Electricity and magnetism 10: Electromagnetism

solenoid coils - design and uses of electromagnets

Doc Brown's Physics Revision Notes

Suitable for GCSE/IGCSE Physics/Science courses or their equivalent

 

This page will help you answer questions such as ...

How do you make a magnet using electricity?

What factors affect the strength of an electromagnet?

What do use electromagnets for?


Introduction to electromagnetism

When a current flows through a wire (or any conductor) a magnetic field is created around the wire.

The field (of the magnetic flux) can be imagined as a series of concentric circles at right-angles (perpendicular) to the wire - which is at the centre of the magnetic field (see the diagram below).

The direction of the magnetic field can be predicted from the 'right-hand thumb' rule.

If the current is flowing 'up' through the wire, the magnetic field runs anticlockwise and perpendicular to the wire.

You can show the direction of the field with a small plotting compass - two shown on the diagram - and you can trace out the circular pattern of the magnetic field.

A few simple rules (apart from the right-hand thumb rule)

(i) If you reverse the direction of current flow, you also reverse the direction of the magnetic field.

(ii) The strength of the magnetic field is increased overall by increasing the current.

(Don't say by 'increasing the p.d.' without saying to increase the current!)

(iii) For any current carrying wire, the closer you are to the wire, the greater the strength of the magnetic field. The magnetic field lines get closer and closer together the nearer you are to the wire.

 


A solenoid - electromagnets

We have seen that a single current carrying wire produces a magnetic field of concentric lines of force.

This in itself is of little use, but, there are ways of increasing the magnetic field effect to produce something of use in many 'electromagnetic' applications.

The principles of the solenoid.

If you coil the wire in a compact way (as in the diagram above) you can greatly intensify the magnetic field effect.

The resulting current carrying coil is called a solenoid.

The increase in field strength is due to all the lines of force lining up with each other and close together too - intensifying the magnetic field effect.

Remember - the closer the lines of force the greater the strength of the magnetic field at that point.

Due to the alignment of the lines of force the magnetic field inside the solenoid is very uniform and very strong.

The lines of force inside the solenoid are linear and parallel giving the magnetic field the same strength and direction.

Note: The magnetic field pattern outside the solenoid is just the same as a bar magnet.

 


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

(a) Increasing the current flow

The strength of the magnetic field is increased overall by increasing the current.

Any stream of moving electrically charged particles naturally creates a magnetic field.

The more charged particles moving through the wire, the greater the magnetic field effect.

(Don't say by 'increasing the p.d.' without saying to increase the current!)

(b) Increasing the number of coils of wire

Many solenoids consist of hundreds of coils of thin insulated copper wire.

The more coils packed tightly together, the greater the strength of the magnetic field.

(c) Using an iron core

If you place a rod of magnetic material like iron, inside the solenoid, the iron becomes an induced magnet and the magnetic lines of force are intensified through it.

As long as the current is flowing the electromagnetic effect will work.

Switch off the current and the magnetic effect goes.

This means you can use this system as an on/off temporary electromagnet that has many useful applications.

 

In most applications it is factors (b) and (c) that are employed to increase the effectiveness of the solenoid.

 


 

Coil 1. Just a plain solenoid coil, producing a relatively weak magnetic field.

Coil 2. This solenoid produces a much greater strength of magnetic field due to the addition of the iron rod.

Coil 3. Using two iron rods, or one thicker one, the filed strength is increased.

Coil 5. Unlike coils 2. and 3., which are temporary electromagnets (on/off with current), coil 5 would make a steel rod a permanent magnet.

Coil 5. could not be used as an on/off electromagnet, but it is a way of making permanent magnets.


Uses of electromagnets

(a) An electromagnet for picking up things

A magnet that can be switched on and off has many uses.

A good examples is picking up scrap iron or steel in a recycling yard.

The 'magnetic' crane can pick up these items and dump them down wherever you want by switching the current to the electromagnet on and off.

 

(b) Relay switch system

You can use an electromagnet in one circuit to operate another circuit.

Consider the relay system in the diagram below.

 

When you switch on the input circuit (closing switch (1)) the current flows through the solenoid (2).

Inside the solenoid coil is a soft iron core which becomes magnetised only when the current flows.

The solenoid electromagnet attracts the soft iron armature (the pivoted 'rocker') which is rotated anticlockwise.

When the 'rocker' rotates it pushes the contacts at (4) together to close the output circuit.

In this case the output circuit drives an electric motor.

 

Uses of a two circuit relay system

(i) This system is used where the output circuit might be operating with a potentially dangerous high p.d. or current.

This is how the starter motor of car is operated. You don't want lots of current through a circuit where you put the ignition key in.

(ii) The output circuit might be in a hazard zone e.g. remote control systems in a nuclear power plant where machinery is operating where there i potentially or actually, radioactive materials - obvious dangers!

 

 

(c) Electric bell

When you press the doorbell you close a circuit that magnetises the soft iron core of 1 or 2 solenoids.

The magnetised soft iron core of the solenoid attracts the striker to hit and ring the bell.

In moving, the striker also breaks the circuit switching off the electromagnet of the solenoid.

The 'sprung' striker then returns to close the circuit, so the striker is attracted again.

As long as you press the doorbell, the circuit keeps on being opened and closed to give the bell ringing effect.

 

 (d)


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Electricity and magnetism revision notes index

1. Usefulness of electricity, safety, energy transfer, cost & power calculations, P = IV = I2R, E = Pt, E=IVt

2. Electrical circuits and how to draw them, circuit symbols, parallel circuits, series circuits explained

3. Ohm's Law, experimental investigations of resistance, I-V graphs, calculations V = IR, Q = It, E = QV

4. Circuit devices and how are they used? (e.g. thermistor and LDR), relevant graphs gcse physics revision

5. More on series and parallel circuits, circuit diagrams, measurements and calculations gcse physics revision

6. The 'National Grid' power supply, environmental issues, use of transformers gcse physics revision notes

7. Comparison of methods of generating electricity gcse physics revision notes (energy 6)

8. Static electricity and electric fields, uses and dangers of static electricity gcse physics revision notes

9. Magnetism - magnetic materials - temporary (induced) and permanent magnets - uses gcse physics revision

10. Electromagnetism, solenoid coils, uses of electromagnets  gcse physics revision notes

11. Motor effect of an electric current, electric motor, loudspeaker, Fleming's left-hand rule, F = BIL gcse physics

12. Generator effect, applications e.g. generators generating electricity and microphone gcse physics revision


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