Short Questions and Answers on DC Circuits

Question 1: Why does a cell possess internal resistance?

Answer : When current flows through a cell, it meets opposition from the electrodes and electrolyte. This opposition to current offered by the cell is called internal resistance.

Question 2 : Why are domestic appliances connected in parallel?

Answer : The domestic appliances are connected in parallel because of the following reasons:

• The domestic appliances are rated for the same voltage (i.e. 230 V) but different powers. When connected in parallel, the performance of each appliance becomes independent of the other. For Example, a 230v, 250 W TV receiver can be operated independently in parallel with a 230V, 100 W lamp.
• If a breakdown occurs in any one of the branch circuits, it will have no effect on the other branch circuits.

Question 3: Why is Wheatstone circuit called a bridge ?

Answer : The circuit is called a bridge because the galvanometer bridges the opposite junctions of the circuit.

Question 4 : Why a Wheatstone bridge method preferred to voltmeter-ammeter method for resistance?

Answer : Wheatstone bridge method is preferred to voltmeter-ammeter method for measuring resistance due to the following reasons :

• The Wheatstone bridge method is independent of the fluctuations and variations in the supply voltage.
• It removes the objectionable feature of voltmeter-ammeter method where  the accuracy of measurement is limited by the accuracy of calibration of instruments.

Question 5 : Which type of grouping of cells will be suitable to obtain higher current and voltage?

Answer : Series-parallel grouping of cells is suitable for obtaining higher current and voltage. To produce a large voltage, a number of cells are connected in series and to produce a large current, a number of sets of such series-connected cells are connected in parallel. Such a connection is called series-parallel grouping.

Question 6 : When we require a D.C. supply,  why do we look for a D.C. source other than a battery? 

Answer : Because battery are costly and requires frequent replacement.

Question 7: What is the  lamps in a house-lighting circuit should not be connected in series?

Answer : The lamps in a house-lighting circuit are not connected in series due to the following reasons :

• If break occurs in any part of the circuit, no current will flow and the entire circuit becomes useless.
• A high supply voltage is required if the lamps (or other devices) are to be connected in series. For example, if five 230V lamps are to be connected in series, then the supply voltage would have to  be 5 x 230 V = 1150 V. Therefore, series connection is not practicable for lighting circuits.
• For efficient operation, only those lamps or devices should be connected on series that have the same current rating. However, electrical devices have different current ratings. Obviously, they can not be connected in series for efficient operation.

Question 8 : Can you measure the e.m.f. of a cell with a voltmeter ?

Answer : Not accurately. Because, when voltmeter is connected across the cell, current starts flowing through it. This causes a voltage drop across the internal resistance of the cell. Consequently, the voltmeter will not indicate the e.m.f. of the cell. As we know, e.m.f. of a cell is the voltage across the open-circuit terminals of the cell i.e. voltage across the terminals of the cell when it carries no current.

Question 9: When the resistance connected in series with a cell is halved, the current is not exactly doubled but is slightly less. Why ?

Answer : This is because of the internal resistance r of the cell. Th current I delivered by the cell is given by :

I = E/(R+r)

When, external resistance R is made R/2, the current will be slightly less than 2I.

Question 10 : Why is it easier to start a car engine on a warm day than on a chilly day ?

Answer : It is because the internal resistance of a car battery on a chilly day is much more than a warm day.

Question 11 : Why should a low voltage supply should have low internal resistance?

Answer : I = E/(R+r)

The maximum current that can be drawn is Imax = E/r

We can obtain a large maximum current from a low voltage supply if its internal resistance is small.

Question 12 : How can three resistances of values 2 Ω, 3 Ω and 6Ω be connected to produce an effective resistance of 4 Ω ?

Answer :

R_AB = [(6 × 3)/ (6+3) ] + 2

= 2+2

= 4 Ω

Question 13 : Why should a  high voltage supply should have high internal resistance ?

Answer : If the internal resistance of a high voltage supply is small, then on accidental short-circuit, a damaging large current will flow through the source of e.m.f. This may damage the high voltage source. For this reason, a high voltage supply should have high internal resistance.

Question 14 : The resistance of human body is quite high. Why does one experience a strong shock from 230V supply ?

Answer : The human body is very sensitive to even minute current. A current as low as a few mA can greatly affect the nervous system.

Question 15 : You are given wires, each of resistance R. Find out the ration of maximum to minimum resistance available from there wires .

Answer : The resistance obtained will be maximum when the wires are connected in series. However, when the wires are connected in parallel, the resistance will be minimum.

R_s = n R

R_p = R/n

Thus, R_s/R_p = (nR)/(R/n)

= n²

Question 16 : When resistances are connected in parallel, the total resistance decreases. Why ?

Answer : When resistances are connected in parallel, the effective area of cross-section of the conductor increases. For this reason, the total resistance decreases.

Question 17 : It is general belief that a person touching a high power line gets stuck to the line. Is it true ?

Answer : This is not true. The fact is that a current of even a few mA is enough to make the nervous system ineffective for a moment. As a result, the affected person has difficulty to remove his hand off  the live wire. We think that the person has got stuck to the line.

Question 18 : Why does the glow of lamps become weaker when a heavy current appliance is switched on in the house ?

Answer : Domestic appliances are connected in parallel across the supply. When heavy current appliance is switched on, the total resistance of the system in the room is decreased as the appliance is connected in parallel with other loads. This increases the line current. This in turn causes greater voltage drop in the line. Therefore, the voltage available across the lamps decreases and so is their glow.

Question 19 : Is  current a scalar or vector ?

Answer : Current is scalar. It is because currents at a junction in an electric circuit do not obey the laws of vector algebra.

Question 20 : When is the efficiency of a cell high ?

Answer : Efficiency of a cell, η = R/ (R+ r)

= 1/(1+(r/R))

When the external resistance R is high compared to the internal resistance r of the cell, the efficiency of the cell is high.

Question 21 : A battery of e.m.f. 6V is connected to a resistance of 100 Ω through an ammeter of resistance 2 Ω, if the circuit current is 50 mA, what is the internal resistance of the battery ?

Answer : Here, E = 6V, R = 100 Ω, R_A = 2 Ω, I = 0.05 A, r =?

Now, I = E/(R + R_A + r )

or, r = (E/I ) – (R + R_A)

= (6/0.05) – (100+ 2)

= 18 Ω