SP12: Magnetism and the Motor Effect

Question 1

SP12b
1) What is meant by an electromagnet?
2) Relate the shape and direction of the magnetic field around a straight wire to the direction of the current.
3) Recall the factors that affect the strength of the magnetic field around a wire.

Answer 1

1) An electromagnet is a magnet which can be controlled (turned on and off) by an electric current. (A current can create a magnetic effect.)
2) The magnetic field made by a current in a straight wire curls around the wire in a ring. You can find it by pointing your right thumb in the direction of the current in the wire and curling your fingers. Your fingers will be curled in the same direction as the magnetic field around the wire. Reversing the direction in which the current flows through the wire will reverse the direction of the magnetic field.
3) - The size of the current going through the wire. The larger the current, the stronger the magnetic field and vice versa.
- The length of the wire inside the magnetic field. The greater the distance from the conductor (such as a wire), the weaker the magnetic field and vice versa.
- The magnitude of the force acting on the wire. The larger the magnitude of the force, the stronger the magnetic field and vice versa.

Question 2

SP12b
1) Describe the magnetic field inside and outside a coil of wire carrying a current.
2) Explain the shape and strength of the magnetic field around a solenoid.

Answer 2

1) Inside the centre of a solenoid, the magnetic field is strong and almost uniform.
Outside of a solenoid, the magnetic field is weak and is a similar shape to that of a bar magnet.
2) A solenoid consists of a wire coiled up into a spiral shape. When an electric current flows, the solenoid acts as an electromagnet. The shape of the magnetic field is very similar to the field of a bar magnet. The field inside a solenoid is strong and uniform. The small magnetic fields caused by the current in each coil add together to make a stronger overall magnetic field. Outside the solenoid, the small magnetic fields from each wire cancel each other out and the outside field is much weaker.

Question 3

SP12c
1) What is produced when a current flows in a magnetic field, and what causes it to be produced when a current flows in a magnetic field?
2) Recall Fleming’s left-hand rule.

Answer 3

1) Forces are produced when a current flows in a magnetic field. The current creates a magnetic field around the coil. The magnetic field produced around the coil interacts with the field produced by the magnets. This results in a force being exerted on the coil. The direction of the force can be determined using Fleming’s left-hand rule. The forces are equal in size and in the opposite direction.
2) Fleming’s left hand rule: the first finger, second finger and thumb should be held mutually perpendicular. Point your first finger in the direction of the field (ie. from the North Pole to the south pole of the magnets). Point your second finger in the direction of the current. Your thumb will then show the direction of motion in the wire.

Question 4

SP12c
1) Use the formula relating force, magnetic flux density, current and length (you are given this in the exam).
2) Explain how the force on a conductor in a magnetic field is used to cause rotation in electric motors.

Answer 4

1) The equation for calculating the force on a wire is: Force (N) = magnetic flux density (T) × current (A) × length (m).
2) The motor effect is when the current in the wire creates its own magnetic field which interacts with the magnetic field between the poles of the magnets (which results in a force).
The coil of wire in a motor rotates because the interacting fields (of the coil and the magnets) causes a force on each arm of the coil. As the current is travelling in opposite directions in each arm of the coil, the forces created are in opposite directions (which causes the coil to rotate).
In order to make the coil keep rotating the same direction, a split-ring commutator swaps the contacts every half turn to reverse the direction of the current. This swaps the direction of the forces for each arm and keeps the direction of rotation constant.

Question 5

SP12a
1) Describe how magnets affect each other.
2) Explain the difference between permanent and induced magnets.
3) Describe the uses of permanent and temporary magnetic materials, and what are the four magnetic materials

Answer 5

1) Magnets exert forces on one another when they are brought together: a non-contact force. If like poles (N- N or S-S) are brought together, the force is of repulsion. If unlike poles are brought together (N-S), the force is of attraction.
2) A permanent magnet produces its own magnetic field at all times. An induced magnet only produces a magnetic field when it is close to another magnet.
3) Four magnetic materials are: iron, cobalt, nickel and steel. One everyday use of induced magnets is cranes in scrapyards. They use induced electromagnets to pick up, move and put down scrap metal.

Question 6

SP12a
1) Describe the shapes of magnetic fields, including variations in strength.
2) Describe how the shape of magnetic fields can be shown using plotting compasses.
3) Explain how a magnetic compass can be used as evidence for the Earth’s magnetic core.

Answer 6

1) A drawing of bar magnet field lines will typically show the magnetic field around the bar magnet. The field lines are usually illustrated as curved lines that extend outward from one pole of the magnet, loop around, and curve back into the other pole. This creates a continuous path that represents the direction and strength of the magnetic field.
The field lines are closer together near the poles, indicating a stronger magnetic field in those regions. As the field lines extend further away from the magnet, they become more spread out, indicating a weaker magnetic field. The drawing will usually show the north pole of the magnet as an arrow pointing outwards, while the south pole is represented by an arrow pointing inwards.
2) The needle of a compass points in the direction of the magnetic field it is in. Put the magnet on a sheet of paper and place a compass near to it. Mark where the North Pole of the compass is pointing. Move the compass so its South Pole is next to the mark, and again mark where the North Pole of the compass is pointing. Repeat this until you’ve moved the compass along the entire magnet. Join up these marks to create a digress of the magnet field lines.
3) The needle of a plotting compass is a very small magnet. Compasses can be used to help people to find their way, as the needle always points to a position near the Earth’s North Pole. A magnet suspended on a string will tilt relative to the horizontal by different amounts in different places. Compass needles are weighted at one end to keep them level.
This behaviour of compasses is evidence that the Earth has a magnetic field, which is similar in shape to the magnetic field of a bar magnet. The Earth’s magnetic field is thought to be caused by electric currents in the molten outer core, which is made from a mixture of iron and nickel.

Question 7

SP12a
What direction does plotting compasses point to?

Answer 7

The plotting compass needle points in the direction of the Earth’s magnetic field, or the magnetic field of a magnet. The needle of a plotting compass points to the south pole of the magnet.

Question 8

SP12a
What would the force of two magnets with two opposite poles next to each other be compared to two magnets with two like poles next to each other?

Answer 8

The forces would be the same magnitude and the opposite direction.