Simple Phenomena Of Magnetism

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Magnetism

Magnets have two poles: north and south.
Magnetic forces are strongest at the poles.
Like poles repel (N–N or S–S).
Opposite poles attract (N–S).
Attraction/repulsion between magnetic poles is a non-contact force.

Magnetic field lines showing attraction and repulsion between bar magnets.

Magnetic Materials

Only a few metals are magnetic: iron, cobalt, nickel.
Steel (an alloy containing iron) is also magnetic.
Magnetic materials are attracted to both poles of a magnet.
Non-magnetic materials (e.g., aluminium, plastic) are not affected by a magnet.
A material is a magnet if it repels a known magnet; it is magnetic if it attracts; non-magnetic if neither.

Magnetic materials are attracted to a magnet; non-magnetic materials are not

Permanent Magnets

Made from hard magnetic materials such as steel.
Produces its own magnetic field.
Does not lose its magnetism easily.
Used in: compasses, school lab magnets, toys, fridge magnets.

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

Induced (Temporary) Magnets

Made from soft magnetic materials such as soft iron.
Becomes magnetised only when placed in a magnetic field.
The end closest to the magnet gains the opposite pole.
Loses magnetism quickly when removed from the field.
Used in: electromagnets, transformers.

Inducing magnetism in a magnetic material

Magnetic Fields

A magnetic field is a region where a magnetic pole experiences a force.
Field lines go from north pole to south pole.
Arrows show direction of force on a north pole.
Field is strongest at the poles (lines are closer together).
Field weakens with distance (lines spread apart).

Magnetic field lines around a bar magnet

Plotting Magnetic Fields

Iron filings method: Place paper over magnet, sprinkle filings, tap gently to reveal field pattern.
Plotting compass method: Place compass near magnet, mark needle ends, move compass step by step to trace field lines.
Compass needle points from north to south (north arrow points to south pole of magnet).

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

Uses of Electromagnets

MRI scanners in hospitals use powerful electromagnets for imaging.
Speakers and earphones use electromagnets to produce sound.
Recycling: Electromagnets separate steel from other scrap.
Mag-Lev trains use repulsion between electromagnets to hover and reduce friction.

Uses of electromagnets in recycling and MRI scanners

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練習題

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1.Which of the following lists of metals contains only metals that are non-ferrous?
Easy
- AIron, steel, nickel
- BCobalt, copper, steel
- CAluminium, brass, copper
- DIron, tin, aluminium
2.Which metal is suitable for use as a permanent magnet?
Easy
- ASteel
- BSoft iron
- CCopper
- DAluminium
3.Which of the following lists contains only ferrous metals?
Easy
- AAluminium and iron
- BSteel and iron
- CSteel and lead
- DTin and iron
4.Which statement about a permanent bar magnet is incorrect?
Medium
- AThe S-pole attracts the N-pole of another magnet
- BThe S-pole repels the S-pole of another magnet
- CIt cannot repel a non-magnetic material
- DIt is made from a soft magnetic material (such as soft iron)
5.Which magnet has the strongest magnetic field?
Easy
- AA bar magnet with widely spaced field lines
- BA bar magnet with closely spaced field lines
- CA bar magnet with no field lines
- DA bar magnet with field lines that cross
6.What material can a temporary magnet be made from?
Easy
- APlastic
- BSoft iron
- CSteel
- DWood
7.Which compass correctly shows the direction of the magnetic field due to a bar magnet?
Medium
- AA compass with its north pole pointing towards the magnet's south pole
- BA compass with its north pole pointing towards the magnet's north pole
- CA compass with its south pole pointing towards the magnet's south pole
- DA compass with its needle aligned perpendicular to the magnet
8.Which of the following statements about a permanent bar magnet is true?
Medium
- AThe north pole of a permanent bar magnet will repel the north pole of another magnet.
- BA permanent bar magnet must be made from a soft magnetic material.
- CA permanent bar magnet can repel a magnetic material.
- DThe field lines of a permanent bar magnet cross each other where the field is strongest.

