ICSE Class 9 Physics Sample Paper with Solutions Set 04

SECTION A (40 Marks)

Attempt all Questions from this Section

Question 1

(i) The pitch of a screw gauge means [1 Mark]
a) The distance moved ahead by it in rotation of circular scale by one division.
b) The length of screw in it.
c) The thickness in it.
d) The separation between two consecutive threads on its screw

Answer: (d) The separation between two consecutive threads on its screw

The pitch is the linear distance moved by the screw along its axis in one complete rotation.

Teacher's Note:
a) The pitch of a screw gauge is defined as the linear distance moved by the screw per complete rotation of the head.
b) Students often confuse pitch with least count; remember that least count is pitch divided by the total number of divisions on the circular scale.

(ii) If an object is dropped from a height and it hits the ground with a velocity of \( 50\text{ m/s} \), then its initial velocity is taken as [1 Mark]
a) Zero
b) \( 50\text{ m/s} \)
c) \( -50\text{ m/s} \)
d) \( 500\text{ m/s} \)

Answer: (a) Zero

When an object is dropped from rest, its initial velocity is zero.

Teacher's Note:
a) The word "dropped" always implies that the initial velocity (\( u \)) of the body is zero.
b) Do not confuse the final velocity with initial velocity when solving numerical problems based on free fall.

(iii) If a body covers equal distances in equal intervals of time, its motion is: [1 Mark]
a) uniform motion
b) non uniform motion
c) static
d) none of the above.

Answer: (a) uniform motion

Covering equal distances in equal intervals of time defines uniform motion.

Teacher's Note:
a) Uniform motion implies constant speed along a straight line.
b) Ensure students check both distance and time intervals to classify motion correctly.

(iv) The physical quantity that remains zero for an object moving with a uniform velocity (in ideal case) is [1 Mark]
a) momentum
b) velocity
c) external force
d) mass

Answer: (c) external force

According to Newton's first law, a body continues to move with uniform velocity when the net external force acting on it is zero.

Teacher's Note:
a) Constant velocity means acceleration is zero, which requires zero net external force.
b) Momentum and mass are non-zero for any moving body with mass.

(v) Assertion: An object can change its state by itself.
Reason: The property of an object by virtue of which it tends to retain its state of rest or of motion, is called inertia. [1 Mark]
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true and R is not the correct explanation of A
c) Assertion is false but reason is true.
d) Assertion is true reason is false.

Answer: (c) Assertion is false but reason is true.

An object cannot change its state by itself due to inertia; thus the assertion is false while the reason correctly defines inertia.

Teacher's Note:
a) Inertia is the inherent property of bodies to resist any change in their state of rest or uniform motion.
b) Read assertion and reason statements independently to check for factual correctness before linking them.

(vi) Calculate the mass of air in a room of dimensions \( 4.5\text{ m} \times 3.5\text{ m} \times 2.5\text{ m} \) if the density of air at N.T.P is \( 1.3\text{ kgm}^{-3} \). [1 Mark]
a) \( 51.19\text{ kg} \)
b) \( 0.03\text{ kg} \)
c) \( 31.29\text{ kg} \)
d) \( 50\text{ kg} \)

Answer: (a) \( 51.19\text{ kg} \)

\( \text{Volume} = 4.5 \times 3.5 \times 2.5 = 39.375\text{ m}^3 \). \( \text{Mass} = \text{Volume} \times \text{Density} = 39.375 \times 1.3 = 51.1875\text{ kg} \approx 51.19\text{ kg} \).

Teacher's Note:
a) Always calculate the volume first by multiplying length, breadth, and height.
b) Multiply volume by density with careful decimal placement to avoid rounding errors.

(vii) Food chains generally consist of only three or four step. Why? [1 Mark]
a) Only four trophic levels can be formed.
b) All organisms are covered in these steps
c) Very little usable energy is left after fourth step
d) The food chain will break down

Answer: (c) Very little usable energy is left after fourth step

Due to the 10 percent law of energy transfer, very little energy remains available after the fourth trophic level.

