1. Magnetic Effect of a Current Remember the electromagnet - a soft-iron bar can be magnetised by putting it in a current carrying solenoid. This is an example of magnetism from electricity.

Direction of the Magnetic Field   A current carrying wire has a magnetic field set up around it. This can easily be investigated using a plotting compass.

Figure 1 shows the magnetic field of a wire being investigated using a plotting compass.

Figure 2 shows Maxwell’s Right Hand Grip Rule for determining the direction of the magnetic field around the wire. The hand grips the wire with the thumb pointing in the direction of the current. The fingers point in the direction of the magnetic fields.

Note – –If current is reversed, the direction of the magnetic field lines will also be reversed as well.

The Magnetic Field Pattern due to a Flat Coil A flat coil has a magnetic field pattern as shown.

Characteristics of the Magnetic Field The strength of the magnetic field is stronger along the inside of the coil than on the outside. Thus, you should see more magnetic field lines per unit area lying on the inside region of the coil. The field lines at the centre are straight and perpendicular to the plane of the coil.

The Magnetic Field Pattern of a Solenoid The magnetic field pattern of a solenoid resembles that of the bar magnet. The solenoid can thus be said to have 2 magnetic poles. There are 2 ways to predict the direction of the magnetic field in a solenoid.

Using the Right-Hand Grip Rule, the hand grips the wires with the fingers pointing in the direction of the current. The thumb will then point to the end of the solenoid that is the North-pole. The other end of the solenoid is then the South-pole. The magnetic field pattern outside the solenoid is similar to that of a bar magnet. The direction of the magnetic field inside the solenoid is from the South-pole to the North-pole.

When viewing one end of the coil, it will be a North-pole if the current is flowing in the aNticlockwise direction, and a South-pole if the current is flowing in a clockwiSe direction.

Characteristics of the Magnetic Field The magnetic field is stronger inside the solenoid. The strength of the magnetic field is about uniform inside the solenoid (almost parallel field lines).

Three ways to increase the strength of the magnetic field at the centre of the flat coil: Increase the number of turns of the solenoid Wind the coil more closer Increase the current

Some Common Uses of Electromagnets

The Electric Bell

The Reed Switch

The Reed Relay

2. Force on a Current-Carrying Conductor in a Magnetic Field Previously, we learnt that a current- carrying wire has a magnetic field around it. –If we place this same wire in another magnetic field: –The 2 magnetic fields may interact. –This interaction will produce a force on the wire. –This effect is known as the Motor Effect.

Fleming's Left-Hand Rule To deduce the direction of the force on the current- carrying wire, we may use Fleming's Left-Hand Rule.

  Hold the thuMb, Forefinger & seCond finger (of Your Left Hand) at right angles to each other.   Point the Forefinger in the direction of the magnetic Field (N to S).   Point the seCond finger in the direction of the Current.   The thuMb will then point in the direction of the Motion of the wire.

Is the direction of motion of BC indicated correctly in the diagram below?

Why does a Force exist on a Current-carrying wire when placed in a Magnetic Field?

 When you put a current-carrying wire (which has a magnetic field around it) in a magnetic field, both magnetic fields will interact with each other.  Following figures are the magnetic fields due to magnets & current in a wire.

 When the current-carrying wire is put into the magnetic field due to the magnet, the 2 magnetic fields interact with each other. The resulting combined magnetic field is as shown:

 Did you notice that the magnetic field is stronger at A than at B?  Due to the difference in magnetic field strengths at A & B, a force will then act on the wire. This force will act on the wire in the direction of the stronger field to the weaker field as shown in the above diagram.

One useful application of the force on a current- carrying conductor in a magnetic field is the Moving Coil Loudspeaker.

3. Force between Two Parallel Current- Carrying Wires Previously, we learnt that a current- carrying wire has a magnetic field around it. If we place 2 such wires parallel to each other, the 2 magnetic fields will then interact. A force will act on each of the wires.

The diagrams above illustrate what happens if we combine the magnetic fields due to 2 wires carrying currents flowing in opposite directions.

The diagrams above illustrate what happens if we combine the magnetic fields due to 2 wires carrying currents flowing in the same direction.

4. Force on a Current-Carrying Rectangular Coil in a Magnetic Field Study the diagram below.

 Note that the stiff wire loop ABCD is placed in between the poles of a strong magnet. Pass a current through the loop. A turning force on the wire loop results.  If current flows in a clockwise direction in ABCD, the loop experiences a clockwise turning moment (as shown in the diagram).  If the current flows in an anticlockwise direction in ABCD, will the loop experience a turning moment? If there is a turning moment, what then is its direction?

To see why there is a turning effect, let’s look at the combined magnetic fields due to the current-carrying loop & the magnets. From the diagrams above, we see that wire AB will have a force acting on one side while the force on wire CD is acting on the other side. Thus ABCD turns.

The D.C. Motor  In the previous section, we saw that a current- carrying loop that is placed in a magnetic field experiences a turning effect. This turning effect on a loop carrying a current has a very important application – the D.C. Motor.

The D.C. Motor

ABCD is mounted on an axle PQ. The ends of the wire are connected to a split rings X & Y (also known as commutators). The commutators rotate with the loop. 2 carbon brushes are made to press lightly against the commutators.

When current runs through ABCD as shown in the diagram, a downward force would act on AB. An upward force would act on CD. The loop rotates in an anticlockwise manner until it reaches the vertical position.

At this position, the current is cut off. However, the momentum of the loop carries it past the vertical position. The current in the wire arm CD is now reversed. A downward force acts on it. An upward force also acts on AB. Therefore, the loop ABCD continues to rotate in an anticlockwise manner.

What is the purpose of the commutator? To reverse the current in the coil for every half cycle

Three ways to increase the turning effect on the wire loop: Increase the current in the coil Increase the number of turns in the coil Use stronger magnets

Electromagnetic Induction  In 1831, Michael Faraday found out that electricity can be obtained from magnets.

Faraday’s Experiments  1. Faraday’s Iron Ring Experiment

 Compass needle only deflects only when switch S is closed and opened.  No deflection was noted when current in coil A was steady.  Current in coil B is called induced current.  At steady current, magnetic field does not change. No current is induced. This is the basis of the transformer.

2. Faraday’s Solenoid Experiment

 The galvanometer flickered when the magnet is inserted.  E.m.f. is induced with relative movement of solenoid and magnet.

Magnitude (size) of e.m.f. induced depends on:  The number of turns in the solenoid  The movement of the magnet  The strength of the magnet Let us consider Faraday’s Solenoid experiment in more detail.

When the bar magnet moves inside, the field lines will be cut by the solenoid as a result a current is produced in the solenoid in a clockwise direction. It is also noticed that the polarity of the solenoid near the magnet is opposing the change caused by the magnet.

When the bar magnet is stationary inside the field lines will not be cut by the solenoid so there is no current produced in the solenoid.

When the bar magnet moves outside, the field lines will be cut by the solenoid as a result a current is produced in the solenoid in an anticlockwise direction. It is also noticed that the polarity of the solenoid near the magnet is opposing the change caused by the magnet. N

THE LAWS OF ELECTROMAGNETISM There are two laws of electromagnetism known as Faraday’s Law and Lenz’s Law. They are derived from the above experimental results.