An electric heater boils 1 kg of water in a time t₁. Another heater boils the same amount of water in a time t₂. When the two heaters are connected in parallel, the time required by them together to boil the same amount of water is

• t₁ + t₂

• t₁t₂

• $\frac{{\mathrm{t}}_1{\mathrm{t}}_2}{{\mathrm{t}}_{1 }+ {\mathrm{t}}_2}$

• $\frac{{\mathrm{t}}_1 - {\mathrm{t}}_2}{2}$

122.

If 96500 C of electricity liberates 1 g equivalent of any substance, the time taken for a current of 0.15A to deposit 20 mg of copper from a solution of copper sulphate is (chemical equivalent of copper = 32)

• 5 min 20 s

• 6 min 42 s

• 4 min 40 s

• 5 min 50 s

An electric motor operates on a 50 V supply and a current of 12 A. If the efficiency of the motor, is 30%, what is the resistance of the winding of the motor

• 6 Ω

• 4 Ω

• 2.9 Ω

• 3.1 Ω

There are three voltmeters of the same range but of resistance 10000 Ω, 8000 Ω and 4000 Ω respectively. The best voltmeter among these is the one whose resistance is

• 10000 Ω

• 8000 Ω

• 4000 Ω

• all are equally good

A wire of length 100 cm is connected to a cell of emf 2V and negligible internal resistance. The resistance of the wire is 3 Ω, the additional resistance required to produce a PD of 1 mV/cm is

• 60 Ω

• 47 Ω

• 57 Ω

• 35 Ω

In the figure a carbon resistor has bands of different colours on its body as mentioned in the figure. The value of the resistance is

• 2.2 KΩ

• 3.3 KΩ

• 5.6 KΩ

• 9.1 KΩ

Two wires of the same dimensions but resistivities ρ₁ and ρ₂ are connected in series. The equivalent resistivity of the combination is

• $\frac{{\mathrm{\rho }}_1 + {\mathrm{\rho }}_2}{2}$

• ρ₁ + ρ₂

• 2 (ρ₁ + ρ₂)

• $\sqrt{{\mathrm{\rho }}_1 {\mathrm{\rho }}_2}$

I£ a 30V, 90W bulb is to be worked in 120V line, the resistance to be connected in series with the bulb is

• 20 Ω

• 10 Ω

• 40 Ω

• 30 Ω

The potential difference between the terminals of a cell in open circuit is 2.2 volt with resistance of 5 ohm across the terminals of a cell, the terminal potential difference is 1.8 volt. The internal resistance of the cell is

• $\frac{9}{10} \mathrm{ohm}$

• $\frac{10}{9} \mathrm{ohm}$

• $\frac{7}{12} \mathrm{ohm}$

• $\frac{12}{7} \mathrm{ohm}$

Thirteen resistances each of resistance R ohm are connected in the circuit as shown in the figure. The effective resistance between A and B is

• $\frac{4\mathrm{R}}{3} \mathrm{\Omega }$

• 2R Ω

• R Ω

• $\frac{2\mathrm{R}}{3} \mathrm{\Omega }$