Permanent and induced magnetism, magnetic forces and fields

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レッスンノート

Poles of a Magnet and the Law of Magnetism

The ends of a magnet are called poles; every magnet has a north pole and a south pole.
The Law of Magnetism states that two like poles (N and N, or S and S) repel each other, while two unlike poles (N and S) attract each other.
The force between two magnetic poles is a non-contact force – the magnets do not need to touch to exert a force.
The magnetic force is strongest at the poles of a magnet.
When two magnets are held close together, they exert an attractive or repulsive force depending on which poles face each other.

Diagram showing two attracting bar magnets and two repelling bar magnets, illustrating the interaction of magnetic poles and magnetic fields.

Magnetic Materials

Magnetic materials are attracted to a magnet; non-magnetic materials are not.
Only a few metals are magnetic: iron, cobalt and nickel.
Steel is an alloy that contains iron, so it is also magnetic.
A magnetic material that is not itself a magnet will always be attracted to a magnet, regardless of which pole is brought close to it.
To test whether an object is a magnet, bring it close to a known magnet: if it is repelled, it is a magnet; if it is only attracted, it is a magnetic material.

Magnetic materials (iron, steel, cobalt, nickel) are attracted to a magnet; non-magnetic materials (zinc, copper, gold, silver, magnesium, aluminium) are not.

Permanent Magnets

A permanent magnet produces its own magnetic field.
Permanent magnets are made from permanent magnetic materials, for example steel.
A permanent magnet does not lose its magnetism.
Permanent magnets can attract or repel other permanent magnets, depending on the poles that face each other.

Compasses and toy trains are two examples of the use of permanent magnets.

Induced Magnets

Induced magnetism occurs when a magnetic material is placed in a magnetic field and temporarily becomes a magnet.
When magnetism is induced, one end of the material becomes a north pole and the other end becomes a south pole.
The end of the induced magnet closest to the permanent magnet will have the opposite pole to the pole of the permanent magnet nearest to it.
Induced magnetism always causes a force of attraction between the permanent magnet and the magnetic material.
When the magnetic material is removed from the magnetic field, it loses most or all of its magnetism quickly.
An induced magnet is temporary, unlike a permanent magnet which keeps its magnetism.

Magnetic Fields

A magnetic field is the region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt and nickel).
All magnets are surrounded by a magnetic field.
Magnetic field lines are used to represent the strength and direction of a magnetic field.
The direction of the magnetic field is shown by arrows on the field lines; the field always goes from north to south.
The strength of the magnetic field is shown by the spacing of the field lines: close together means a strong field, far apart means a weak field.
Magnetic field lines must never touch or cross each other.
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.

Magnetic field lines

Magnetic Field Around a Bar Magnet

The magnetic field is strongest at the poles, where the field lines are closest together.
The magnetic field becomes weaker as the distance from the magnet increases, because the field lines get further apart.
When drawing the field around a bar magnet, add arrows pointing away from the north pole and towards the south pole.
Make sure the field lines are drawn further apart as the distance from the magnet increases to show the field getting weaker.
Two bar magnets can attract or repel each other, and the field line patterns look different for each case.

Magnetic field lines around a bar magnet shown using iron filings.

Uniform Magnetic Fields

A uniform magnetic field has the same strength and direction at all points.
A uniform field is produced in the gap between two opposite poles held close together.
In a uniform field, the magnetic field lines are equally spaced and parallel.
Each field line has an arrow pointing from the north pole to the south pole.
Outside the gap between the poles, the field is not uniform.

Plotting Magnetic Fields with a Compass

A plotting compass is a small bar magnet with a north and south pole; the arrow of the compass represents the north pole.
To plot a field line: place the magnet on paper, draw a dot near one pole, then place the compass so one end of the needle points away from the dot.
Draw a new dot at the other side of the compass needle, then move the compass so it points away from the new dot and repeat.
Continue until you have a chain of dots from one end of the magnet to the other, then remove the compass and join the dots with a smooth curve.
Repeat the whole process several times to plot several magnetic field lines.
The compass needle always points in the direction of the magnetic field at that point.

