Physics: 12. Magnetism

Syllabus

OP45Carry out simple experiments to show attraction and repulsion between magnets, and test a variety of materials for magnetism

OP46Plot the magnetic field of a bar magnet

OP47Demonstrate that the Earth has a magnetic field, and locate North and South.

To show attraction and repulsion between magnets

  1. Hang a magnet as shown (the dark half represents the north-pole).
  2. Bring another magnet up close to it.
  3. Notice that if you bring up a north-pole it repels the north of the first magnet, but if you bring up the south-pole end it attracts the north-pole of the first magnet.

Conclusion:

Similar poles repel, opposite poles attract

To test a variety of materials for magnetism

To test a material for magnetism bring a magnet up to it and see if the material becomes attracted to the magnet.

To plot the magnetic field of a bar magnet

Equipment: Magnet, sheet of paper, compasses

Procedure:

  • Place the plotting compass beside the magnet and mark the position of the north end.
  • Move the position of the compass and repeat a number of times on both sides of the magnet.
  • Join the dots.

Result: a pattern is formed on the paper representing the magnetic field of the magnet.

The lines joining the North and South poles are called magnetic field lines; they are most concentrated at the poles.

Magnetic field lines are drawn going from north to south.

To demonstrate that the Earth has a magnetic field, and locate North and South.

When we allow magnets to hang free, they come to rest with one end facing north and the other facing south.

We therefore label the north-facing end ‘the North Pole’, and the south-facing end ‘the South-Pole’.

‘pole’ means ‘end’.

Uses of magnets

Fridge magnets, in motors, in speakers

Exam Questions

Repel
Attract
  1. [2007 OL]

The diagram shows a magnet freely suspended from a wooden stand.

Complete the statements below using the correct word from the list on the right in each case.

(i)When the north pole of another magnet is brought close to the north pole of the hanging magnet they will ______each other.

(ii)When the south pole of another magnet is brought close to the north pole of the hanging magnet they would ______each other.

  1. [2006 OL][2009 OL]

The diagram shows a bar magnet.

(i)Draw the pattern made if iron filings or plotting compasses were placed around the bar magnet.

(ii)Give one use of a magnet.

  1. [2007]

The diagram shows the outline of a bar magnet.

(i)Draw two magnetic field lines one on each side of the bar magnet.

(ii)What are the parts labelled N and S in the diagram called?

  1. [2008 OL]

Describe, with the help of a labelled diagram, how you could carry out an experiment to plot the magnetic field of a bar magnet.

Use the following headings: Labelled diagram, Equipment, Procedure, Result.

Other Test Questions

  1. Complete the following in your answerbook: Like poles ______, unlike poles ______
  2. Describe an experiment to plot the magnetic field around a bar magnet and illustrate the magnetic field.
  3. List two uses of magnets.
  4. What is a compass and how does it work?
  5. What is a space around a magnet called?
  6. Draw a diagram of a bar magnet and sketch some magnetic field lines around it.
  7. Show by means of arrows the directions of the lines of force.

Physics: 13. Static Electricity

Syllabus

OP48Use simple materials to generate static electricity; demonstrate the force between charged objects and the effects of earthing.

Student Notes

When one object is rubbed against another, charges (electrons) often get transferred from one object to the other.

Remember that electrons have a negative charge.

An object becomes positively charged if it loses electrons, and negatively charged if it gains electrons.

Demonstration: using simple materials to generate static electricity

Option 1: Rub a biro or a balloon with a cloth to charge it and then use it to attract pieces of paper.

Option 2: A charged balloon can attract an empty coke can which is lying on a table.

Conclusion: Neutral objects are attracted to charged objects

To demonstrate the force between charged objects

Charge a plastic rod by rubbing it with a cloth and then hang it from a retort stand.

Rub another rod with the same cloth (so that it will have the same charge) and bring it up to the first rod.

The first rod will be repelled by the second rod because they both have the same charge and similar charges repel.

Next bring up a different type of charged rod (which has an opposite charge and notice that it attracts the first rod because they both have opposite charges and opposite charges attract.

Earthing

Earthing means connecting a charged object to the earth by means of a conductor, so that most of the charge which was on the object flows to the earth.

If a rod is charged then all the charges on the rod are repelled from each other and will try to escape from the material if they can. If the material is an insulator like plastic then the charges are not able to move and so remain on the material.

However if you touch the material with your fingers then the points of contact become ‘earthed’ because the charges get transferred to earth via your body.When an object loses charge in this manner we say it gets ‘earthed’.

Similarly the charges can transfer into the air if there is a lot of moisture in the air because water is a conductor.

Useful effects of static electricity

  1. Removing soot from chimneys
  2. Spray-painting

Nuisance effects of static electricity

  1. Television screens attracting dust
  2. Lightning

Exam Questions

  1. [2006 OL]

A student set up the circuit drawn on the right to investigate different materials to see which were electrical conductors and which were electrical insulators.

(i)What would you expect to observe when an electrical conductor is connected between the contact points A and B? Give a reason for your answer.

(ii)What would you expect to observe when an electrical insulator is connected between the contact points A and B? Give a reason for your answer.

