Magnets and magnetic fields

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शिक्षकों के लिए: Magnets and magnetic fields (MYP Physics, Year 1) के लिए इस्तेमाल के लिए तैयार लेसन स्लाइड्स, रिवीज़न नोट्स — इन्हें अपने लेसन में इस्तेमाल करें, या टॉपिक को एक इंटरैक्टिव क्लास एक्टिविटी की तरह चलाएं जिसे आपके शिक्षार्थी लाइव गेम की तरह खेलें।

लेसन नोट्स

Big idea: forces without touching

Big idea. Key concept: Relationships. Magnets show how two objects can affect each other from a distance.
Related concept: Interaction. Two magnets interact: each one pushes or pulls the other.
Global context: Scientific and technical innovation. Magnets are in compasses, motors, speakers, door catches, hard drives and many more machines.
A magnet is an object that attracts some metals. Every magnet has two poles, a north pole and a south pole, near its ends. The magnetic force is strongest at the poles.
Magnetic force is a non-contact force. A magnet can pull a paper clip without touching it.
Unlike poles attract (north and south). Like poles repel (north and north, or south and south).
If you cut a bar magnet in half, you do not get a north-only piece and a south-only piece. You get two smaller magnets, each with a north and a south pole.

Attraction and repulsion between magnets

Magnetic materials and induced magnets

A magnetic material is attracted to a magnet. The magnetic materials you need to know are iron, steel, nickel and cobalt. Steel is mostly iron, so it is magnetic too.
Most other materials are not magnetic: copper, aluminium, gold, silver, wood, plastic and glass. Not every metal is magnetic.
When a magnetic material such as an iron nail is held near a magnet, it becomes a magnet too. We say that magnetism is induced. The end nearest the magnet becomes the opposite pole, so the nail is attracted.
Iron is a soft magnetic material. It is easily magnetised and loses its magnetism quickly. It makes a temporary magnet.
Steel is a hard magnetic material. It is harder to magnetise, but it keeps its magnetism. It makes a permanent magnet.
A permanent magnet can lose its magnetism if it is dropped, hit hard or heated. The only sure test for a magnet is that it repels another magnet. Attraction alone is not enough, because a magnet also attracts unmagnetised iron.

Magnetic and non-magnetic materials

Magnetic fields and field lines

The magnetic field is the region around a magnet where it can push or pull on magnetic materials and other magnets. You cannot see it, but you can show it.
We draw the field using field lines. Outside the magnet, the lines run from the north pole to the south pole. The arrows show the direction a north pole would be pushed.
The field is strongest where the lines are closest together, near the poles. It gets weaker further away, where the lines are far apart.
Field lines never cross each other.
Sprinkling iron filings on paper above a magnet shows the shape of the field. The filings become tiny magnets and line up along the field.
Between two unlike poles that face each other, the field lines run straight from one magnet to the other and the magnets are pulled together. Between two like poles, the lines push apart and the magnets are pushed away.

Field lines around a bar magnet

Plotting compasses and the Earth's magnetic field

A plotting compass is a tiny magnet on a pivot, so it is free to turn. It lines up with the magnetic field where it is placed. The north-seeking end of the needle points along the field, away from a north pole and towards a south pole.
To plot a field line, place a bar magnet on paper. Put the compass near one pole and mark a dot at each end of the needle. Move the compass so that one end sits on the last dot, and repeat. Join the dots, then add an arrow from north to south.
The Earth has its own magnetic field. It behaves as if there is a giant bar magnet inside it, tilted slightly from the Earth's spin axis.
Scientists think the field is made by the slow movement of molten iron and nickel in the Earth's outer core.
A compass needle lines up with the Earth's field, so its north-seeking end points towards the Earth's north. Since opposite poles attract, the magnetic pole in the Arctic must really be a magnetic south pole.
Animals such as some birds and turtles are thought to use the Earth's field to find their way.

Plotting compasses round a bar magnet

Electromagnets

An electric current produces a magnetic field. An electromagnet is a coil of wire wrapped round a soft iron core. When current flows, the coil and the core act as a magnet.
The big advantage is that an electromagnet can be switched on and off. When the current stops, the soft iron loses its magnetism.
You can make an electromagnet stronger by adding more turns of wire, by using a bigger current, or by using an iron core.
If you reverse the current, the north and south poles swap over.
Electromagnets are used in scrapyard cranes that lift and drop iron and steel, in electric bells and in door locks. Very strong ones are used in medical MRI scanners.
For safety, use low-voltage cells in school. The wire can get warm, so switch the current off between readings.

An electromagnet

Think like a scientist: how strong is the electromagnet?

