Magnetic Effects of Electric Current
NCERT Solutions
NCERT Exercises
1. 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 centered on the wire.
(d) The field consists of concentric circles centered on the wire.
2. 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.
(c) producing induced current in a coil due to relative motion between a magnet and the coil.
3. The device used for producing electric current is called a
(a) generator
(b) galvanometer
(c) ammeter
(d) motor
(a) generator
4. 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.
(d) AC generator has slip rings while the DC generator has a commutator.
5. At the time of short circuit, the current in the circuit
(a) reduces substantially.
(b) does not change.
(c) increases heavily.
(d) vary continuously.
(c) increases heavily
6. State whether the following statements are true or false.
(a) An electric motor converts mechanical energy into electrical energy.
False
(b) An electric generator works on the principle of electromagnetic induction.
True
(c) The field at the center of a long circular coil carrying current will be parallel straight lines.
True
(d) A wire with a green insulation is usually the live wire of an electric supply.
False
7. List two methods of producing magnetic fields.
- By using a current carrying straight conductor or current carrying coil.
- By using a permanent magnet
8. How does a solenoid behave like a magnet? Can you determine the north and south poles of a current–carrying solenoid with the help of a bar magnet? Explain.
A solenoid is a coil of copper wire. When current is passed through the coil, magnetic field is produced. One end of the solenoid act as a magnetic north pole and other end as a magnetic south pole.
If we bring a bar magnet’s north pole to one end of solenoid and if it shows a repulsion, then that end is the solenoid’s north pole and other end is the south pole.
9. When is the force experienced by a current–carrying conductor placed in a magnetic field largest?
The force experienced by a current–carrying conductor placed in a magnetic field largest when the direction of the current is perpendicular to the direction of the magnetic field.
10. 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?
By using the fleming’s left hand rule we can determine the direction of magnetic field. The direction of flow of current and direction of flow of electrons are in opposite direction. It is clear from the question that electron beam, moving horizontally from back wall towards the front wall so that the direction of electric current will be from front wall to back wall. The direction oof magnetic force is rightwards, so we can conclude that the magnetic field inside the chamber is in downward direction.
11. Draw a labelled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Motor principle: A conductor which can move freely, and which is kept in a magnetic field, experiences a force when current passes through it and it moves. The working of electric motor, fan, mixie etc is based on this principle.
Electric Motor consists of following parts
- Armature
- Split ring
- Magnet
- Graphite Brushes
- Axis of rotation of motors
Armature: It is a part of motor having an important role. It consists of a rectangular iron core with copper coil wrapped around it. It is placed in a magnetic field and rotates when electric current is passed.
Split ring commutator: It changes the direction of current flowing through coil after every half rotation so that the motor will continue to rotate in the same direction. If the split ring was not used, then coil would have rotated half in clock wise direction and half in anti-clock wise direction.
12. Name some devices in which electric motors are used.
- Electric Mixers
- Water pumps
- Electric fans
- Washing machines
13. 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?
(i) When a bar magnet is pushed into the coil, current is induced in the coil momentarily. As a result the galvanometer deflects in a particular direction momentarily.
(ii) When the bar magnet is withdrawn from inside the coil, current is induced momentarily but in the opposite direction and the galvanometer deflects in the opposite direction momentarily.
(iii) When the bar magnet is held stationary inside the coil, no current will be induced as a result there will be no deflection in the galvanometer.
14. 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.
If there is a change in current in the coil A, then the magnetic field associated with it also changes. As a result there is a change in the magnetic field around coil B. The change in the magnetic field of coil B induces current in it.
15. 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 which is perpendicular to it, and (iii) current induced in a coil due to its rotation in a magnetic field.
(i) Maxwell’s right hand thumb rule.
(ii) Fleming’s left hand rule.
(iii) Fleming’s right-hand rule.
16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?
Electric Generator
A device that converts mechanical energy to electrical energy is called an electric generator. It works on the phenomenon of electromagnetic induction.
Working of an electric generator
An electric generator has a rectangular coil ABCD. The coil is kept between the two poles of a permanent magnet. The two ends of the coil are attached to the rings R1 and R2. Two conducting brushes B1 and B2 are kept on the rings R1 and R2 respectively. The two rings are attached to the axle. To rotate the coil, the axle is mechanically rotated. The outer ends of the two brushes are joined to the galvanometer. This shows the flow of current in the external circuit.
