Science (English Medium) Class 11 - CBSE Question Bank Solutions for Physics

The thermal radiation emitted by a body is proportional to Tⁿ where T is its absolute temperature. The value of n is exactly 4 for 

A blackbody does not

(a) emit radiation
(b) absorb radiation
(c) reflect radiation
(d) refract radiation

The specific heat capacities of hydrogen at constant volume and at constant pressure are 2.4 cal g⁻¹ °C⁻¹ and 3.4 cal g⁻¹ °C⁻¹ respectively. The molecular weight of hydrogen is 2 g mol⁻¹ and the gas constant, R = 8.3 × 10⁷ erg °C⁻¹ mol⁻¹. Calculate the value of J.

The normal body-temperature of a person is 97°F. Calculate the rate at which heat is flowing out of his body through the clothes assuming the following values. Room temperature = 47°F, surface of the body under clothes = 1.6 m², conductivity of the cloth = 0.04 J s⁻¹ m⁻¹°C⁻¹, thickness of the cloth = 0.5 cm.

A cubical block of mass 1.0 kg and edge 5.0 cm is heated to 227°C. It is kept in an evacuated chamber maintained at 27°C. Assuming that the block emits radiation like a blackbody, find the rate at which the temperature of the block will decrease. Specific heat capacity of the material of the block is 400 J Kg^-1 K-^1.

A copper sphere is suspended in an evacuated chamber maintained at 300 K. The sphere is maintained at a constant temperature of 500 K by heating it electrically. A total of 210 W of electric power is needed to do it. When the surface of the copper sphere is completely blackened, 700 W is needed to maintain the same temperature of the sphere. Calculate the emissivity of copper.

One end of a rod of length 20 cm is inserted in a furnace at 800 K. The sides of the rod are covered with an insulating material and the other end emits radiation like a blackbody. The temperature of this end is 750 K in the steady state. The temperature of the surrounding air is 300 K. Assuming radiation to be the only important mode of energy transfer between the surrounding and the open end of the rod, find the thermal conductivity of the rod. Stefan constant σ = 6.0 × 10⁻⁸ W m⁻² K⁻⁴.

A body cools down from 50°C to 45°C in 5 mintues and to 40°C in another 8 minutes. Find the temperature of the surrounding.

A metal ball of mass 1 kg is heated by means of a 20 W heater in a room at 20°C. The temperature of the ball becomes steady at 50°C. (a) Find the rate of loss of heat to the surrounding when the ball is at 50°C. (b) Assuming Newton's law of cooling, calculate the rate of loss of heat to the surrounding when the ball rises 30°C. (c) Assume that the temperature of the ball rises uniformly from 20°C to 30°C in 5 minutes. Find the total loss of heat to the surrounding during this period. (d) Calculate the specific heat capacity of the metal.

A hot body placed in a surrounding of temperature θ₀ obeys Newton's law of cooling `(d theta)/(dt) = -K(theta - theta_0)`  . Its temperature at t = 0 is θ₁. The specific heat capacity of the body is sand its mass is m. Find (a) the maximum heat that the body can lose and (b) the time starting from t = 0 in which it will lose 90% of this maximum heat.

Which of the following is not a unit of time?

1. Second
2. Parsec
3. Year
4. Light year

An astronaut inside a small spaceship orbiting around the earth cannot detect gravity. If the space station orbiting around the earth has a large size, can he hope to detect gravity?

A tall cylinder is filled with viscous oil. A round pebble is dropped from the top with zero initial velocity. From the plot shown in figure, indicate the one that represents the velocity (v) of the pebble as a function of time (t).

Consider a Carnot’s cycle operating between T₁ = 500 K and T₂ = 300 K producing 1 k J of mechanical work per cycle. Find the heat transferred to the engine by the reservoirs.

In a refrigerator one removes heat from a lower temperature and deposits to the surroundings at a higher temperature. In this process, mechanical work has to be done, which is provided by an electric motor. If the motor is of 1 kW power, and heat is transferred from – 3°C to 27°C, find the heat taken out of the refrigerator per second assuming its efficiency is 50% of a perfect engine.

A vessel of volume V contains a mixture of 1 mole of Hydrogen and 1 mole of Oxygen (both considered as ideal). Let f₁(v)dv, denote the fraction of molecules with speed between v and (v + dv) with f₂(v)dv, similarly for oxygen. Then ______.

Consider an ideal gas with following distribution of speeds.

Calculate V_rms and hence T. (m = 3.0 × 10⁻²⁶ kg)

Consider an ideal gas with following distribution of speeds.

If all the molecules with speed 1000 m/s escape from the system, calculate new V_rms and hence T.

A man stands on a rotating platform, with his arms stretched horizontally holding a 5 kg weight in each hand. The angular speed of the platform is 30 revolutions per minute. The man then brings his arms close to his body with the distance of each weight from the axis changing from 90cm to 20cm. The moment of inertia of the man together with the platform may be taken to be constant and equal to 7.6 kg m².

(a) What is his new angular speed? (Neglect friction.)

(b) Is kinetic energy conserved in the process? If not, from where does the change come about?

A ball is whirled in a circle by attaching it to a fixed point with a string. Is there an angular rotation of the ball about its centre? If yes, is this angular velocity equal to the angular velocity of the ball about the fixed point?