School Physics notes: Electromagnetism & its applications

Electricity and magnetism 10: Electromagnetism

solenoid coils - design & uses of electromagnets

Doc Brown's school physics revision notes: GCSE physics, IGCSE physics, O level physics,  ~US grades 8, 9 and 10 school science courses or equivalent for ~14-16 year old students of physics

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 we use electromagnets for?

1. Introduction to electromagnetism

(See other page for )

2. The solenoid and electromagnet applications

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

4. Electromagnet uses: magnet, relay switch, bell, loudspeaker, microphone, Maglev train

1. Introduction to electromagnetism

This is all about the connection between electricity and magnetism.

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 diagrams below).

You can demonstrate the presence of the magnetic field with iron filings and thickish wire carrying a relatively high current.

The direction of the magnetic field can be predicted using Fleming's left-hand rule (illustrated above).

For the above diagram - imagine the current flowing through a straight wire and the magnetic field can be envisaged as a series of concentric rings about the axis of the conducting wire.

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 above - 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 flux lines get closer and closer together the nearer you are to the wire - meaning the magnetic field strength increases the nearer you are to the wire.

The strength of the magnetic field falls away quite rapidly at first as you get further from the wire, then the reduction rate slows down with increasing distance.

Its a non-linear graph.

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2. The solenoid and its electromagnet applications

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.

Left: The magnetic field produced a solenoid coil, note the linear and denser concentration of the lines of force down the centre of the coil.

Right diagram above: A solenoid coil containing a soft iron core, around which is coiled insulated copper wire.

An effective solenoid needs to consist of hundreds of coils of finely wound insulated copper wire.

Such an iron-cored solenoid coil can act as a temporary magnet.

The principles of a functioning solenoid

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

The stretched out resulting current carrying coil is called a solenoid and can act as an electromagnet which can be switched on and off depending whether current is flowing or not i.e. acts as a temporary magnet.

You can 'construct' this magnetic field diagram using a plotting compass to map the magnetic field of a steel bar permanent magnet.

Inside the coils, 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 at what is effectively another north-south pole situation.

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

So, note the uniformity and intensity of the magnetic field inside the coil, which is much weaker outside the coil because lots of overlapping lines of force around each coil cancel each other out.

The magnetic field is overall weak except at the ends of the solenoid where it is very strong.

Note: The magnetic field pattern outside the solenoid is just the same as a with a north and south pole and the magnetic flux lines flowing from north to south.

Polarity of a solenoid coil

You can work out the polarity of a solenoid by viewing the end of the solenoid and observing the way the current is flowing.

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

As we have seen, due to the alignment of the parallel lines of force, the magnetic field inside the solenoid is very uniform and very strong, but only in the coil and the poles at the ends of the solenoid, so how can we increase the magnetic effect i.e. make a stronger electromagnet?

(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 turns of thin insulated copper wire.

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

You can do this by (i) increase the number of coils using the same length of wire OR (ii) you can both increase the number of coils AND the total length of wire.

(c) Using an soft iron core - this makes a practical electromagnet

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 with the 'empty' solenoid its self, the magnetic flux is greatest at the ends of the solenoid, which now coincides with the ends of the magnetically 'soft' iron rod (left diagram).

The solenoid plus the iron rod are effectively make a strong 'bar magnet' (right diagram).

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.

(d) Decreasing the length of the solenoid

If you can compact the solenoid to a shorter length for the same number of insulated coils of wire you increase the intensity of the magnetic field.

I don't consider this an important factor since the insulated coils of wire are usually packed as tightly together as possible and length might be determined by how it fits into some device.

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.

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4. Uses of electromagnets

As we have seen, due to the alignment of the parallel lines of force, the magnetic field inside the solenoid is very uniform and very strong, but only at pole at the ends of the solenoid.

Therefore in the applications of a solenoid electromagnet, the mechanical actions is centred on the poles.

(a) An electromagnet for picking up things

An electromagnet 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. Pressing the  'on' switch induces a magnet field in the iron 'pickup'. You move the scrap iron or steel to another location, then switch 'off' the current. The iron 'pickup' loses its magnetism and the scrap falls to the ground.

(b) Relay switch system

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

A switch in a primary circuit automatically controls the 2nd circuit via an electromagnetic switch system.

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, but could be anything you want to switch on remotely using a low voltage-current circuit.

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 the high current needed by the starter motor moving 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

The electric bell circuit

The d.c. power supply is not shown in the diagram, but the terminal connections are on the left.

When you press the doorbell you close a circuit that allows current to flow to magnetise the soft iron core of the 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 current and so the electromagnetism of the solenoid.

Therefore the 'sprung' striker then returns to close the circuit, re-magnetising the solenoid soft iron core, so the striker is attracted again to strike the bell.

This happens quite quickly to give a continuous ringing sound.

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

(d) Magnetic separators in a recycling plant

A magnet can be used to pick out scrap iron and steel from a conveyer belt of rubbish.

The items might be cans or steel grills etc. An electromagnet is used for the process.

(e) Maglev trains

(maglev is shorthand for 'magnetic levitation' but not of the spirit world!)

Maglev trains use magnetic repulsion to literally float a train a short height above the guidance track. A magnetic field can be manipulated to move the train along at high speeds with virtually no friction except for air resistance.

Maglev (derived from magnetic levitation) is a system of high train transportation that uses two sets of magnets: one set to repel and push the train up off the track, and another set to move the elevated train ahead, taking advantage of the lack of friction.

With maglev technology, there is just one moving part: the train itself. The train travels along a guideway of magnets which control the train's stability and speed.

Since the propulsion and levitation require no moving parts Maglev trains are quieter and smoother than conventional trains and have the potential for much higher speeds.

Note that electromagnets do not produce a permanent magnetic force. The magnetism they produce is temporary, and can be switched off and on depending upon what is required. Engineers use electromagnetism in the design and construction of maglev trains.

(f) Loudspeakers and microphones

Loudspeakers and microphones use an oscillating electromagnet system running off alternating current (a.c.).

For more details see

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(g) MRI scanners magnetic resonance imaging

MRI scanners use powerful electromagnets to create detailed images of the inside of your body.

It is a relatively safe technique that does not use ionising radiation, instead it uses safer EM radio waves.

The high frequency radio waves resonate with protons (H atoms) in you body and this resonance is detected and used to build up an image based on where the protons are and their density or concentration - and there are a lot of them in your body e.g. water, fat, protein etc.

(h)

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