Question: does the number of turns of wire change how strong an electromagnet is?
The independent variable is the number of turns. The dependent variable is the strength of the electromagnet, which you can measure by counting how many identical paper clips it picks up.
Control variables include the same cell (same current), the same iron nail and the same size of paper clip.
Repeat each test three times and find a mean, because the number of clips can change between tries.
To evaluate your results, ask what could be wrong. The cell may have run down, the clips may have been tangled, or the wire may have heated up.
Inquiry task: plan a fair test to find out whether the distance between a magnet and a paper clip changes the force on it. List your variables and say how you will measure the distance and the effect.

स्लाइड्स

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साइन अप करें — फ्री लॉग इन

प्रैक्टिस सवाल

फ्री प्रीव्यू — 51 में से 8 सवाल। सभी देखने के लिए साइन अप करें।

1.Where is the pull of a bar magnet strongest?
Easy
- AIn the middle of the magnet
- BIt is the same everywhere along the magnet
- CNear the poles, at the ends
- DOnly at the north pole, not at the south pole
2.What happens when the north poles of two magnets are brought close together?
Easy
- AThey push each other away
- BThey pull towards each other
- CThey have no effect on each other
- DThey swap poles with each other
3.Two magnets are pulling towards each other. Which poles must be facing?
Easy
- ATwo north poles
- BTwo south poles
- CIt could be any two poles
- DA north pole and a south pole
4.Which of these objects would a magnet pick up?
Easy
- AA piece of aluminium foil
- BA steel paper clip
- CA plastic spoon
- DA wooden pencil
5.What is a magnetic field?
Easy
- AThe metal inside the magnet that makes it magnetic in the first place
- BThe region around a magnet where it can push or pull on magnetic materials
- CThe strongest pole at one end of the magnet, where the force acts
- DA kind of force that only ever works when the objects are touching
6.A magnet pulls a paper clip towards it without touching it. What kind of force is this?
Easy
- AA contact force
- BA friction force
- CA force that only acts on plastic
- DA non-contact force
7.Which way do magnetic field lines point outside a bar magnet?
Easy
- AFrom the north pole to the south pole
- BFrom the south pole to the north pole
- CAway from both poles at the same time
- DTowards the middle of the magnet only
8.Where are the field lines closest together around a bar magnet?
Easy
- AIn the middle of the magnet
- BFar away from the magnet
- CNear the poles, where the field is strongest
- DThey are always spaced evenly