When the axle is rotated, the arm AB moves upward and the arm CD moves downward in the magnetic field produced by the permanent magnet. The coil ABCD is rotated clockwise. On applying Fleming’s right-hand rule, the induced current is set in the arms in the directions AB and CD. The induced current flows in the direction of ABCD. In the external circuit, the current flows from B2 to B1.
After half rotation, AB moves upward (earlier downward) and CD moves downward (earlier upward). Thus, the direction of induced current in both arms changes. The net induced current flows in the direction of DCBA. In the external circuit, the current flows from B1 to B2.
Thus, after every half rotation the polarity of the current in the arms changes. The current that changes direction after an equal interval of time is called an alternating current (AC). The device is called an AC generator.
The current that does not change its direction with time is called direct current (DC). To get a direct current, a split-ring type commutator is used. In this arrangement, one brush is always in contact with the arm moving up while the other brush is in contact with the arm moving down. Thus, the current is produced in one direction only. The generator used is called the DC generator.
17. When does an electric short circuit occur?
Short circuit occurs when live wire and neutral wire comes in direct contact.
18. What is the function of an earth wire? Why is it necessary to earth metallic appliances?
Earth wire protects the appliances and human life from the damage due to the leakage of current. It is possible by connecting the metallic body of electric appliances to earth wire. By doing this any leakage of electric current is transferred to the ground.
Intext Exercises
Pg. No. 224
1. Why does a compass needle get deflected when brought near a bar magnet?
Compass needle is a type of magnet. Compass needle get deflected when brought near a bar magnet because the magnetic field lines of the compass needle interact with the magnetic field lines of bar magnet.
Pg. No. 228
1. Draw magnetic field lines around a bar magnet.
2. List the properties of magnetic field lines.
- Magnetic field lines can never cross.
- Magnetic field lines are continuous.
- They all have the same strength.
3. Why don’t two magnetic field lines intersect each other?
If two magnetic field lines intersect each other at a point, then the point at which it intersect, the compass needle shows two different directions which is not possible.
Pg. No. 229
1. 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.
Apply right hand thumb rule to the right side and left side of the loop. When we hold the right hand in the left side of the loop, we can find the direction of magnetic field lines inside the loop are going into the table while outside the loop they are coming out of the table. When we apply right hand thumb rule in the right side of loop the direction of the magnetic field is like it emerging from the table outside the loop and merging with the table inside the loop.
2. The magnetic field in a given region is uniform. Draw a diagram to represent it.
3. 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) is the same at all points.
Pg. No. 231
1. 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
Both (c) and (d)
2. 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?
(i) The deflection of the rod will be more if there is an increase in current in the rod.
(ii) As the magnetic field increases using a stronger horseshoe magnet, there will be more force on the rod, and it will be deflected more.
(iii) The deflection of the rod will be also more if the length of the rod is increased.
3. A positively-charged particle (alpha-particle) projected towards the 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) upward
Pg. No. 233
1. State Fleming’s left-hand rule.
Fleming’s Left hand rule states that if we arrange our thumb, forefinger and middle finger of the left hand at right angles to each other, then the thumb points towards the direction of the magnetic force, the forefinger points towards the direction of magnetic field and the middle finger points towards the direction of current.
2. What is the principle of an electric motor?
Working principle of electric motor is based on the magnetic effect of current.
3. What is the role of split ring in an electric motor?
In a circuit, the device that reverses the direction of flow of current is known as a commutator. In electric motor the split ring acts as the commutator.
Pg. No. 236
1. Explain different ways to induce current in a coil.
- Bringing and taking away a bar magnet from the coil.
- By moving the coil rapidly between a horse shoe magnet
Pg. No. 237
1. State the principle of an electric generator.
It works on the phenomenon of electromagnetic induction.
2. Name some sources of direct current.
DC generator and cells
3. Which sources produce alternating current?
AC generator and power stations.
4. 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) half revolution
Pg. No. 238
1. Name two safety measures commonly used in electric circuits and appliances.
Fuse and Earthing
2. 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.
Given
P = 2 kW = 2000 W
V = 220 V
I = 5A
We know the formula
P= \frac{V}{I}
\therefore I= \frac{P}{V}
\therefore I= \frac{2000}{220}=9.09 A
I = 9.09 A which exceeds the safe limit of the circuit and causes the fuse to melt and break the circuit.
3. What precaution should be taken to avoid the overloading of domestic electric circuits?
- Avoid connecting too many appliances in one socket
- Avoid faulty appliances
- Proper earthing and using fuse.
- Avoid using too many appliances at the same time