Science (English Medium) Class 12 - CBSE Important Questions for Physics

What is its relation with relaxation time?

When 5 V potential difference is applied across a wire of length 0.1 m, the drift speed of electrons is 2.5 x 10^-4 m/s. If the electron density in the wire is 8 x 10²⁸ m^-3, calculate the resistivity of the material of the wire.

Two identical cells of emf 1.5 V each joined in parallel, supply energy to an external circuit consisting of two resistances of 7 Ω each joined in parallel. A very high resistance voltmeter reads the terminal voltage of cells to be 1.4 V. Calculate the internal resistance of each cell.

Estimate the average drift speed of conduction electrons in a copper wire of cross-sectional area 2.5 × 10⁻⁷ m² carrying a current of 1.8 A. Assume the density of conduction electrons to be 9 × 10²⁸ m⁻³.

Estimate the average drift speed of conduction electrons in a copper wire of cross-sectional area 1.0 × 10⁻⁷ m² carrying a current of 1.5 A. Assume the density of conduction electrons to be 9 × 10²⁸ m⁻³

Estimate the average drift speed of conduction electrons in a copper wire of cross-sectional area 2·5 × 10⁻⁷ m² carrying a current of 2·7 A. Assume the density of conduction electrons to be 9 × 10²⁸ m⁻³

A heating element using nichrome connected to a 230 V supply draws an initial current of 3.2 A which settles after a few seconds to a steady value of 2.8 A. What is the steady temperature of the heating element if the room temperature is 27.0°C? The temperature coefficient of resistance of nichrome averaged over the temperature range involved is 1.70 × 10⁻⁴ °C⁻¹.

In a potentiometer arrangement, a cell of emf 1.25 V gives a balance point at 35.0 cm length of the wire. If the cell is replaced by another cell and the balance point shifts to 63.0 cm, what is the emf of the second cell?

How does drift velocity of electrons in a metallic conductor vary with increase in temperature? Explain.

In the given circuit, with steady current, calculate the potential drop across the capacitor and the charge stored in it.

A charge Q is distributed uniformly over a metallic sphere of radius R. Obtain the expressions for the electric field (E) and electric potential (V) at a point 0 < x < R.
Show on a plot the variation of E and V with x for 0 < x < 2R.

(i) State the principle on which a potentiometer works. How can a given potentiometer be made more sensitive?

Two metallic wires, P₁ and P₂ of the same material and same length but different cross-sectional areas, A₁ and A₂ are joined together and connected to a source of emf. Find the ratio of the drift velocities of free electrons in the two wires when they are connected (i) in series, and (ii) in parallel.

Describe the working principle of a solar cell. Mention three basic processes involved in the generation of emf.

Why are Si and GaAs preferred materials for solar cells?

Why is potentiometer preferred over a voltmeter for comparison of emf. of cells?

 Draw a circuit diagram to determine internal resistance of a cell in the laboratory?

Plot a graph showing variation of voltage vs the current drawn from the cell. How can one get information from this plot about the emf of the cell and its internal resistance?

State Kirchhoff's rules and explain on what basis they are justified.

Given the resistances of 1 Ω, 2 Ω, 3 Ω, how will be combine them to get an equivalent resistance of (11/3) Ω?