Evaluate — 3 marks
A scrapyard uses an electromagnet suspended from a crane to lift ferrous metal waste. The electromagnet consists of an iron core wrapped with copper wire connected to a power supply. A worker suggests that once the power is switched off, the electromagnet will lose all its magnetism instantly. However, the supervisor notices that some metal scraps occasionally stick to the electromagnet briefly after the power is turned off.
-
(a) Explain why some metal scraps remain briefly attached to the electromagnet after the power supply is switched off.
[1 mark]
-
(b) The iron core retains some magnetism after the power is switched off, while a steel core would retain significantly more. Evaluate which material would be more suitable for this scrapyard application and justify your answer with reference to permanent and induced magnetism.
[2 marks]
Show mark scheme
- (a) The iron core becomes magnetised (induced magnetism) when current flows and retains some of this magnetism briefly after the current stops / the iron domains remain partially aligned after the power is switched off
- (b) Iron is more suitable because it loses its magnetism quickly / has low remanence / does not retain permanent magnetism
- (b) Steel would retain too much magnetism (high remanence/permanent magnetism), making it difficult to release the scrap metal safely / the metal would stick even when power is off, creating a safety hazard or operational problem
Compare — 4 marks
A scrapyard uses two different magnetic devices to separate metal waste. Device A is a permanent magnet made from iron, while Device B is an electromagnet that can be switched on and off. Both devices are used to pick up steel objects from a pile of mixed waste.
-
(a) Compare the magnetic properties of the permanent magnet (Device A) and the electromagnet (Device B). [2 marks]
[2 marks]
-
(b) Explain why the electromagnet (Device B) is more suitable for use in a scrapyard than the permanent magnet (Device A). [1 mark]
[1 mark]
-
(c) When Device B is switched off, some steel objects remain attached to it. Explain what has happened to the steel and why this occurs. [1 mark]
[1 mark]
Show mark scheme
- (a) The permanent magnet produces its own magnetic field continuously / always has a magnetic field.
- (a) The electromagnet only produces a magnetic field when current flows through it / when switched on.
- (a) The electromagnet's magnetic field can be switched on and off, whereas the permanent magnet's cannot be controlled.
- (a) The electromagnet's magnetic field strength can be varied by changing the current, but the permanent magnet's field strength is fixed.
- (b) The electromagnet can be switched off to release the steel objects easily.
- (b) It allows waste to be separated / dropped from the magnet when switched off.
- (b) It provides control over the magnetic field, making it safer and more efficient in a scrapyard environment.
- (c) The steel objects have become magnetised / induced magnetism.
- (c) The steel acts as an induced magnet because it was in the magnetic field of the electromagnet.
- (c) The domains in the steel have been aligned by the electromagnet's field and remain partially aligned even after the electromagnet is switched off.
Evaluate — 5 marks
A scrapyard manager needs to choose between two methods for separating iron from a pile of mixed metals. Method A uses a permanent magnet attached to a crane. Method B uses an electromagnet powered by a battery. The manager wants the system that is most efficient and practical for daily use.
-
(a) Explain the difference between how a permanent magnet and an electromagnet produce their magnetic fields.
[2 marks]
-
(b) Evaluate which method (A or B) would be more suitable for the scrapyard. Consider the advantages and disadvantages of each approach.
[2 marks]
-
(c) The electromagnet in Method B can be switched off to release the metal. Explain why a permanent magnet cannot achieve this, and suggest one modification that could make a permanent magnet system more practical for this application.
[1 mark]
Show mark scheme
- (a) Permanent magnet: magnetic field produced by aligned electron/atomic magnets within the material (or equivalent description of atomic magnetic domains)
- (a) Electromagnet: magnetic field produced by electric current flowing through a coil of wire (or solenoid)
- (b) Method B (electromagnet) is more suitable because the magnetic field can be switched on and off, allowing easy release of metal / Method A disadvantage: permanent magnet cannot easily release metal once attracted
- (b) Method B advantage: strength can be controlled by adjusting current / Method A disadvantage: fixed magnetic strength
- (c) Permanent magnet cannot be switched off because its magnetic field is always present / the alignment of domains cannot be easily reversed; One suitable modification: use a mechanical system (e.g. slider/plate) to move the permanent magnet away from the metal to release it, OR use an electromagnet to demagnetize the permanent magnet, OR use a magnetic shield to block the field
Compare — 3 marks
A student is investigating two magnets in a school laboratory. One magnet is a permanent bar magnet made from iron. The other is an electromagnet made by wrapping wire around an iron core and connecting it to a power supply.
-
(a) Compare the magnetic properties of the permanent bar magnet and the electromagnet when the power supply is switched on.
[2 marks]
-
(b) Explain why the electromagnet loses its magnetic properties when the power supply is switched off, but the permanent bar magnet does not.
[1 mark]
Show mark scheme
- (a) Both magnets have a north and south pole (or both attract magnetic materials/both produce a magnetic field)
- (a) The permanent magnet's magnetism cannot be switched off but the electromagnet's magnetism can be switched off (or electromagnet's strength can be varied/controlled)
- (b) The electromagnet requires an electric current flowing through the coil to produce a magnetic field; when the power supply is switched off there is no current so no magnetic field is produced (or permanent magnets have their atoms/domains permanently aligned whereas electromagnets only have aligned domains while current flows)
Calculate — 2 marks
A steel screwdriver becomes temporarily magnetized when the north pole of a permanent magnet is stroked along its shaft several times. The screwdriver can now attract and hold steel screws. The weight of one steel screw is 0.3 N. The maximum magnetic force the temporarily magnetized screwdriver can exert is 1.5 N.
-
(a) Calculate the mass of one steel screw. Use g = 10 N/kg.
[1 mark]
-
(b) Calculate the maximum number of screws the temporarily magnetized screwdriver can hold vertically at one time.
[1 mark]
Show mark scheme
- (a) 0.3 ÷ 10 OR 0.3 / 10 = 0.03 kg (accept 30 g)
- (b) 1.5 ÷ 0.3 OR 1.5 / 0.3 = 5 (screws)
GCSE Perfect