Teacher's Note:
a) Energy diminishes drastically at each successive trophic level.
b) Long food chains are unsustainable because higher-level consumers would expend more energy catching food than they acquire.

(viii) What will be the angle of reflection if light rays fall normally on a reflecting surface? [1 Mark]
a) \( -180^{\circ} \)
b) \( 180^{\circ} \)
c) \( 90^{\circ} \)
d) \( 0^{\circ} \)

Answer: (d) \( 0^{\circ} \)

For normal incidence, the angle of incidence is \( 0^{\circ} \), and by the laws of reflection, the angle of reflection is also \( 0^{\circ} \).

Teacher's Note:
a) Normal incidence means the ray retraces its path.
b) Always measure angles of incidence and reflection with respect to the normal, not the reflecting surface.

(ix) Which one of the following correctly defines the amplitude of a wave: [1 Mark]
a) The distance the wave moves in one second.
b) The maximum distance moved by the particles of a medium on either side of the mean position
c) The distance equal to one wavelength.
d) The distance the wave moves in one time period of the wave.

Answer: (b) The maximum distance moved by the particles of a medium on either side of the mean position

Amplitude represents the peak displacement of oscillating particles from their central equilibrium position.

Teacher's Note:
a) Amplitude determines the loudness of a sound wave.
b) Distinguish clearly between wavelength (spatial period) and amplitude (maximum displacement).

(x) An ammeter is a resistance device and it is always connected in with the circuit. [1 Mark]
a) high, series
b) high, parallel
c) low, series
d) low, parallel

Answer: (c) low, series

An ammeter has very low resistance and must be connected in series to measure current accurately without altering it.

Teacher's Note:
a) An ideal ammeter has zero resistance.
b) Connecting an ammeter in parallel would cause a short circuit due to its low resistance.

(xi) Which amongst the following is best insulator [1 Mark]
a) paper
b) carbon
c) graphite
d) ebonite

Answer: (d) ebonite

Ebonite is a hard, highly vulcanized rubber possessing exceptionally high electrical resistance, making it an excellent insulator.

Teacher's Note:
a) Graphite and carbon conduct electricity, while paper is a poor insulator compared to ebonite.
b) Memorize common electrical conductors and insulators.

(xii) A small magnet is suspended by a silk thread from a rigid support such that magnet can freely swing. How will it rest? [1 Mark]
a) It will rest with its north pole towards geographic north in a perfect straight line
b) It will rest with its north pole towards geographic south, making some angle with the horizontal.
c) It will rest with its south pole towards geographic north, making some angle with the horizontal.
d) It will rest in geographic north-south direction with north pole towards geographic north, making some angle with the horizontal.

Answer: (d) It will rest in geographic north-south direction with north pole towards geographic north, making some angle with the horizontal.

Earth's magnetic field causes a freely suspended magnet to align roughly north-south while exhibiting a dip angle with the horizontal.

Teacher's Note:
a) The angle made with the horizontal is called the angle of dip.
b) A freely suspended magnet does not point exactly along the geographic meridian due to magnetic declination.

(xiii) Which of the following statement concerning magnetic field is correct?
(1) The part of a bar magnet, at which the magnetic field is the strongest, is called its pole.
(2) A magnetic field is present near a compass needle.
(3) There is no magnetic field inside a current-carrying solenoid. [1 Mark]
a) (1) only
b) (2) only
c) (1) and (2) only
d) (2) and (3) only

Answer: (c) (1) and (2) only

Statement 1 and Statement 2 are correct because magnetic strength is maximum at poles and a compass detects magnetic fields. Statement 3 is incorrect because a uniform magnetic field exists inside a solenoid.

Teacher's Note:
a) The magnetic field inside a long current-carrying solenoid is strong and uniform.
b) Evaluate each statement individually before choosing combined options.

(xiv) A substance floats in water, but sinks in coconut oil. The density of the substance [1 Mark]
a) is less than the density of water and equal to the density of oil
b) is greater than the density of oil and equal to the density of water
c) is less than the density of water and greater than the density of oil
d) cannot be decided based on the given information

Answer: (c) is less than the density of water and greater than the density of oil

An object floats if its density is less than the liquid's density and sinks if greater, meaning density of substance is less than water and greater than coconut oil.

