Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
·  that like poles on magnets repel while unlike poles attract
·  that magnetic field lines indicate the strength and direction of a magnetic field around a magnet
·  how to make electromagnets
·  some uses of electromagnets. .Learning outcomes / Most students should be able to:
·  describe the effect that magnets have on each other and on magnetic materials
·  draw basic magnetic field patterns for a bar magnet
·  describe the key elements of an electromagnet including the coil and iron core
·  explain the function of basic electromagnetic devices.
Some students should also be able to:
·  interpret diagrams of electromagnetic appliances in order to explain how they work. / When a current flows through a wire a magnetic field is produced around the wire. [P3.3.1 a)]
Interpret diagrams of electromagnetic appliances in order to explain how they work. [P3.3]
Controlled Assessment: P4.1 Plan practical ways to develop and test candidates’ own scientific ideas. [P4.1.1 a) b) c)]; P4.2 Assess and manage risks when carrying out practical work. [P4.2.1 a) b)] / Chapter map: Using magnetic fields to keep things moving
Lesson structure / Support, Extend and Practical notes
Starters
Electricity and magnetism recap – Use true/false/don’t know cards and a set of electricity questions to establish the students’ prior knowledge. To support students these can be relatively simple like ‘north poles attract south poles’ or to extend students give more difficult examples like ‘the size of an electric current depends on the rate of flow of electrons’. (5 minutes)
Magnetic fields – The students should draw the shape of a magnetic field around a bar magnet and explain how they can find this shape. If they cannot do this, then demonstrate the shape of the field using a magnet and a compass or iron filings. You might want to support students’ understanding by allowing them to find the field, or extend some students by demonstrating the field around magnets of other shapes (horseshoe, ring or spherical magnetic toys). (10 minutes)
Main
·  Most students will be familiar with basic magnetic fields but their understanding of field lines tends to be limited and diagrams can be very inaccurate. It is important that they recognise the correct shape for a bar magnet and understand that the field lines have a direction indicated by arrows (from north pole to south pole). Let the students investigate the basics using the practical task in the Student Book.
·  Move on to electromagnets by demonstrating a basic one; a coil, nail and power supply. You can show that this produces a field by turning it off and on to lift objects or making it affect some compasses placed nearby. Once the basics are covered, you can move on to discuss how to make the magnetic field stronger or leave this for the possible investigation later. Either way, the students need to understand that the strength can be increased by increasing the current.
·  A scrapyard crane uses a powerful electromagnet to lift objects but similar lifting electromagnets are used whenever large ferric metal objects need to be moved easily; they can be found in many factories and foundries. You can demonstrate the action by simply using an electromagnet to lift and drop some small iron blocks.
·  All of the other devices can be demonstrated in the laboratory, see ‘Practical support’ for further details. You could choose to let the students test the devices themselves if you have enough equipment.
·  You can finish the lesson by allowing the students to plan for an electromagnetic investigation. This is a good way of building up a range of skills needed to understand ‘How Science Works’.
Plenaries
Space boots – In space, astronauts are weightless but need some way of walking around the outer surface of a space station. Can the students design a system to do this? There would need to be a way for the astronaut to switch the boots on and off individually so that they could still walk. (5 minutes)
More power – Stretch some of the students by asking them to design a fair test to see what affects the strength of an electromagnet. They should choose one possible factor (current, number of loops in coil, type of core) and form a full plan to see if there is a qualitative or quantitative relationship that can be determined. If time permits, the plans can be carried out as a full investigation. To support students, you can provide plans for them to use in the investigation. (10 minutes) / Support
Students may need some extra time to revise the basic properties of electromagnets. It is certainly worth getting the students to plot out the shape of the fields if they have not done so before.
Extend
Students can attempt to determine the shape of magnetic fields around other magnets or to see if they can detect the field around an electromagnet. Does increasing the current supplied to the electromagnet change the shape of the field lines? They may be able to find that the field lines become more closely packed, indicating an increased magnetic field. They can also see the effect of removing the iron core from the coil.
Practical support
Investigating magnetic fields
Equipment and materials required
Bar magnets, paper clips, plotting compasses, cling film, iron filings in sprinklers. Optional: horseshoe and ceramic magnets.
Details
The students will just recap their basic knowledge of magnetic fields and identify magnetic field lines. They can test to see how magnets affect each other in terms of repulsion and attraction and then use the iron filings and plotting compasses to find the shape and direction of the magnetic field lines.
Electromagnets: Demonstrations
Lifting electromagnet
A lifting electromagnet can be built from a coil of wire wrapped around an iron core (large nail). Use about thirty loops of wire and connect to a low-voltage power supply. Set this to 5 V but make sure that the coil does not start to get warm; if it does, then you will have to reduce the pd.
Circuit breaker
You can set a circuit breaker in a simple lamp circuit. Use a low current one that cuts out before the lamp melts. Don’t forget to demonstrate that the circuit breaker can be re-set unlike a fuse.
Electric bell
There is a range of electric bells designed to demonstrate the function. Set this up according to the instructions and then explain the process that causes the hammer to move back and forth. The key concept is that when the hammer moves to strike the bell, the circuit is broken and so the electromagnet turns off, the hammer then falls back to the start position and the electromagnet turns back on. This cycle continues until you release the main switch.
