List of top Physics Questions asked in CBSE Class XI

A cylinder with a movable piston contains 3 moles of hydrogen at standard temperature and pressure. The walls of the cylinder are made of a heat insulator, and the piston is insulated by having a pile of sand on it. By what factor does the pressure of the gas increase if the gas is compressed to half its original volume?

Two cylinders A and B of equal capacity are connected to each other via a stopcock. A contains a gas at standard temperature and pressure. B is completely evacuated. The entire system is thermally insulated. The stopcock is suddenly opened. Answer the following : 

(a) What is the final pressure of the gas in A and B ? 

(b) What is the change in internal energy of the gas ? 

(c) What is the change in the temperature of the gas ? 

Two cylinders A and B of equal capacity are connected to each other via a stopcock. A contains a gas at standard temperature and pressure. B is completely evacuated. The entire system is thermally insulated. The stopcock is suddenly opened. Answer the following 1) What is the final pressure of the gas in A and B? 2) What is the change in internal energy of the gas? 3) What is the change in the temperature of the gas? 

Two cylinders A and B of equal capacity are connected to each other via a stopcock. A contains a gas at standard temperature and pressure. B is completely evacuated. The entire system is thermally insulated. The stopcock is suddenly opened. Answer the following : 

(a) What is the final pressure of the gas in A and B ? 

(b) What is the change in internal energy of the gas ? 

(c) What is the change in the temperature of the gas ? 

Explain clearly, with examples, the distinction between : 
  1. magnitude of displacement (sometimes called distance) over an interval of time, and the total length of path covered by a particle over the same interval; 
  2. magnitude of average velocity over an interval of time, and the average speed over the same interval. [Average speed of a particle over an interval of time is defined as the total path length divided by the time interval]. Show in both (a) and (b) that the second quantity is either greater than or equal to the first. When is the equality sign true ? [For simplicity, consider one-dimensional motion only].
An electric heater supplies heat to a system at a rate of 100W. If system performs work at a rate of 75 joules per second. At what rate is the internal energy increasing?

A thermodynamic system is taken from an original state to an intermediate state by the linear process shown in Fig. (11.13). 

A thermodynamic system

Its volume is then reduced to the original value from E to F by an isobaric process. Calculate the total work done by the gas from D to E to F

A man walks on a straight road from his home to a market 2.5 \(\text {km}\) away with a speed of 5 \(\text {km}\) \(\text h^{-1}\). Finding the market closed, he instantly turns and walks back home with a speed of 7.5 \(\text {km}\) \(\text h^{-1}\) What is the 
  1. magnitude of average velocity, and 
  2. average speed of the man over the interval of time (i) 0 to 30 min, (ii) 0 to 50 min, (iii) 0 to 40 min ? [Note: You will appreciate from this exercise why it is better to define average speed as total path length divided by time, and not as magnitude of average velocity. You would not like to tell the tired man on his return home that his average speed was zero !].
Estimate the fraction of molecular volume to the actual volume occupied by oxygen gas at STP. Take the diameter of an oxygen molecule to be 3 Å.
Molar volume is the volume occupied by 1 mol of any (ideal) gas at standard temperature and pressure (STP : 1 atmospheric pressure, 0 °C). Show that it is 22.4 litres

Figure 12.8 shows plot of \(\frac{PV}{T}\) versus P for 1.00×10–3 kg of oxygen gas at two different temperatures. 

oxygen gas at two different temperatures.

(a) What does the dotted plot signify? 

(b) Which is true: T1>T2 or T1<T2 ? 

(c) What is the value of  \(\frac{PV}{T}\) where the curves meet on the y-axis? 

(d) If we obtained similar plots for \(1.00×10^–3\) kg of hydrogen, would we get the same value of \(\frac{PV}{T}\) at the point where the curves meet on the y-axis? If not, what mass of hydrogen yields the same value of  \(\frac{PV}{T}\) (for low pressure high temperature region of the plot) ? (Molecular mass of H2 = 2.02 u, of O2 = 32.0 u, R = 8.31 J mo1–1 K–1.)

