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

Why alloys like constantan and manganin are used for making standard resistors?

State the underlying principle of a potentiometer ?

State the principle of a potentiometer. Define potential gradient. Obtain an expression for potential gradient in terms of resistivity of the potentiometer wire.

Figure shows a long potentiometer wire AB having a constant potential gradient. The null points for the two primary cells of emfs ε₁ and ε₂ connected in the manner shown are obtained at a distance of l₁ = 120 cm and l₂ = 300 cm from the end A. Determine (i) ε₁/ε₂ and (ii) position of null point for the cell ε₁ only.

A wheel with 8 metallic spokes each 50 cm long is rotated with a speed of 120 rev/min in a plane normal to the horizontal component of the Earth’s magnetic field. The Earth’s magnetic field at the place is 0.4 G and the angle of dip is 60°. Calculate the emf induced between the axle and the rim of the wheel. How will the value of emf be affected if the number of spokes were increased?

Two wires of equal length, one of copper and the other of manganin have the same resistance. Which wire is thicker?

Define relaxation time of the free electrons drifting in a conductor. How is it related to the drift velocity of free electrons? Use this relation to deduce the expression for the electrical resistivity of the material.

A conductor of length ‘l’ is connected to a dc source of potential ‘V’. If the length of the conductor is tripled by gradually stretching it, keeping ‘V’ constant, how will (i) drift speed of electrons and (ii) resistance of the conductor be affected? Justify your answer.

Two students ‘X’ and ‘Y’ perform an experiment on potentiometer separately using the circuit given below:

Keeping other parameters unchanged, how will the position of the null point be affected if

(i) ‘X’ increases the value of resistance R in the set-up by keeping the key K₁ closed and the Key K₂ opens?

(ii) ‘Y’ decreases the value of resistance S in the set-up, while the key K₂ remains open and they K₁ closed?

Justify.

Two cells of emf E₁, E₂ and internal resistance r₁ and r₂ respectively are connected in parallel as shown in the figure.

Deduce the expressions for

(1) the equivalent e.m.f of the combination

(2) the equivalent resistance of the combination, and

(3) the potential difference between the point A and B.

Two metallic spheres A and B kept on insulating stands are in contact with each other. A positively charged  rod P is brought near the sphere A as shown in the figure. The two spheres are separated from each other,  and the rod P is removed. What will be the nature of charges on spheres A and B?

A metal sphere is kept on an insulting stands. A negatively charged rod is brought near it, then the sphere is  earthed as shown. On removing the earthing, and taking the negatively charged rod away, what will be the  nature of charge on the sphere? Give reason for your answer. 

Answer the following question.
Two bulbs are rated (P₁, V) and (P₂, V). If they are connected (i) in series and (ii) in parallel across a supply V, find the power dissipated in the two combinations in terms of P₁ and P₂.

Solve the following question.
Using Kirchhoff’s rules, calculate the current through the 40 Ω and 20 Ω  resistors in the following circuit. 

Answer the following question.
What is the end error in a meter bridge? How is it overcome? The resistances in the two arms of the metre bridge are R = Ω and S respectively.  When the resistance S is shunted with equal resistance, the new balance length found to be 1.5 l₁, where l₂ is the initial balancing length. calculate the value of s.

The ______, a property of materials C, Si, and Ge depends upon the energy gap between their conduction and valence bands.

A thin circular ring of radius r is charged uniformly so that its linear charge density becomes λ. Derive an expression for the electric field at a point P at a distance x from it along the axis of the ring. Hence, prove that at large distances (x >> r), the ring behaves as a point charge.

A conductor of length ‘l’ is rotated about one of its ends at a constant angular speed ‘ω’ in a plane perpendicular to a uniform magnetic field B. Plot graphs to show variations of the emf induced across the ends of the conductor with

1. angular speed ω and
2. length of the conductor l.

Define Current density.

A current of 0.8 A flows in a conductor of 40 Ω for 1 minute. The heat produced in the conductor will be ______.