Cls 10 Sci Chptr 13 Slusn

Class 10 Science

Chapter 13 Magnetic effect of Electric current

Solutions 

Page:224

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

Ans:

The needle of a compass is a small magnet and its magnetic field lines interact with that of the bar magnet. Hence, a compass needle gets deflected.

Page Number: 228

1. Draw magnetic field lines around a bar magnet.

Ans:

2. List the properties of magnetic lines of force.

Ans:

> Magnetic field lines do not intersect with each other.

> They emerge from the North Pole and terminate at the South Pole.

> Inside the magnet, the direction of the field lines is from the South Pole to the North Pole.

> Magnetic field lines are curved loop.

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

Ans:

If two magnetic field lines intersect then at the point of intersection the compass needle shows two different directions, which is not possible, hence they do not intersect with each other.

Page Number: 229 – 230

1. Consider a circular loop of wire lying on 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.

Ans:

Inside the loop = Pierce inside the tableOutside the loop = Appear to emerge out from the tableFor downward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop. Similarly, for upward direction of current flowing in the circular loop, the direction of magnetic field lines will be as if they are emerging from the table outside the loop and merging in the table inside the loop, as shown in the given figure.

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

Ans:

3. Choose the correct option.

The magnetic field inside a long straight solenoid-carrying current

(i) is zero

(ii) decreases as we move towards its end

(iii) increases as we move towards its end

(iv) is the same at all points

Ans:

(iv) Is the same at all points.

Page Number: 231 – 232

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.)

(i) Mass

(ii) Speed

(iii) Velocity

(iv) Momentum

Ans:

The correct options are (iii) velocity, (iv) momentum.

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 increasedif

Ans:

the strength of the magnetic force exerted on the rod AB and its displacement will increase if

> The current in rod AB is increased

> Stronger horse shoe magnet is used

> When the length of the rod AB increases

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

The direction of magnetic field is :

(i) towards south

(ii) towards east

(iii) downward

(iv) upward

Ans:

(iv) Upward.

Page Number: 233

1. State Fleming’s left hand rule. 

Ans:

Fleming’s left hand rule : Stretch the first finger, the middle finger and the thumb of your left hand mutually perpendicular to each other in such a way that the first finger represents the direction of the magnetic field, the middle finger represents the direction of the current in the conductor, then the thumb will represent the direction of motion of the conductor.

2. What is the principle of an electric motor ? 

Ans:

A motor works on the principle of magnetic effect of current. When a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously.

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

Ans:

The split ring reverses the direction of current in the armature coil after every half rotation.

Page Number: 236

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

Ans: 

Different ways to induce current in a coil are :

> moving a magnet towards or away from the coil or vice-versa, and

> changing current in the neighbouring coil.

Page Number: 237

1. State the principle of an electric generator.

Ans:

Electric generator works on the principle of electromagnetic induction. Electricity is generated by rotating a coil inside magnetic field.

2.'Name some sources of direct current.

Ans:

dry cells, button cells, DC generator

3. Which sources produce alternating current ?

Ans:

Power plants and AC generators

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:

(i) two revolution

(ii) one revolution

(iii) half revolution

(iv) one-fourth revolution

Ans:

(iii) Half revolution.

Page Number: 238

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

Ans:

(i) Earthing and

(ii) Electric fuse.

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.

Ans:

The electric oven draws a current given by

The current drawn by the electric oven is 9.09 A which exceeds the safe limit of the circuit. This causes the fuse to melt and break the circuit.

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

Ans:

> Connecting too many devices to a single socket should be avoided

> Using too many appliances at the same time should be avoided

> Faulty appliances should not be connected to the circuit

Exercises Questions 

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

(i) the field consists of straight lines perpendicular to the wire

(ii) the field consists of straight lines parallel to the wire

(iii) the field consists of radial lines originating from the wire

(iv) the field consists of concentric circles centred on the wire

Ans:

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

2. The phenomenon of electromagnetic induction is

(i) the process of charging a body

(ii) the process of generating magnetic field due to a current passing through a coil

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

(iv) the process of rotating a coil of an electric motor

Ans:

(iii) 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

(i) generator

(ii) galvanometer

(iii) ammeter

(iv) motor

Ans:

(i) Generator.

