WEEK 6

TOPIC: ELECTROMAGNETISM II

CONTENT:

i. Interaction between magnetic field and current

ii. The motor effect of current
iii. Application of motor effect of current.

LEARNING OBJECTIVES

By the end of the class, students should be able to:

i. State Fleming’s left-hand rule and use it to demonstrate the directions of current,
magnetic field and force in an electromagnetic field.
ii. Explain the working principle of the galvanometer and the electric motor.
CONTENT DEVELOPMENT

Interaction between Magnetic Field and Current: Flemings Left-Hand Rule:

When a current-carrying wire is placed between the poles of a strong permanent magnet, it
experiences a force due to the interaction between fields of the magnet and its own magnetic
field.

The force on the wire is either upwards or downwards depending on the direction of current
but never attracted to either poles of the permanent magnet. The direction of the force is
predicted using Flemings Left-Hand Rule.
Fleming’s Left Hand Rule

The rule states that “If the thumb, the forefinger and the middle finger are held mutually at
right angles to one another with the fore-finger pointing in the direction of the magnetic field,
and the second finger in the direction of the current, then thumb points in the direction of
motion or force producing motion.

This rule is also known as the Motor Rule. It is used in situations where electrical energy is
converted to mechanical energy, similar to the energy conversion in a motor.

Key points to Remember on applying Fleming’s Left-Hand Rule

i. The field direction is from North pole of the magnet to South Pole
ii. The current direction is from the positive terminal of the battery to the negative
terminal which is called the conventional current flow.
iii. Fleming’s left-hand rule only applies if the current and field directions are at right
angles. If they are at any other angle other than 900, though Force exists, but its
direction becomes more difficult to predict.

The Size or Magnitude of the Force

The magnitude of force F acting on the conductor of length L, inside the magnetic field is
given by:

F = BILsinθ.

F = force acting on the conductor,

B = the strength or intensity of the magnetic field (Magnetic flux density),

I = current flowing through the conductor,

L = length of the material conductor and

θ = the angle between the conductor and the magnetic field.

The magnitude of the force is increased if the current is increased or a stronger magnet is
used or the number of turns of the wire is increased(ie Increasing the length of the wire) or
increasing the size of the angle the conductor makes with the field.

Applications of Fleming’s left-hand rule are found in the working of a moving-coil

instruments and also turning effect of a coil. (e.g, in moving coil galvanometer)

Illustrative Examples (To be solved in the class with the students)

1. A straight conductor of length 0.45m moves in a magnetic field of flux density 1.2T when
a current of 10A flows through it.
i. Calculate the force moving the conductor when it is inclined at an angle of 60 0 to the
magnetic field.
ii. At what angle is the maximum force on the conductor in the same magnetic field?

Class Work

A force of 12.5N acts on a straight conductor 0.5m long placed perpendicularly in a

magnet when a current of 24A flows through it. Calculate the magnetic field density of the
field.

Applications of Motor Effect of Current

The motor effect of current is used in the construction of the following devices:

i. Moving coil meters (galvanometer, ammeter and voltmeter)

ii. Electric motor
iii. Loudspeakers and
iv. Relays.
The Sensitivity of a Moving Coil Meter

This is the ability of the instrument to detect a very small amount of current or voltage
flowing through the circuit. It is defined as the deflection (angle turned) per unit current or
voltage.

The sensitivity of a moving coil meter is increased by using a:

i. Stronger magnet
ii. Weaker hair spring (Spring with small force constant)
iii. Coil with large area and more (large) turns of wire on it.
iv. Beam of light reflected from a mirror attached to the coil as a pointer.

Moving coil instruments are suitable for the measurement of direct currents and voltages
only. It can be modified to measure alternating current by using diodes as rectifiers.

Advantages of a Moving Coil Meter

i. They are very sensitive to small changes in current and voltages

ii. Deflection of a moving coil meters is proportional to current: hence, they have a
uniform or linear scale.
iii. They can be upgraded to measure higher currents and voltages.

Disadvantages of moving Coil Meter

i. They can measure only direct current

ii. The internal resistance of the metering device affects the current or voltage being
measured.

The Electric Motor

An electric motor is a transducer whose energy conversion is from electrical to mechanical.

A DC motor is defined as a class of electrical motors that convert direct current

electrical energy into mechanical energy.

Different Parts of a D.C Motor

i. Armature or rotor.
ii. Field coil or stator
iii. Commutator brushes.
The armature is a cylinder of magnetic laminations that are insulated from one another. The
armature is perpendicular to the axis of the cylinder. The armature is a rotating part that
rotates on its axis and is separated from the field coil by an air gap.

The field coil or stator is a non-moving part on which winding is wound to produce
a magnetic field. This electro-magnet has a cylindrical cavity between its poles.

The commutator is a cylindrical structure made of copper segments stacked together but
insulated from each other using mica. The primary function of a commutator is to supply
electrical current to the armature winding.

The brushes conduct electric current from the external circuit to the rotating commutator.
The brush and the commutator unit are concerned with transmitting the power from
the static electrical circuit to the mechanically rotating region or the rotor.
For practical d.c motors, the power or efficiency is increased by:

i. Increasing the number of loops of coil in the laminated soft iron drum
 ii. Connecting each loop of wire to a split-ring commutator segment to improve its
efficiency and make it run smoothly.
iii. Using an electromagnet against a permanent magnet to produce a stronger magnetic
field in it turns
iv. Increasing the amount of current flowing in the coil.

Evaluation

State Fleming’s left hand rule.

Assignment

1. State 2 advantages and 2 disadvantages of the moving coil meter.

2. (a) What is an electric motor? (b) Mention 4 of its parts.

Answer questions 1 to 10 on Exercise 6 (Book 3, Page 433) on New School Physics

Textbook.