NCERT Solutions for Class 10 Science Chapter 12 "Magnetic Effects of Electric Current"

Topic 12.2, Page Number- 200

Q.1 Draw magnetic field lines around a bar magnet.

Answer:

The magnetic field lines emerge from the North pole of the bar magnet and merge into the South pole, that is inside the bar magnet the field lines are from South to North, and outside it is from North to South.

Q.2 List the properties of magnetic field lines

Answer:

Some properties of magnetic field lines are

1) They never intersect with each other. If they intersect there must be two directions of the magnetic field which is not possible

2) The direction of magnetic field lines is from North pole to South pole outside the magnet and from the South pole to the North pole inside the magnet.

3) The field lines emerge from the north pole and merge at the south pole.

Q.3 Why don’t two magnetic field lines intersect each other?

Answer:

Two magnetic field lines do not intersect each other because the resultant magnetic field due to both poles can be in only one direction and if they intersect there will be two directions of the magnetic field which is impossible.

Topic 12.2, Page Numbers- 201-202

Q.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.

Answer:

When we pass the current clockwise, the magnetic field lines will emerge from outside the loop and will merge inside the loop.

Q.2 The magnetic field in a given region is uniform. Draw a diagram to represent it.

Answer:

The Uniform Magnetic Field can be represented as:

Q.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.

Answer:

The magnetic field inside a long straight solenoid-carrying current is Uniform in nature, So the magnetic field is the same at all points.

Hence, option (d) is the correct answer.

Topic 12.3, Page Number- 203

Q.1 Which of the following properties 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

Answer:

When a proton moves in the magnetic field, it experiences a force. This force will always be in a direction perpendicular to the direction of motion, so, the proton will always perform the circular motion. therefore, the velocity and momentum can change,

Hence, options (c) and (d) are the correct answers.

Q.2 a) In Activity 12.7, how do we think the displacement of rod AB will be affected if current in rod AB is increased

Answer :

When a current-carrying wire is placed in the magnetic field, it experiences a force.

This force is proportional to the length of the wire, the strength of the magnetic field and the magnitude of the current passing through the wire.

Hence, when the current in rod AB is increased, it will experience a greater force and hence will be more displaced.

Q.2 b) In Activity 12.7, how do we think the displacement of rod AB will be affected if a stronger horseshoe magnet is used?

Answer:

When a current-carrying wire is placed in the magnetic field, it experiences a force; this force is proportional to the length of the wire, the strength of the magnetic field and the current through the wire.

Hence, when a stronger horseshoe magnet is used, the strength of the magnetic field is high, and it will experience a greater force and hence will be more displaced.

Q. 2. c) In Activity 12.7, how do we think the displacement of rod AB will be affected if the length of the rod AB is increased?

Answer:

When a current-carrying wire is placed in the magnetic field, it experiences a force.

This force is proportional to the length of the wire, the strength of the magnetic field and the current through the wire.

Hence, when the length of the rod AB is increased, it will experience a greater force and hence will be more displaced.

Q.3 A positively charged particle (alpha-particle) projected towards the west is deflected towards the north by a magnetic field. The direction of the magnetic field

(a) towards south

(b) towards east

(c) downward

(d) upward

Answer:

By Fleming's Left-hand rule,

Keeping the thumb, forefinger and middle finger in mutually perpendicular directions. The middle finger represents the direction of the current (in the direction of motion of positive charge: towards the West), the thumb represents the force (towards the North), and then the forefinger represents the direction of the field (upward)

The direction of the Magnetic field will be upward.

Hence, option (d) is the correct answer.

Topic 12.4, Page Number - 205

Q.1 State Fleming’s left-hand rule

Answer:

Fleming's Left-Hand Rule:

Whenever a current-carrying conductor is placed in a magnetic field, the conductor experiences a force that is perpendicular to both the magnetic field and the direction of the current.

in other words,

if the first three fingers(Thumb, forefinger and middle finger) of the left hand are held mutually at right angles to each other if the forefinger indicates the direction of the original field, and if the middle finger indicates the direction of the current flowing through the conductor, then the thumb indicates the direction of the force exerted on the conductor

Q.2 What is the principle of an electric motor?

Answer: When a current-carrying conductor is placed in a magnetic field, it experiences a force that is perpendicular to both the magnetic field and the direction of current through the conductor. So, the force helps the conductor in performing the rotation. The electric motor uses this principle to rotate.

Q.3 What is the role of the split ring in an electric motor?

Answer: The split ring is also called a commutator because it commutes (transfers) the current from the outside of the motor to the inside of the motor. It changes the direction of the current after every half rotation so that the motor will continue to rotate in the same direction.

Q.2 Name some sources of direct current.

