Simple Phenomena Of Magnetism

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Magnetic and Non-Magnetic Materials

Only iron, cobalt, nickel, and steel (an alloy containing iron) are magnetic.
Magnetic materials are attracted to a magnet; non-magnetic materials are not.
A magnetic material (not a magnet) is attracted to both poles of a permanent magnet.
To test a material: if repelled by a known magnet, it is a magnet; if attracted, it is a magnetic material; if neither, it is non-magnetic.

Magnetic materials

Forces Between Magnetic Poles

Every magnet has a north pole and a south pole; magnetic forces are strongest at the poles.
Like poles repel: north–north or south–south push apart.
Opposite poles attract: north–south pull together.
The force between a magnet and a magnetic material is always attractive due to induced magnetism.

Poles of a Magnet

Induced Magnetism

An induced magnet is a material (e.g., soft iron) that becomes a magnet temporarily in a magnetic field.
The end of the material nearest the magnet acquires the opposite pole to the magnet’s pole.
When removed from the field, the material loses its magnetism quickly.
Induced magnetism always causes attraction between the magnet and the material.

Inducing magnetism in a magnetic material

Permanent and Temporary Magnets

Permanent magnets (e.g., steel) produce their own magnetic field and do not lose magnetism easily.
Temporary (induced) magnets (e.g., soft iron) are magnetic only while in an external field.
An electromagnet consists of coils of wire around a soft iron core; it can be switched on/off.
Strength of an electromagnet can be increased by: increasing current, increasing number of turns, or adding a soft iron core.

Electromagnet diagram

Uses of Magnets

Permanent magnets are used in compasses, fridge magnets, toys, and school experiments.
Electromagnets are used in MRI scanners, speakers, recycling (lifting scrap metal), and Mag-Lev trains.

Uses of permanent magnets

Magnetic Fields

A magnetic field is a region where a magnetic pole experiences a force.
Field lines go from north to south; arrows show the direction of force on a north pole.
Field is strongest at the poles (lines are closer together) and weakens with distance (lines spread out).
The direction of the field at a point is the direction of the force on a north pole placed there.

The magnetic field around a bar magnet

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练习题

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1.Which of the following metals is magnetic?
Easy
- ACopper
- BAluminium
- CNickel
- DZinc
2.Two magnets are brought close together. The north pole of one magnet is placed near the north pole of the other magnet. What happens?
Easy
- AThey attract each other
- BThey repel each other
- CNo force is exerted
- DThey become demagnetised
3.A bar magnet is brought near a piece of metal. The metal is attracted to the magnet. Which of the following could be the metal?
Easy
- AAluminium
- BCopper
- CSteel
- DZinc
4.Magnetic materials are attracted to both poles of a permanent magnet.
Easy
True or false?
5.An induced magnet can be switched on and off.
Medium
True or false?
6.State the two magnetic metals (other than steel).
Easy
7.Explain why a magnetic material is always attracted to a magnet, regardless of which pole is held close to it.
Medium
8.Complete the sentence:
Easy
Like magnetic poles ____ and opposite magnetic poles ____.

