CURRENT ELECTRICITY AND MAGNETISM (3)

1. The field inside a looped current-carrying conductor

     

If a steady current flows through a conductor which forms a loop, the loop will be surrounded by magnetic lines of force, as shown in the diagram on the left. The corkscrew rule will give the direction of the magnetic field which results from any point in the conductor.

The magnetic lines of force which result from a steady current flowing through a looped conductor are shown in the diagram on the right. Only the lines in the horizontal plane are shown.

2. Field produced by a solenoid

A solenoid is a conducting wire looped to form a coil. If a steady current flows through such a device, the magnetic fields produced by the loops reinforce one another in certain places, and cancel one another at other places. The result is that the solenoid produces an overall magnetic field similar to that produced by a bar magnet, as illustrated in the diagram below.


2.1 Polarity of the magnetic field of a solenoid

The polarity of the magnetic field (that is, which end is magnetic "north" or "south") can easily be worked out from the following rule, which is illustrated in the diagram on the left:

If you grasp the solenoid in your right hand, so that your fingers curl around it in the direction of the conventional current flow, then your thumb will point to the north pole of the magnet.

Alternately, imagine that you are facing the coil end-on. If the current flows in a clockwise direction, you are looking at the south (blue) pole. If the current flows in an anticlockwise direction, you are looking at the north (red) pole.


3. Electromagnets

If a soft iron bar is placed inside the solenoid, and the current switched on, the iron bar becomes temporarily magnetised. This is the principle of the ELECTROMAGNET. The strength of an electromagnet depends on:

The diagram on the right simulates a simple electromagnet. Insulated copper wire is wound around the arms of an iron bar bent in the shape of a horseshoe. The coils are wound in opposite directions on each arm. When the switch is closed, the two solenoids create magnetic fields of opposite polarity in the two arms, and the iron acts as a magnet, easily attracting the iron block.

As soon as the switch is opened, the current stops flowing, and the magnetic fields collapse.

In practice, powerful electromagnets are used for lifting iron or steel masses, such as in the scrap-iron industry.


4. Some devices based on electromagnets:

Many useful devices are based on electromagnets produced by currents passing through solenoids. A few are listed below:

5. Additional questions


Maxwell's Corkscrew Rule

This rule predicts the direction of the magnetic field lines around a straight conductor through which a steady electric current is flowing.

Imagine that you are using a right-handed corkscrew in such a way that it points in the direction of the current flow. The direction of rotation of the handle then gives the direction of the magnetic field lines.

























Why do parallel current-carrying conductors attract one another?

Current attraction

Conductor B produces a magnetic field which is perpendicular to the current flow in A.

The force acting on A will be at right angles to the current flow and at right angles to the magnetic field.

Now apply Fleming's left Hand rule: point the first finger in the direction of the field, the second finger in the direction of the current, and then your thumb will point in the direction of the direction of the force experienced by the conductor A.

A similar argument will show that the field due to conductor A will cause a force on B towards A. Hence the conductors are attracted to each other.


Soft and hard magnetic materials

Materials which are easily magnetised - such as soft iron or mumetal (an alloy of ) - are called SOFT MAGNETIC materials. They tend to lose their acquired magnetism rather quickly. On the other hand, materials which are relatively difficult to magnetise - such as steel or alnico (an alloy of aluminium, cobalt and nickel) are called HARD MAGNETIC MATERIALS. They are used for making PERMANENT MAGNETS, that is, magnets that can keep their magnetised states for very long periods of time.








The electric bell

Parts of an electric bell
P - Push button
E - Electromagnet
A - Soft iron armature
S - Flexible metal strip
C - Contact screw
H - Hammer
G - Gong

When the push button P is depressed, current will flow through the circuit, and this will energise the electromagnet, E. It will attract the soft iron armature, A, causing the hammer to strike the gong. Contact at the screw C, will be broken, de-energising the electromagnet. The flexible metal strip, S acts as a spring, pulling back the armature and the hammer, and restoring contact at the screw. This cycle is repeated as long as the push button is depressed.








Relays

A relay is a device whereby a secondary circuit can be switched on by a primary circuit.

Referring to the diagram above, when the switch S is closed, the iron core inside the solenoid becomes magnetised. It attracts the armature, which is pivoted. The upper limb of the armature causes the contacts in the secondary circuit XY to close, thus activating that circuit.









The telephone

A typical telephone handset consists of two parts - the receiver (that you place next to your ear) utilises an electromagnet, and a microphone, into which you speak.

The receiver has an electromagnet, B, connected to the circuit. Varying strengths of the current cause different amount of magnetisation of the electromagnet. The electromagnet causes deformation of the steel membrane, AB. This results in sound waves that are picked up by the user's ear.

The microphone consists of a steel membrane, D, lying across a small container of carbon powder, C. Two electrical contacts, E are embedded in the carbon powder, whose resistance depends on the density of the powder particles. Sound waves compress the powder to varying degrees, thus altering the resistance of the carbon powder. This in turn causes small changes in the current flowing through the circuit.









Loudspeakers

A loudspeaker consists of a central pole, A, surrounded by a ring magnet, B. A movable coil, C, is wound around the central pole. Attached to this coil, which carries the current, is a cardboard cone, D.

When a current passes through the coil, a force acts on the coil forcing it to move in and out. The paper cone which is attached to it moves with it, and sets up sound waves, which reproduce the sounds which a microphone originally converted to a current.