और सवाल

9.Which statement about magnetic field lines is correct?
Medium
- AThey always cross over at the poles
- BThey start at the south pole of a magnet
- CThey are all exactly the same length
- DThey never cross each other
10.What is the needle of a compass?
Medium
- AA piece of iron that has not been magnetised
- BA small magnet that is free to turn
- CA tiny arrow pointing to the nearest metal
- DA thin wire that the Earth's gravity pulls
11.Why does a compass needle point north?
Medium
- AIt is attracted by the light of the Sun
- BIt is pulled by the force of gravity
- CIt lines up with the Earth's magnetic field
- DIt points to the nearest large magnet it finds
12.The north-seeking end of a compass points towards the Arctic. What does this tell us about the Earth's magnetic pole there?
Hard
- AIt behaves like a magnetic south pole
- BIt behaves like a magnetic north pole
- CIt is not a magnetic pole at all
- DIt is the same as an electric pole
13.A steel nail is held close to a strong magnet and then picks up a pin. What has happened to the nail?
Medium
- AIt has melted slightly because of the magnet
- BIt has gained an electric charge from the magnet
- CIt has become a magnet because of the nearby magnet
- DIt has become heavier because of the magnet
14.Which of these is a temporary magnet?
Medium
- AA soft iron nail held next to a magnet
- BA bar magnet made of steel
- CA magnetised steel compass needle
- DA horseshoe magnet in a toy
15.A student has two bars. One attracts the end of a magnet, and she wants to know if it is a magnet itself. Which test is certain?
Hard
- ASee whether it also attracts a steel paper clip
- BSee whether it attracts the pole of the magnet
- CSee whether it feels colder than the other bar
- DSee whether it repels the same pole of the magnet
16.What is an electromagnet?
Medium
- AA magnet that is made from a coil of plastic instead of metal
- BA coil of wire that acts like a magnet when a current flows through it
- CA magnet that works without needing any current to flow at all
- DA magnet that can only ever attract objects made of copper
17.What is one advantage of an electromagnet over a permanent magnet?
Medium
- AIt never needs a power supply to work
- BIt can be switched on and off
- CIt is always stronger than any permanent magnet
- DIt works the same way without any wire
18.Which change would make an electromagnet stronger?
Medium
- ATaking the iron core out of the coil
- BReplacing the iron core with a plastic rod
- CUsing a longer lead between the cell and the coil
- DWinding more turns of wire round the core
19.Why is soft iron used for the core of an electromagnet?
Medium
- AIt is easily magnetised and loses its magnetism when the current stops
- BIt conducts electricity much better than copper does
- CIt stops the wire from getting warm when it is used
- DIt keeps its magnetism for years after the current stops
20.A bar magnet is cut into two equal pieces. What do you have?
Medium
- AOne piece with only a north pole and one with only a south pole
- BTwo pieces of metal that are no longer magnets
- CTwo smaller magnets, each with a north and a south pole
- DOne strong magnet and one very weak magnet
21.A student tests whether the number of turns on an electromagnet changes how many paper clips it picks up. Which variable should she keep the same?
Hard
- AThe number of turns of wire that are wound on the coil
- BThe number of paper clips that the nail manages to pick up
- CThe strength of the electromagnet that she is testing
- DThe size of the current, using the same cell each time
22.Which is the best way to compare the strength of two magnets fairly?
Medium
- AHold each one and decide which one feels the strongest to you
- BCount how many identical paper clips each magnet can pick up
- CLook at which magnet seems to be the shiniest and newest
- DAsk a friend which magnet looks to be the most powerful
23.
A student moves a paper clip towards a magnet and records whether it is pulled across.
Distance from magnet (cm) Paper clip pulled to magnet?
1 Yes
2 Yes
3 Yes
4 No
5 No
Which conclusion matches these results?
Hard
- AThe magnet pulls the clip at any distance if you wait long enough
- BThe magnet only pulls the clip when the two of them touch
- CThe magnet pulled the clip at 3 cm or less, but not at 4 cm or more
- DThe magnet pulled the clip harder as the distance grew larger
24.A plotting compass is placed near the south pole of a bar magnet. Which way does the north-seeking end of the needle point?
Hard
- ATowards the south pole of the magnet
- BAway from the south pole of the magnet
- CAlong the side of the magnet, not at the pole
- DTowards the north pole at the far end of the magnet
25.Which of these materials are magnetic? (select all that apply)
Easy
- AIron
- BNickel
- CPlastic
- DSteel
- ECopper
26.Which statements about magnetic field lines are true? (select all that apply)
Medium
- AThey are the same distance apart everywhere
- BThey are closest together where the field is strongest
- CThey leave the north pole and enter the south pole
- DThey never cross each other
- EThey stop at the edge of the paper
27.Which of these can weaken a permanent magnet? (select all that apply)
Medium
- AHitting it hard with a hammer
- BDropping it onto a hard floor
- CHeating it strongly
- DKeeping it in a drawer with a plastic ruler
- ELeaving it on a wooden table
28.Which of these use an electromagnet? (select all that apply)
Hard
- AAn electric door lock
- BA fridge magnet
- CAn electric bell
- DA scrapyard crane lifting old cars
- EA compass needle
29.Every magnet has a north pole and a south pole.
Easy
True or false?
30.Two like poles attract each other.
Easy
True or false?
31.Iron, steel, nickel and cobalt are all magnetic materials.
Easy
True or false?
32.Outside a bar magnet, field lines are drawn from the south pole to the north pole.
Easy
True or false?
33.Aluminium is attracted to a magnet because it is a metal.
Medium
True or false?
34.Scientists think the Earth's magnetic field is produced by the movement of molten iron and nickel in its outer core.
Hard
True or false?
35.Match each term to its meaning.
Easy
- Pole
- Magnetic field
- Compass
- Electromagnet
- A coil of wire with a current that acts as a magnet
- The region around a magnet where it acts
- A small magnet that lines up with the field
- A place near the end of a magnet where the force is strongest
36.Match each material to a description of how it behaves near a magnet.
Medium
- Iron
- Steel
- Copper
- Plastic
- Magnetic, and keeps its magnetism
- A conductor that is not magnetic
- Magnetic, and easily magnetised and demagnetised
- An insulator that is not magnetic
37.Match each change to what happens to an electromagnet.
Medium
- Add more turns of wire
- Switch off the current
- Reverse the current
- Remove the iron core
- It gets weaker
- It gets stronger
- It stops being a magnet
- Its north and south poles swap
38.Put these electromagnets in order from the weakest to the strongest. They use the same cell and nail.
Easy
- 5 turns of wire
- 40 turns of wire
- 20 turns of wire
- 10 turns of wire
39.Put these steps for magnetising a steel needle in order.
Medium
- Lay the steel needle on the table
- Lift the magnet well away and repeat in the same direction
- Test the needle by seeing if it picks up a pin
- Stroke the needle along its length with one pole of a magnet
40.Put these steps for plotting a field line with a compass in order.
Hard
- Put the compass near one pole and mark a dot at each end of the needle
- Move the compass so one end of the needle sits on the last dot
- Repeat, then join the dots into a smooth line
- Add an arrow from the north pole towards the south pole
- Draw round a bar magnet on a sheet of paper
41.Complete the sentence about magnetic poles.
Easy
Unlike poles ____ each other.
42.Complete the sentence about like poles.
Easy
Like poles ____ each other.
43.Complete the sentence about field lines.
Easy
Outside a magnet, field lines point from the ____ pole to the south pole.
44.Complete the sentence about magnets.
Medium
A magnet that works only while a magnet is near it, or while a current flows, is called a ____ magnet.
45.Complete the sentence about electromagnets.
Hard
A coil of wire wrapped round an iron core, which is a magnet only while a current flows, is called an ____.
46.Describe how to plot the magnetic field of a bar magnet using a plotting compass.
Medium
47.Explain why an iron nail is attracted to a magnet and can pick up a pin.
Medium
48.Plan a fair test to find out how the number of turns of wire affects the strength of an electromagnet. Use low-voltage cells only.
Hard
49.A magnet picks up 11, 13, 12 and 12 paper clips in four trials. Calculate the mean number of paper clips picked up.
Medium
50.A magnet picks up 15 paper clips when it is 1 cm from the pile and 6 paper clips when it is 3 cm from the pile. Calculate how many fewer paper clips it picks up at 3 cm.
Easy
51.An electromagnet with 20 turns picks up 6, 9 and 12 paper clips in three repeats. Calculate the mean number of paper clips.
Medium

