Subject: 

PHYSICS

[mediator_tech]

Term:

FIRST TERM

Week:

WEEK 3

Class:

SS 3

Topic:

ELECTROMAGNETIC WAVES

CONTENT

1. The Concept of Electromagnetic Waves
2. The Concept of Electromagnetic Field
3. Fleming’s Left Hand Rule
4. Force between Two Parallel Curent-carrying Conductors
5. Applications of Electromagnetic Field
6. Electromagnetic Induction
7. The Laws of Electromagnetic Induction
8. Eddy Currents
9. Advantages and Disadvantages of Eddy Currents

The Concept of Electromagnetic Waves

Unlike mechanical waves, electromagnetic waves are waves that require no material medium for propagation. Thus, they can propagate through a vacuum or free space.

The human eye can only see light of which the wavelength lies between 3.8 ×10^{– 7}m and 7.2 ×10^{– 7}m. Other examples of electromagnetic radiation differ from visible light  only in wavelength and in the ways of producing them. However, they all have the same velocity of value 3.0 ×10⁸m/s.

EVALUATION

1. What are electromagnetic waves?
2. Differentiate electromagnetic waves from mechanical waves.
3. What is the common speed value of all electromagnetic waves?
4. List the components in the electromagnetic spectrum.
5. What is the wavelength range of the radio waves?
6. Mention two uses of the x-rays.

The Concept of Electromagnetic Field

An electromagnetic field is that field that results from the interaction of the electric and magnetic fields.

When a current carrying conductor is passed in a magnetic field, the conductor will experience a force.

To identify the directions of the force (motion), current and field, Fleming’s Left Hand Rule is employed.

Fleming’s Left Hand Rule

It states that, “If the first three fingers are mutually at right angles to each other, with the Forefinger in the direction of the field, B and the mIddle finger in the direction of the convectional  current, I, then, the thuMb is in the direction of the motion, M or force on the conductor”.

Fleming’s Left Hand Rule is for current-carrying conductor, used to determine the direction of  current in a conductor. The rule can only be used if the magnetic field and current are  perpendicular or inclined to each other but not when they are parallel.

Force between Two Parallel Curent-carrying Conductors

The below rules govern the direction of the magnetic field round the current-carrying conductors.

1. Maxwell: If a right handed cork screw is turned so that it travels along the direction of the  current, the direction of the cork screw is the direction of the magnetic field.
2. Clenched Fist Rule: If the wire is grasped with the right hand so that the thumb point in the  direction of the current, the Clenched Fist direction is the direction of the magnetic field.

Case I: Two parallel current-carrying conductors with current in opposite directions.

Here, a repulsive force is created between the two conductors. Hence, they tend to move away  from each other.

Case II: Two parallel current-carrying conductors with current in the same direction.

Here, an attractive force is created between the two conductors. Hence, they tend to move towards each other.

EVALUATION

1. What is an electromagnetic field?
2. State Fleming’s left hand rule.
3. Where is the rule applicable?

Applications of Electromagnetic Field

1. Moving-Coil Galvanometer

Components of a Moving-Coil Galvanometer

1. A rectangular coil with many turns.
2. A powerful magnetic field between the curved pole N & S,and a soft iron cylinder.
3. Spring to control the angle of rotation of the coil.
4. A pointer and a linear scale.

Parts of a moving coil galvanometer

Moving Coil Galvanometer

Workability of a Moving-Coil Galvanometer

The coil is suspended so that it can rotate freely in the magnetic field. When a current flows, it  enters the meter from the terminal connected to the spring, goes round the coil and returns back.

As the circuit is completed, the coil then rotates about the V-axis between the jewelled bearings.

The rotation is due to the two opposite forces of the current and the magnetic field.

This rotation is then opposed by the forces due to the springs. The pointer attains equilibrium  position when the forces causing the rotation are equal to the forces due to the springs.

The angle of rotation is proportional to the current strength, that is, increase in current increases  the angle of rotation.

The uniform scale is calibrated in a way that equal divisions represent equal changes in current.  The value of the current can then be read on the scale.

2. The Electric Motor

Parts of an electric motor

Parts of an electric motor

Components of an Electric Motor

1. A coil of wire called the armature, which turns about a fixed axis.
2. A powerful magnetic field in which the coil turns.
3. A cummutator (split rings).

Workability of an Electric Motor

When the armature is horizontal and current flows through the coil, the direction is clockwise. This  produces S-pole on top of the armature, which then is attracted to the N-pole of the permanent  magnet; likewise the N-pole of the armature at the bottom, which also gets attracted to the S-pole  of the permanent magnet. Hence, the armature rotates through 90° about its axis to the vertical  position.

As the armature rotates, the cummutator also rotates with it, making the split rings to interchange contact. This therefore reverses the polarity because the battery connection to the coil is reversed, thus, reversing the current’s direction.

This reversal in current produces N-pole in place of the S-pole. This created N-pole will now be repelled by the N-pole of the permanent magnet, making the armature to rotates another 90° about its axis to attains its horizontal position.

As the current continues to flow, the process is repeated, making the armature (coil) to rotate continuously.

EVALUATION

1. What are the applications of the electromagnetic field?
2. State three components of the moving coil galvanometer.
3. What is another name for the coil in the electric motor?