Uses of magnetism (P4.2) | OCR GCSE Physics A (Gateway Science) — Higher (J249)

P4.2 Uses of magnetism

Summary Forces show the existence of fields and how they interact with one another but here the force itself is discussed in more depth and then quantified. These forces also lead to the use of magnetic fields to induce electrical currents and the applications of this electromagnetic induction in motors, dynamos and transformers. Underlying knowledge and understanding This topic will predominantly be new content for learners with some understanding of D.C. motors. Learners will have looked at fields in the previous subtopic and now this knowledge will be built on to give learners the understanding of the application.

Common misconceptions Learners find understanding the manner in which electric and magnetic fields interact to produce a force challenging. Learners commonly have difficulty with the right angles and three-dimensional requirements of Fleming’s left-hand rule. Their ability to visualise this will impact how they deal with this concept. Learners find the action of a commutator difficult to apply in the D.C. motor. The application of changing direction of field in the transformer is found challenging by many learners and hence often leads to a superficial grasp of the working of the transformer.

Reference	Mathematical learning outcomes	Mathematical skills
$PM4.2i$

Learning outcomes	To include	Maths	Working scientifically	Practical suggestions
$P4.2a describe how a magnet and a current- carrying conductor exert a force on one another$			$WS1.1b, WS1.1e, WS1.2a, WS1.3e$	$Demonstration of the jumping wire experiment.$
$P4.2b show that Fleming’s left-hand rule represents the relative orientations of the force, the current and the magnetic field$
$P4.2c apply the equation that links the force on a conductor to the magnetic flux density, the current and the length of conductor to calculate the forces involved$		$M1a, M1b, M1d, M2a, M3a, M3b, M3c, M3d$
$P4.2d explain how the force exerted from a magnet and a current-carrying conductor is used to cause rotation in electric motors$	$an understanding of how electric motors work but knowledge of the structure of a motor is not expected$		$WS1.1e, WS1.3e, WS2a$	$Construction of simple motors.$
$P4.2e ☑ recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change$			$WS1.1e, WS1.3e, WS2a$	$Examination of wind up radios or torches to investigate how dynamos work. Demonstration of induction using a strong magnet and a wire using a zero point galvanometer.$
$P4.2f ☑ explain how this effect is used in an alternator to generate a.c., and in a dynamo to generate d.c.$			$WS1.1a, WS1.1e, WS1.4a$	$Research the structure of dynamos and compare with DC motors.$
$P4.2g ☑ explain how the effect of an alternating current in one circuit, in inducing a current in another, is used in transformers$
$P4.2h ☑ explain how the ratio of the potential differences across the two coils in a transformer depends on the ratio of the numbers of turns in each$		$M1c$	$WS1.1e, WS1.2a, WS1.2b, WS1.3a, WS1.3b, WS1.3e, WS1.3h, WS2a, WS2b$	$Building of a step-up and step- down transformer to investigate their effects.$
$P4.2i ☑ apply the equations linking the potential differences and numbers of turns in the two coils of a transformer$		$M1c, M3b, M3c$
$P4.2j ☑ explain the action of the microphone in converting the pressure variations in sound waves into variations in current in electrical circuits, and the reverse effect as used in loudspeakers and headphones$	$an understanding of how dynamic microphones work using electromagnetic induction$		$WS1.1e, WS1.2a, WS1.3e, WS1.3h, WS2a, WS2b$	$Examination of the construction of a loudspeaker. Building of a loud speaker.$

Learning outcomes

To include

Maths

Working scientifically

Practical suggestions

P4.2a describe how a magnet and a current- carrying conductor exert a force on one another

WS1.1b, WS1.1e, WS1.2a, WS1.3e

Demonstration of the jumping wire experiment.

P4.2b show that Fleming’s left-hand rule represents the relative orientations of the force, the current and the magnetic field

P4.2c apply the equation that links the force on a conductor to the magnetic flux density, the current and the length of conductor to calculate the forces involved

M1a, M1b, M1d, M2a, M3a, M3b, M3c, M3d

P4.2d explain how the force exerted from a magnet and a current-carrying conductor is used to cause rotation in electric motors

an understanding of how electric motors work but knowledge of the structure of a motor is not expected

WS1.1e, WS1.3e, WS2a

Construction of simple motors.

P4.2e ☑ recall that a change in the magnetic field around a conductor can give rise to an induced potential difference across its ends, which could drive a current, generating a magnetic field that would oppose the original change

WS1.1e, WS1.3e, WS2a

Examination of wind up radios or torches to investigate how dynamos work. Demonstration of induction using a strong magnet and a wire using a zero point galvanometer.

P4.2f ☑ explain how this effect is used in an alternator to generate a.c., and in a dynamo to generate d.c.

WS1.1a, WS1.1e, WS1.4a

Research the structure of dynamos and compare with DC motors.

P4.2g ☑ explain how the effect of an alternating current in one circuit, in inducing a current in another, is used in transformers

P4.2h ☑ explain how the ratio of the potential differences across the two coils in a transformer depends on the ratio of the numbers of turns in each

M1c

WS1.1e, WS1.2a, WS1.2b, WS1.3a, WS1.3b, WS1.3e, WS1.3h, WS2a, WS2b

Building of a step-up and step- down transformer to investigate their effects.

P4.2i ☑ apply the equations linking the potential differences and numbers of turns in the two coils of a transformer

M1c, M3b, M3c

P4.2j ☑ explain the action of the microphone in converting the pressure variations in sound waves into variations in current in electrical circuits, and the reverse effect as used in loudspeakers and headphones

an understanding of how dynamic microphones work using electromagnetic induction

WS1.1e, WS1.2a, WS1.3e, WS1.3h, WS2a, WS2b

Examination of the construction of a loudspeaker. Building of a loud speaker.

Uses of magnetism

Next Steps

Learning Objectives