CBSE 10th Science Light : Reflection and Refraction - New Important Questions And Answers Study Material - QB365 Set A

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Questions:

Generic 3

3 x 3 = 9

Manju has two convex lenses P and Q made of the same material as shown in the figure.
She is looking at printed text on a page using lens P. She notices that when she slowly takes the lens away from the page, the text turns upside down when the lens crosses a distance of 10 cm from the page.
(i) Which characteristic of lens P does the distance 10 cm signify?
(ii) If Manju does the same with lens Q, at what distance will the inversion of the image happen-less than, equal to or more than 10 cm? Justify your answer.

The magnetic field associated with a current-carrying straight conductor is in anticlockwise direction. If the conductor was held along the east-west direction, what will be the direction of current through it? Name and state the rule applied to determine the direction of current.

What is an electromagnet? Draw a circuit diagram to show how a soft iron piece can be transformed into an electromagnet.

Case Study Questions

1 x 4 = 4

A current carrying wire produces magnetic field around it. The phenomena in which an electromotive force and current (if the conductor is in the form of a closed circuit) is induced by changing magnetic field (or by passing magnetic field lines) through it is called electromagnetic induction. The emf so developed is called induced emf and current made to flow is called induced current. The cause of induced emf carried out by Faraday and Henry. It can be concluded that the induced current flows in a conductor as long as the magnetic lines of force change within the conductor. In case of relative motion i.e., motion of coil w.r.t to magnet or vice versa, the direction of the current flowing in the conductor is determined by the direction of the relative motion of the conductor with respect to the magnetic field. The induced emf or current is directly proportional to the rate of change in magnetic field.

(i) A student connects a coil of wire with a sensitive galvanometer as shown in figure. He will observe the deflection in the galvanometer if bar magnet is

(a) placed near one of the faces of the coil and parallel to the axis of the coil
(b) placed near one of the faces of the coil and perpendicular to the axis of the coil
(c) placed inside the coil
(d) moved towards or away from the coil parallel to the axis of the coil.
(ii) A conducting rod AB moves across two magnets as shown in figure and the needle in the galvanometer deflects momentarily. What is the name of this physical phenomenon?

(a) Electromagnetism	(b) Induced magnetism
(c) Electromagnetic induction	(d) Static induction

(iii) A bar magnet is pushed steadily into a long solenoid connected to a sensitive meter.

Which of the following would affect the magnitude of the deflection of the meter?
(a) How fast the magnet is pushed into the coil.
(b) The direction in which the coil is wound.
(c) The end of the solenoid the magnet enters.
(d) The pole of the magnet enters the coil first.
(iv) What is the condition of an electromagnetic induction?
(a) There must be a relative motion between galvanometer and coil of wire.
(b) There must be a relative motion between galvanometer and a magnet.
(c) There must be a relative motion between galvanometer and electric motor.
(d) There must be a relative motion between the coil of wire and a magnet.
(v) An induced emf is produced when a magnet is plunged into a coil. The magnitude of induced emf does not depend on

(a) the number of turns in the coil	(b) the speed with which the magnet is moved
(c) the strength of the magnet	(d) the resistivity of the material of the coil

Multiple Choice Question

2 x 1 = 2

Which of these devices works due to the magnetic effect of electric current?

(a)

LED bulb

(b)

Electric bell

(c)

Electric heater

(d)

Mobile charger

A current flows In a straight wire In the direction of the arrow, shown below.

Choose the correct directions of the magnetic field produced by the current at X and Y.

(a)

(b)

(c)

(d)

Assertion and reason

2 x 1 = 2

Assertion (A) To avoid risk of electrical shock, the metal body of electric appliances is earthed.
Reason (R) Earthing saves us from electrical shocks.
Codes:
(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
(c) If Assertion is true, but Reason is false.
(d) If Assertion is false, but Reason is true.

Assertion (A) A current carrying straight conductor experiences a force when placed perpendicular to the direction of magnetic field.
Reason (R) The net charge on a current carrying conductor is always zero.
Codes:
(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.
(b) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
(c) If Assertion is true, but Reason is false.
(d) If Assertion is false, but Reason is true.

Short Answer Type Questions

3 x 2 = 6

An object 5 cm in length is placed 25 cm away from a converging lens of focal length 10 cm. Draw the ray diagram and find position, size and nature of image formed.

10.

What is short circuiting? State its possible causes. What is likely to happen if a domestic circuit gets short circuited? Give reason for the justification of your answer.

11.

The following diagram shows two parallel straight conductors carrying same current. Copy the diagram and draw the pattern of the magnetic field lines around them showing their directions. What is the magnitude of magnetic field at a point X which is equidistant from the conductors? Give justification for your answer.

Long Answer Type Questions

2 x 5 = 10

12.

In a domestic circuit five LED bulbs are arranged as shown. The source voltage is 220 V and the power rating of each bulb is marked in the circuit diagram. Based of the following circuit diagram, answer the following questions.

(a) State what happens when
(i) key K₁ is closed.
(ii) key k₂is closed.
(b) Find the current drawn by the bulb B when it glows.
(c) Calculate
(i) the resistance of bulb B, and
(ii) total resistance of the combination of four bulbs B, C, D and E.
Or
(c) What would happen to the glow of all the bulbs in the circuit when keys K₁ and K₂ both are closed and the bulb C suddenly get fused? Give reason to justify your
answer.