Relay
You can put a relay in a simple circuit to demonstrate its operation. Quite often the relay is used to turn on a high current circuit from a low current (safer) one. However, it is sufficient to build a basic circuit including a relay and trigger it with an electromagnet or even a bar magnet. The students may be able to see the armature move.
Course / Subject / Topic / Pages
Physics / Physics / P3 3.1 Electromagnets / Pages 242–243
Learning objectives / Learning outcomes / Specification link-up / Kerboodle
Students should learn:
·  that the force on a current-carrying conductor in a magnetic field can be increased by increasing the current or magnetic field strength
·  that the direction of the force can be reversed by reversing the current
·  how this force can be used to make objects move. / Most students should be able to:
·  describe the effect of increasing the current or magnetic field strength on a current-carrying wire
·  identify the direction of the force using Fleming’s left-hand rule
·  describe how an electric motor and loudspeaker work.
Some students should also be
able to:
·  describe and explain the effect of making changes to the design or operation of an electric motor. / The motor effect and its use. [P3.3.1 b)]
The size of the force can be increased by:
– increasing the strength of the magnetic field
– increasing the size of the current. [P3.3.1 c)]
The conductor will not experience a force if it is parallel to the magnetic field. [P3.3.1 d)]
The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed. [P3.3.1 e)] / How science works: Current through a motor
Animation: The motor effect
Lesson structure / Support, Extend and Practical notes
Starters
Magnetic magic – Some magicians use magnetic effects to levitate. You can show some footage of this levitation and ask the students to explain the magic to see if they realise there is a scientific principle behind the mystery (electromagnets lifting a person wearing a magnetic under-suit). Identify the direction of the force using Fleming’s left-hand rule. (5 minutes)
Motor demonstration – Start by showing a functioning electric motor lifting a small load from the floor. Ask the students to explain what you can do to increase the force the motor can provide. They should be able to identify increasing the current. Support students by giving some suggestions and allowing them to select which would and which would not have an effect and what that effect would be. Extend the students by asking them to determine the power of the motor by giving them the load it lifts, the height and the time taken. Does this power match the electrical power provided (P = VI)? Why not? (10 minutes)
Main
·  Before the first experiment, make sure that the students are aware of what magnetic field lines are and where they would be in the arrangement you are going to use. The experiment itself yields obvious results. When discussing the size of the force, most students intuitively realise that if you make the current ‘stronger’ or the magnet ‘stronger’ then the effect will be ‘stronger’ too. You can use diagrams to explain what is meant by perpendicular or parallel to the magnetic field lines, so that the students can understand why there is no force when the wire is parallel to the magnetic field lines.
·  Fleming’s left-hand rule is necessary for the specification, and helps solidify some students’ understanding of the motor effect. To find the direction of the force acting on a current-carrying wire, the students can use Fleming’s left-hand rule. They hold out their left hand with the thumb and first two fingers perpendicular to each other. Point the first finger in the direction of the magnetic field (from north to south); point the second finger in the direction of the current (from positive to negative). The thumb will point in the direction of the movement.
·  A real motor, as used in a drill, will have several separate coils. Using their understanding of the motor effect, the students should be able to explain why this is necessary.
·  Use a very large loudspeaker to show its action. If you can, strip away all of the paper including the central part to expose the magnet and the coil. You may be able to use a direct current to levitate the magnet, showing the students that there is no vibration and hence no sound.
Plenaries
Loudspeaker links – Show the students an electrical waveform that is to be fed into a loudspeaker. Ask them to describe the motion of the magnet in the speaker, including when it will be moving up or down and when its movement will be fastest. You can support students by using simple patterns or extend others by asking them to describe the movements in terms of acceleration (this will be linked to the gradient of the signal). This links back to their study of sound and oscilloscopes. (5 minutes)
Motor competition – All of the students should have completed motors. Get them to turn them on at the same time and see which ones are stable and which ones stop working quite quickly. Select the motor that is smoothest and most stable as the winner and give a prize. (10 minutes) / Support
For students who have difficulty manipulating small objects, you will need to provide some scaled‑up motors that are partially pre-assembled.
Extend
There are devices called linear motors that can be used to move objects without the rotation associated with normal motors. The students could find out about the invention of these devices and the uses they have been put to.
Practical support
Investigating the motor effect
The motor effect is easy to demonstrate or to let the students find out about.
Equipment and materials required
Battery, length of wire (stiff wire works best), variable
resistor, leads, two magnets mounted on U frame (or a U-shaped magnet).
Details
The students place the wire between the two magnets and pass a small current through it. The variable resistor allows them to control the current to see if increasing it has the expected effect. Make sure that the students see the effect of reversing the current and changing the angle between the magnets and the wire. It can be hard to get the wire to run in the same direction as the field lines, but a piece of stiff wire can be bent into shape. Alternatively, as a simple demonstration of the effect, a wider strip of foil can be used instead of the stiff wire. This will deflect or bend when a current is passed through it, demonstrating that a force is experienced.