Current Electricity

An electrical cable having a resistance of 0.2 Ω delivers 10 kW at 200 V DC to a factory. What is the efficiency of transmission ?

• 65 %

• 75 %

• 85 %

• 95 %

A wire of resistance 5 Ω is drawn out so that its new length is 3 times its original length. What is the resistance of the new wire ?

• 45 Ω

• 15 Ω

• 5/3 Ω

• 5 Ω

Two identical cells each of emf E and internal resistance r are connected in parallel with an external resistance R. To get maximum power developed across R, the value of R is

• $\mathrm{R} = \frac{\mathrm{r}}{2}$

• R = r

• $\mathrm{R} = \frac{\mathrm{r}}{3}$

• R = 2r

The current flowing through a wire depends on time as I = 3t² + 2t + 5. The charge flowing through the cross-section of the wire in time from t = 0 to t = 2 s is 

• 22 C

• 20 C

• 18 C

• 5 C

Two cells each of same emf e but of internal resistances r₁ and r₂ are connected in series through an external resistance R. If the potential difference between the ends of the first cell is zero, what will be the value of R in terms of r₁ and r₂ ?

The reading of the ammeter in the following figure will be

• 0.8 A

• 0.6 A

• 0.4 A

• 0.2 A

A wire of resistance R is elongated n-fold to make a new uniform wire. The resistance of new wire

• nR

• n²R

• 2nR

• 2n²R

A battery of emf E and internal resistance r is connected across a pure resistive device (such as an electric heater) of resistance R. Prove that the power output of the device will be maximum if R = r.

The resistance across A and B in the figure below will be 

• 3R

• R

• $\frac{\mathrm{R}}{3}$

• None of the above

Five equal resistance, each of resistance R are connected as shown in figure below. A battery of V volt is connected between A and B. The current flowing in FC will be

• $\frac{3\mathrm{V}}{\mathrm{R}}$

• $\frac{\mathrm{V}}{\mathrm{R}}$

• $\frac{\mathrm{V}}{2\mathrm{R}}$

• $\frac{2\mathrm{V}}{\mathrm{R}}$