook, draw the bar magnet and the pattern formed by the iron filings.

Investigation : Field around a Pair of Bar Magnets

• Take two bar magnets and place them a short distance apart such that they are repelling each other.

• Place a sheet of white paper over the bar magnets and sprinkle some iron filings onto the paper.

• Give the paper a shake to evenly distribute the iron filings.

• In your workbook, draw both the bar magnets and the pattern formed by the iron filings.

• Repeat the procedure for two bar magnets attracting each other and draw what the pattern looks like for this situation.

Like repels like…Like repels like…

Opposites attract!Opposites attract!

• Take a magnet and stroke it along the nail in ONE DIRECTION ONLY from thick to pointed end.

• Repeat this about 10 times.• Now try to pick up the paper clip with the nail.

Demonstration

• Place two magnets with opposite poles facing each other in a test tube.

• Note what happens.

lab• Use compass to determine the north,

south, east and west directions.• Taks a dry needle and Magnetize it by

stroking with a magnet.• Drop the needle in the container of water• The needle won't sink if you drop it

carefully. • In what direction does it point?

• Hold magnet above the water with the needle in the water.

• Vary the distance of the magnet above the water and move it around

• Observe what happens to the needle.• Repeat using the magnet above a compass.• Repeat using a paper clip.

• If you dip a bar magnet into a cup of nails, nails will stick to it. But exactly, why? You know that magnets attract iron, but then you also note, some nails stick to other nails. Why?

• Ordinary iron turned into a magnet whenever it touched another magnet.

• allow two small nails to attach themselves to adjacent spots on one of the poles of a magnet, with your fingers holding them parallel to each other.

• Both nails are now temporary magnets with the same polarity--say, north-seeking or N--at the ends next to the magnet. The polarities of their other ends therefore must be the same--here, south-seeking or S, and those ends should repel each other. By spreading your fingers and allowing the ends of the nails to move apart, you can show that in fact they do.

• Use a compass, a D-cell and a short insulated wire. (insulated--just in case it gets hot).

• The D-cell should be fresh: you will have to draw a large current from it, a short circuit really, though only for a very short time.

• The compass should point north. Then with your thumb press one end of the wire against the bottom of the D-cell. The wire should form a short loop, coming back to the other terminal of the battery, but not touching it.

• Move the wire so that the middle of the wire passes over the compass needle and is parallel to it.

• Then touch the other end of the wire to the other end of the cell--just a short touch (1-2 seconds), it's a short circuit and not good for the cell, also it generates a lot of heat at the contacts. The needle will immediately pivot to stand at 90 degrees to the wire.

• Reverse the electrical contacts by turning the D-cell around.

Electromagnets

• electromagnet – temporary magnet made by wrapping a current-carrying wire around an iron core

– the center of an electromagnet is called the core• it is often made of iron

Electromagnets

• as long as current is flowing, an electromagnet has a magnetic field

• when current is turned off, there is no longer a magnetic field

Electromagnets

• there are two ways to make an electromagnet stronger

– increasing the number of coils

– increasing the amount of current

Electromagnets

• electromagnets are useful because they can be turned on and off

• electromagnets have many important uses– ex. radios, telephones, computers

The ends of a magnet are where the magnetic effect is the strongest.

Magnetic FieldsMagnetic Fields

The region where the magnetic forcesThe region where the magnetic forcesact is called the “magnetic field”act is called the “magnetic field”

An unmagnetized substance An unmagnetized substance looks likelooks likethis…this…

While a magnetized substance looksWhile a magnetized substance lookslike this…like this…

What are magnetic domains?Magnetic substances like iron, cobalt, and nickel are composed of small areas where the groups of atoms are aligned like the poles of a magnet. These regions are called domains. All of the domains of a magnetic substance tend to align themselves in the same direction when placed in a magnetic field. These domains are typically composed of billions of atoms.

Atoms themselves have magnetic properties due to the spin of the atom’s electrons.

These areas of atoms are called “domains”

Groups of atoms join so that their magnetic fields are all going in the same direction

When an unmagnetized substance is placed in a magneticWhen an unmagnetized substance is placed in a magneticfield, the substance can become magnetized.field, the substance can become magnetized.