Answer- The DC current is the current which does not change direction with time. The sources of DC current are a DC battery and a DC generator.

Q.3 Which sources produce alternating current?

Answer- Alternating current is the current which alters the direction at some time interval. Some sources of this type of current are AC generators and AC-producing power plants.

Q. 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

Answer: The direction of the induced current changes once in each half-revolution, since the direction of the relative motion of the coil and the magnetic field, changes in every half-cycle

Hence the correct option is (c).

Q.5 Name two safety measures commonly used in electric circuits and appliances.

Answer:

Two safety measures commonly used in electric circuits and appliances are

1) Use of an Electric Fuse: We add an electric fuse in series with our circuit to protect our circuit in the event when the current through the circuit becomes extremely high. When the current is high fuse breaks and the circuit gets isolated from the supply.

2) Earthing to prevent shock: We connect a proper ground connection so that whenever there is any leakage current in any appliance, it directly gets transferred to the ground without giving a shock to the person using the appliance.

Q.6 An electric oven with a 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.

Answer: As given in the question, the maximum current which can pass through an electric oven is 5 A. So let's find out how much current will pass through the device in given power ratings.

So for power, we have a relation

Power (P) = Voltage(V) X Current (I)

So, putting given values, we get

$2\mathrm{kW}=220\mathrm{V}\times \text{Current}(\mathrm{I})$

$\mathrm{Current}(I)=\frac{2000}{220}A=\frac{100}{11}A=9.09A$

As we can see the current flowing in the electric oven is 9.09 A which is greater than 5 A. This can be a dangerous situation. The insulation on the wire may melt and cause a short circuit problem. we use a fuse to protect us from this situation. A fuse is just a wire that melts and breaks the circuit in the case of an overcurrent situation like the above.

Q.7 What precautions should be taken to avoid the overloading of domestic electric circuits?

Answer: Some precautions that should be taken to avoid the overloading of domestic electric circuits are:

1) We should not use too many appliances at the same time.

2) We should not connect too many appliances to a single socket.

3) We should use the fuse for precaution when there is an overload.

4) We should not connect the faulty appliances in the circuit.

5) Proper earthing should be there

Q. 2. At the time of the short circuit, the current in the circuit

(a) reduces substantially.

(b) does not change.

(c) increases heavily.

(d) vary continuously.

Answer:

At the time of the short circuit, the current in the circuit increases abruptly.

Hence, option (c) is correct.

Q. 3. State whether the following statements are true or false.

(a) The field at the centre of a long circular coil carrying current will be parallel straight lines.

(b) A wire with green insulation is usually the live wire of an electric supply.

Answer:

a)True, as the long circular coil is equivalent to a solenoid. and hence the field at the centre of a long circular coil carrying current will be parallel straight lines.

b) False, As the live wire in domestic circuits is usually red in colour and ground wire is in green colour, NORMALLY.

Q. 4. List two methods of producing magnetic fields.

Answer:

The two methods of producing the magnetic field are:

1) Passing the current through any conductor.

2) The permanent magnet also produces the magnetic field around it.

Q. 5. When is the force experienced by a current-carrying conductor placed in a magnetic field largest?

Answer:

In the current-carrying conductor, the force experienced will be maximum when the direction of the current in the conductor is perpendicular to the magnetic field.

Q. 6. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from the back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of a magnetic field?

Answer:

Suppose you are leaning towards the back wall then the electron beam is moving away from your eyes which means the direction of current is towards you (direction of current is taken opposite to the flow of electrons) and the deflection is to the right. Then by Fleming's left-hand rule, the field is in the downward direction.

Q. 8. When does an electric short circuit occur?

Answer: The short circuit happens when the positive voltage point of the wire comes in contact with the negative voltage point in the wire. As the resistance of wires is very small a very high current flows through the circuit which can potentially damage the circuit. This phenomenon is called a short circuit.

When neutral wire and live wire come in contact, a short circuit happens.

Also, when we connect too many appliances in a single circuit, the resistance of the circuit can get very small up to the point of the possibility of having a short circuit.

Q. 9. What is the function of an earth wire? Why is it necessary to earth metallic appliances?

Answer: The main function of the earth wire is to prevent the live wire from overloading and absorb the excess electrons & flow beneath the ground. It is necessary because it prevents electrical appliances from getting damaged and absorbs the excess electricity from appliances.

In other words, whenever we have a connection to the ground the leakage current or the overload current can be transferred to the ground directly and the events of shock can be prevented in which the current passes through the body of the person touching the metallic appliances.

Q. 5. 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?

Answer:

A simple Electric Motor :

An electric motor converts electrical energy into mechanical energy.

Principle: It works on the principle of the magnetic effect of the current. when a current-carrying conductor is placed in the magnetic field, it experiences a force and starts moving.

