What you'll learn
Magnetism and electromagnetism is the final unit of AQA GCSE Combined Science: Trilogy physics, and it connects two things that at first appear unrelated: magnets and electric current. The connecting discovery is that a current-carrying wire produces a magnetic field, which means magnetism can be switched on and off and its strength controlled — the basis of an enormous amount of modern technology, from door locks to loudspeakers to electric motors. By the end of this unit you should be able to describe the forces between magnetic poles, distinguish permanent from induced magnets, describe the magnetic field around a bar magnet and around a current-carrying wire, use the right-hand grip rule and the left-hand rule, explain how a solenoid concentrates a field and how an electromagnet is made stronger, describe the motor effect, and explain how a simple electric motor works. This unit is assessed on Physics Paper 2.
Key terms and definitions
Magnetic field — the region around a magnet where a force acts on another magnet or on a magnetic material
Magnetic pole — the region of a magnet where the magnetic forces are strongest, either north seeking or south seeking
Permanent magnet — a magnet that produces its own magnetic field all the time
Induced magnet — a material that becomes magnetic when placed in a magnetic field and loses most or all of its magnetism when removed from it
Magnetic material — a material that experiences a force in a magnetic field; iron, steel, cobalt and nickel
Solenoid — a coil of wire that produces a strong, uniform magnetic field inside it when a current flows
Electromagnet — a solenoid with an iron core, producing a magnetic field that can be switched on and off
Motor effect — the force experienced by a current-carrying conductor placed in a magnetic field
Magnetic flux density — a measure of the strength of a magnetic field, measured in tesla
Right-hand grip rule — the rule giving the direction of the magnetic field around a straight current-carrying wire
Left-hand rule — the rule relating the directions of field, current and force in the motor effect
Core concepts
Poles and the forces between magnets
Every magnet has two poles, a north seeking pole and a south seeking pole, and these are the regions where the magnetic forces are strongest.
When two magnets are brought close together they exert a force on each other. Two like poles repel; two unlike poles attract. Both attraction and repulsion are non-contact forces, acting without the magnets touching.
Repulsion is the only definitive test for a magnet. A magnet will attract a piece of unmagnetised magnetic material, and so will another magnet, so attraction alone proves nothing. Only repulsion shows that both objects are magnets, and this is a classic exam question.
Permanent and induced magnets
A permanent magnet produces its own magnetic field continuously. An induced magnet is a magnetic material that becomes a magnet only when placed in a magnetic field.
Induced magnetism always causes attraction, which explains why a magnet picks up paper clips: each clip becomes an induced magnet with its nearest pole opposite to the magnet's pole. When removed from the magnetic field, an induced magnet loses most or all of its magnetism quickly.
The magnetic materials are iron, steel, cobalt and nickel. Aluminium, copper and brass are not magnetic, which is why a magnet will not pick up a copper coin.
Magnetic fields
The magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material. The force between a magnet and a magnetic material is always one of attraction.
The strength of the field depends on the distance from the magnet: it is strongest at the poles and gets weaker as the distance increases.
The direction of the magnetic field at any point is the direction of the force that would act on a north pole placed at that point, so field lines are always drawn from north to south outside the magnet. The closer together the field lines, the stronger the field.
A magnetic compass contains a small bar magnet that lines up with the field it is in, so compasses can be used to plot the shape of a field. The fact that a compass points north when away from other magnets is evidence that the Earth's core generates its own magnetic field.
The field around a current-carrying wire
When a current flows through a conducting wire, a magnetic field is produced around it. The field consists of concentric circles centred on the wire and perpendicular to it.
Two things determine the field strength: the size of the current, and the distance from the wire. A larger current gives a stronger field, and the field becomes weaker with distance.
The direction of the field is given by the right-hand grip rule. Point the thumb of the right hand in the direction of the conventional current, and the fingers curl in the direction of the magnetic field.
Solenoids and electromagnets
Shaping a wire into a solenoid, which is a coil, increases the strength of the magnetic field considerably. Inside the solenoid the field is strong and uniform, with the field lines parallel and evenly spaced. Outside, the field is similar to that around a bar magnet, with the solenoid having a north pole at one end and a south pole at the other.
The field is made still stronger by placing an iron core inside the solenoid. This arrangement is an electromagnet.
The strength of an electromagnet is increased by increasing the current, by increasing the number of turns on the coil, and by using an iron core. The great advantage over a permanent magnet is that the field can be switched on and off, and varied in strength, simply by controlling the current. This is why electromagnets are used in scrapyard cranes, in electric bells and in door locks.
The motor effect
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 the motor effect.
For the force to be at its maximum, the conductor must be at right angles to the magnetic field. If the wire lies parallel to the field, there is no force at all.
The size of the force depends on the magnetic flux density, the current and the length of conductor in the field. The force equals magnetic flux density multiplied by current multiplied by length, with magnetic flux density in tesla, current in amperes and length in metres. Magnetic flux density is a measure of field strength, and one tesla is a large field.