This happens when the spinning electrons line up in theThis happens when the spinning electrons line up in thesame direction.same direction.

Making a Magnet

• some magnets occur in nature

• these magnets are called natural magnets

– ex. magnetite (also called lodestone)

The Earth is a magnet:The Earth is a magnet:

Magnetic South Pole

Magnetic North Pole

It exerts magnetic forces and is surrounded by amagnetic field that is strongest near theNorth and South magnetic poles

Geographic North Pole

Geographic South Pole

The Earth as a Magnet

• the Earth is surrounded by a magnetic field which extends far into space

• magnetosphere – region of the Earth’s magnetic field

The Earth as a Magnet

• the magnetosphere traps charged particles from the sun

• when these particles enter the atmosphere, an aurora is formed

• auroras are also called the northern and southern lights

• Magnetic fields are produced by magnets and by an electric current in a wire.

Static Electricity

Static electricity is the charge that stays on an object.

Unlike charges attract each other, and like charges repel each other.

• Static electricity is the build up of electric charges on an object. (can occur by rubbing).

• For example: if you rub a balloon against your head, then electrons from the atoms that make up your hair get transferred to the balloon.

• The balloon becomes negatively charged and your hair becomes positively charged. If you hold the balloon next to your hair, your hair will stand on end.

Experiment : Electrostatic Force• You can easily test that like charges repel and

unlike charges attract each other by doing a very simple experiment.

• Take a glass rod and rub it with a piece of silk, then hang it from its middle with a piece string so that it is free to move.

• If you then bring another glass rod which you have also charged in the same way next to it, you will see the rod on the string turn away from the rod in your hand i.e. it is repelled.

• If, however, you take a plastic rod, rub it with a piece of fur and then bring it close to the rod on the string, you will see the rod on the string turn towards the rod in your hand i.e. it is attracted.

• This happens because when you rub the glass with silk, tiny amounts of negative charge are transferred from the glass onto the silk, which causes the glass to have less negative charge than positive charge, making it positively charged.

• When you rub the plastic rod with the fur, you transfer tiny amounts of negative charge onto the rod and so it has more negative charge than positive charge on it, making it negatively charged.

• Going down the list materials have an increased tendency to lose electrons.

• brass• copper• silk• lead• fur• wool• glass

Static Electricity

• Occurs with materials which are insulators• Rubbing adds or removes electrons• Object becomes charged• Like objects repel, unlike attract

What is static electricity?

When two objects rub against each other electrons transfer and build up on an object causing it to have a different charge from its surroundings.

Like the shoes rubbing against the carpet. Electrons are transferred from the carpet to the shoes.

As electrons collect on an object, it becomes negatively charged. As electrons leave an object it attains a positive charges. Charges interact with each other:

Often when you remove clothes from the clothes dryer, they seem to stick together. This is because some of the clothes have gained electrons by rubbing against other clothes. The clothes losing electrons become positive. The negative clothes are attracted to the positive clothes.

What causes you to be shocked when you rub your feet across carpet?

An electrical discharge is the passing of an electric current through the air from a negatively charged object to a positively charge object. This is what causes lightning!

How are static charges detected?

What is the difference between static electricity and current electricity?

Static electricity is stationary or collects on the surface of an object, whereas current electricity is flowing very rapidly through a conductor.

The flow of electricity in current electricity has electrical pressure or voltage. Electric charges flow from an area of high voltage to an area of low voltage.

Water pressure and voltage behave in similar ways.

What are batteries?Batteries are composed of a chemical substance which can generate voltage which can be used in a circuit.

There are two kinds of batteries: dry cell and wet cell batteries. Below is an example of a dry cell.

The zinc container of the dry cell contains a moist chemical paste surrounding a carbon rod suspended in the middle.

Wet cell batteries are most commonly associated with automobile batteries.

A wet cell contains two connected plates made of different metals or metal compounds in a conducting solution. Most car batteries have a series of six cells, each containing lead and lead oxide in a sulfuric acid solution.