Look at the graphs (a) to (d) carefully and state, with reasons, which of these cannot possibly represent one-dimensional motion of a particle.
 
Graph a to d possibly representing one-dimensional motion of a particle
Figure 2.11shows the x-t plot of one dimensional motion of a particle. Is it correct to say from the graph that the particle moves in a straight line for t < 0 and on a parabolic path for t >0 ? If not, suggest a suitable physical context for this graph.
x-t plot of one dimensional motion of a particle
Figure 2.11
A large steel wheel is to be fitted on to a shaft of the same material. At 27 °C, the outer diameter of the shaft is 8.70 cm and the diameter of the central hole in the wheel is 8.69 cm. The shaft is cooled using ‘dry ice’. At what temperature of the shaft does the wheel slip on the shaft? Assume coefficient of linear expansion of the steel to be constant over the required temperature range : α\(_{steel}\)= 1.20 × 10\(^{-5}\) K\(^{-1}\).
Figure 2.15 gives a speed-time graph of a particle in motion along a constant direction. Three equal intervals of time are shown. In which interval is the average acceleration greatest in magnitude? In which interval is the average speed greatest ? Choosing the positive direction as the constant direction of motion, give the signs of v and a in the three intervals. What are the accelerations at the points A, B, C and D ?
speed-time graph of a particle in motion along a constant direction
Suggest a suitable physical situation for each of the following graphs : 
figure a
figure b
figure c
Three girls skating on a circular ice ground of radius 200 m start from a point P on the edge of the ground and reach a point Q diametrically opposite to P following different paths as shown in Fig. 3.19. What is the magnitude of the displacement vector for each? For which girl is this equal to the actual length of the path skated?
Figure 3.19
Figure 3.19
A cyclist starts from the centre O of a circular park of radius 1 km, reaches the edge P of the park, then cycles along the circumference, and returns to the centre along QO as shown in Fig. 3.20. If the round trip takes 10 min, what is the 
  1. net displacement, 
  2. average velocity, and  
  3. average speed of the cyclist?
    a circular park with Centre O
On an open ground, a motorist follows a track that turns to his left by an angle of 60° after every 500 m. Starting from a given turn, specify the displacement of the motorist at the third, sixth and eighth turn. Compare the magnitude of the displacement with the total path length covered by the motorist in each case.
A hole is drilled in a copper sheet. The diameter of the hole is 4.24 cm at 27.0 °C. What is the change in the diameter of the hole when the sheet is heated to 227 °C? Coefficient of linear expansion of copper = 1.70 × \(10^{-5} K^{-1}.\)
State, for each of the following physical quantities, if it is a scalar or a vector: 
Volume, mass, speed, acceleration, density, number of moles, velocity, angular frequency, displacement, angular velocity.
Pick out the two scalar quantities in the following list:
Force, angular momentum, work, current, linear momentum, electric field, average velocity, magnetic moment, relative velocity.
Pick out the only vector quantity in the following list:
Temperature, pressure, impulse, time, power, total path length, energy, gravitational potential, coefficient of friction, charge.

State with reasons, whether the following algebraic operations with scalar and vector physical quantities are meaningful:

(a) adding any two scalars, 

(b) adding a scalar to a vector of the same dimensions, 

(c) multiplying any vector by any scalar, 

(d) multiplying any two scalars, 

(e) adding any two vectors, 

(f) adding a component of a vector to the same vector

Read each statement below carefully and state with reasons, if it is true or false:
(a) The magnitude of a vector is always a scalar.
(b) each component of a vector is always a scalar
(c) the total path length is always equal to the magnitude of the displacement vector of a particle.
(d) the average speed of a particle (defined as total path length divided by the time taken to cover the path) is either greater or equal to the magnitude of average velocity of the particle over the same interval of time,
(e) Three vectors not lying in a plane can never add up to give a null vector