Five rods of same dimensions are arranged as shown in the figure. They have thermal conductivities k₁, k₂, k₃, k₄ and k₅ when points A and B are maintained at different temperatures. No heat flows through the central rod if

• k₁k₄ = k₂k₃

• k₁ = k₄ and k₂ = k₃

• $\frac{{\mathrm{k}}_1}{{\mathrm{k}}_4} = \frac{{\mathrm{k}}_2}{{\mathrm{k}}_3}$

• k₁k₂ = k₃k₄

Out of the following statements which is not true ?

• Infrared radiations arise due to minor electron transitions in atoms

• Infrared radiations are used for long distance photography

• Sun is the natural source of infrared radiation

• Infrared radiations are detected by using a bolometer

23.

64 small drops of mercury, each of radius r and charge q coalesce to form a big drop. The ratio of the surface density of charge of each small drop with of the big drop is

• 64 : 1

• 1 : 64

• 1 : 4

• 4 : 1

Two capacitors of capacitances 3 µF and 6 µF are charged to a potential of 12 V each. They are now connected to each other, with the positive plate to each joined to the negative plate to the other. The potential difference across each will be

• 4V

• 6V

• zero

• 3V

The resistance of an incandescent lamp is

• smaller when switched on

• greater when switched off

• the same whether it is switch off or switch on

• greater when switched on

A superconductor exhibits perfect

• ferromagnetism

• ferrimagnetism

• diamagnetism

• paramagnetism

A magnet is dropped down an infinitely long vertical copper tube. Then

• the magnet moves with continuously decreasing velocity and ultimately comes to rest.

• the magnet moves with continuosly increasing velocity and ultimately acquires a constant terminal velocity.

• the magnet moves with continuously increasing velocity and acceleration.

• the magnet moves with continuosly increasing velocity but constant acceleration.

An ammeter and a voltmeter are joined in series to a cell. Their readings are A and V respectively. If a resistance is now joined in parallel with the voltmeter, then

• A will decrease, V will increase

• A will increase, V will decrease

• both A and V will increase

• both A and V will decrease

If each of the resistance of the network shown in figure is R, the equivalent resistance between A and B is

• 3R

• 5R

• $\frac{\mathrm{R}}{2}$

• R

A cell supplies a current of 0.9 A through a 2 Ω resistor and a current of 0.3 A through a 7 Ω resistor. The internal resistance of the cell is

• 1.2 Ω

• 2.0 Ω

• 0.5 Ω

• 1.0 Ω