Lesson: Magnetic effect of electric current

Why does a compass needle get deflected when brought near a bar magnet?

A needle of a compass is also a magnet. It is a fact that, like poles of two magnets repels each other and unlike poles attract. Due to this repulsion and attraction between the poles, a magnetic needle deflects when brought near a bar magnet.

 Draw magnetic field lines around a bar magnet.

List the properties of magnetic lines of force.

The properties of magnetic lines of force are as follows:

·         These lines are closed curves emerging from the north pole and merging at the south pole.

·         The direction of field lines inside the magnet is from the south pole to the north pole.

·         The field lines are dense near a magnet and their density decreases as we move away from the magnet.

·         The field lines are most intense near the poles, thereby indicating that the magnetic force is strongest near the poles.

·         Any two field lines never intersect each other. This means that no point in the magnetic field will ever have more than one direction.

Why don't two magnetic lines of force intersect each other?

Magnetic lines of forces represent the direction of the magnetic field. If two magnetic field lines intersect each other, it would mean there are two directions of the magnetic field at a same point, which is impossible. Thus, magnetic field lines never intersect each other.

Consider a circular loop of wire lying in the plane of the table. Let the current pass through the loop clockwise. Apply the right-hand rule to find out the direction of the magnetic field inside and outside the loop.

For the current (I) flowing inside a wire, the direction of the magnetic field (B) is shown the diagram. The direction of the magnetic field when the current passes through a circular loop is shown in the diagram below.

The direction of the magnetic field inside the loop appears as if the magnetic fields pierce through the table. Whereas, the direction of the magnetic field outside the loop appears as if the magnetic fields have emerged out of the table.

The magnetic field in a given region is uniform. Draw a diagram to represent it.

The magnetic field lines inside a current-carrying solenoid are uniform.

Choose the correct option.

The magnetic field inside a long straight solenoid-carrying current

(a) is zero
(b) decreases as we move towards its end
(c) increases as we move towards its end
(d) is the same at all points

d

Which of the following property of a proton can change while it moves freely in a magnetic field? (There may be more than one correct answer.)
(a) Mass
(b) Speed
(c) Velocity
(d) Momentum

c, d

In activity displayed below, how do we think the displacement of rod AB will be affected if

(i) Current in rod AB is increased;

(ii) A stronger horse-shoe magnet is used; and

(iii) length of the rod AB is increased?

(i) If the current in the rod is increased, the magnetic force exerted on the conductor and the deflection of the rod will increase.

(ii) If a stronger horse-shoe magnet is used, the magnetic force exerted on the conductor and the deflection of the rod will increase.

(iii) If the length of the rod AB is increased, there is no effect on the displacement of the rod.

A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
(a) Towards south 
(b) Towards east
(c) Downward 
(d) Upward

d

State Fleming's left-hand rule.

Fleming's left hand rule: If we arrange the thumb, the forefinger, and the middle finger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the middle finger gives the direction of the current, and the forefinger points in the direction of the magnetic field.

What is the principle of an electric motor?

The principle of an electric motor is based on the magnetic effect of electric current. A motor works on the principle that when a current-carrying rectangular coil is placed in a magnetic field; it experiences a force and rotates. The direction of rotation of the loop is given by the Fleming’s left-hand rule.

What is the role of the split ring in an electric motor?

The split-ring commutator in the electric motor reverses the direction of current flowing through the coil after every half rotation of the coil. This makes the coil rotate in the same direction continuously.

Explain different ways to induce current in a coil.

Induced current is produced in a coil which is placed in a region where the magnetic field changes with time. The magnetic field can be changed when either the coil is rotated in the magnetic field or the magnet is rotated around a static coil.

State the principle of an electric generator.

Electric generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor.

Name some sources of direct current.

Some sources of direct current are:

·         Cell,

·         DC generator, etc.

Which sources produce alternating current?

Some sources producing alternating current are:

·         AC generators,

·         Power plants, etc.

Choose the correct option.

A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each:

(a) Two revolutions
(b) One revolution
(c) Half revolution
(d) One-fourth revolution

c

Name two safety measures commonly used in electric circuits and appliances.

·         Electric fuse

·         Proper earthling of all electric circuits

An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.

The current drawn by the electric oven can be obtained by the expression

$I=\frac{P}{V}$

where, the power of the oven, $P=2\mathrm{kW}=2000\text{ W}$ and the voltage supplied, 

$V =220 \text{V}$

Therefore, $I=\frac{2000}{220}=9.09A$
Since, the current drawn by the electric oven (9.09 A) exceeds the safe limit of the circuit (5A), the fuse element of the electric fuse will melt and break the circuit.

What precaution should be taken to avoid the overloading of domestic electric circuits?

Some of the precautions that can be taken to avoid the overloading of domestic circuits are as follows:

·         Connecting fuse in the circuit

·         Not using faulty appliances in the circuit

·         Not connecting too many appliances to a single socket

·         Not using too many heavy appliances at the same time