  1. [2008]

Two rods A and B, made from different plastics, were given the static electrical charges shown in the diagram.

How could you have charged the rods as shown?

  1. [2008]

Describe with the help of a labelled diagram how the force between the two charged rods A and B could be investigated.

What result would you expect from this investigation?

  1. [2008]

In dry weather you can sometimes get an electric shock from a supermarket trolley.

This is caused by the build-up of static electricity on the trolley.

Explain clearly why this only happens in dry weather.

Answer

In wet weather moisture allows electric charge to escape.

  1. [2009]

A plastic pen when rubbed with a dry cloth can attract small pieces of paper which ‘stick’ to it.

(i)Why does this happen?

(ii)Explain why the pieces of paper fall from the pen after some time.

  1. [2006 OL]

The picture shows a flash of lightning.

(i)What type of energy generates lightning?

(ii)The flash of lightning is seen before the thunder is heard.

What does this tell us about the speed of light?

Other Test Questions

  1. What is the difference between a conductor and an insulator?
  1. Write out the following sentence, filling in the missing words: similarly-charged objects ______while oppositely-charged objects ______.
  1. Describe briefly how you would investigate the relationship between two similarly charged objects.
  1. Sometimes when you touch a metal object (like a shopping trolley) you can get a small shock. Why is this more likely to happen on a dry day than on a wet day?
  1. Give one example of when static electricity is useful and one example of when static electricity can be a nuisance (apart from getting a shock)?
  1. When a Perspex rod is rubbed with a cloth it becomes positively charged. Explain in terms of electron transfer how this occurs.

Physics: 14. Current Electricity

Syllabus

OP49 Test electrical conduction in a variety of materials, and classify each material as a conductor or insulator

OP50 Set up a simple electric circuit, use appropriate instruments to measure current, potential difference (voltage) and resistance, and establish the relationship between them

OP51 Demonstrate simple series and parallel circuits containing a switch and two bulbs

OP52 Perform simple calculations based on the relationship between current, potential difference (voltage), and resistance

OP53 Describe the heating effect, the chemical effect, and the magnetic effect of an electric current, and identify everyday applications of these, including the action of a fuse

OP54 Distinguish between direct and alternating current; recall that the voltage of the mains supply is 230 volts a.c.

OP55 Recall that the unit of electrical energy used by electricity supply companies is the kilowatt-hour, and calculate the cost of using common electrical appliances, based on their power rating

OP56 Describe how to wire a plug correctly, and explain the safety role of a fuse or circuit breaker in domestic electrical circuits.

Student Notes

Conductors and Insulators

Remember our definitions of conductors and insulators from the chapter on Static Electricity:

A conductor is a substance that allows charge to flow through it easily (metals are conductors).

An insulator is a substance that does not allow charge to flow through it (plastics are insulators).

Experiment: Identify materials as conductors or insulators

  1. Set up the circuit as shown.

Label / Circuit component
A / Switch
B / Power supply
C / Resistor
D / Bulb
  1. Place various different materials between points X and Y in the circuit and turn on the switch.
  2. If the bulb lights then the material is a conductor and if it doesn’t then the material is an insulator.

Resistors

A resistor is used to reduce the flow of current in a circuit.

A series circuit

This is where the two bulbs are connected one after the other.

All the current coming from the battery goes through both bulbs.

Advantage: Uses less electricity than if the bulbs are connected in parallel.

Disadvantage: If one bulb blows the circuit is broken and so no current flows, e.g. lights in a Christmas tree.

A parallel circuit

The current coming from the battery splits up and some goes through each bulb.

Advantage: If one bulb blows there will still be a complete circuit through the other bulb so it will remain lit.

Light bulbs in a house are generally connected in parallel for this reason.

Disadvantage: It uses more electricity than if connected in series.

Current, voltage and resistance

  1. Current
  • The unit of current is the amp (the symbol for the amp is A).
  • The symbol for current is I.
  • Current is measured with an ammeter or multimeter.

For current to flow we need two things:

1. There has to be a complete circuit.

2. There has to be a source of potential difference (power supplies and batteries both act as a source of potential difference).

  1. Potential difference (commonly called ‘voltage’)

Current will flow between two points if there is a potential difference between the two points.

This is a bit like saying that water will flow between two points if there is a height difference between the two points.

Another way of thinking about potential difference is that it provides the ‘push’ to move the electrons around a circuit.

  • The unit of potential difference is the volt (the symbol for the volt is V)
  • The symbol for potential difference is V.
  • Potential difference is measured with a voltmeter or multimeter.
  1. Resistance
  • The unit of resistance is the ohm (the symbol for the ohm is Ω).
  • The symbol for resistance is R.
  • Resistance is measured with an ohmmeter or mulitmeter.