更多題目

9.A student lines up some permanent magnets with soft iron nails in between. What would the magnetic poles of locations X and Y be? (Assume the north pole of a magnet is on the left and south on the right, with nails between magnets.)
Medium
- AX: north, Y: north
- BX: north, Y: south
- CX: south, Y: north
- DX: south, Y: south
10.Which of the following statements describes something a permanent magnet can do?
Easy
- AIt will repel non-magnetic materials.
- BIt will repel non-ferrous materials.
- CIt will attract ferrous materials.
- DIt will have a single magnetic pole.
11.Which test could be used to find which end of a magnet is the south pole?
Medium
- APutting it near a ferrous metal
- BPutting it near a compass needle and seeing whether the needle points towards the pole
- CPutting it near a compass needle and seeing whether the needle points away from the pole
- DPutting it near a non-ferrous metal
12.A student has two different materials. Material 1 magnetises easily and loses its magnetism as soon as it is removed from a magnetic field. Material 2 is difficult to magnetise but retains its magnetism for a long time. Which row shows the most suitable use for these materials?
Medium
- AMaterial 1: core of an electromagnet, Material 2: core of an electromagnet
- BMaterial 1: bar magnet, Material 2: bar magnet
- CMaterial 1: bar magnet, Material 2: core of an electromagnet
- DMaterial 1: core of an electromagnet, Material 2: bar magnet
13.A student sprinkles iron filings on a sheet of paper placed over a bar magnet and taps the paper gently. Which statement about the pattern formed by the iron filings is correct?
Medium
- AThe filings cluster at the poles and form lines that curve from north to south.
- BThe filings cluster at the centre and form straight lines from north to south.
- CThe filings spread evenly over the paper with no pattern.
- DThe filings form concentric circles around the magnet.
14.A student has a bar magnet and a metal bar. She brings the north pole of the magnet near one end of the metal bar. The metal bar is attracted to the magnet. She then brings the south pole of the magnet near the same end of the metal bar. What will happen?
Medium
- AThe metal bar will be repelled.
- BThe metal bar will be attracted.
- CThe metal bar will not move.
- DThe metal bar will rotate.
15.Two bar magnets are placed with their north poles facing each other, 5 cm apart. Which statement best describes the magnetic field pattern between them?
Medium
- AField lines curving from one north pole to the other north pole.
- BField lines going straight from one north pole to the other north pole.
- CField lines spreading out from each north pole and curving away, with a neutral point in the middle.
- DField lines going from each north pole to the south pole of the same magnet only.
16.Which of the following is a non-magnetic metal?
Easy
- AIron
- BCobalt
- CNickel
- DCopper
17.An electromagnet is made by winding a coil of wire around a core and connecting it to a d.c. power supply. Which change would decrease the strength of the electromagnet?
Medium
- AIncreasing the number of turns on the coil.
- BIncreasing the current in the coil.
- CUsing a core made of steel instead of soft iron.
- DUsing a core made of soft iron instead of air.
18.A student has four metal rods: W, X, Y, Z. She brings the north pole of a bar magnet near each end of each rod. Her results are: W attracts both ends, X repels one end and attracts the other, Y attracts both ends, Z shows no effect. Which rod(s) is/are permanent magnets?
Hard
- AW only
- BX only
- CY only
- DW and Y
19.The diagram shows a bar magnet placed under a sheet of paper. Iron filings are sprinkled on the paper and the paper is tapped. Which position on the paper has the strongest magnetic field?
Medium
- ADirectly above the centre of the magnet.
- BDirectly above the north pole of the magnet.
- CAt a point 5 cm from the north pole along the magnet's axis.
- DAt a point 5 cm from the centre perpendicular to the magnet.
20.Which material is most suitable for the core of an electromagnet that is switched on and off frequently?
Easy
- ASteel
- BSoft iron
- CCobalt
- DNickel
