 # Magnetic Effects of Electric Current | Q & A

Why does a compass needle get deflected when brought near a bar magnet ?
Ans. The magnetic field lines of compass needle interact with magnetic field lines of a bar magnet. Thus, a force acts on both of them. However, magnetic needle being free to move, gets deflected.

State Fleming’s left-hand rule.
Ans. Stretch the thumb, the forefinger and middle finger of your left hand mutually at right angles to each other, such that forefinger points in the direction of the magnetic field and middle finger in the direction of flow of current. Then, the thumb gives the direction of motion of conductor.

What is the principle of an electric motor ?
Ans. It is based on the principle that when a coil carrying current is placed in a magnetic field, it always experiences a couple due to interaction of the magnetic fields.

What is the role of the split ring in an electric motor?
Ans. The split rings help in changing the direction of current in the coil after every half rotation and hence help the coil to move in the same direction.

List three sources of magnetic fields?
Ans.
(i) Earth’s magnetism produces magnetic field.
(ii) Permanent, natural and artificial magnets produce magnetic fields.
(iii) Conductors carrying current produce magnetic fields.

When is the force experienced by a currentcarrying conductor placed in a magnetic field largest?
Ans. When the conductor is placed parallel to the magnetic field such that magnetic field set up by the current is at right angles to it.

Name some devices in which electric motors are used.
Ans. (i) Mixer-grinders, (ii) electric fans, (iii) washing machine (iv) electric water pumps.

List the properties of magnetic lines of force.
Ans.

• They are closed continuous curves which originate from the north pole and end at the south pole of a bar magnet.
• They mutually repel each other.
• No two magnetic lines of force cross each other.

Why don’t two magnetic lines of force intersect each other?
Ans. If we imagine that two magnetic lines of force cancel each other at some point, then the tracing magnetic compass, when placed at that point will point in two different directions at the same time, which is impossible. Thus, we can say that magnetic lines of force don’t 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 magnetic field inside and outside the loop.
Ans
. The magnetic field lines will move at right angles to the plane of table, such that they move downward inside the loop and upward outside the loop.

Think you are sitting in a chamber with your back to one of the walls. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right. What is the direction of the magnetic field ?
Ans. As the electron beam is moving horizontally from back wall, towards the front wall, therefore, conventional current is moving horizontally from the front wall, towards the back wall. Applying Fleming’s left-hand rule, the forefinger points vertically downward. Thus, the direction of magnetic field is vertically downward at right angles to the direction of the electron beam.

State the rule to determine the direction of a :
(i) Magnetic field produced around a straight conductor-carrying current.
(ii) Force experienced by a current-carrying straight conductor placed in a magnetic field perpendicular to it.

Ans.

(i)
Right-hand thumb rule : Imagine you are holding the conductor with the palm of your right hand, such that the fingers encircle the conductor and the thumb points in the direction of current. Then the direction of fingers encircling the conductor gives the direction of magnetic lines of force around it.
(ii)
Fleming’s left-hand rule : Stretch the thumb, the middle finger and the forefinger of your left hand mutually at right angles to each other, such that the forefinger points in the direction of magnetic field and middle finger in the direction of flow of current. Then, the thumb gives the direction of motion of conductor.

A metallic conductor is suspended freely between the poles of horse-shoe magnet and current is passed. How do you think the displacement of conductor will be affected, if :
(i) current in the conductor is increased?
(ii) a stronger horse-shoe magnet is used ?
(iii) length of the conductor is increased?

Ans.

(i) The displacement of conductor will increase with the increase in current.
(ii) The displacement of conductor will increase with the increase in magnetic strength.
(iii) The displacement of conductor will increase with the increase in the length of conductor.

How does a solenoid behaves like a magnet? Can you determine the north and south poles of a current-carrying solenoid with the help of a bar magnet? Explain.
Ans. When the current flows through a solenoid, each turn of it sets up a magnetic field which is at right angles to the plane of the coil. Thus, each turn in its own right behaves like a magnet, such that its one face is north pole, and the other face is south pole. As these tiny magnets in the form of turns are arranged along the same axis, therefore, solenoid on the whole behaves like a magnet.

In order to find the polarity of the solenoid suspend it free so that it is pointing in north-south direction. Now bring the north pole of a permanent bar magnet near one end of the solenoid. If this end get repelled then this is north pole of solenoid.

State the principle of an electric generator.
Ans. It is based on the phenomenon of electromagnetic induction i.e., when a closed coil is moved in a magnetic field or vice-versa, an induced current is produced in the coil.

Name some sources of direct electric current.
Ans.
Dry cell, lead acid accumulators, NiFe cells, solar cells and d.c. dynamo are the sources of direct electric current.

Which sources produce alternating current?
Ans.
An a.c. dynamo produces alternating current.

Name two safety measures commonly used in electric circuits and appliances.
Ans.
(i) An appropriate fuse is put in each circuit.
(ii) All appliances which are movable or have metal body are earthed.

What precaution should be taken to avoid overloading of domestic circuits?
Ans
. An appropriate fuse should be placed in each of the domestic circuits. This will ensure that fuse melts, when circuit is overloaded.

Two circular coils A and B are placed closed to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.
Ans.
Yes, induced current is produced in coil B. It is because, when we change current in coil A, it changes magnetic flux. This changing magnetic flux induces current in coil B.