What are electric circuits?Circuits typically contain a voltage source, a wire conductor, and one or more devices which use the electrical energy.A conductor is any material that allows electrons to flow through it easily. The term conductor is also used to refer to objects that are good conductors of electricity, such as copper wire.

An insulator is a material which does not allow an electric current to pass. Nonmetals are good conductors of electricity. Plastic, glass, wood, and rubber are good insulatorsA resistor is a material that resists, but doesn’t stop the flow of current.

• Voltage is the electrical potential energy and is measured in volts.

• A good analogy is to think of a water hose. There is water pressure or potential energy on the other side of the faucet or outlet valve. Once you open the faucet, the pressure causes the water to rush through the hose.

• The unit symbol for volts is V, as in 110V.

Current•Current indicates the amount of electrons passing through the wire and is measured in amperes or amps for short. For some reason, they use I to indicate current instead of a different letter. The unit symbol for amps is A, as in 2.0A.

Resistance•Electrical resistance can be thought of as the "friction" on the movement of electrons in a wire. Resistance is measured in ohms, and the unit symbol for it is the Greek letter omega, Ω. Thus 3 ohms is often written as 3 Ω.•Following the water hose analogy, resistance is similar to the friction inside the hose. But also, the resistance increases with a narrower hose, just like a thin copper wire has more electrical resistance than a thick wire.

DC circuit•The power source for a DC circuit could be a battery or DC generator. The (+) and (−) indicate the direction of the current.

AC circuit•A simple AC circuit is illustrated below. A circle with the sine wave symbolizes an AC generator with some given voltage.

What is the difference between an open circuit and a closed circuit?

A closed circuit is one in which the pathway of the electrical current is complete and unbroken.

An open circuit is one in which the pathway of the electrical current is broken. A switch is a device in the circuit in which the circuit can be closed (turned on) or open (turned off).

The steady flow of electricity is called an electric current. A current will move along a wire or a path called a circuit.

Circuit means to “go around.”

CIRCUIT TYPESThe simplest type of circuit involves electricity going around with no “choices” (electrons don’t really choose).

This is called a Series circuit.

Draw the path the electrons travel.

The other main type of circuit has two or more branches.

This is called a Parallel circuit.

Draw on the electron flow.

What sort of circuit is this?

A parallel… but, more importantly…

A short circuit.

Circuits

• A series circuit is a circuit that has only one path for the current.

• A parallel circuit has more than one path for current to travel.

• Lights in our homes are wired in parallel circuits.

Series circuit

• Has a single loop for electrons to travel round

• Components are connected one after another

• Current has to travel through all components

• Current is the same at all points• Voltage is shared between components

• What is a series circuit? • A series circuit is one which provides a single

pathway for the current to flow. If the circuit breaks, all devices using the circuit will fail.

• In series:• I = I1 = I2 = I3 ...

• V = V1 + V2 + V3...

• RTotal = R1 + R2 + R3...

What is a parallel circuit?A parallel circuit has multiple pathways for the current to flow. If the circuit is broken the current may pass through other pathways and other devices will continue to work.

Parallel circuit

• Has two or more paths for electrons to flow down

• Current is shared between the branches• Sum of the current in each branch = total

current• Voltage loss is the same across all components

• In a parallel circuit :• I = I1 + I2 + I3 ...

• V = V1 = V2 = V3...

• 1/ RTotal =1/ R1 + 1/R2 + 1/ R3...

Current (I)

Current is the flow of electronsaround a circuit• DC = direct current like battery• Electrons flow in one direction• AC = Alternating current like mains• Electron flow changes direction 50x per

second

Ammeter

• Measures CURRENT(I)• Unit = Amp (A)• Current is flow of electrons• Connect in series at the point you wish to

measure• RED to RED and BLACK to BLACK

Voltmeter

• Measures voltage• Unit = Volt (V)• Connect in parallel around a component

The unit for measuring resistance is the ohm ohm ((ΩΩ).).

Resistance (R)

The amount that a component slows the current

•As the electrons are slowed by a resistor, energy is lost in the form of heat.

•This means that current, resistance and voltage must be linked.