21.A student investigates the number of paper clips an electromagnet can pick up. She uses a soft iron core and a d.c. power supply. She varies the current and counts the clips picked up. Which graph best shows the relationship between current and number of clips?
Medium
- AA straight line through the origin.
- BA curve that increases and then levels off.
- CA curve that increases steadily.
- DA horizontal line.
22.A bar magnet is cut into two equal pieces perpendicular to its length. What happens to the magnetic poles of the pieces?
Medium
- AEach piece has only a north pole or only a south pole.
- BEach piece becomes a complete magnet with both a north and a south pole.
- CThe pieces lose their magnetism completely.
- DOne piece has two north poles, the other has two south poles.
23.A student uses a plotting compass to map the magnetic field around a bar magnet. She places the compass at point P and the north pole of the compass needle points towards the south pole of the magnet. What does this indicate about the direction of the magnetic field at point P?
Medium
- AThe magnetic field at P points towards the south pole of the magnet.
- BThe magnetic field at P points towards the north pole of the magnet.
- CThe magnetic field at P is perpendicular to the line joining P and the south pole.
- DThe magnetic field at P is zero.
24.A student wants to make an electromagnet that retains its magnetism after the power supply is switched off. Which combination of core material and power supply type should she use?
Medium
- ASoft iron core and a.c. supply
- BSteel core and a.c. supply
- CSoft iron core and d.c. supply
- DSteel core and d.c. supply
25.The diagram shows several plotting compasses placed around a bar magnet, each needle aligned along a magnetic field line. Overall, which direction do the compass needles indicate for the magnetic field?
Medium
- AFrom the north pole to the south pole of the magnet
- BFrom the south pole to the north pole of the magnet
- CDirectly between the two poles only
- DPerpendicular to the magnet's length everywhere
26.A student has two unlabeled metal rods. She brings the north pole of a bar magnet near end X of rod 1. End X is attracted to the magnet. She then brings the north pole near end Y of rod 2. End Y is repelled by the magnet. What can she conclude?
Medium
- ARod 1 is a magnet and rod 2 is a magnetic material.
- BRod 1 is a magnetic material and rod 2 is a magnet.
- CBoth rods are magnets.
- DBoth rods are magnetic materials.
27.An unmagnetised 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?
Medium
- AThe end of the iron bar closest to the magnet becomes a north pole.
- BThe end of the iron bar closest to the magnet becomes a south pole.
- CThe iron bar becomes permanently magnetised.
- DThe iron bar is repelled by the magnet.
28.A student has four metal rods: W, X, Y, Z. She brings the north pole of a bar magnet near each end of each rod. Her results are: W attracts both ends, X repels both ends, Y attracts one end and repels the other, Z attracts both ends. Which rod is definitely a magnet?
Hard
- AW
- BX
- CY
- DZ
29.What is the definition of a magnetic field?
Easy
- AA region in which a magnetic pole experiences a force
- BA region where iron filings align
- CThe area around a magnet where it can attract paper clips
- DThe space between the poles of a magnet
30.A student uses the method of plotting compasses to draw magnetic field lines around a bar magnet. She places the compass at a point and marks the positions of the north and south ends of the needle. She then moves the compass so that the south end of the needle points to the previous north mark. Which statement about the direction of the field line is correct?
Hard
- AThe field line direction is from the north mark to the south mark.
- BThe field line direction is from the south mark to the north mark.
- CThe field line direction is perpendicular to the line joining the marks.
- DThe field line direction cannot be determined from this method.
31.Which of the following is a ferrous metal?
Easy
- AAluminium
- BCopper
- CSteel
- DBrass
32.A student places a bar magnet under a sheet of paper and sprinkles iron filings on top. After tapping the paper, the filings form a pattern. Which statement about the pattern is correct?