Teacher's Note:
a) Floating requires object density to be strictly less than fluid density.
b) Sinking occurs when object density exceeds fluid density.

(xv) The expression for the magnification of a spherical mirror in terms of focal length (f) and the distance of the object from the mirror (u) is: [1 Mark]
a) -f/(u-f)
b) f (u + f)
c) -f/ (u + f)
d) f/ (u - f)

Answer: (a) -f/(u-f)

Magnification \( m = -\frac{v}{u} \). Using mirror formula \( \frac{1}{f} = \frac{1}{v} + \frac{1}{u} \), we get \( v = \frac{uf}{u-f} \), substituting gives \( m = -\frac{f}{u-f} \).

Teacher's Note:
a) Derive this formula systematically using the mirror formula and linear magnification expression.
b) Pay close attention to sign conventions for object and image distances.

Question 2

(i) Complete the following by choosing the correct answers from the bracket: [6 Marks]
a) ________ [Watt/Newton meter\( ^2 \)/Joule] is the unit of power.
b) SONAR stands for ________.
c) ________ are obtained on plotting the field line of the magnetic field of a magnet.
d) The length of a seconds pendulum where g = \( 9.8\text{ m/s}^2 \) is nearly ________ [\( 1\text{ m} / 10\text{ m} / 1\text{ cm} \)].
e) Sound is a ________ wave and it travels through ________.

Answer:
a) Watt
b) sound Navigation and Ranging.
c) Neutral points
d) 1 m
e) mechanical, any material medium.

Teacher's Note:
a) Ensure exact terminology from standard definitions is used for fill-in-the-blank questions.
b) Double-check units and acronym expansions for accuracy.

(ii) During a military drill Raju saw a cannon fired at some distance, he noticed that the sound was heard after 10 seconds after seeing the flash. [2 Marks]

[Figure: Cartoon child observing a cannon firing in a field with distance labelled as x and speed of sound 320 m/s]

Assume the speed of sound in air \( 320\text{ m/s} \) and x is the distance between the body and cannon.
a) Why do we see flash of cannon fire before hearing its sound?
b) Compared to the speed of sound how fast does light travel in the air?

Answer:
a) The speed of light is enormously greater than the speed of sound, so light reaches our eyes instantaneously while sound takes time to travel through the air.
b) Light travels in the air at about \( 3 \times 10^8\text{ m/s} \), which is roughly a million times faster than sound.

Teacher's Note:
a) This phenomenon is a classic illustration of the vast difference between optical and acoustic wave speeds.
b) Mention numerical values clearly for complete credit.

(iii) It is said that "virtual image cannot be caught on a screen." Yet, when we are seeing a virtual image, we are obviously bringing it onto the screen i.e., the retina of our eyes. Is the said statement wrong? [2 Marks]

Answer:
No, the statement is not wrong because the eye lens refracts diverging rays coming from a virtual image and converges them to form a real image on the retina.

Teacher's Note:
a) A virtual image itself cannot be projected onto a physical screen placed in space.
b) The eye acts as an optical system that converts diverging virtual beams into a real image on the light-sensitive retina.

(iv) What is the difference between the virtual images produced by [2 Marks]
a) plane mirror
b) concave mirror
c) convex mirror

Answer:
a) Plane mirror produces a virtual image of the same size as the object.
b) Concave mirror produces a magnified (enlarged) virtual image when the object is placed close to it.
c) Convex mirror always produces a diminished (smaller) virtual image for any real position of the object.

Teacher's Note:
a) Differentiate images based on size relative to the object.
b) Ensure all three mirror types are addressed clearly.

Question 3

(i) Give one example of each of the following forces which [3 Marks]
a) Attract
b) squeeze
c) Stretch

Answer:
a) A magnet attracting iron nails.
b) Squeezing a toothpaste tube.
c) Stretching a spring.

