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Electromagnetism Learning Objectives

Candidates should use their skills, knowledge and understanding of how science works:

• The poles of a magnet are the places where the magnetic forces are strongest.
• When two magnets are brought close together they exert a force on each other. Two like poles repel each other. Two unlike poles attract each other. Attraction and repulsion between two magnetic poles are examples of non-contact force.
• A permanent magnet produces its own magnetic field. An induced magnet is a material that becomes a magnet when it is placed in a magnetic field. Induced magnetism always causes a force of attraction. When removed from the magnetic field an induced magnet loses most/all of its magnetism quickly.
• Students should be able to describe:
  • the attraction and repulsion between unlike and like poles for permanent magnets
  • the difference between permanent and induced magnets.
• The region around a magnet where a force acts on another magnet or on a magnetic material (iron, steel, cobalt and nickel) is called the magnetic field.
• The force between a magnet and a magnetic material is always one of attraction.
• The strength of the magnetic field depends on the distance from the magnet.
• The field is strongest at the poles of the magnet.
• The direction of the magnetic field at any point is given by the direction of the force that would act on another north pole placed at that point.
• The direction of a magnetic field line is from the north (seeking) pole of a magnet to the south (seeking) pole of the magnet.
• A magnetic compass contains a small bar magnet. The Earth has a magnetic field. The compass needle points in the direction of the Earth’s magnetic field.
• Students should be able to:
  • describe how to plot the magnetic field pattern of a magnet using a compass
  • draw the magnetic field pattern of a bar magnet showing how strength and direction change from one point to another
  • explain how the behaviour of a magnetic compass is related to evidence that the core of the Earth must be magnetic.
• When a current flows through a conducting wire a magnetic field is produced around the wire.
• The strength of the magnetic field depends on the current through the wire and the distance from the wire.
• Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire.
• The magnetic field inside a solenoid is strong and uniform.
• The magnetic field around a solenoid has a similar shape to that of a bar magnet.
• Adding an iron core increases the strength of the magnetic field of a solenoid. An electromagnet is a solenoid with an iron core.
• Students should be able to:
  • describe how the magnetic effect of a current can be demonstrated
  • draw the magnetic field pattern for a straight wire carrying a current and for a solenoid (showing the direction of the field)
  • explain how a solenoid arrangement can increase the magnetic effect of the current.
  • Students should be able to interpret diagrams of electromagnetic devices in order to explain how they work.
• When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other. This is called the motor effect.
• Students should be able to show that Fleming’s left-hand rule represents the relative orientation of the force, the current in the conductor and the magnetic field.
• Students should be able to recall the factors that affect the size of the force on the conductor.
• For a conductor at right angles to a magnetic field and carrying a current:
  • force = magnetic flux density × current × length (F = B I l)
• A coil of wire carrying a current in a magnetic field tends to rotate.
• This is the basis of an electric motor.
• Students should be able to explain how the force on a conductor in a magnetic field causes the rotation of the coil in an electric motor.
• Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves.
• Students should be able to explain how a moving-coil loudspeaker and headphones work.
• If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor, a potential difference is induced across the ends of the conductor.
• If the conductor is part of a complete circuit, a current is induced in the conductor. This is called the generator effect.
• An induced current generates a magnetic field that opposes the original change, either the movement of the conductor or the change in magnetic field.
• Students should be able to recall the factors that affect the size of the induced potential difference/induced current.
• Students should be able to recall the factors that affect the direction of the induced potential difference/induced current.
• Students should be able to apply the principles of the generator effect in a given context.
• The generator effect is used in an alternator to generate ac and in a dynamo to generate dc.
• Students should be able to:
  • explain how the generator effect is used in an alternator to generate ac and in a dynamo to generate dc
  •  draw/interpret graphs of potential difference generated in the coil against time.
• Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits.
• Students should be able to explain how a moving-coil microphone works.
• A basic transformer consists of a primary coil and a secondary coil wound on an iron core.
• Iron is used as it is easily magnetised.
• Knowledge of laminations and eddy currents in the core is not required.
• The ratio of the potential differences across the primary and secondary coils of a transformer Vp and Vs depends on the ratio of the number of turns on each coil, np and ns.
  • Vp/Vs=Np/Ns
• In a step-up transformer Vs > Vp
• In a step-down transformer Vs < Vp
• If transformers were 100 % efficient, the electrical power output would equal the electrical power input.
• Vs × Is = Vp × Ip
  • Where Vs × Is is the power output (secondary coil) and Vp × Ip is the power input (primary coil).
• Students should be able to:
  • explain how the effect of an alternating current in one coil in inducing a current in another is used in transformers
  • explain how the ratio of the potential differences across the two coils depends on the ratio of the number of turns on each
  • calculate the current drawn from the input supply to provide a particular power output
  • apply the equation linking the p.d.s and number of turns in the two coils of a transformer to the currents and the power transfer involved, and relate these to the advantages of power transmission at high potential differences.
• Electrical power is transferred from power stations to consumers using the National Grid.
• The National Grid is a system of cables and transformers linking power stations to consumers.
• Step-up transformers are used to increase the potential difference from the power station to the transmission cables then step-down transformers are used to decrease, to a much lower value, the potential difference for domestic use.
• Students should be able to explain why the National Grid system is an efficient way to transfer energy.

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