Working :

An electric motor, as shown, consists of a rectangular coil ABCD of insulated copper wire. The coil is placed between the two poles of a magnetic field such that the arm AB and CD are perpendicular to the direction of the magnetic field. The ends of the coil are connected to the two halves P and Q of a split ring. The inner sides of these halves are insulated and attached to an axle. The external conducting edges of P and Q touch two conducting stationary brushes X and Y, respectively.

Current in the coil ABCD enters from the source battery through conducting brush X and flows back to the battery through brush Y. Notice that the current in arm AB of the coil flows from A to B. In arm CD it flows from C to D, that is, opposite to the direction of current through arm AB. On applying Fleming’s left-hand rule for the direction of the force on a current-carrying conductor in a magnetic field. We find that the force acting on arm AB pushes it downwards while the force acting on arm CD pushes it upwards. Thus the coil and the axle O are mounted free to turn about an axis and rotate anti-clockwise. At half rotation, Q makes contact with brush X and P with brush Y. Therefore the current in the coil gets reversed and flows along the path DCBA.

A device that reverses the direction of flow of current through a circuit is called a commutator. In electric motors, the split ring acts as a commutator. The reversal of current also reverses the direction of force acting on the two arms AB and CD. Thus the arm AB of the coil that was earlier pushed down is now pushed up and the arm CD previously pushed up is now pushed down. Therefore the coil and the axle rotate half a turn more in the same direction. The reversing of the current is repeated at each half-rotation, giving rise to a continuous rotation of the coil and the axle.

The split ring helps in reversing the direction of current in the armature so that it rotates continuously in one direction.

Q. 6. Name some devices in which electric motors are used.

Answer: - We use electric motors in many devices like fans, pumps, grinders, washing machines, vehicles and many more.

Q. 7. 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?

Answer:

(i) When a bar magnet is pushed into the coil, there is a relative motion between the magnetic field and the coil hence a current will get induced which will deflect the galvanometer. when the bar magnet is inside the coil and not moving, the flux won't change so the galvanometer won't show deflection. And hence Overall galvanometer will deflect momentarily.

(ii) When the bar magnet is being removed, the flux through the coil will change which results in inducing the current and deflecting the galvanometer. after bar magnet is completely removed and is at a sufficiently large distance for its magnetic field to affect the coil, there won't be any flux change in the coil and hence no current will be induced. and hence overall the galvanometer will deflect momentarily.

(iii) If a bar magnet is held stationary inside the coil, the flux through the coil is not changing with time and hence there won't be any induced current. therefore the galvanometer won't show any deflection.

Q. 8. Two circular coils A and B are placed close to each other. If the current in coil A is changed, will some current be induced in coil B? Give a reason.

Answer- When the two circular coils A and B are placed close to each other and current in the coil A is changed, the magnetic field coil A is producing will change which leads to inducing the current in coil B.

Q. 9. Explain the underlying principle and working of an electric generator by drawing a labelled diagram. What is the function of brushes?

Answer: The Electric Generator:

An electric generator converts mechanical energy into electrical energy.

Principle: It works on the principle of electromagnetic induction. whenever there is a change in the flux through any coil, there would be the induction of current.

Working:

An electric generator, as shown consists of a rotating rectangular coil ABCD placed between the poles of a permanent magnet. The two ends of this coil are connected to the two rings R1 and R2. The inner side of the ring is insulated. The two conducting stationary brushes B1 and B2 are kept pressed separately on the rings R1 and R2, respectively. The two rings R1 and R2 are internally attached to an axle. The axle may be mechanically rotated from outside to rotate the coil inside the magnetic field. The outer ends of the two brushes are connected to the galvanometer to show the direction of the flow of current in the external circuit. When the axle attached to the two rings is rotated such that the arm AB moves up (and the arm CD moves down) in the magnetic field produced by the permanent magnet. Let us say the coil ABCD is rotated clockwise in the arrangement shown. By applying Fleming’s right-hand rule, the induced currents are set up in these arms along the directions AB and CD.

Thus an induced current flows in the direction ABCD. This means that the current in the external circuit flows from B2 to B1. After half a rotation, arm CD starts moving up and AB moving down. As a result, the directions of the induced currents in both arms change, giving rise to the net induced current in the direction of DCBA. The current in the external circuit now flows from B1 to B2. Thus after every half rotation the polarity of the current in the respective arms changes. Such a current, which changes direction periodically, is called alternating current (AC). This device is called an AC generator.

To get a direct current (DC, which does not change its direction with time), a split-ring type commutator must be used. With this arrangement, one brush is at all times in contact with the arm moving up in the field, while the other is in contact with the arm moving down

The function of Brushes: Brushes help in transferring the current from inside the generator to the external circuit.