•This is Ohms law

•The unit of resistance is the ohm, symbol V

I R×

• Ohm’s Law• The volt, ohm, and ampere are related to each

other in a simple formula known as Ohm’s law:

Voltage = current resistance, or E = I R• (1) Voltage = current x resistance• (2) Current = voltage / resistance• (3) Resistance = voltage/ current

• If a current of 5 amps flows through a resistance of 40 ohms, the voltage across that resistor, according to the formula is:

Volts = amps x ohms = 5 amps x 40 ohms = 200 volts

Electrical Calculations – What is Ohm’s Law?

I = 3 V2 Ω

I = 1.5 amps

• Using the information given in this diagram determine the reading on the ammeter.

• If the current of the circuit is 10 amps, what is the voltage of the battery?

Solve for the current of the circuit

• In this circuit what is the reading on the ammeter?

• When a conductor has a potential difference of 100 volts placed across it, the current through it is 5 ampere. What is the resistance of the conductor?

• If the potential difference across a 50- ohm resistor is 5 volts, what is the current through the resistor?

• (1)10A (2) .5A (3) 5A (4) .1A

• A generator supplies current in a circuit. If the resistance in the circuit is increased, the force required to keep the generator turning at the same speed is

• (1) decreased (2) increased (3) unchanged

• If the voltage across a 4-ohm resistor is 12 volts, the current through the resistor is

• (1) .25 A (2) .48 A (3) 3.0A (4) 4.0A

• A resistor carries a current of .1 ampere when the potential difference across it is 5 volts. The resistance of the resistor is

• (1) .02 Ω (2) .5Ω (3) 5Ω (4) 50Ω

• Draw a circuit diagram to include a 60-V battery, an ammeter, and a resistance of 12.5 Ω in series. Determine the reading on the ammeter.

• (a) What is the total resistance of this circuit?• (b) What is the current of the circuit?

Power

Energy used by component per second• Unit of power is the Watt, symbol is W• One watt means that 1 joule of electrical energy is being used

up per second.• Current, voltage and power

are linked

P

I V×

How is Electrical Power calculated?

Electrical Power is the product of the current (I) and the voltage (v)

The unit for electrical power is the same as that for mechanical power in the previous module – the watt (W)

Example Problem: How much power is used in a circuit which is 110 volts and has a current of 1.36 amps?

P = I V

Power = (1.36 amps) (110 V) = 150 W

How is electrical energy determined?

Electrical energy is a measure of the amount of power used and the time of use.

Electrical energy is the product of the power and the time.

Example problem:

E = P X time

P = I V

P = (2A) (120 V) = 240 W

E = (240 W) (4 h) = 960Wh = 0.96 kWh

Multimeter

• DC Voltage Measurement Procedure• 1. Insert the black test lead plug into the COM

jack and the red test lead plug into the V/ jack.• 2. Select a higher VDC range than you

anticipate measuring.• For example, to measure 700 VDC, select the• 1000 VDC range. If the magnitude of voltage

isn’t known, select the highest range (1000 V).

• Note: If you tried to measure 700 VDC on the 200 VDC range, an overrange indication of “1” would be displayed.

• Conversely, you wouldn’t measure 1.5 VDC on the 1000 VDC range because accuracy would suffer.

DC Current Measurement Procedure

• Insert the red test lead plug into the A jack and the black test lead plug into the COM jack for a maximum measurement of current up to 200 mA. Turn OFF the power to the device being measured.

• Select a higher DCA range than you anticipate measuring.

• If the magnitude of current isn’t known, select the highest range (200 mA) and reduce the

setting until a satisfactory reading is obtained

• In this circuit, three resistors receive the same amount of voltage (24 volts) from a single source. Calculate the amount of current "drawn" by each resistor, as well as the amount of power dissipated by each resistor:

• I1 Ω = 24 ampsI2 Ω = 12 ampsI3 Ω = 8 ampsP1 Ω = 576 wattsP2 Ω = 288 wattsP3 Ω = 192 watts

Each resistor has 15 volts across it in this circuit.

• Determine the amount of voltage impressed across each resistor in this circuit:

•

IR(2.2k) = 10.91 mAIR(4.7k) = 5.11 mA

• According to Ohm's Law, how much current goes through each of the two resistors in this circuit?