Teacher's Note:
a) Provide common everyday examples for mechanical and non-contact forces.
b) Ensure examples correctly map to attraction, compression, and tension.

(ii) Let two objects A and B of masses M and m lie at a distance 'd' from each other. Let the force of attraction between two objects be F. [2 Marks]

[Figure: Two point masses M and m separated by distance d with a connecting line segment]

According to the universal gravitational law, what will be the force between two objects? Give equation.

Answer:
\( F = G \frac{Mm}{d^2} \), where \( G \) is the universal gravitational constant.

Teacher's Note:
a) State Newton's Law of Gravitation clearly along with the mathematical formula.
b) Define every symbol used in the equation to secure full marks.

(iii) Why one cannot suck lemonade on the surface of the moon with a soda straw? [2 Marks]

Answer:
There is no atmosphere on the moon, so there is no atmospheric pressure to push the lemonade up the straw when air is sucked out.

Teacher's Note:
a) Sucking through a straw relies on creating a pressure difference driven by external atmospheric pressure.
b) Absence of air on the moon eliminates this ambient pressure.

(iv) State the use of graph in physics. [2 Marks]

Answer:
Graphs are used to find and confirm the relationship between different physical variables, such as stretching of a metal wire versus the applied load.

Teacher's Note:
a) Graphs provide a visual representation of experimental data.
b) Mention how gradients and areas under graphs help determine derived physical quantities.

(v) A boy throws a ball vertically upwards. It rises to a height 'h' and then returns to the point from where it was thrown. What is the total distance moved by the ball? Also, find its displacement. [2 Marks]

Answer:
Total distance = \( h + h = 2h \). Displacement = \( 0 \).

Teacher's Note:
a) Distance is scalar and accounts for the entire path length covered (\( \text{up} + \text{down} \)).
b) Displacement is vector and evaluates to zero since the final position coincides with the initial position.

(vi) On what factors does the time period of a simple pendulum depend? [2 Marks]

Answer:
a) Directly proportional to the square root of its length (\( l \)), i.e., \( T \propto \sqrt{l} \).
b) Inversely proportional to the square root of acceleration due to gravity (\( g \)), i.e., \( T \propto \frac{1}{\sqrt{g}} \).

Teacher's Note:
a) State the dependencies clearly using proportionality relations.
b) Note that time period is independent of the mass of the bob and amplitude (for small oscillations).

(vii) Observe the below image and answer the following questions based on it. [2 Marks]

[Figure: Simple mercury barometer setup showing vertical glass tube of length h filled with mercury inverted in a mercury trough with a vacuum space at the top labelled B]

a) What does the above diagram represent? What is it used for?
b) What is the empty space above the empty column called?

Answer:
a) It represents a simple barometer used to measure atmospheric pressure.
b) The empty space above the mercury column is called the Torricellian vacuum.

Teacher's Note:
a) Barometers rely on balancing atmospheric pressure against the hydrostatic pressure of a mercury column.
b) Emphasize that the space above mercury contains negligible mercury vapor and acts as a near-vacuum.

SECTION B (40 Marks)

Attempt any four Questions from this Section

Question 4

(i) The focal length of a convex lens obtained by six students is \( 38.3\text{ cm} \), \( 37.8\text{ cm} \), \( 38.0\text{ cm} \), \( 37.9\text{ cm} \), \( 38.1\text{ cm} \), and \( 37.2\text{ cm} \). Express the mean focal length of the convex lens up to one place of the decimal. [3 Marks]

Answer:
\( \text{Mean focal length} = \frac{38.3 + 37.8 + 38.0 + 37.9 + 38.1 + 37.2}{6} = \frac{227.3}{6} = 37.883\text{ cm} \approx 37.9\text{ cm} \).

Teacher's Note:
a) To find the mean, sum all individual observations and divide by the total number of observations.
b) Round off the final result to the correct number of decimal places as requested.