रिवीज़न नोट्स

## Poles and forces - Every magnet has a **north** and a **south pole**. The force is strongest at the poles. - **Unlike poles attract. Like poles repel.** Magnetic force is a **non-contact force**. ## Materials - **Magnetic:** iron, steel, nickel, cobalt. **Not magnetic:** copper, aluminium, gold, silver, wood, plastic. - A magnetic material near a magnet becomes a magnet: **induced magnetism**. Iron is temporary, steel is permanent. - Only **repulsion** proves something is a magnet. ## Fields - The **magnetic field** is the region where a magnet acts. **Field lines** run from **north to south**, never cross and are closest where the field is strongest. - A **plotting compass** lines up with the field. The **Earth** has its own magnetic field. ## Electromagnets - A **coil** of wire with a **current**, round an **iron core**. It can be switched on and off. - Stronger with **more turns**, **more current** and an iron core. Reversing the current swaps the poles. ## Inquiry - Identify the independent, dependent and control variables, repeat and evaluate.

Magnet: An object that attracts magnetic materials and has a north pole and a south pole.
Pole: A place near the end of a magnet where the magnetic force is strongest.
Non-contact force: A force that acts across a gap, without the objects touching.
Magnetic material: A material attracted to a magnet: iron, steel, nickel and cobalt.
Induced magnetism: Magnetism that a magnetic material gains when it is near a magnet.
Permanent magnet: A magnet that keeps its magnetism, such as one made of steel.
Temporary magnet: A magnet that is only magnetic while a magnet is near it or a current flows, such as soft iron.
Magnetic field: The region around a magnet where it can push or pull on magnetic materials.
Field line: A line showing the direction of a magnetic field, from north to south outside the magnet.
Plotting compass: A small magnet on a pivot that lines up with a magnetic field.
Earth's magnetic field: The field around the Earth, which makes a compass needle point north.
Electromagnet: A coil of wire, usually round an iron core, that is a magnet while a current flows.
Soft iron: A magnetic material that is easily magnetised and demagnetised.

फ्लैशकार्ड

What two poles does every magnet have?

A north pole and a south pole.

Where is the force of a magnet strongest?

Near the poles, at the ends.

What happens between unlike poles?

They attract.

What happens between like poles?

They repel.

Why is magnetic force called a non-contact force?

It acts across a gap, without the objects touching.

Name four magnetic materials.

Iron, steel, nickel and cobalt.

Name three materials that are not magnetic.

For example copper, aluminium and plastic.

What is the only sure test for a magnet?

It repels another magnet.

What is induced magnetism?

A magnetic material becomes a magnet when it is near a magnet.

How do iron and steel differ as magnets?

Iron is a temporary magnet. Steel keeps its magnetism, so it makes a permanent magnet.

What is a magnetic field?

The region around a magnet where it can push or pull on magnetic materials.

Which way do field lines point outside a magnet?

From the north pole to the south pole.

Where are field lines closest together?

Where the field is strongest, near the poles.

What is a plotting compass?

A small magnet on a pivot that lines up with the magnetic field.

Why does a compass needle point north?

It lines up with the Earth's magnetic field.

What makes up an electromagnet?

A coil of wire round a soft iron core, with a current flowing.

How can you make an electromagnet stronger?

More turns of wire, a bigger current or an iron core.

What happens if you reverse the current in an electromagnet?

The north and south poles swap over.

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जल्द आ रहा है

Distance from magnet (cm)	Paper clip pulled to magnet?
1	Yes
2	Yes
3	Yes
4	No
5	No