Summary

Quantity / Symbol / Unit / Symbol / Measured with / Symbol
Current / I / Amps / A / Ammeter /
Potential
difference / V / Volts / V / Voltmeter /
Resistance / R / Ohms / Ω / Ohmmeter

Relationship between current, potential difference and resistance

Experiment: To establish the relationship between potential difference and current

  1. Set up the circuit as shown and note the current (I) and potential difference (V)
  2. Adjust the variable resistor (rheostat) to get a new set of values.
  3. Repeat about 6 times and then plot a graph of potential difference against current.
  4. The fact that we get a straight line shows that the potential difference is proportional to the current (this means that if we double the potential difference, the current will double also).
  5. Note that the slope of the graph corresponds to the resistance of the component.

Maths problems

Calculate the resistance of a resistor when 20 V produces a current of 4 A.

Answer

R = V÷I = 20 ÷ 4 = 5 Ohms

Effects of an electric current

There are three effects of an electric current; a heating effect, a magnetic effect and a chemical effect

Effect / Demonstration / Everyday application
Heating Effect / An electric current will cause a light-bulb to heat up and emit light / Electric kettle, electric fire etc.
Magnetic Effect / An electric current will deflect a magnetic compass / Electromagnets
Chemical Effect / Electrolysis occurs when an electric current splits water into hydrogen and oxygen / Electroplating

Alternating Current (a.c.) and Direct Current (d.c.)

Mains Electricity

Electricity which comes through the sockets in your house is referred to as ‘mains’ electricity’.

It changes direction 50 times per second and so is called alternating current (a.c.).

Look at the back of your electrical appliance and it will state this voltage and also the power.

Fuses

The 3-pin plug

Cost of electricity - the kilowatt-hour

The ESB charges for electricity at a rate of 11 cent per kW h.

A hair-drier of power rating 1.5 kW is used for 20 minutes each day.

(i) How many units of electricity are used?

(ii) What is the cost, in cent, of using the hair- drier for six days?

Answer

(i)Six days corresponds to 120 minutes or 2 hours.

The number of kWh = kW × hours = 1.5 × 2 = 3 kWh

(ii)Cost = 3 kWh × 11 cent per kWh = 33 cent

Fill in the following table using the formula V = IR:

Potential Difference
(V) / Current (I) / Resistance
(R)
10 / 5
2 / 200
120 / 30
100 / 2
0.5 / 20
120 / 10

Fill in the following table:

Watts
(W) / kilowatts
(kW) / Hours
(Hr) / kWhr / Cost
(12 cent per unit)
50 / 0.05 / 60 / 3 / 24
20 / 5
1000 / 1 day
500 / 10 minutes
60 / 1 week

Activity:

Check appliances at home (e.g. hairdryers, lamps, heaters etc) to find the power rating and use this to fill in the table below.

Appliance / Power-rating (kW) / Average hrs used in a week / kWhr per week / Cost over 1 week @ 9p per unit (kWhr)

The power rating of various appliances

You don’t have to remember the numbers – just remember which machines use a lot of power and which use relatively little.

In general, if an appliance has a motor in it (something which moves) then it will have a high power rating.

Appliance / Power (in watts)
Washing machine / 2500
Microwave / 1000
Light-bulb / 40

Exam questions

  1. [2008 OL]

The diagram shows a simple electrical circuit.

Label / Circuit component
Bulb
Power supply
Resistor
Switch

Complete the table below correctly matching each of the names of the components in the circuit with one of the labels A, B, C or D.

  1. [2008 OL]

You are given a piece of copper metal and a piece of timber.

Which piece, metal or wood, should you connect between X and Y in order that the bulb would light when the switch is closed?Give a reason for your choice.

  1. [2007 OL]

A student set up the circuit shown to investigate the relationship between thepotential difference (voltage), the current andthe resistance of a wire conductor.

Gaps are left in the diagram in the places where the ammeter and voltmeter should be placed. The symbols for these devices are given on the right.

Complete the circuit inserting the symbols for the ammeter and the voltmeter in their correct positions.

  1. [2007]

The symbols for two electrical meters are given in the diagram.

The symbol is for a meter that measures potential difference, often called ‘voltage’.

What electrical quantity can be measured using the meter with the symbol ?

  1. [2006]

Components, e.g. bulbs, in electrical circuits can be connected in series or in parallel.

It is noticed that, when one headlight fails (blows) in a car, the second remains lighting.

(i)State the way the headlights are connected and give a reason whythis mode of connection is used.

(ii)All of the bulbs go out in an old set of Christmas tree lights, when one of bulbs fails (blows).

In what way are the bulbsconnected in this set of lights?

(iii)Explain why, when one bulb blows, they all go out.

  1. [2006]

Calculate the resistance of the filament of a car headlamp when 12 V produces a current of 5 A in it.

In what unit is resistance measured?

  1. [2007 OL]

The student used the variable voltage supply to apply different voltages across the resistor. She measured the voltage across the resistor and the current passing through it several times. She collected the following data.

Voltage (V) / 0 / 2 / 4 / 6 / 8
Current (A) / 0 / 0.5 / 1.0 / 1.5 / 2.0

(i)Draw a graph of the voltage (y-axis) against the current (x-axis).

(ii)What conclusion can you draw from the graph about the relationshipbetween the potentialdifference (voltage) andthe current passing through the wire conductor?

  1. [2007]

Meters and are used in the circuit shown.