The left-hand rule
The direction of the force is given by Fleming's left-hand rule. Hold the thumb, first finger and second finger of the left hand at right angles to one another. The First finger points in the direction of the magnetic Field, from north to south. The seCond finger points in the direction of the Conventional Current, from positive to negative. The Thumb then points in the direction of the Thrust, which is the force on the conductor.
The letters are the memory aid: First finger Field, seCond finger Current, thuMb Motion. Using the right hand by mistake reverses the answer, so it is worth writing left in the margin before starting.
The electric motor
A simple electric motor uses the motor effect. A coil of wire carrying a current sits inside a magnetic field. The current flows in opposite directions on the two opposite sides of the coil, so by the left-hand rule the forces on those two sides act in opposite directions: one side is pushed up while the other is pushed down. This produces a turning effect and the coil rotates.
To keep the coil rotating in the same direction, the current must reverse every half turn. Without this, the coil would turn half a revolution and then be pushed back the other way, so it would simply oscillate.
The speed of rotation is increased by increasing the current, by increasing the strength of the magnetic field, and by increasing the number of turns on the coil.
Worked examples
Example 1: Testing for a magnet (3 marks)
A student has two identical-looking bars. One is a magnet and one is unmagnetised steel. Describe how the student could identify the magnet.
The student should bring one end of the first bar close to one end of the second and observe. If they attract, this proves nothing, because a magnet attracts unmagnetised magnetic material as well as the opposite pole of another magnet. The student should then reverse one bar and try again. If repulsion is observed in either orientation, both bars are magnets; if only attraction occurs in both orientations, one bar is unmagnetised. Repulsion is the only definitive test.
Example 2: Calculating the force on a conductor (3 marks)
A wire of length 0.25 metres carries a current of 3.0 amperes at right angles to a magnetic field of flux density 0.40 tesla. Calculate the force on the wire.
The force equals magnetic flux density multiplied by current multiplied by length, which is 0.40 multiplied by 3.0 multiplied by 0.25. That gives 0.30 newtons. The wire is at right angles to the field, so the full force acts; had it been parallel to the field, the force would have been zero.
Example 3: Explaining a motor (4 marks)
Explain why the coil in a simple electric motor rotates continuously rather than stopping after half a turn.
The current flows in opposite directions along the two opposite sides of the coil, so by the left-hand rule the forces on them act in opposite directions, producing a turning effect. After half a rotation, the two sides have swapped positions in the field, so without any change the forces would now turn the coil back the way it came and it would oscillate. Reversing the direction of the current every half turn reverses the forces at the same moment, so the turning effect continues to act in the same rotational direction and the coil keeps rotating.
Common mistakes and how to avoid them
The most frequent error in this unit is using the right hand for the motor effect. The left-hand rule applies to the force on a current-carrying conductor; the right-hand grip rule applies only to the field around a wire.
Students often state that attraction proves an object is a magnet. Only repulsion does, because attraction also occurs between a magnet and an unmagnetised magnetic material.
Another routine slip is drawing magnetic field lines from south to north, or without arrows. Field lines run from north to south outside the magnet and must be arrowed.
In electromagnet questions, many answers say adding more coils makes it stronger without mentioning the current or the iron core. Questions usually expect at least two ways of increasing the strength.
Finally, students frequently forget that the force is zero when the conductor is parallel to the field. If a question says the wire lies along the field direction, the answer is zero, not a calculation.
Exam technique for "Physics: Magnetism and Electromagnetism"
Physically use your left hand in the exam for motor effect questions. It looks awkward and it is the single most reliable way to get the direction right.
When drawing a field, mark the poles first, then draw lines from north to south with arrows, keeping them closer together near the poles to show the field is stronger there.
For calculations, check whether the conductor is at right angles to the field before substituting. The equation given assumes it is.
Where a question asks how to increase the strength of an electromagnet or the speed of a motor, give all the available factors rather than one. Current, number of turns and the iron core cover the electromagnet; current, field strength and number of turns cover the motor.
Quick revision summary
Like poles repel and unlike poles attract, both non-contact forces, and repulsion is the only definitive test for a magnet. Permanent magnets produce their own field; induced magnets become magnetic in a field, always attract, and lose magnetism when removed. Magnetic materials are iron, steel, cobalt and nickel. Field lines run from north to south, are closer together where the field is stronger, and can be plotted with a compass; the Earth's core generates its own field. A current-carrying wire produces circular field lines whose direction follows the right-hand grip rule, stronger with a larger current and weaker with distance. A solenoid gives a strong uniform internal field, and adding an iron core makes an electromagnet that can be switched and controlled. The motor effect gives a force equal to magnetic flux density times current times length, maximum at right angles and zero when parallel, with direction from Fleming's left-hand rule. A motor rotates continuously because the current reverses every half turn.