(ii) A particle moves from P to Q. Find the ratio of the distance to the displacement of the particle. [3 Marks]

[Figure: Semicircular path from point P to Q with center O]

Answer:
Distance = Circumference of semi-circular path = \( \pi \times r \).
Displacement = Diameter of semi-circle = \( 2 \times r \).
\( \text{Ratio} = \frac{\pi \times r}{2 \times r} = \frac{\pi}{2} \).

Teacher's Note:
a) Distance along a semi-circular arc is half the full circle circumference (\( \pi r \)).
b) Displacement is the shortest straight line joining initial and final points, which is the diameter (\( 2r \)).

(iii) What do you mean by order of magnitude? A person takes 15 breaths per minute. Find the order of magnitude of number of breaths taken by him in his life, if the person survives for 70 years. [4 Marks]

Answer:
Order of magnitude is the value of a physical quantity expressed as an integral power of 10 when rounded to the nearest power of 10.
Total minutes in 70 years = \( 70 \times 365 \times 24 \times 60 = 36792000\text{ min} \).
Number of breaths in 70 years = \( 36792000 \times 15 = 551880000 = 5.5188 \times 10^8 \).
Since \( 5.5188 \gt \sqrt{10} \) (or 3.16), the order of magnitude is \( 10^9 \).

Teacher's Note:
a) Convert time units carefully from years to minutes before multiplying by breathing frequency.
b) When rounding a number in standard form \( a \times 10^n \), if \( a \gt \sqrt{10} \), the order of magnitude rounds up to \( 10^{n+1} \).

Question 5

(i) Velocity - time graph of a body is shown in the figure. [4 Marks]

[Figure: Velocity-time graph with time intervals marked as OA, AB, BC, CD, and DE along the time axis]

Locate the time intervals during which the body moves with
i. Acceleration
ii. Retardation
iii. Uniform velocity
iv. Positive velocity
v. Negative velocity.

Answer:
a) Acceleration is shown by time interval OA.
b) Retardation is shown by time interval BC.
c) Uniform velocity is shown by time interval AB.
d) Positive velocity is shown by time interval OC.
e) Negative velocity is shown by time interval CE.

Teacher's Note:
a) Slope of velocity-time graph represents acceleration; positive slope means acceleration, negative slope means retardation.
b) Regions above the time axis indicate positive velocity, while regions below indicate negative velocity.

(ii) What do you mean by acceleration due to gravity? What is its value in SI and CGS units? [3 Marks]

Answer:
Acceleration due to gravity is the uniform acceleration produced in a body when it falls freely under the sole influence of gravity. Its value in SI units is \( 9.8\text{ m/s}^2 \) and in CGS units is \( 980\text{ cm/s}^2 \).

Teacher's Note:
a) Emphasize that free fall acceleration is independent of the mass of the falling object.
b) Always include proper units when stating physical constants.

(iii) From the top of a building, a ball is dropped, while another is thrown horizontally, at the same time. Which ball will hit the ground first? [3 Marks]

Answer:
Both balls will hit the ground at the same time because vertical motion is independent of horizontal motion. Both start with zero initial vertical velocity and fall through the same vertical height under the same acceleration due to gravity.

Teacher's Note:
a) Horizontal velocity does not affect the time taken to fall vertically.
b) This is a classic conceptual physics problem demonstrating independence of perpendicular motion components.

Question 6

(i) A and B are the forces acting on a body floating in water with its some part submerged. [3 Marks]

[Figure: Floating object in water showing downward force A at center of gravity and upward buoyant force B at center of buoyancy]

a) Name these forces (A and B) which are acting on the object.
b) How will the weight of water be displaced by the floating body related to the weight of the body itself?

Answer:
a) Force A is the weight of the body acting downwards, and Force B is the upthrust (buoyant force) acting upwards.
b) The weight of water displaced by the floating body is equal to the weight of the body itself according to the Law of Floatation.

Teacher's Note:
a) For a floating body, the downward gravitational force equals the upward buoyant force.
b) Archimedes' principle ensures that displaced fluid weight equals the buoyant force.