更多题目

9.Complete the sentence:
Easy
The magnetic field is strongest at the ____ of a magnet, where the field lines are closest together.
10.Match each material with its magnetic property:
Medium
- Iron
- Steel
- Aluminium
- Permanent magnetic material
- Temporary magnetic material
- Non-magnetic material
11.Arrange the following steps in the correct order to show how an electromagnet can be made stronger:
Medium
- Increase the current
- Add a soft iron core
- Increase the number of turns of the coil
12.A magnetic material that is not a magnet will always be attracted to a magnet, regardless of which pole is held close to it.
Medium
True or false?
13.A student brings a bar magnet near a suspended metal rod. The rod is repelled by the north pole of the magnet. What can be concluded about the rod?
Medium
- AIt is a magnetic material.
- BIt is a non-magnetic material.
- CIt is a magnet with its north pole facing the magnet.
- DIt is a magnet with its south pole facing the magnet.
14.Like magnetic poles attract each other.
Easy
True or false?
15.Which of the following is a property of a permanent magnet?
Easy
- AIt can be switched on and off.
- BIts strength can be easily changed.
- CIt produces its own magnetic field.
- DIt is made from soft iron.
16.Which of the following can be used to increase the strength of an electromagnet? (Select all that apply)
Medium
- AIncrease the current through the coil
- BDecrease the number of turns of wire
- CAdd a soft iron core
- DUse a longer wire
17.Match each material to its magnetic property.
Medium
- Steel
- Soft iron
- Aluminium
- Permanent magnetic material
- Temporary magnetic material
- Non-magnetic material
18.Arrange the following steps in the correct order to make an electromagnet that can be turned on and off.
Medium
- Connect the ends of the wire to a battery
- Wrap a coil of wire around a soft iron core
- Switch on the current to magnetise the core
- Switch off the current to demagnetise the core
19.Magnetic field lines are closer together where the field is weaker.
Easy
True or false?
20.Arrange the following steps in the correct order to magnetise a steel bar using a permanent magnet (stroke method).
Medium
- Stroke the steel bar repeatedly in one direction with one pole of the magnet.
- Lift the magnet away from the steel bar at the end of each stroke.
- Place the steel bar on a table.
- Bring the magnet pole to the starting point again.
21.Induced magnetism always causes a force of attraction between a magnet and an induced magnet.
Medium
True or false?
22.A soft iron bar is placed near the north pole of a strong permanent magnet. Which statement about the induced magnetism in the iron bar is correct?
Hard
- AThe end near the magnet becomes a south pole and the bar remains magnetised after the magnet is removed.
- BThe end near the magnet becomes a south pole and the bar loses its magnetism quickly after the magnet is removed.
- CThe end near the magnet becomes a north pole and the bar remains magnetised after the magnet is removed.
- DThe end near the magnet becomes a north pole and the bar loses its magnetism quickly after the magnet is removed.
23.Which of the following materials are magnetic? (Select all that apply)
Easy
- ANickel
- BCopper
- CCobalt
- DZinc
24.The magnetic field is strongest where the field lines are furthest apart.
Easy
True or false?
25.A student brings the north pole of a bar magnet near one end of a steel rod. The rod is attracted to the magnet. Which statement about the steel rod is correct?
Medium
- AThe rod is a permanent magnet with its south pole at the end near the magnet.
- BThe rod is a permanent magnet with its north pole at the end near the magnet.
- CThe rod is a magnetic material that has been temporarily magnetised by induction.
- DThe rod is a non-magnetic material that becomes polarised by the magnetic field.
26.Which of the following metals are magnetic? (Select all that apply)
Easy
- AIron
- BCopper
- CCobalt
- DAluminium
- ENickel
27.The magnetic field lines around a bar magnet go from the south pole to the north pole outside the magnet.
Easy
True or false?
28.Match each material with its magnetic property.
Medium
- Steel
- Soft iron
- Copper
- Permanent magnetic material
- Temporary magnetic material
- Non-magnetic material
29.Arrange the following steps in the correct order to magnetise a steel bar using the stroking method with a permanent magnet.
Medium
- Stroke the steel bar from one end to the other with the north pole of the magnet.
- Lift the magnet away from the steel bar after each stroke.
- Repeat the stroking process many times in the same direction.
- Place the steel bar on a table.
30.A student has four rods: one is a permanent magnet, one is a magnetic material, one is non-magnetic, and one is an unmagnetised steel bar. Using only a known bar magnet, which test correctly identifies the permanent magnet?
Medium
- AThe rod that is repelled by the known magnet is the permanent magnet.
