7.Magnetic Effects of Electric Current

Q1. Why does a compass needle get deflected when brought near a bar magnet?

Ans:

A compass needle behave as a small bar magnet. When it is brought near a bar magnet, its magnetic field lines interact with that of the bar magnet.

Hence, It shows a deflection when brought near the bar magnet.

 

Q1. Draw magnetic field lines around a bar magnet.

Ans:

Magnetic field lines of a bar magnet emerge from the north pole and terminate at the south pole. Inside the magnet, the field lines emerge from the south pole and terminate at the north pole.

 

Q2. List the properties of magnetic lines of force.

Ans:

The properties of magnetic lines of force are as follows.

(1) Magnetic field lines emerge from the north pole.

(2) They merge at the south pole.

(3) The direction of field lines inside the magnet is from the south pole to the north pole.

(4) Magnetic lines do not intersect with each other.

 

Q3. Why don’t two magnetic lines of force intersect each other?

Ans:

If two field lines of a magnet intersect, then at the point of intersection, the compass needle points in two different directions which is not possible. Hence, two field lines do not intersect each other.

Q3. 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.

Ans:

(d) is the same at all points.

 

Q1. 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

Ans:

(c) and (d)

When a proton enters in a region of magnetic field, it experiences a magnetic force and  the path of the proton becomes circular.Thus changing its velocity and momentum as well.

 

Q1. 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

Ans:

(c) and (d)

When a proton enters in a region of magnetic field, it experiences a magnetic force and  the path of the proton becomes circular.Thus changing its velocity and momentum as well.

 

Q2. In Activity 13.7, 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?

Ans:

A current-carrying conductor placed in a magnetic field experiences a force. The magnitude of force increases with the amount of current, strength of the magnetic field, and the length of the conductor. Hence, the magnetic force exerted on rod AB and its deflection will increase if

(i) current in rod AB is increased

(ii) a stronger horse-shoe magnet is used

(iii) length of rod AB is increased

 

Q3. 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

Ans:

(d) upward

 

Q1. State Fleming’s left-hand rule.

Ans:

It states that

if we arrange the thumb, the centre finger, and the forefinger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the centre finger gives the direction of current, and the forefinger points in the direction of magnetic field.

 

Q2. What is the principle of an electric motor?

Ans:

The working principle is based on the magnetic effect of current.

A current-carrying loop experiences a force and rotates when placed in a magnetic field. The direction of rotation of the loop is given by the Fleming’s left-hand rule.

 

Q3. What is the role of the split ring in an electric motor?

Ans:

It acts as a commutator.The commutator reverses the direction of current flowing through the coil after each half rotation of the coil. Due to this reversal of the current, the coil continues to rotate in the same direction.

 

Q1. Explain different ways to induce current in a coil.

Ans:

The different ways to induce current in a coil are as follows:

(a) If a coil is moved rapidly between the two poles of a horse-shoe magnet, then an electric current is induced in the coil.

(b) If a magnet is moved relative to a coil, then an electric current is induced in the coil.

 

Q1. State the principle of an electric generator.

Ans:

It works on the principle of electromagnetic induction. It generates electricity by rotating a coil in a magnetic field.

 

Q2. Name some sources of direct current.

Ans:

The different sources of direct current are cell, DC generator, etc.

 

Q3. Which sources produce alternating current?

Ans:

AC generators, power plants, etc. 

 

Q4. 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

Ans:

(c) half revolution

 

Q1. Name two safety measures commonly used in electric circuits and
appliances.

Ans:

Two safety measures commonly used are as follows:

(i) Each circuit must be connected with an electric fuse.

This checks the flow of excessive current through the circuit. When the current passing through the wire exceeds the maximum limit value of the fuse element,it melts to stop the flow of current through that circuit,thus protects the appliances connected to the circuit.

(ii) Earthing should be done to prevent electric shocks.

Any leakage of current in an electric appliance is transferred to the ground and people using the appliance do not get the shock.

 

Q2. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect? Explain.