(ii) By drawing a ray diagram, show why shaving mirrors are concave while rear view mirrors are convex. [3 Marks]

[Figure: Ray diagram for concave mirror showing enlarged virtual image when object is within focus, and ray diagram for convex mirror showing diminished erect image with wide field of view]

Answer:
Concave mirrors are used as shaving mirrors because when the face is placed close to the mirror (within its focus), it produces a magnified and erect image of the face, making shaving easier.
Convex mirrors are used as rear-view mirrors because they always produce a diminished, erect image of objects and offer a much wider field of view.

Teacher's Note:
a) Clearly draw incoming and reflected rays obeying standard rules of reflection.
b) Highlight the specific property (magnification vs. wide field of view) that dictates the choice of mirror.

(iii) Why are forests considered as one of the most important carbon dioxide sinks? [4 Marks]

Answer:
Forests absorb large amounts of carbon dioxide from the atmosphere during photosynthesis, converting it into biomass and oxygen. This process helps regulate atmospheric carbon dioxide levels, mitigating greenhouse gas accumulation and stabilizing global temperatures.

Teacher's Note:
a) Trees act as natural carbon sequestration agents.
b) Deforestation reduces this capacity, increasing atmospheric carbon dioxide concentration.

Question 7

(i) During a military drill Raju saw a cannon fired at some distance, he noticed that the sound was heard after 10 seconds after seeing the flash. [4 Marks]

[Figure: Military drill scene with cannon flash and sound propagation delay]

Assume the speed of sound in air \( 320\text{ m/s} \) and x is the distance between the body and cannon.
a) Why do we see flash of cannon fire before hearing its sound?
b) Compared to the speed of sound how fast does light travel in the air?
c) Find the distance x for the given case.
d) Will there be difference in speed of sound if cannon is fired during day and night? When will sound travel fastest through air? (Assume that temperature during day is \( 40^{\circ}\text{C} \) and night is \( 25^{\circ}\text{C} \))

Answer:
a) Light travels infinitely faster than sound, so its travel time is negligible.
b) Light travels about a million times faster than sound in air.
c) \( \text{Distance } x = \text{Speed} \times \text{Time} = 320\text{ m/s} \times 10\text{ s} = 3200\text{ m} = 3.2\text{ km} \).
d) Yes, sound travels faster during the day because temperature is higher (\( 40^{\circ}\text{C} \)), and the speed of sound increases with temperature.

Teacher's Note:
a) Use standard kinematic relation \( \text{Distance} = \text{Speed} \times \text{Time} \).
b) Temperature directly affects the speed of sound in gases.

(ii) Why are mud houses with thatched roofs more comfortable than concrete houses? [3 Marks]

Answer:
Mud and thatched roofs are poor conductors of heat and trap air in their structures. In summer, they prevent outside heat from entering, keeping interiors cool. In winter, they prevent internal heat from escaping, keeping interiors warm, unlike concrete which conducts heat readily.

Teacher's Note:
a) Emphasize the role of insulating materials in regulating indoor thermal comfort.
b) Concrete structures absorb and radiate heat much faster than mud walls.

(iii) Complete the following:
(i) \( l_t = l_0 (......) \) (ii) \( \alpha = .......\beta \) (iii) \( \gamma = .....\alpha \) [3 Marks]

Answer:
(i) \( l_t = l_0 (1 + \alpha T) \)
(ii) \( \alpha = \frac{1}{2}\beta $
(iii) \( \gamma = 3\alpha \)

Teacher's Note:
a) Memorize coefficients of linear, superficial, and cubical expansion (\( \alpha, \beta, \gamma \)).
b) Recall their standard inter-relations: \( \alpha : \beta : \gamma = 1 : 2 : 3 \).

Question 8

(i) Why does the interior of a car become too hot when parked in the sun as compared to the temperature outside? [3 Marks]

Answer:
Sunlight enters through car glass windows and is absorbed by interior surfaces, which then re-emit heat as infrared radiation. Glass is opaque to infrared radiation, trapping heat inside due to the greenhouse effect.

Teacher's Note:
a) Explain transmission of short-wave solar radiation and trapping of long-wave thermal radiation.
b) This is a classic everyday example of the greenhouse effect.

