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physics Physics For High Schools (KS4)

Solved problems for the chapter:

The Design and Use of Permanent Magnets and Electromagnets

The chapter does not contain theoretical materials.

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Detailed solution steps are available at:

https://en.openprof.com/wb/chapter:the_design_and_use_of_permanent_magnets_and_electromagnets/3628/

Example 1.

Answer the following questions:

Which poles make up a horseshoe magnet? What about a rod magnet?
Two bar magnets are placed close together. Show using a diagram, how their poles must align so that the magnets repel each other.
If we take the plotting compass in our hands and turn west. Which way does the compass needle point? If we then turn east through the south. Where does the compass needle point in the end?
A small iron filling is approaching the north pole of a magnet. What happens to the iron filling. What about when we approach the north pole of the magnet with a heavy iron anchor. Does the anchor move?

Example 2.

An electromagnet is used to separate magnetic metals from non-magnetic metals. Why is steel not suitable as the core of the electromagnet?

It forms a permanent magnet.
It has a high density.
It has a high thermal capacity.
It is a good conductor of electricity.

An electromagnet with a soft-iron core is connected to a battery and an open switch. The soft-iron core is just above some small soft-iron nails.

The switch is now closed, left closed for a few seconds, and then opened. What do the soft-iron nails do as the switch is closed, and what do they do when the switch is then opened?

	as switch is closed	as switch is opened
A	nails jump up	nails fall down
B	nails jump up	nails stay up
C	nails stay down	nails jump up
D	nails stay down	nails stay down

Which metal could be used for a permanent magnet and which metal could be used for the core of an electromagnet?

	permanent magnet	core of electromagnet
A	iron	copper
B	iron	steel
C	steel	copper
D	steel	iron

Which row gives the metal used to make the core of an electromagnet and one property of the electromagnet?

	metal	property
A	iron	permanent magnet
B	iron	temporary magnet
C	steel	permanent magnet
D	steel	temporary magnet

A student counts how many iron pins an electromagnet picks up when its power supply is switched on. Then, she counts how many pins are picked up when the power supply is switched off.

She repeats the experiment using cores made of different materials. The results are shown. Which core is made out of soft iron?

	pins picked up with the power supply on	pins picked up with the power supply off
A	0	0
B	2	7
C	8	5
D	12	0

A permanent magnet is to be made. Which row gives the correct material to use and the correct reason for choosing this material?

	material	reason
A	soft iron	easy to demagnetise
B	soft iron	hard to demagnetise
C	steel	easy to demagnetise
D	steel	hard to demagnetise

Why is soft iron used for the core of an electromagnet?

Soft iron easily becomes a permanent magnet.
Soft iron is a good electrical conductor.
Soft iron is a poor thermal conductor.
Soft iron loses its magnetism when the current in the coil is switched off.

A metal bar is placed inside a current-carrying coil, as shown in diagram 1. There is a small current in the coil. The bar holds a few nails, as shown in diagram 2.

When there is no current in the coil, the nails drop off. Which row is correct?

	metal from which the bar is made	effect of a larger current in the coil
A	soft iron	it makes no difference
B	soft iron	the bar holds more nails
C	steel	it makes no difference
D	steel	the bar holds more nails

10.

Iron is used for the core of a transformer and steel is used to make a bar magnet. Which statement explains these uses of iron and of steel?

Iron is a magnetic material and steel is a non-magnetic material.
Iron is a permanent magnetic material and steel is a temporary magnetic material.
Iron is a temporary magnetic material and steel is a permanent magnetic material.
Iron is a non-magnetic material and steel is a magnetic material.

11.

Which metal is used for the core of an electromagnet?

copper
iron
magnesium
steel

12.

An electromagnet is used to remove a splinter from an eye. What material is the splinter made from?

aluminium
glass
iron
wood

13.

The diagram shows an electromagnet.

Which material is part X made from?

copper
iron
steel
tin

14.

What is wrong with this labelled diagram of a permanent magnet?

The cross-section should be rectangular.
The length should be greater than the diameter.
The magnet should be made of steel.
The N-pole and the S-pole should be reversed.

15.

From which materials are the coil and the core of an electromagnet made?

	coil	core
A	copper	soft iron
B	copper	steel
C	soft iron	copper
D	steel	soft iron

16.

In which way are a bar magnet and an electromagnet similar?

A bar magnet and an electromagnet are always magnetised when stored.
A bar magnet and an electromagnet can both be used to separate magnetic and non-magnetic materials.
A bar magnet can be made of steel and an electromagnet uses a steel core.
The magnetic field strength of a bar magnet and of an electromagnet can both be varied.

17.

A student wishes to make a permanent magnet. She has an iron rod and a steel rod. Which rod should she use to make the permanent magnet, and is this rod a hard magnetic material or a soft magnetic material?

	rod	type of magnetic material
A	iron	hard
B	iron	soft
C	steel	hard
D	steel	soft

18.

A strong electromagnet is used to attract pins.

What happens when the current in the coil is halved?

No pins are attracted.
Some pins are attracted, but not as many.
The same number of pins is attracted.
More pins are attracted.

19.

Which statement about a permanent bar magnet is correct?

It is made from a soft magnetic material.
It repels a non-magnetic material.
Its field lines cross each other where the magnetic field is strong.
Its N-pole repels the N-pole of another magnet.

20.

A permanent magnet is made from metal and an electromagnet uses a metal core. Which metal is suitable for each of these purposes?

	permanent magnet	core of electromagnet
A	iron	iron
B	iron	steel
C	steel	iron
D	steel	steel

21.

A hard magnetic material can be used to make a permanent magnet. A soft magnetic material can be used to make a temporary magnet. Which row shows whether iron and steel are hard or soft magnetic materials?

	iron	steel
A	hard	hard
B	hard	soft
C	soft	hard
D	soft	soft