- BThe rod that is attracted to the known magnet is the permanent magnet.
- CThe rod that is not affected by the known magnet is the permanent magnet.
- DThe rod that is only attracted when placed near the north pole is the permanent magnet.
31.The strength of an electromagnet can be increased by increasing the number of turns of wire on the coil.
Easy
True or false?
32.A piece of aluminium is brought near a strong bar magnet. What will happen?
Easy
- AIt will be strongly attracted.
- BIt will be weakly attracted.
- CIt will be repelled.
- DIt will not be affected.
33.A student has four unlabelled rods: P, Q, R, and S. The student brings the north pole of a known bar magnet near each rod and observes the following: P is attracted, Q is repelled, R is attracted, S shows no effect. The student then brings the south pole of the magnet near each rod: P is attracted, Q is attracted, R is attracted, S shows no effect. Which rod is a permanent magnet?
Hard
- AP
- BQ
- CR
- DS
34.An induced magnet always produces a force of repulsion when brought near a permanent magnet.
Easy
True or false?
35.A student sets up an electromagnet by winding a coil of wire around a soft iron core and connecting it to a battery. Which change would cause the electromagnet to attract more paper clips?
Medium
- AUsing a shorter piece of wire for the coil
- BReversing the connections to the battery
- CIncreasing the number of turns of wire on the core
- DRemoving the iron core
36.The diagram shows the magnetic field around a bar magnet. A plotting compass is placed at a point near the magnet, outside it. In which direction does the north pole of the compass needle point?
Medium
- AIn the direction shown by the nearby field line
- BDirectly towards the centre of the magnet
- CPerpendicular to the field line at that point
- DThe needle stays pointing towards true north, unaffected by the magnet
37.A student wants to demagnetise a steel bar. Which of the following methods can be used? (Select all that apply)
Medium
- AHeating the bar to a high temperature
- BStroking the bar repeatedly with a magnet in one direction
- CHammering the bar repeatedly
- DPlacing the bar in an alternating magnetic field and slowly withdrawing it
- EPlacing the bar in a strong magnetic field for a long time
38.Steel is a magnetic material because it contains iron.
Easy
True or false?
39.Magnetic field lines go from the south pole to the north pole outside the magnet.
Easy
True or false?
40.A student wants to increase the strength of an electromagnet. Which of the following changes will achieve this? (Select all that apply)
Medium
- AIncrease the number of turns of wire on the coil
- BDecrease the current in the coil
- CReplace the soft iron core with a steel core
- DIncrease the current in the coil
- ERemove the core entirely
41.A student has a steel rod and a bar magnet. The student strokes the rod repeatedly with one pole of the magnet, always in the same direction. Which statement describes the result?
Medium
- AThe rod becomes a permanent magnet and keeps its magnetism after the bar magnet is removed.
- BThe rod becomes a temporary magnet that loses its magnetism as soon as the bar magnet is removed.
- CThe rod does not become magnetised because steel is not a magnetic material.
- DThe rod is magnetised only while the bar magnet is touching it.
42.A student sets up an electromagnet with a soft iron core. She connects the coil to a variable power supply and an ammeter. She wants to increase the strength of the electromagnet. Which of the following changes will increase the strength?
Medium
- ADecreasing the number of turns of wire on the coil.
- BReplacing the soft iron core with a steel core.
- CIncreasing the current through the coil.
- DDecreasing the current through the coil.
43.State one use of a permanent magnet.
Easy
44.Complete the sentence about induced magnetism.
Medium
When a magnetic material is placed in a magnetic field, the end nearest to the magnet becomes a ____ pole, and the material is said to be ____.
45.Complete the sentence about magnetic materials.
Easy
The three magnetic metals are ____, ____, and ____.
46.Complete the sentence about magnetic field lines.
Easy
Outside a bar magnet, magnetic field lines go from the ____ pole to the ____ pole.
47.Explain why a magnetic material that is not a magnet is always attracted to a magnet, regardless of which pole is held close to it.
Medium
48.Describe how you would use a known bar magnet to determine whether an unmarked metal bar is a magnetic material, a permanent magnet, or a non-magnetic material.
Hard
49.Explain why an induced magnet is always attracted to a permanent magnet, regardless of which pole of the permanent magnet is brought near it.
Medium
50.Describe how you would use a plotting compass to map the magnetic field around a bar magnet. Include how you determine the direction of the field lines.
Medium

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