Magnetism and electromagnetism

Question 1

What is magnetism?

Answer 1

The ability to pick up other magnetic substances.

Question 2

What is a magnetically hard material?

Answer 2

One that will stay magnetised once it has been magnetised. The domains (regions that the atoms are grouped into, in which all the tiny atomic magnets are aligned) are locked into position by the material’s structure once they have been aligned.
These are good for making permanent magnets.

Question 3

What is a magnetically soft material?

Answer 3

They will lose their magnetism more-or-less straight after it has been magnetised. The domains quickly become jumbled up again once the magnetising influence has been removed.
These are good for making electromagnets or the cores of transformers

Question 4

Simple rules of magnetism:

Answer 4

Like poles repel

Unlike poles attract

Question 5

What is a magnetic field?

Answer 5

A region where magnetic materials (e.g iron) experience a force. It has a strength and a direction. It can be represented by magnetic field lines

Question 6

What are magnetic field lines used to show?

Answer 6

The direction of magnetic fields. They ALWAYS point from north to south. The density of the lines gives an indication of the strength of the field

Question 7

How can you look at magnetic field lines?

Answer 7

Using iron filings or compasses as they align themselves with magnetic fields. Multiple compasses show you the field lines coming out, (if you only have one just move it around) or iron filings on top of a piece of paper.

Question 8

What does an electric current produce? How do the size and direction effect this?

Answer 8

A magnetic field. The larger the current, the stronger the magnetic field. The direction of the magnetic field depends on the direction of the current.

Question 9

What materials are notable for their display of magnetism?

Answer 9

Iron and iron compounds

Question 10

When does a wire have a magnetic field around it? What is this called?

Answer 10

When current is flowing through it a current carrying wire will have a field around it. This is called electromagnetism.

Question 11

What are examples of magnetic materials?

Answer 11

Iron, steel, nickel and cobalt.

Question 12

How can magnetism be induced in magnetic material?

Answer 12

A magnetic field can line up the domains in an unmagnetised object, making it into a temporary magnet (if it is made of a magnetically soft material).
This is why iron filing can be used to display magnetic field patterns - they become tiny temporary magnets when sprinkled in a magnetic field, and line up along the field lines

Question 13

Learn the diagrams of magnets and their magnetic field lines on page 181

Answer 13

Will do!

Question 14

How does Flemings right hand rule work? (FOR FIELDS)

Answer 14

With your thumb (of your right hand) pointing in the direction of the current, the way your fingers curl is the direction of the field.

Question 15

The strength of the field around a current carrying wire is quite weak, how can this be increased?

Answer 15

1) increasing the current in the wire

2) wrapping the wire into a coil or solenoid (a long coil)

Question 16

What is the shape of the magnetic field around a solenoid like?

Answer 16

A solenoid is wire that is coiled multiple times. The shape of the magnetic field is the same as the shape of the magnetic field around a single bar magnet (p. 183).

Question 17

How can you work out the polarity of a solenoid? (The right hand grip rule for poles)

Answer 17

Imagine your hand it wrapped around the solenoid. Your fingers curl in the direction of the current and your thump will point to the North Pole of the solenoid.

Question 18

How can you increase the strength of the magnetic field in a solenoid?

Answer 18

1) increase the current flowing through it.
2) increase the number of turns on the solenoid.
3) wrap the solenoid around a magnetically soft core such as iron - this combination of a soft iron core and solenoid is known as an electromagnet.

Question 19

Explain how an electric bell works using electromagnets:

Answer 19

When the bell push switch is closed the circuit is complete and current flows (p. 183 for diagram). The soft iron core of the electromagnet becomes magnetised and attracts the iron armature. When the armature moves, the hammer strikes the bell. This causes the contact switch to open, the electromagnet switches off and the circuit is incomplete and current stops flowing. The electromagnet is turned off so the springs armature returns to the original position. The circuit is again complete and the whole process begins again. (i.e. the bell gets hit many many times)

Question 20

How does a circuit breaker use electromagnetism?

Answer 20

The circuit breaker cuts off current if it exceeds a certain value. If the current is too high the electromagnet becomes strong enough to pull the iron catch out of position so that the contacts open and the circuit breaks. Once the problem in the circuit has been corrected the catch is repositioned by pressing the button.

Question 21

What is a relay switch?

Answer 21

It is a way of using a low voltage circuit to remotely switch on a high voltage (and possibly dangerous) circuit

Question 22

How does a relay switch work using electromagnets?

Answer 22

When switch A is closed, the small current flows around the circuit on the left (p. 185 for reference). The current passes through the solenoid, the soft iron core becomes magnetised and attracts the iron armature as it is pivoted and L-shaped. This causes the springy contact switch B to close the high voltage circuit. This completes the second circuit without the user having to come into contact with the second circuit.

Question 23

Example of when relay switches are used?

Answer 23

Car ignition circuits.

Question 24

How can you use two permanent magnets to produce a uniform magnetic field?

Answer 24

A uniform magnetic field exerts a constant force over a region
Such a field will consist of parallel, equally spaced magnetic field lines
This type of field can almost be found between a north and south magnetic pole (the middle line between two bar magnets)

Question 25

What is a solenoid?

Answer 25

A coil of wire carrying an electric current

Question 26

What is an electromagnet?

Answer 26

A coil of wire carrying a current creates a magnetic field that resembles that of a bar magnet. The strength of the field can be increased substantially by inserting a magnetically soft iron rod into the coil.
Electromagnets can be made much stronger than permanent magnets and can be turned on and off (by turning the current on and off)
They can also produce an alternating magnetic field if an alternating current flows in the coil.

Question 27

If a wire carrying a current in a magnetic field experiences a force then what must be the case?

Answer 27

As current is just a flow of charged particles (electrons if we’re talking about wires), it must also be the case that a charged particle moving in a magnetic field experiences a force (as long as it is not moving parallel to the field direction)

Question 28

What do you have to remember about predicting the direction of of the force on a charged particle in a magnetic field?

Answer 28

You have to be careful about the direction of current - as this is the direction of conventional current, but electrons go the other way
So if you are using Fleming’s left hand rule, you have ti turn your seCond finger round

Question 29

Describe an experiment to show the shape of the magnetic field around a bar magnet

Answer 29

Place a magnet under paper and sprinkle iron filings over

Tap the paper gently to reveal the pattern (the shape of the magnetic field)

Question 30

How does a loudspeaker use electromagnetism?

Answer 30

There is a current in the coil
This alternating current produces a magnetic field which is constantly changing
This interacts with the field of the permanent magnet
So, because of Fleming’s LHR there is a force on the coil and the vibration causes longitudinal sound waves