Science (English Medium) कक्षा १२ - CBSE Important Questions

The emf of a cell is always greater than its terminal voltage. Why? Give reason.

A heating element is marked 210 V, 630 W. Find the resistance of the element when connected to a 210 V dc source.

Calculate the value of the resistance R in the circuit shown in the figure so that the current in the circuit is 0.2 A. What would b the potential difference between points B and E?

Calculate the value of the resistance R in the circuit shown in the figure so that the current in the circuit is 0.2 A. What would b the potential difference between points A and B?

In the given circuit, assuming point A to be at zero potential, use Kirchhoff’s rules to determine the potential at point B.

Plot a graph showing the variation of stopping potential with the frequency of incident radiation for two different photosensitive materials having work functions W₁ and W₂ (W₁ > W₂). On what factors does the (i) slope and (ii) intercept of the lines depend?

A cell of emf ‘E’ and internal resistance ‘r’ is connected across a variable resistor ‘R’. Plot a graph showing the variation of terminal potential ‘V’ with resistance R. Predict from the graph the condition under which ‘V’ becomes equal to ‘E’.

Derive an expression for drift velocity of free electrons in a conductor in terms of relaxation time.

Obtain the expression for the current flowing through a conductor having number density of the electron n, area of cross-section A in terms of the drift velocity v_d . 

How does the resistivity of a semiconductor change with rise of temperature ? Explain.

Answer the following question.
How is the drift velocity in a conductor affected with the rise in temperature?

A carbon resistor is shown in the figure. Using color code, write the value of the resistance.

Twelve wires each having a resistance of 3 Ω are connected to form a cubical network. A battery of 10 V and negligible internal resistance is connected across the diagonally opposite corners of this network. Determine its equivalent resistance and the current along each edge of the cube.

V-I graph for a metallic wire at two different temperatures T1 and T2 is as shown in the figure. Which of the two temperatures is higher and why?

A variable resistor R is connected across a cell of emf ε and internal resistance r as shown in the figure. Draw a plot showing the variation of
(i) Terminal voltage V and
(ii) the current I, as a function of R.

Sunita and her friends visited the exhibition. The policeman asked them to pass through a metal detector. Sunita's friends were initially scared of it. Sunita, however, explained to them the purpose and working of the metal detector.

Answer the following questions :
(a) On what principle does a metal detector work?
(b) Why does the detector emit a sound when a person carrying any metallic object walks through it?
(c) State any two qualities which Sunita displayed while explaining the purpose of walking through the detector.

Define internal resistance of a cell.

The temperature (T) dependence of resistivity of materials A and material B is represented by fig (i) and fig (ii) respectively. Identify material A and material B.

fig. (i)

fig. (ii)

Two cells of emfs E₁ and E₂ and internal resistances r₁ and r₂ respectively are connected in parallel as shown in the figure. Deduce the expression for the

1. equivalent emf of the combination
2. equivalent internal resistance of the combination
3. potential difference between the points A and B.

State the two Kirchhoff’s rules used in the analysis of electric circuits and explain them.