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

(i) AC generator has an electromagnet while a DC generator has permanent magnet

(ii) DC generator will generate a higher voltage

(iii) AC generator will generate a higher voltage

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

Ans:

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

5. At the time of short circuit, the current in the circuit

(i) reduces substantially

(ii) does not change

(iii) increases heavily

(iv) varies continuously

Ans:

(iii) Increases heavily.

6. State whether the following statements are True or False.

(i) An electric motor converts mechanical energy into electrical energy.

(ii) An electric generator works on the principle of electromagnetic induction.

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

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

Ans:

(i) False

(ii) True

(iii) True

(iv) False.

7. List three sources of magnetic fields.

Ans:

(i) Current carrying conductor

(ii) Electromagnets

(iii) 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.

Ans:

A solenoid behaves like a magnet in the following ways.

> The magnetic field produced by a current carrying solenoid is very much similar to that of a bar magnet.

> Like a bar magnet, one end of the solenoid has N-polarity while the other end has S-polarity.

To determine the north and south poles, we bring N-pole of the bar magnet near one end of the solenoid. If there is an attraction, then that end of the solenoid has south polarity and the other has north polarity. If there is a repulsion, then that end of the solenoid has north polarity and the other end has south polarity because similar poles repel each other.

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

Ans:

When the direction of the current is perpendicular to the direction of the magnetic field is when the force experienced is the largest.

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 ?

Ans:

The direction of the magnetic field is vertically downwards.

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 ?

Ans:

Principle: It works on the principle of the magnetic effect of current. A current-carrying coil rotates in a magnetic field.

Working: When current starts to flow in the coil ABCD, the direction of current  from A to B  and from C to D. The direction of force exerted on the coil can be found through Fleming’s left hand law.

According to this law, it is found that the force exerted on the part AB, pushes the coil downwards. While the force exerted on the part CD pushes it upwards. that rotates the coil in anticlockwise direction.

After half rotation, the polarity of the commutator also changes,  now the force exerts downwards on the arm AB and upwards on the arm CD. This process is repeated again and again and the coil rotate continuously.

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.

Ans:

• Electric fans
• Water pumps
• Mixers
• Washing machines

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

Ans:

(i) The needle of the galvanometer shows a momentary deflection in a particular direction.

(ii) The needle of the galvanometer shows a momentarily in the opposite direction.

(iii) The needle of the galvanometer shows no deflection.

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.

Ans:

When the current in coil A changes, the magnetic field associated with it also changes. As a result the magnetic field around coil B undergoes change. 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 (it) force experienced by a current-carrying straight conductor placed in a magnetic field which is perpendicular to it, and (in) current induced in a coil due to its rotation in a magnetic field.

Ans:

(i) Right hand thumb rule : If the current carrying conductor is held in the right hand such that the thumb points in the direction of the current, then the direction of the curl of the fingers will give the direction of the magnetic field.

(ii) Fleming’s left hand rule : Stretch the forefinger, the central finger and the thumb of the left hand mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field, the middle finger in the direction of current, then the thumb points in the direction of force in the conductor.

(iii) Fleming’s right hand rule : Stretch the thumb, forefinger and the central finger of the right hand mutually perpendicular to each other. If the forefinger points in the direction of magnetic field, thumb in the direction of motion of the conductor, then the middle finger points in the direction of current induced in the conductor.

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

Ans:

Principle: The working principle of the electric generator is the electromagnetic induction. It generates electricity by rotating a coil in the magnetic field.

Working : When the armature coil ABCD rotates in a magnetic field produced by the permanent magnets, it cuts through the magnetic lines of force.

Due to the rotation of armature coil, the associated magnetic field changes and an induced electromagnetic force is produced in it. The direction of this induced electromotive force or current can be determined by using Fleming’s right hand rule.

In first half cycle the current flows in one direction by brush B1 and in second it flows in opposite direction by brush B2. This process continues. So the current produced is alternating in nature.

Functions of Brushes : Brushes in contact with rings provide the current for external use

17. When does an electric short circuit occur ?

Ans:

short-circuit occurs when live and neutral wire come in direct contact with each other without any resistance. The resistance of the circuit becomes zero and excessive current starts to flow through it.

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

Ans:

> The metallic body of electric appliances is earthed by means of earth wire. Any leakage of electric wire is transferred to the ground by means of earth wire.

> This prevents any electric shock to the user.