Science (English Medium) Class 12 - CBSE Question Bank Solutions for Physics

A light bulb is rated at 125 W for a 250 V a.c. supply. Calculate the resistance of the bulb.

A cell of emf E and internal resistance r is connected to two external resistance R₁ and R₂ and a perfect ammeter. The current in the circuit is measured in four different situations:

(i) without any external resistance in the circuit

(ii) with resistance R₁ only

(iii) with R₁ and R₂ in series combination

(iv) with R₁ and R₂ in parallel combination

The currents measured in the four cases are 0.42 A, 1.05 A, 1.4 A and 4.2 A, but not necessarily in the order. Identify the currents corresponding to the four cases mentioned above.

Describe briefly with the help of a circuit diagram, how the flow of current carriers in a p-n-p transistor is regulated with emitter-base junction forward biased and base-collector junction reverse biased.

A light bulb is rated 100 W for 220 V ac supply of 50 Hz. Calculate

(i) The resistance of the bulb;

(ii) The rms current through the bulb.

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.

Write any two characteristic properties of nuclear force.

A compound microscope uses an objective lens of focal length 4 cm and eyepiece lens of focal length 10 cm. An object is placed at 6 cm from the objective lens. Calculate the magnifying power of the compound microscope. Also calculate the length of the microscope.

With the help of circuit diagram explain the working principle of a transistor amplifier as an oscillator.

Draw a ray diagram to show the working of a compound microscope. Deduce an expression for the total magnification when the final image is formed at the near point.

In a compound microscope, an object is placed at a distance of 1.5 cm from the objective of focal length 1.25 cm. If the eye piece has a focal length of 5 cm and the final image is formed at the near point, estimate the magnifying power of the microscope.

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’.

By what percentage will the transmission ranges of TV tower be affected when the height of the tower is increased by 21%?

Why are high frequency carrier waves used for transmission?

Derive the expression for force per unit length between two long straight parallel current carrying conductors. Hence define one ampere.

Draw the labelled ray diagram for the formation of image by a compound microscope.

Derive the expression for the total magnification of a compound microscope. Explain why both the objective and the eyepiece of a compound microscope must have short focal lengths.

An object is placed at a distance u from a simple microscope of focal length f. The angular magnification obtained depends

A man is looking at a small object placed at his near point. Without altering the position of his eye or the object, he puts a simple microscope of magnifying power 5 X before his eyes. The angular magnification achieved is

In which of the following the final image is erect?
(a) Simple microscope
(b) Compound microscope
(c) Astronomical telescope
(d) Galilean telescope

The focal length of the objective of a compound microscope if f_o and its distance from the eyepiece is L. The object is placed at a distance u from the objective. For proper working of the instrument,
(a) L < u
(b) L > u
(c) f_o < L < 2f_o
(d) L > 2f_o

A compound microscope forms an inverted image of an object. In which of the following cases it it likely to create difficulties? 

An object is to be seen through a simple microscope of focal length 12 cm. Where should the object be placed so as to produce maximum angular magnification? The least distance for clear vision is 25 cm.