Electromagnetism

under construction

The various syllabus statements cover very different quantities of material. I'm sorry there aren't any pictures yet. You need to read this page in conjunction with block 6  of the syllabus specification.  xxx  represents a page number in England, (3rd edition).

See below the table for a general introduction that applies to most of these syllabus statements. Points relating to particular syllabus statements are given in the hyperlinks, but those hyperlinks will assume that you have the general introduction at your fingertips.

1 use the following units: ampere (A), volt (V), watt (W)       258-259
2 recall that magnets repel and attract other magnets, and attract magnetic substances         258-259
3 recall the properties of magnetically hard and soft materials        262-263
4 understand the term ‘magnetic field line’        258-261
5

understand that magnetism is induced in some materials when they are placed in a magnetic field )       262-263

6

sketch and recognise the magnetic field pattern for a permanent bar magnet and that between two bar magnets       258-259

7 know how to use two permanent magnets to produce a uniform magnetic field pattern       260-261
8 recall that an electric current in a conductor produces a magnetic field round it        260-261
9 describe the construction of electromagnets        262-263
10

sketch and recognise magnetic field patterns for a straight wire, a flat circular coil and a solenoid when each is carrying a current      260-261

11

appreciate that there is a force on a charged particle when it moves in a magnetic field as long as its motion is not parallel to the field        -

12

recall that a force is exerted on a current-carrying wire in a magnetic field, and, how this effect is applied in simple d.c. electric motors and  loudspeakers         264-265, 268-269

13

predict the direction of the resulting force when a wire carries a current perpendicular to a magnetic field      264-265

14

recall that the force on a current-carrying conductor in a magnetic field increases with the strength of the field and with the current        264-265

15

recall that a voltage is induced in a conductor when it moves through a magnetic field or when a magnetic field changes through a coil; also recall the factors which affect the size of the induced voltage        270-272   

16

describe the generation of electricity by the rotation of a magnet within a coil of wire and of a coil of wire within a magnetic field; also describe the factors which affect the size of the induced voltage                                  273-275

17

recall the structure of a transformer, and understand that a transformer changes the size of an alternating voltage by having different numbers of turns on the input and output sides                     276-277

18

explain the use of step-up and step-down transformers in the large-scale generation and transmission of electrical energy       276-277, 278-279

19

recall and use the relationship between input (primary) and output (secondary) voltages and the turns ratio for a transformer secondary turns

input (primary) voltage/output (secondary) voltage = primary turns / secondary turns

Vp/Vs = np / ns

    276-277, 278-279

20

recall and use the relationship

input power = output power

Vp Ip = Vs Is

for 100% efficiency

    276-277, 278-279

General introduction

You need to grasp three basic ideas:

Basic idea Rule used Notes
current  ®  magnetic field Right hand grip rule thumb  ®  current, fingers  ®  field
current + magnetic field  ®  motion Left hand Motor rule First finger  ®  Field

seCond finger  ®  Current

thuMb  ®  Motion  (or THumb  ®  THrust)
motion + magnetic field  ®  induced current Right hand Dynamo rule

Which rule to use ?

All currents produce magnetic fields. The strength of the field at any given instant is proportional to the size of the current at that instant, and also to the number of turns of wire on your coil. If you have two equal currents in roughly the same place, but flowing in opposite directions, then their magnetic fields cancel: this happens, for instance, in a two-core cable connecting a device to a supply, regardless of whether the supply is d.c. or a.c. It also happens in an RCCB, used nowadays where fuses used to be employed. Use the right hand grip rule to determine the direction of the field produced by the current.

An electromagnet* placed near to a magnet** will be attracted to, repelled by or deflected by the magnetic field. This is the principle of the electric motor. If you start with a current and a magnet, and get motion as a result, then you should use the Left hand Motor rule. Notice that L and M come next to each other in the alphabet. This rule applies to obvious things like electric motors, but also to less obvious situations like loudspeakers (where the initial current comes from an amplifier and the resulting movement makes a sound wave) and electric bells (in which the resulting movement causes a hammer to hit a gong). Notice that the 'magnet' referred to above might itself be an electromagnet.

* or a current-carrying wire

** which might itself be another electromagnet or even another current carrying wire

Induced currents occur when you change the magnetic environment of a coil of wire. For example

In the first five of these cases you start with motion and get a current as a result, so everything is the other way round to the situation in the preceding paragraph. The appropriate rule to use here is the Right hand Dynamo rule.

In deciding which rule to use, it's the bits in purple and yellow that you have to think about.

Faraday's Law

This law is enormously useful, and you need to learn it word-perfectly

 

The induced emf is proportional to the rate of change of magnetic flux linkage.

  

'emf' stands for 'electro-motive force', which effectively means 'electron-moving force'. Bizarrely, it is measured in volts. The emf then generates an induced current, whose size depends on the resistance of the coils and whatever is connected to them, as well as on the size of the emf itself.

Using the law to explain why there is an induced emf.

You have to show that all three coloured phrases apply. So you have to state the law and then establish that

Using the law to explain how to increase or decrease the induced emf.

Changing any one of the three coloured phrases will change the emf, because of the  'is proportional to'  bit. Thus, to double the emf in a generator you could