Easy
- AThe filings cluster at the poles where the field is strongest.
- BThe filings are evenly distributed across the paper.
- CThe filings form straight lines parallel to the magnet.
- DThe filings form circles around the magnet.
33.What are the two poles of a magnet called?
Easy
- ANorth and south
- BPositive and negative
- CEast and west
- DUp and down
34.Which of the following metals is magnetic?
Easy
- ACopper
- BAluminium
- CNickel
- DZinc
35.What is the direction of magnetic field lines outside a bar magnet?
Easy
- AFrom north to south
- BFrom south to north
- CFrom east to west
- DFrom west to east
36.A student brings the north pole of a bar magnet near one end of a metal rod. The rod is attracted to the magnet. Which conclusion is correct?
Medium
- AThe rod is definitely a magnet.
- BThe rod is definitely a magnetic material.
- CThe rod is definitely a non-magnetic material.
- DThe rod could be either a magnet or a magnetic material.
37.Like magnetic poles attract each other.
Easy
True or false?
38.A magnetic material that is not a magnet is brought near the north pole of a bar magnet. What will happen?
Easy
- AIt will be attracted to the magnet.
- BIt will be repelled by the magnet.
- CIt will not be affected.
- DIt will become a permanent magnet.
39.Soft iron retains its magnetism after being removed from a magnetic field.
Easy
True or false?
40.What is a magnetic field?
Easy
- AA region in which a magnetic pole experiences a force
- BA region where electric charges experience a force
- CA region where gravitational force acts
- DA region where there are no forces
41.Where is the magnetic field of a bar magnet strongest?
Easy
- AAt the poles
- BAt the middle
- CFar away from the magnet
- DEqually strong everywhere
42.Which apparatus is used to plot the magnetic field lines around a bar magnet?
Easy
- AA plotting compass
- BA voltmeter
- CA thermometer
- DA balance
43.Arrange the following steps in the correct order to plot magnetic field lines using a plotting compass.
Medium
- Place the magnet on a piece of paper.
- Draw a dot near one pole of the magnet.
- Place the compass next to the dot and draw a new dot at the other end of the needle.
- Move the compass so it points to the new dot and repeat.
- Link the dots to form a smooth curve.
44.Match each use of a magnet with the correct type of magnet (permanent or electromagnet).
Medium
- Compass needle
- MRI scanner
- Speakers and earphones
- Fridge magnets
- Permanent magnet
- Electromagnet
45.Which of the following materials are magnetic? (Select all that apply)
Easy
- AIron
- BAluminium
- CCobalt
- DNickel
- ECopper
46.Magnetic field lines outside a bar magnet go from the south pole to the north pole.
Easy
True or false?
47.Complete the sentence: The ends of a magnet are called ___.
Easy
The ends of a magnet are called ____.
48.Complete the sentence: The direction of magnetic field lines outside a bar magnet is from ___ to ___.
Easy
The direction of magnetic field lines outside a bar magnet is from ____ to ____.
49.State one use of a permanent magnet.
Easy
50.Complete the sentence about induced magnetism.
Easy
When a magnetic material is placed in a magnetic field, the end closest to the magnet becomes the ____ pole.
51.State one use of an electromagnet.
Easy
52.A student uses a plotting compass to map the magnetic field around a bar magnet. At one point, the north pole of the compass needle points towards the south pole of the magnet. How many degrees does the compass needle turn if the student moves the compass to a point where the north pole points directly away from the north pole of the magnet? (Assume the compass needle rotates in a horizontal plane.)
Medium
53.A student uses a plotting compass to map the magnetic field around a bar magnet. At one point, the north pole of the compass needle points towards the south pole of the magnet. How many degrees does the compass needle turn if the student moves the compass to a point where the north pole points directly away from the north pole of the magnet? (Assume the compass needle rotates in a horizontal plane.)
Medium
- A216 °
- B198 °
- C162 °
- D180 °

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