Electro Magnets

Electro Magnetic Effect

Electromagnets are temporary magnets created by wrapping a length of wire around an iron core. When a current is passed through the wire coil the iron becomes magnetized.

All current carrying conductors, such as a wire, have a weak localized magnetic field wrapped around them. By surrounding the iron core with many wires in a coil, the weak fields combine to magnetize the iron temporarily. This is known as the electromagnetic effect. This effect can be used in electromagnets, solenoids and electric motors.

All wires carrying a current have a small, localized magnetic field wrapped around them

The magnetic field around an iron core , which is part of an electromagnet, is similar in shape to that of the magnetic field around a permanent bar magnet. The location of the poles are determined by the direction of flow of the current. This means that the magnet can be reversed by reversing the current flow. The strength of the electromagnet is controlled by:

1. The size of the current - more current equals a stronger magnet.
2. The number of coils of wire - more coils equals a stronger magnet.

Like all magnets the magnetic field strength will decrease with the distance from the magnet, in the case of an electromagnet it is the distance from the iron core that matters.

Solenoids

The term solenoid was first used by Monsieur Ampere to mean any helical tube of wire. The coil of wire around an electromagnet is an example of a solenoid. A solenoid is an example itself of an electromagnetic device. The shape of the coil creates a controlled and concentrated magnetic field within the coils of the wire. The external part of the solenoid has a much weaker magnetic field.

A simple solenoid

Magnetic Field of a Solenoid

Within the coils of the solenoid the lines of magnetism are concentrated and form a strong uniform magnetic field along the core, at the ends the field becomes weaker as the field spreads out. Along the outside of the solenoid the field from each wire has a cancelling out effect, resulting in a weak diffuse field that does not extend far from the coil itself.

The field is strong and uniform within the solenoid. The direction of the field depends upon the direction of the current. In this example the magnetic field is indicated by the blue arrow and the lines of magnetism are in blue.

Cross section of a solenoid and the associated magnetic field.

The solenoid results in a highly concentrated and easy to control magnetic field, this can used to create simple electronically controlled switches. The solenoid is used to pull open or closes a metal switch connected to a seperate circuit. These are found in cars to allow a low current connection from the ignition switch to operate a high current switch to the starter motor.

Fleming’s Left Hand Rule

The magnetic field produced by any conductor carrying an electric current can be used to produce movement. The magnet created by the flow of current behaves in the same way as any other type of magnet. If it comes close to any magnet there will be movement due to the repulsion or attraction of the magnetic fields. This induced motion in known as the motor effect.

The force of attraction or repulsion is felt by both the magnet and the wire, however, as normally the wire is light and the magnet heavy the movement is only observed in the wire.

The direction of the magnetic field around the wire is controlled by the direction of flow of current. If the current direction is known and so is the direction of the main magnetic field it is possible to predict the direction of moment of the wire. This is achieved by using Fleming’s Left Hand Rule.

The magnetic field and the current are represented by the fingers as shown,using the left hand, the thumb indicates the direction of movement of the wire.

Try it: If the wire in the middle of this magnet has a current flow into the page, the wire will move down. Try it with the current flowing out of the page.

__F__irst finger : __F__ield

__C__entre finger : __C__urrent

__Thumb__s up for movement

Calculating Force

The force created by the interaction of the magnetic field around the wire and the permanent magnetic field is affected by

● The strength of the main magnetic field

● The current in the wire

● The length of wire inside the main magnetic field.

The relationship can be expressed as an equation.

Force (Newtons) = Magnetic Flux Density (Tesla (T)) x Current (Amps) x Length (meters).

Or F = B x I x l

Magnetic Flux Density is a way of stating how strong the magnetic field is. A standard bar magnet has a flux density of around 0.01 T, the Earth’s flux density is 0.0005 T, whereas a strong lab magnet might be as high as 10 T

The force on the wire will increase if:

● The magnets is stronger

● The current is increased

● More wire is used.

Example: What force is applied to 0.5 m of wire if it is sitting in a magnetic field of 4 T and a 2 A current is passing through the wire?

F = Bx I x l _= 4 x 2 x 0.5 = 4 _Newtons

What force would be applied to the wire in a 15A motor using a 10 T magnet and 200 m of wire in the coil?

F = B x I x l = 10 x 15 x 200 = 30,000 Newtons

Electric Motors

Electric motors make use of the motor effect of a wire in a magnetic field. The motor contains a magnet surrounding a coil of wire on a spindle so that the coil can rotate. In larger motors the magnet is an electromagnet to provide more flux and therefore more force.

As the coil of wire in the motor is spinning, this presents two main problems.

1. How to prevent the connecting wire tangling up as the coil spins.
2. How to ensure that the direction of flow of current remains to same as the coil spins.

The solution to these problems is called a split ring commutator. This is a metal ring split into two halves. One half being connected to either side of the coil. These touch up against small blocks of carbon, called brushes that are then connects to the power supply. This separates the external power supply wires from the wires of the internal coil, and ensures that the current flow in the wire remains flowing in one direction only. If this did not happen the motor would not spin freely, it would vibrate backwards and forwards instead.

Within the motor the magnetic field from the coils, caused by the current flowing in the wire, interacts with the magnetic field around it to produce movement. The spindle in the middle of the coil always the coil to rotate and this motion can be used to drive a machine, including cars and trains. Each side of the coil follows the motion as described by Fleming’s Left Hand rule. The current on each side is flowing in the opposite direction so on one side the wire is forced up and on the other it is forced downwards.

If you wanted a wire to move upwards and the magnetic field was heading in towards the screen, what direction is the current flowing?

Your answer should include: Fleming’s / Fleming / Left Hand rule
Explanation: Using Fleming’s Left Hand rule the current must be flowing from Left to Right.

Describe the structure of a magnetic field inside a solenoid.

Your answer should include: strong / uniform / shape
Explanation: The field is strong and uniform in shape with the core of the solenoid, it becomes weaker towards the end of the coil.

In electric motor in a train has coil of wire made from 1 Km of wire which is surrounded by an electromagnet producing 15 T of flux. Assuming the electrical supply is 25 A , what force is applied to the spinning coil?

Your answer should include: 375000N / 375000
Explanation: F = B x I x l = 15 x 25 x 1000 = 375,000 Newtons