Electromagnetism- O Level Physics

Posted on Tuesday, September 27, 2016 by Mysterious QUIZ!

ELECTROMAGNETISM!

Magnetic effect of current:

Remember that a current-carrying conductor produces a magnetic field around it. If the current direction is reversed, the magnetic field direction will be reversed too, as shown in the diagram above.

A dot in the wire, as shown above, shows the current coming out of the plane whereas a cross in the wire shows the current moving into the plane.

Now how to determine the direction, shown above, of the magnetic field?

The right hand grip rule! Grip the wire with your right hand in such a way that the thumb is pointing to the direction of the current. The curl of your fingers, in turn, will show the direction of the magnetic field. Easy?

The strength of the magnetic field in a long, straight current carrying wire depends on

● the magnitude of the current. A larger current will produce a stronger magnetic field around the wire.

● the distance from the wire. The strength of the field decreases as you move further out.

Magnetic field pattern around a flat coil:

The direction of the field can be determined by the Right Hand Grip Rule. Grip the wire at one side of the coil with your right hand, with thumb pointing along the direction of the current. Your other fingers will be pointing in the direction of the field. Then do the same with the other side of the coil.

Plane view of the flat coil:

In a flat coil the strength of the magnetic field is closer in the centre of the wire as you can see in the diagram above (closer field lines). To increase the strength,

● increase the current ,

● increase the number of turns of the coil.

Magnetic field pattern of a solenoid:

A solenoid is a long coil made up of a numbers of turns of wire. The magnetic field of a solenoid resembles that of the long bar magnet, and it behaves as if it has a North Pole at one end and a South Pole at the other.

You would have noticed that it’s equal to increasing the turns of wire of flat coil

To determine the poles in this case, again, apply right hand rule as shown below. The fingers point towards the current direction while the thumb will show North pole direction, as shown:

The strength of the magnetic field in this case can be increased by

● Increasing the current,

● Increasing the number of turns per unit length of the solenoid,

● Using a soft-iron core within the solenoid.

Force on current carrying conductor:

Now if you place the same current carrying wire in a magnetic field, the wire will experience a force. The force is experienced due to the interactions between the two magnetic fields (Which two? The one which we learned is produced by the current carrying wire itself, and the one in which it is placed) which produces a force on the conductor.

The direction of the force should be known, and this can be found using the Fleming’s left hand rule as shown in the diagram:

Notice that the fore finger, middle finger and the thumb are perpendicularly to each other. The forefinger points along the direction of the magnetic field, middle finger points in the current direction and the thumb points along the direction of the force.

The strength of the force can be increased by

1. Increase the current

2. Using a stronger magnet.

And the force direction can be controlled easily. It can be reversed by reversing the direction of the current or the magnetic field.

Now let’s see how exactly a force is experienced on the current-carrying conductor when placed in a magnetic field. The interaction between the two magnetic fields is as shown below:

We know that the field lines will always be from North to South. The direction of the current is shown by the cross, which is into the plane. Now apply the left hand rule and you’ll get the direction of the force which is 1 in the diagram.

Force on a moving charge in magnetic field:

Now if we consider a charge entering a magnetic field with direction into the plane, the direction of current will be the convectional current direction i.e. from positive to negative (the direction of current is shown). Now if you apply left hand rule, you’ll get the direction of force it will experience, which will be upwards.

Forces between two parallel current-carrying conductors:

If two parallel wires are placed together, we know they will generate a magnetic field around them, right? The wires will experience a force. Why? Let’s see!

If you right hand rule on the current carrying wires, you’ll get the direction of the fields around them. Now if the direction of the current in both wires is the same, they will attract. If, however, the direction is opposite, they will repel. This is because the field direction in the wires with same current direction will be the same. See the diagram carefully and draw it yourself, you’ll understand how this happens.

Force on current-carrying rectangular coil in a Magnetic field:

If a current carrying coil is placed in a magnetic field (As shown in diagram above), a pair of forces will be produced on the coil. This is due to the interaction of the magnetic field of the permanent magnet and the magnetic filed of the current carrying coil.

The direction of the force can be determined by Fleming's left hand rule. Since the current in both sides of the coil flow in opposite direction, the forces produced are also in opposite direction. The 2 forces in opposite direction constitute a couple which produces a turning effect to make the coil rotate.

This phenomenon is used in a d.c. (direct current) motor. What does an electric motor do? It simple converts electric energy to kinetic energy. The d.c. motor consists a rectangular coil of wire placed between 2 permanent magnets. The coil are soldered to a copper split ring known as commutator. 2 carbon brushes are held against the commutator.

This is shown below:

The function of the brush is to conduct electricity from the external circuit to the coil and allow the commutator to rotate continuously (since the brush is of carbon, it conducts electricity).

The function of the commutator is to change the direction of the current in the coil and hence change the direction of the couple (the 2 forces in opposite direction) in every half revolution. This is to make sure that the coil can rotate continuously.

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