The Earth's Magnetic Field

In the absence of any magnet or magnetic material, a magnetic compass always points north.
This is evidence that the core of the Earth is magnetic and creates its own magnetic field.
The Earth's magnetic field is similar to that of a bar magnet.
The geographic North Pole (in the Arctic Ocean) acts like a magnetic south pole, so the north pole of a compass is attracted to it.
The geographic South Pole (in Antarctica) acts like a magnetic north pole, so the north pole of a compass is repelled from it.
The north pole of a compass points towards the geographic North Pole because it is attracted to the Earth's magnetic south pole.

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練習問題

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1.Which of the following is the correct definition of the poles of a magnet?
Easy
- AThe places where the magnetic forces are strongest
- BThe places where the magnetic forces are weakest
- CThe centre of the magnet where there is no magnetic force
- DThe places where the magnetic field lines cross
2.Two magnets are held close together. Which statement describes the force between two like poles?
Easy
- AThey attract each other
- BThey repel each other
- CThere is no force between them
- DThey attract if one is a permanent magnet and the other is induced
3.The attraction or repulsion between two magnetic poles is an example of a non-contact force.
Easy
True or false?
4.Which of the following materials are magnetic? (Select all that apply.)
Medium
- AIron
- BCobalt
- CNickel
- DCopper
- EAluminium
5.A student wants to test whether an unknown metal bar is a magnet. Which test is correct?
Medium
- ABring it close to a known magnet; if it is repelled, it is a magnet
- BBring it close to a known magnet; if it is attracted, it is a magnet
- CBring it close to a known magnet; if it is attracted, it is a magnetic material
- DBring it close to a known magnet; if it is repelled, it is a magnetic material
6.Which statement correctly describes the difference between a permanent magnet and an induced magnet?
Medium
- AA permanent magnet produces its own magnetic field and does not lose its magnetism, while an induced magnet only becomes magnetic in a magnetic field and loses most/all of its magnetism when removed
- BA permanent magnet only becomes magnetic in a magnetic field, while an induced magnet produces its own magnetic field
- CBoth permanent and induced magnets lose their magnetism quickly when removed from a magnetic field
- DA permanent magnet is always made of soft iron, while an induced magnet is always made of steel
7.An induced magnet always causes a force of attraction.
Easy
True or false?
8.What is the definition of a magnetic field?
Easy
- AThe region around a magnet where a force acts on another magnet or on a magnetic material
- BThe region around a magnet where the magnetic force is zero
- CThe space inside a magnet where the magnetic poles are located
- DThe path that a magnetic material takes when attracted to a magnet