Ans:

The current drawn by an electric oven i calculated as:

P=VI

I=P/V

Given that,

Power of the oven=2kww=2x103 W

Voltage applied=220V

Therefore,current(I)=(2000/220)=9.09A

The current drawn by the electric oven is 9.09 A, which exceeds the safe limit of the circuit. Fuse element of the electric fuse will melt and break the circuit.

 

Q3. What precaution should be taken to avoid the overloading of domestic
electric circuits?

Ans:

The precautions that should be taken to avoid the overloading of domestic circuits are as follows:

i) Fuse should be connected in the circuit.

ii)Multiple appliances should not be connected to a single socket.

iii) Too many appliances should not be used at the same time.

iv) Faulty appliances should  be replaced in the circuit.

 

Q1. Which of the following correctly describes the magnetic field near a long straight wire?
(a) The field consists of straight lines perpendicular to the wire.
(b) The field consists of straight lines parallel to the wire.
(c) The field consists of radial lines originating from the wire.
(d) The field consists of concentric circles centred on the wire.

Ans:

(d) The field consists of concentric circles centred on the wire.

 

Q2. The phenomenon of electromagnetic induction is
(a) the process of charging a body.
(b) the process of generating magnetic field due to a current passing through a coil.
(c) producing induced current in a coil due to relative motion between a magnet and the coil.
(d) the process of rotating a coil of an electric motor.

Ans:

(c) producing induced current in a coil due to relative motion between a magnet and the coil.

 

Q3. The device used for producing electric current is called a
(a) generator.
(b) galvanometer.
(c) ammeter.
(d) motor.

Ans:

(a) generator.

 

Q4. The essential difference between an AC generator and a DC generator is that
(a) AC generator has an electromagnet while a DC generator has
permanent magnet.
(b) DC generator will generate a higher voltage.
(c) AC generator will generate a higher voltage.
(d) AC generator has slip rings while the DC generator has a commutator.

Ans:

(d) AC generator has slip rings while the DC generator has a commutator.

 

Q5. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.

Ans:

(c) increases heavily.

 

Q6. State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
(b) An electric generator works on the principle of electromagnetic induction.
(c) The field at the centre of a long circular coil carrying current will be
parallel straight lines.
(d) A wire with a green insulation is usually the live wire of an electric supply.

Ans:

a)-----------------(False

b)-----------------(True

c)-----------------(True

d)-----------------(False

Q7. List three sources of magnetic fields.

Ans:

i) Magnet

ii) A current carrying conductor

iii) A current carrying solenoid

 

Q9. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?

Ans:

The force experienced by a current-currying conductor placed in a magnetic field is the largest when the direction of current is perpendicular to the direction of the magnetic field.

 

Q10. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field?

Ans:

The direction of current is from the front wall to the back wall because negatively charged electrons beam are moving from back wall to the front wall.The beam deflets towards right side so that the direction of the force is towards our rightside.

According to Fleming’s left hand rule:thumb indicates direction of the force.Middle finger indicate direction of the current.The fore finger indicates vertically downward. 

Hence, using Fleming’s left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward.

 

Q12. Name some devices in which electric motors are used.

Ans:

i)Refrigrators

ii)Electric fans

iii)washing machines

 

Q13. A coil of insulated copper wire is connected to a galvanometer. What will happen if a bar magnet is (i) pushed into the coil, (ii) withdrawn from inside the coil, (iii) held stationary inside the coil?

Ans:

i) When the bar magnet is pushed into the coil,the galvanometer deflects momentarily.

ii)When the bar magnet is withdrawn ,the galvanometer deflects momentarily again but in opposite direction.

iii)There would be no deflection when the bar magnet is stationary.

 

Q14. 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,some current would be induced in the coil B when the current in the coil A changes due to the change in magnetic field.

 

Q17. When does an electric short circuit occur?

Ans:

Short circuit occurs when the live wire touches the neutral wire.As the current through the wire becomes quite large which heats the wire and it temperature increases to a very high level,sometimes may cause fire.

 

 

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