# NCERT Solutions

## Class 10 science

###### magnetic effect of electric current 1. Which of the following correctly describes the magnetic field near a long straight wire?

1. The field consists of straight lines perpendicular to the wire
2. The field consists of straight lines parallel to the wire
3. The field consists of radial lines originating from the wire
4. The field consists of concentric circles centred on the wire

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

2. The phenomenon of electromagnetic induction is

1. the process of charging a body
2. the process of generating magnetic field due to a current passing through a coil
3. producing induced current in a coil due to relative motion between a magnet and the coil
4. 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

1. generator
2. galvanometer
3. ammeter
4. motor

(a) generator

4. The essential difference between an AC generator and a DC generator is that

1. AC generator has an electromagnet while a DC generator has permanent magnet.
2. DC generator will generate a higher voltage.
3. AC generator will generate a higher voltage.
4. 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

1. reduces substantially
2. does not change
3. increases heavily
4. vary continuously

(c) increases heavily

6. State whether the following statements are true or false.

1. An electric motor converts mechanical energy into electrical energy.
2. (b) An electric generator works on the principle of electromagnetic induction.
3. (c) The field at the centre of a long circular coil carrying current will be parallel straight lines.
4. (d) A wire with a green insulation is usually the live wire of an electric supply.

1. False
An electric motor converts electrical energy into mechanical energy.
2. True
A generator is an electric device that generates electricity by rotating a coil in a magnetic field. It works on the principle of electromagnetic induction.
3. True
The field at the centre of a long circular coil carrying current will be parallel straight lines. A long circular coil is a long solenoid. The magnetic field lines inside the solenoid are parallel.
4. False
A wire with a green insulation is usually the live wire of an electric supply. Live wire has red insulation cover, whereas earth wire has green insulation cover in domestic circuits.

7.List three sources of magnetic fields.

The sources of magnetic fields are following:-
Electromagnets
Current-carrying conductors
Permanent magnets

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 formed of long coil of circular loops of insulated copper wire. When a current is allowed to flow through it, magnetic field lines are produced around the solenoid. The magnetic field produced by it is similar to the magnetic field of a bar magnet. When the north pole of a bar magnet is brought near the end connected to the negative terminal of the battery, the solenoid repels the bar magnet. Since like poles repel each other, the end connected to the negative terminal of the battery behaves as the north pole of the solenoid and the other end behaves as a south pole. Therefore one end of the solenoid behaves as a north pole and the other end behaves as a 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 magnetic field and electric current are perpendicular to each other. 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?

The direction of the magnetic field is vertically downwards. The direction of current is from the front wall to the back wall because negatively charged electrons are moving from back wall to the front wall. The direction of magnetic force is rightward. Hence, using Fleming's left hand rule, it can be concluded that the direction of magnetic field inside the chamber is downward. 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? Principle: It works on the principle of the magnetic effect of current. A current-carrying coil rotates in a magnetic field.
Working: When a current is allowed to flow through the coil MNST by closing the switch, the coil starts rotating anti-clockwise. This happens because a downward force acts on length MN and at the same time, an upward force acts on length ST. As a result, the coil rotates anti-clockwise. Current in the length MN flows from M to N and the magnetic field acts from left to right, normal to length MN. Therefore, according to Fleming's left hand rule, a downward force acts on the length MN. Similarly, current in the length ST flows from S to T and the magnetic field acts from left to right, normal to the flow of current. Therefore, an upward force acts on the length ST. These two forces cause the coil to rotate anti-clockwise. After half a rotation, the position of MN and ST interchange. The half-ring D comes in contact with brush A and half-ring C comes in contact with brush B. Hence, the direction of current in the coil MNST gets reversed. The current flows through the coil in the direction TSNM. The reversal of current through the coil MNST repeats after each half rotation. As a result, the coil rotates unidirectional.
The split rings help to reverse the direction of current in the circuit. These are called the commutator.

12. Name some devices in which electric motors are used?

Some devices in which electric motors are Water pumps, Electric fans, Electric mixers and 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 then a current is induced momentarily in the coil due to electromagnetic induction . As a result galvanometer shows momentarily deflection.
(II) When a bar magnet is withdrawn from inside the coil, a current is again induced momentarily in the coil but in opposite direction. As a result galvanometer shows momentarily deflection but in opposite direction.
(iii) When a bar magnet is held stationary inside the coil, there will be no current induced in the coil. So 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.

Two circular coils A and B are placed close to each other. When the current in coil A is changed, the magnetic field associated with it also changes. As a result, the magnetic field around coil B also changes. This change in magnetic field lines around coil B induces an electric current in it. This is called electromagnetic induction. 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 Maxwell’s right hand rule states that if we are holding a current-carrying conductor in our right hand such that the thumb points in the direction of current, then the direction in which fingers encircle, gives the direction of magnetic lines. This rule is also called as Maxwell’s Corkscrew Rule. (ii) Fleming's left hand rule Fleming’s left hand rule states if the forefinger, thumb and middle finger of the left hand are stretched mutually perpendicular and the forefinger points in the direction of external magnetic field; middle finger indicates the direction of current, then the thumb points along the direction of force acting on the conductor. (iii) Fleming's right hand rule Fleming’s Right hand rule it states that the states that if we stretch the thumb, forefinger and the middle finger of right hand at right angles to one another in such a way that the forefinger points in the direction of magnetic field; then the thumb gives the direction of motion of conductor (force), forefinger indicates direction of magnetic field and the middle finger points the direction of induced current. 16. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Principle: An electric generator works on the principle of electromagnetic induction phenomenon. According to it, whenever a coil is rotated between the poles of a magnet, an induced current is set up in the coil, whose direction is given by Fleming’s right hand rule. Working: Let in the beginning, as shown in Fig. brushes B1 and B2 are kept pressed separately on rings R1 and R2 respectively. Let the axle attached to the rings is rotated such that arm AB of the coil moves up and arm CD moves down in the magnetic field. Due to rotation of arms AB and CD induced currents are set up in them. As per Fleming’s right hand rule induced currents in these arms are along the directions AB and CD. Thus an induced current flows along ABCD and current in the external circuit flows from B2 to B1. After half a rotation, arm AB starts moving down and the arm CD upward. Therefore, directions of induced currents in these arms change. Thus net induced current now becomes in the direction DCBA. In the external circuit now current flows from B1 to B2. Thus after every half rotation current changes its direction and an alternating current is obtained from the generator.
Function of Brushes: Brushes are kept pressed on the two slip rings separately. Outer ends of the brushes are connected to the galvanometer (or the external load). Thus brushes help in transferring current from the coil ABCD to the external circuit.

17. When does an electric short circuit occur?