もっと問題

9.Which of the following correctly describes the direction of magnetic field lines?
Medium
- AFrom the north pole to the south pole
- BFrom the south pole to the north pole
- CFrom the centre of the magnet outwards
- DFrom the south pole to the centre of the magnet
10.Magnetic field lines can cross each other if the magnetic field is very strong.
Easy
True or false?
11.A uniform magnetic field is produced in the gap between two opposite poles. Which statement about a uniform magnetic field is correct?
Medium
- AIt has the same strength and direction at all points
- BIt has the same strength but different directions at different points
- CIt is strongest at the edges of the gap
- DIt only exists around a single bar magnet
12.Which of the following is the correct method to plot the magnetic field lines around a bar magnet?
Medium
- APlace a plotting compass near the magnet, mark the direction of the needle, move the compass to the new position, and repeat
- BSprinkle iron filings on the magnet and draw around them
- CHold a magnetic material near the magnet and observe the direction it points
- DUse a ruler to draw straight lines from the north pole to the south pole
13.The Earth's magnetic field is similar to that of a bar magnet.
Easy
True or false?
14.A magnetic compass is taken to the geographic North Pole. The north-seeking pole of the compass needle is attracted to the Earth's magnetic south pole. Which statement explains this?
Medium
- AThe geographic North Pole is a magnetic south pole
- BThe geographic North Pole is a magnetic north pole
- CThe geographic North Pole has no magnetic field
- DThe compass needle is attracted to the geographic North Pole because it is a magnetic north pole
15.Which of the following statements about the poles of a magnet are correct? (select all that apply)
Medium
- AThe poles are the places where the magnetic forces are strongest.
- BEvery magnet has a north pole and a south pole.
- CThe poles are found in the exact centre of the magnet.
- DTwo like poles repel each other.
- EA magnet can have only one pole.
16.Match each magnet arrangement to the force it produces.
Medium
- North pole near north pole
- North pole near south pole
- South pole near south pole
- Magnet near an unmagnetised iron nail
- Repulsion
- Attraction
17.Place the steps for plotting a magnetic field line with a plotting compass into the correct order.
Medium
- Place the magnet on paper and mark a dot at one end of the magnet.
- Place the compass so one end of its needle points away from the dot, then mark a new dot at the other side of the needle.
- Move the compass so it points away from the new dot and mark another dot.
- Repeat until a chain of dots runs from one end of the magnet to the other.
- Remove the compass and join the dots with a smooth curve.
18.Which of the following are magnetic materials? (select all that apply)
Medium
- AIron
- BCobalt
- CNickel
- DCopper
- ECarbon
19.A magnetic material that is not itself a magnet is always attracted to a magnet, whichever pole is brought close to it.
Easy
True or false?
20.A metal bar is brought towards the north pole of a known magnet and is repelled. What does this show about the bar?
Medium
- AThe bar is a magnet with a north pole at the end nearest the known magnet.
- BThe bar is an unmagnetised magnetic material.
- CThe bar is a non-magnetic material.
- DThe bar is a magnet with a south pole at the end nearest the known magnet.
21.Which statements correctly describe an induced magnet? (select all that apply)
Medium
- AIt becomes a magnet when placed in a magnetic field.
- BIt loses most or all of its magnetism quickly when removed from the magnetic field.
- CIt keeps its magnetism permanently once it has been magnetised.
- DIt produces its own magnetic field at all times, even when no magnet is nearby.
22.Match each type of magnet to its correct property.
Medium
- Permanent magnet
- Induced magnet
- Soft iron
- Steel
- Produces its own magnetic field and does not lose its magnetism
- Loses most or all of its magnetism quickly when removed from a magnetic field
- Used to make temporary magnets
- Used to make permanent magnets
23.A plotting compass is placed near the north pole of a bar magnet. In which direction does the arrow of the compass point?
Medium
- AAway from the north pole of the magnet, along the field line.
- BTowards the north pole of the magnet, against the field line.
- CAt right angles to the field line.
- DIn a random direction, since the compass is unaffected by the magnet.
24.Which statements about a uniform magnetic field are correct? (select all that apply)
Medium
- AIt has the same strength and direction at all points.
- BIt is produced in the gap between two opposite poles held close together.
- CIts field lines are equally spaced and parallel.
- DIts field lines get further apart as you move across the gap.
- EIt is produced between two like poles held close together.
25.Order these stages to show how a plotting compass is used to find the direction of the magnetic field at a point.
Medium
- Place the compass at the chosen point.
- Allow the needle to settle.
- Note the direction the arrow (north-seeking pole) points.
- Record this as the direction of the field line at that point.
26.Which of the following are evidence that the Earth's core is magnetic? (select all that apply)
Hard
- AA magnetic compass points north when no other magnets are nearby.
- BThe Earth's magnetic field behaves like the field of a bar magnet.
- CA compass needle is deflected by a nearby magnet.
- DThe Earth's geographic North Pole acts like a magnetic south pole.
27.The geographic North Pole of the Earth acts like which magnetic pole, and why does a compass needle point towards it?
Medium
- AA magnetic south pole, so the north-seeking pole of the compass is attracted to it.
- BA magnetic north pole, so the north-seeking pole of the compass is attracted to it.
- CA magnetic south pole, so the north-seeking pole of the compass is repelled from it.
- DA magnetic north pole, so the north-seeking pole of the compass is repelled from it.
28.Match each description to the correct term.
Medium
- Region around a magnet where a force acts on another magnet or magnetic material
- Place where the magnetic forces are strongest
- A material that becomes a magnet when placed in a magnetic field