13.

Rishi went to a palmist to show his palm. The palmist used a special lens for this purpose.
(i) State the nature of the lens and reasons for its use.
(ii) Where should the palmist place/hold the lens so as to have a real and magnified image of an object?
(iii) If the focal length of this lens is 10 cm and the lens is held at a distance of 5 cm from the palm, use lens formula to find the position and size of the image.

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Answers Key:

Generic 3

3 x 3 = 9

(i) Focal length / half the radius of curvature.
(ii) The inversion will happen at a distance less than 10 cm. A thicker lens a smaller focal length.

Right hand thumb rule -Imagine holding the current carrying straight conductor in your right hand such that the thumb points towards the direction of current. Then the fingers of right hand wrap around the conductor and shows the direction of the magnetic field formed around it.

The direction of the current would be east-west, this can be determined by the right hand thumb rule.

When a material is placed inside a coil carrying current (may be solenoid) it will get magnetised. A bunch of nails or an iron rod placed along the axis of the coil can be magnetised by the current allowed to pass through the coil.

Once the current is put-off the magnetic field will also be lost. Such magnets are called electromagnets.

Case Study Questions

1 x 4 = 4

(i) (d) : The deflection in galvanometer can be seen if bar magnet moved towards or away from coil parallel to the axis of the coil.
(ii) (c): If the needle of the galvanometer deflects it means there is change in magnetic field and current is induced.
(iii) (a): By Faraday's law of electromagnetic induction, the e.m.f. induced in a conductor is proportional to the rate of change of magnetic lines of force linking the circuit. Hence, by pushing in the magnet faster, the rate of change of magnetic lines will increase. This results in a larger induced e.m.f. and hence, larger deflection of the meter.
(iv) (d)
(v) (d): Resistivity of coil will determine the resistance of the coil and induced current through it, as induced
\(\text { current }=\frac{\mathrm{emf}}{\text { resistance }}\)

Multiple Choice Question

2 x 1 = 2

Electric bell works on the principle of magnetic effects of current. It uses an electromagnet to produce sound.

When we point our right hand thumb in the direction of current, then our curl fingers gives the direction of magnetic field.

Assertion and reason

2 x 1 = 2

(a) If both Assertion and Reason are true and Reason is the correct explanation of Assertion.

(b) If both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
Explanation:
A current carrying conductor experience a force when placed perpendicular to the direction of magnetic field. This force is called Lorentz force. In a current carrying conductor the net charge is always zero.

Short Answer Type Questions

3 x 2 = 6

Given, object size, h_o=5 cm; Image size, h, =?
Object distance, u =-25 cm; Image distance, v =?
Focal length, f = 10 cm
From lens formula,

﻿\(\frac{1}{f}=\frac{1}{v}-\frac{1}{u}\)﻿

﻿\(\frac{1}{v}=\frac{1}{f}+\frac{1}{u}=\frac{1}{10}+\frac{1}{(-25)}=\frac{3}{50}\)﻿

﻿\(v=\frac{50}{3}=16.67\)﻿

cm from the lens
Size of image,

﻿\(h_i=h_o \times \frac{v}{u}=5 \times \frac{50}{3 \times(-25)}=-3.33 \mathrm{~cm}\)﻿

[diminished inverted image]
The image is real in nature.

10.

If the live wire and neutral wire come in contact either directly or via conducting wire, then it is called short circuiting.
Causes
(i) Damaged as worn-out insulation of wires.
(ii) Loose or faulty connections.
(iii) Overloaded circuits.
If a domestic circuit gets short-circuited, a large amount of current will flow suddenly through the wires. This can cause
(i) Overheating of wires. (ii) Fire hazard.
(iii) Damage to applications. (iv) Tripping of fuse or MCB.

11.

Magnetic field lines due to parallel current carrying conductors is shown in figure.

Magnetic field at X = 0, because magnetic fields at X due to both conductors are equal in magnitude but opposite in direction.

Long Answer Type Questions

2 x 5 = 10

12.

(a) (i) When key K₁ is closed

I=\frac{P}{V}=\frac{22}{220}=0.1 \mathrm{~A},

current will flow in the circuit.
(ii) When key K₂ is closed,

I=\frac{P}{V}=\frac{11}{55}=0.2 \mathrm{~A},

current will flow in the circuit.
(b) Current drawn by the bulb Bis same as current flows when the key K₂ is closed.
(c) (i) The resistance of bulb B is R =

=\frac{V}{J}=\frac{55}{0.2}=275 \Omega

(ii) Since the power rating and voltage is same] across all four bulbs B, C, D and E, resistance will be same as

275 \Omega .

All bulbs are connected in series, so, total resistance will be 275 + 275 + 275 + 275 =

1100 \Omega

Or
When keys K₁ and K₂ both are closed, then all the bulbs A, B,C, D and E will glow and when the bulb C suddenly gets fused then, current will not flow in the lower part of circuit consisting bulb C. Bulb A will glow this time only.

13.

(i) A palmist uses convex lens to see the magnified image of palm lines.
(ii) The palmist will hold the lens where palm is in between the focus and centre of curvature or at focus only.
(iii) Given, focal length f = 10 cm
and object distance, u = - 5 cm
From lens formula,