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

How do you make a magnet using electricity?

What factors affect the strength of an electromagnet?

What do we 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).

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 - 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.

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.

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, but only in the coil and the poles at the ends of the solenoid.

Because the lines of force inside the solenoid are linear and parallel it gives the magnetic field the same strength and direction.

However, outside the coil, the overlapping field lines cancel each other out - so the magnetic field is weak except for the ends of the solenoid.

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.

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 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 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.

Uses of electromagnets

(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 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.

This happens quite quickly to give a continuous ringing.

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.

(f) Loudspeakers and microphones

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

For more details see

gcse physics

gcse physics revision

(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)

TOP OF PAGE

Electricity and magnetism revision notes index

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

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 gcse physics revision notes (energy 6)

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

9. gcse physics revision

10  gcse physics revision notes

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

12 gcse physics revision

GCSE physics revision notes on electromagnetism solenoid coils design and uses of electromagnets IGCSE revision notes on electromagnetism solenoid coils design and uses of electromagnets KS4 physics Science notes on electromagnetism solenoid coils design and uses of electromagnets GCSE physics guide notes on electromagnetism solenoid coils design and uses of electromagnets for schools colleges academies science course tutors images pictures diagrams for electromagnetism solenoid coils design and uses of electromagnets science revision notes on electromagnetism solenoid coils design and uses of electromagnets for revising physics modules physics topics notes to help on understanding of electromagnetism solenoid coils design and uses of electromagnets university courses in physics careers in science physics jobs in the engineering industry technical laboratory assistant apprenticeships engineer internships in physics USA US grade 8 grade 9 grade10 AQA GCSE 9-1 physics science revision notes on electromagnetism solenoid coils design and uses of electromagnets GCSE notes on electromagnetism solenoid coils design and uses of electromagnets Edexcel GCSE 9-1 physics science revision notes on electromagnetism solenoid coils design and uses of electromagnets for OCR GCSE 9-1 21st century physics science notes on electromagnetism solenoid coils design and uses of electromagnets OCR GCSE 9-1 Gateway  physics science revision notes on electromagnetism solenoid coils design and uses of electromagnets WJEC gcse science CCEA/CEA gcse science GCSE physics revision notes on electromagnetism solenoid coils design and uses of electromagnets

 KS3 SCIENCE QUIZZES ALPHABETICAL INDEX GCSE grade 9-1 & IGCSE CHEMISTRY Doc Brown's Travel Pictures & Notes ADVANCED LEVEL CHEMISTRY [SEARCH BOX] - see below All website content © Dr Phil Brown 2000 onwards. All copyrights reserved on revision notes, images, quizzes, worksheets etc. Copying of website material is NOT permitted. Exam revision summaries and references to science course specifications are unofficial. Email doc b: chem55555@hotmail.com

Doc Brown's Physics

*