- A magnet that produces its own magnetic field and does not lose its magnetism
- Magnetic field
- Poles
- Induced magnet
- Permanent magnet
29.The force of attraction between a magnet and an unmagnetised iron nail is a non-contact force.
Easy
True or false?
30.A bar magnet is placed near an unmagnetised iron bar. Which statements are correct? (select all that apply)
Hard
- AMagnetism is induced in the iron bar.
- BThe end of the iron bar nearest the magnet becomes the opposite pole to the magnet's nearest pole.
- CThe iron bar is always attracted to the magnet.
- DThe iron bar is always repelled by the magnet.
- EThe iron bar keeps all of its magnetism permanently after the magnet is removed.
31.Which of the following is the correct way to complete the sentence: 'The direction of a magnetic field at a point is given by ...'?
Medium
- Athe direction of the force that would act on a north pole placed at that point.
- Bthe direction of the force that would act on a south pole placed at that point.
- Cthe direction in which a magnetic material moves when placed at that point.
- Dthe direction of the nearest magnetic field line only.
32.Which of the following are correct rules for drawing magnetic field lines around a bar magnet? (select all that apply)
Hard
- AField lines must never touch or cross each other.
- BArrows must point away from the north pole and towards the south pole.
- CField lines should be drawn further apart as the distance from the magnet increases.
- DField lines may cross where the field is very strong.
- EAll field lines must be equally spaced everywhere around the magnet.
33.The Earth's magnetic field is similar to the magnetic field of a bar magnet.
Easy
True or false?
34.A student sprinkles iron filings on paper above a bar magnet to see the field pattern. Why is the plotting compass method preferred in the exam for plotting field lines?
Medium
- AThe plotting compass shows the direction of the field as well as its shape.
- BIron filings are not attracted to magnets.
- CIron filings only work for non-magnetic materials.
- DThe plotting compass method does not need a magnet.
35.Which of the following statements about the force between a magnet and a magnetic material are correct? (select all that apply)
Medium
- AThe force is always one of attraction.
- BThe force is a non-contact force.
- CThe force depends on the distance between the magnet and the material.
- DThe force can be either attraction or repulsion depending on the pole used.
36.Order these statements to show how the direction of a magnetic field is defined and used.
Medium
- Place a north pole at a point in the field.
- The force on that north pole acts in a certain direction.
- This direction is the direction of the magnetic field at that point.
- The field line is drawn with an arrow in that direction.
37.A student places a plotting compass at a point on a magnetic field line. In which direction does the north-seeking pole of the compass needle point?
Medium
- AIn the direction of the force that would act on a north pole placed at that point
- BIn the direction of the force that would act on a south pole placed at that point
- CFrom the south pole of the magnet towards the north pole
- DAt right angles to the magnetic field line
38.The diagram shows two bar magnets placed with their north poles facing each other. Which of the following correctly describes the magnetic field pattern between the two north poles?
Medium
- AThe field lines are equally spaced and parallel, indicating a uniform field.
- BThe field lines curve away from each other, indicating repulsion.
- CThe field lines curve towards each other, indicating attraction.
- DThe field lines cross each other at the midpoint between the poles.
39.A permanent magnet is brought close to an unmagnetised iron nail. The nail becomes an induced magnet. Which statement correctly describes the pole induced at the end of the nail closest to the magnet?
Medium
- AIt is the same pole as the pole of the magnet closest to it.
- BIt is the opposite pole to the pole of the magnet closest to it.
- CIt is always a north pole.
- DIt is always a south pole.
40.A student plots the magnetic field around a bar magnet using a plotting compass. They find that the field lines are very close together near the poles and further apart near the centre of the magnet. What does this indicate about the magnetic field?
Hard
- AThe magnetic field is stronger at the poles than at the centre.
- BThe magnetic field is stronger at the centre than at the poles.
- CThe magnetic field is uniform throughout.
- DThe magnetic field is zero at the poles.
41.Complete the sentence about induced magnets. Choose from: forced, induced, permanent.
Easy
The iron nails have become ____ magnets.
42.Complete the sentences about magnetic field lines using the correct terms.
Medium
The direction of a magnetic field line is from the ____ pole to the ____ pole. The strength of the field is shown by the ____ of the field lines.
43.Complete the sentence about the magnetic field around a bar magnet.
Easy
The magnetic field is strongest at the ____ of the magnet, and it becomes ____ as the distance from the magnet increases.
44.A plotting compass is used to trace a field line step by step. Each step moves the compass 3.0 cm along the field line. How many steps are needed to trace a field line 24 cm long?
Medium
45.Complete the sentence about testing a material with a known magnet.
Easy
If a material is ____ by a known magnet then it is itself a magnet, but if it is only ____ and never repelled then it is a magnetic material.
46.Describe how you would use a plotting compass to plot the magnetic field lines around a bar magnet.
Hard
47.Explain why a compass needle points north when there are no other magnets nearby.
Medium
48.A student says that a compass needle points north because it is attracted to the geographic North Pole. Explain why this reasoning is only partly correct.
Hard
49.A student holds a magnet near a steel paperclip and the paperclip is attracted to it. Explain what happens inside the paperclip and what happens when the magnet is moved away.
Hard
50.A plotting compass is used to trace a field line step by step. Each step moves the compass 3.0 cm along the field line. How many steps are needed to trace a field line 24 cm long?
Medium
- A8 steps
- B9 steps
- C10 steps
- D7 steps

