Intext Questions Solved

Page 224

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

Answer : A compass needle gets deflected due to magnetic force applied by the bar magnet.

Page 228

Question 1 : Draw magnetic filed lines around a bar magnet.

Answer :

Question 2 : List the properties of magnetic lines of force.

Answer :
Properties of Magnetic Lines of Force

  1. Magnetic lines of force are closed continuous curves or loops.
  2. Inside the magnet, lines of force are from south to north whereas outside the magnet these are from north to south.
  3. The direction of magnetic field at a point is given by the tangent drawn to the magnetic lines of force.
  4. No two lines of force are found to intersect each other.
  5. Density of magnetic lines of force in a given region represents the relative strength of field i.e., if lines are crowded, field will be stronger and if lines are not crowded, field will be weaker.

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

Answer : No two lines of force are found to intersect each other because if they did, then there would be two directions at one point, which is not possible.

Page 229-230

Question 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 :

According to the right hand thumb rule the direction of magnetic field is into the plane of paper.

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

Answer : Uniform magnetic field is represented by equidistant parallel lines.

Question 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 ends.
c) increases as we move towards its end.
d) is the same at all points.

Answer : The correct option is (b)

Pages 231-232

Question 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

Answer : When a charged particle enters in a magnetic field its velocity changes (due to change in direction only) as a result momentum will also change. Therefore the correct options are (c) and (d).

Question 2 : In Fig.(a) and (b), how do we think the displacement of wire AB will be affected if (i) current in the wire AB is increased (ii) a stronger horse shoe magnet is used and (iii) length of wire AB is increased.

Answer : When a current-carrying conductor is placed inside magnetic field (B) then force (F) on the conductor is given by
F = I lB,
Where I = Current, l = Length of conductor

i) Current is increased, force will also increase.
ii) When stronger horse-shoe magnet is used then magnetic field
(B) would increase resulting in increase of force as F∝B.
iii) When length (l) of conductor is increased then force will also increase as F∝l.

Question 3 : 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.

Answer : As proton is positively charged its motion can be considered equivalent to electric current flowing towards west. Deflection of proton is towards north therefore force on the proton is towards north, on applying Fleming’s left hand rule magnetic field is outward (Out of page).

Page 233

Question 1 : State Fleming’s left-hand rule.

Answer : If you stretch the forefinger, middle finger and the thumb of left hand such that these three are mutually perpendicular, then according to Fleming’s left hand rule, thumb gives direction of force (F), forefinger gives direction of magnetic field (B) and middle finger would give direction of current (I) as shown in figure.

Question 2 : What is the principle of an electric motor?

Answer : When a current-carrying coil is freely suspended inside uniform magnetic field it experiences torque and rotates.

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

Answer : With the help of split ring the direction of electric current gets reversed in the coil or motor after each half cycle due to which torque keep acting in the same direction therefore the coil keeps rotating in one direction.

Page 236

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

Answer :  Various ways to induce current in a coil are
(i) moving a coil in a magnetic field
(ii) changing the magnetic field around a coil.

Page 237

Question 1 : State the principle of an electric generator.

Answer : It is based on the principle of electromagnetic induction.

Question 2 : Name some sources of direct current.

Answer : Dynamo and battery cells.

Question 3 : Which sources produce alternating current?

Answer :
a) a cell
b) a battery
c) a d.c. generator
d) a solar cell

Question 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 : c) half revolution.

Page 238

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

Answer : i) Earthing and (ii) fuse

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

Answer : We are given, P=2kW, V = 220 volt.

P = VI
I = P/V
I = 2000/220 = 9A

As current in the circuit is more than the current rating, fuse will melt and circuit will be cut off.

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

Answer : We should avoid connecting too many appliance in a single socket.

Text Book Questions Solved

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

a) The filed 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.

Answer : d)

Question 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.

Answer : c)

Question 3 : The device used for producing electric current is called a

a) generator
b) galvanometer
c) ammeter
d) motor.

Answer : (a) generator.

Question 4 : The essential difference between an A.C. generator and a D.C. generator is that

a) A.C. generator has an electromagnet while a D.C. generator has permanent magnet.
b) D.C. generator will generate a higher voltage.
c) A.C. generator will generate a higher voltage.
d) A.C. generator has slip rings while the D.C. generator has a commutator.

Answer : (d)

Question 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

Answer : (c)

Question 6 : 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 on an electric supply.

Answer :
a) False
b) True
c) True
d) False

Question 7 : Lis three sources of magnetic fields.

Answer : i) A bar magnet.
ii) A straight current-carrying conductor.
iii) A solenoid.

Question 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.

Answer : A solenoid consists of many circular turns of an insulated wire in the shape of cylinder. The cylinder on which wire is wound is called ‘core’.

When current is passed through the solenoid, magnetic field is produced, the pattern of magnetic lines of force due to a solenoid is very much similar to that of a bar magnet as shown in figures.

With the help of a compass needle north pole and south pole can be easily determined. North of compass needle will point towards south pole of the solenoid and vice versa.

Question 9 : When the force experienced by a current-carrying conductor placed in a magnetic field largest?

Answer : When the current-carrying conductor is placed in a direction perpendicular to the direction of magnetic field.

Question 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?

Answer : Vertically downwards. (From ceiling to the floor).

Question 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?

Answer : 

Principle : When a current-carrying coil is freely suspended inside uniform magnetic field it experiences torque and rotates.

Working : When current flows in the armature ABCD in such a way that position of armature in the field is as shown in the figure (a), force on AB acts upwards and force on CD acts in the downward direction according to Fleming’s left hand rule. Due to this, armature rotates and it starts rotating. When it completes half turn position of armature in the field as shown in Figure (b). Now brush B1 is in contact with C2 and brush B2 is in contact with C1, due to which direction of current  in the armature reverses but forces on the arm AB and CD are in such a way that the rotation of armature remains in the same direction. This way by the action of commutator (split rings) the armature rotates continuously in one direction.

Question 12 : Name some devices  in which electric motors are used.

Answer : Electric fans, refrigerators, mixers, washing machines, computers, MP3 players, etc.

Question 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?

Answer : (i) Needle of galvanometer will deflect.
ii) It will deflect in the opposite direction.
iii) No deflection.

Question 14 : Two circular coils A and B are placed close to each other. If the current in the coil A is changed, will some current be induced in the coil B? Give reason.

Answer : Yes. Due to electromagnetic induction.

Question 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 to its rotation in a magnetic field.

Answer : (i) Right hand thumb rule.
ii) Fleming’s left hand rule.
iii) Fleming’s right hand rule.

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

Answer : An A.C. generator converts mechanical energy into electrical energy (Alternating current). Alternating current is such a current which changes direction after equal intervals of time. In our houses we get supply of A.C. whose frequency is 50 Hz that means its direction will change 100 times in a second.

Question 17 : When does an electric short circuit occur?

Answer : When the liver wire and neutral wire comes into direct due to naked wires (insulation of the wires is damaged) or fault in an appliance, electric short-circuit occurs.

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

Answer : The earth wire provides a low-resistance conducting path for electric current from the metallic body of an appliance to the earth.

Sometimes leakage of current takes place to the metallic body of an appliance like toaster, electric iron, etc. due to which user may get electric shock. The earth wire ensures that this leakage of current flows to the earth through it and user does not get a severe shock.