(ii) Define the terms:
i. Mechanical waves
ii. Electromagnetic waves
iii. Sound waves. [3 Marks]

Answer:
i. Mechanical waves are waves that require a material medium for propagation.
ii. Electromagnetic waves are periodic oscillations of electric and magnetic fields that do not require a material medium.
iii. Sound waves are longitudinal mechanical waves consisting of pressure oscillations through a solid, liquid, or gas.

Teacher's Note:
a) Clearly distinguish between waves needing a medium and those propagating through vacuum.
b) Mention the nature of oscillations for sound waves.

(iii) A test card of an optician is situated \( 1.5\text{ m} \) behind the eyes of a patient who is looking through a plane mirror \( 2\text{ m} \) away from him. What is the distance between him and the image of the test card?
ii. Define the terms: (1) Principal axis (2) Focus. [4 Marks]

[Figure: Diagram showing patient, plane mirror at 2 m, and test card 1.5 m behind patient's eyes]

Answer:
Distance of test card from mirror = \( 2\text{ m} + 1.5\text{ m} = 3.5\text{ m} \).
Image distance behind the mirror = \( 3.5\text{ m} \).
Distance of patient from mirror = \( 2\text{ m} \).
Total distance between patient and image of test card = \( 2\text{ m} + 3.5\text{ m} = 5.5\text{ m} \).
Definitions:
1. Principal axis: The straight line passing through the pole and the center of curvature of a spherical mirror.
2. Focus: The point on the principal axis where rays parallel to the principal axis converge or appear to diverge after reflection.

Teacher's Note:
a) Account for object distance from the mirror and property of plane mirrors where image distance equals object distance.
b) State precise optical definitions for principal axis and focus.

Question 9

(i) Two metal spheres A and B are suspended from two silk threads and a positively charged glass rod is held near A.
i. Describe what you will observe.
ii. What charges are obtained on A and B?
iii. Explain the appearance of charges on the basis of electron movement. [3 Marks]

Answer:
i. Sphere A is attracted towards the positively charged glass rod.
ii. Negative charge is induced on sphere A and positive charge on sphere B.
iii. Due to attraction by the positive rod, free electrons move towards sphere A, leaving a deficiency of electrons (positive charge) on sphere B and an excess of electrons (negative charge) on sphere A.

Teacher's Note:
a) Electrostatic induction involves redistribution of charges within a conductor.
b) Emphasize that electrons are the mobile charge carriers in metals.

(ii) Three students, Rohan, Aryan and Riya, are conducting an experiment in their science class to learn about electric circuits. They have set up three different circuits using metal wires, metal cylinders, and light bulbs. [3 Marks]

[Figure: Three electrical circuit setups showing: (1) Complete circuit with glowing bulb, (2) Broken circuit with non-glowing bulb, (3) Circuit with thread connection and non-glowing bulb]

In the first setup, Rohan has created a complete circuit and the light bulb is glowing. In the second setup, Aryan has created a broken circuit and the light bulb is not glowing. In the third setup, Riya has used a thread instead of a metal wire to connect the two metal cylinders, so the circuit is incomplete, and the light bulb is not glowing.
a) What is the difference between a complete circuit and an incomplete circuit?
b) Why does the light bulb glow in Rohan's setup but not in Aryan's or Riya's setups?
c) What is the role of the metal wire in completing the circuit?
d) Distinguish between a closed circuit and an open circuit, with the use of suitable labelled diagrams. [4 Marks]

Answer:
a) A complete circuit provides a continuous closed path for electric current, whereas an incomplete circuit has a break or gap interrupting current flow.
b) Rohan's setup is a closed loop of conducting materials allowing current to flow. Aryan's setup has a break, and Riya's setup uses an insulating thread, preventing current flow.
c) The metal wire provides a low-resistance conductive path for free electrons to flow between components.
d) A closed circuit has an unbroken conducting path allowing current flow, while an open circuit has a break preventing current flow.

Teacher's Note:
a) Conduction requires a continuous unbroken pathway of conductors.
b) Insulators like thread interrupt the flow of electric charge.