フラッシュカード

What are the poles of a magnet?

The ends of a magnet, where the magnetic forces are strongest. Every magnet has a north pole and a south pole.

State the Law of Magnetism.

Two like poles (N and N, or S and S) repel each other; two unlike poles (N and S) attract each other.

Is the force between two magnetic poles a contact or non-contact force?

It is a non-contact force — the magnets exert a force on each other without touching.

Which four magnetic materials are named in this topic?

Iron, steel, cobalt and nickel. Steel is magnetic because it is an alloy containing iron.

Why is steel magnetic?

Steel is an alloy that contains iron, so it behaves as a magnetic material.

How can you test whether an object is a magnet or just a magnetic material?

Bring it close to a known magnet. If it is repelled, it is a magnet; if it is only ever attracted, it is a magnetic material.

What is a permanent magnet?

A magnet made from a permanent magnetic material (such as steel) that produces its own magnetic field and does not lose its magnetism.

What is an induced magnet?

A magnetic material that becomes a magnet temporarily when it is placed in a magnetic field.

What happens to an induced magnet when it is removed from the magnetic field?

It loses most or all of its magnetism quickly.

What type of force does induced magnetism always cause?

A force of attraction.

When a magnetic material is placed near a magnet, which pole is induced at the end closest to the magnet?

The opposite pole to the magnet's pole nearest to it, so the material is attracted. One end becomes a north pole and the other a south pole.

Define a magnetic field.

The region around a magnet where a force acts on another magnet or on a magnetic material (such as iron, steel, cobalt or nickel).

What does the spacing of magnetic field lines show?

The strength of the field: lines close together mean a strong field, lines far apart mean a weak field.

In which direction do magnetic field lines point?

From the north pole of a magnet to the south pole.

What two rules must be followed when drawing magnetic field lines?

They always go from north to south (shown by an arrow), and they must never touch or cross each other.

Where is the magnetic field of a bar magnet strongest, and how does it change with distance?

It is strongest at the poles, where the field lines are closest together, and it becomes weaker as the distance from the magnet increases.

What is a uniform magnetic field, and where is it produced?

A field with the same strength and direction at all points. It is produced in the gap between opposite poles held close together, shown by equally spaced parallel field lines.

How is the direction of a magnetic field defined at a point?

It is the direction of the force that would act on another north pole placed at that point.

Describe how to plot the magnetic field pattern of a magnet using a plotting compass.

Place the magnet on paper and mark a dot near one pole. Put the compass by the dot, mark a new dot at the other side of the needle, then move the compass so it points away from the new dot and repeat. Join the chain of dots with a smooth curve, then repeat for other field lines.

What does the arrow of a plotting compass represent?

The north pole of the compass needle, which is a small bar magnet.

What evidence shows that the Earth's core is magnetic?

A magnetic compass, with no other magnet or magnetic material nearby, always points north. This shows the Earth has its own magnetic field, similar to that of a bar magnet.

Why does the north pole of a compass point towards the geographic North Pole?

The geographic North Pole acts as a magnetic south pole, so the compass's north pole is attracted to it.

What is the magnetic polarity of the geographic South Pole?

It is a magnetic north pole, so the north pole of a compass is repelled from it.

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