ADITYA VIDYASHRAM RESIDENTIAL SCHOOL PUDUCHERRY.

CURRENT ELECTRICITY (Board)

PHYSICS

1. What does an electric circuit mean?

Ans. A continuous conducting path consisting of wires and other resistances (like electric bulb, etc.)
and a switch, between the two terminals of a cell or battery, along which an electric current flows, is
called a circuit.
2. Define the unit of current
Ans. The SI unit of electric current is ampere (which is denoted by the letter A). When 1 coulomb of
charge flows through any cross-section of a conductor in 1 second, the current flowing through it is
said to be 1 ampere
3. Calculate the number of electrons constituting one coulomb of charge

4. Name a device that helps to maintain a potential difference across a conductor.

Ans: An electric cell (or a battery).
5. What is meant by saying that the potential difference between two points is 1 V?
Ans. The potential difference between two points is said to be 1 volt (1 V) if 1 joule of work is done in
moving 1 coulomb of electric charge from one point to the other.
6. How much energy is given to each coulomb of charge passing through a 6 V battery?

7. On what factors does the resistance of a conductor depend?

Ans: The electrical resistance of a conductor (or a wire) depends on the following factors :
(i) length of the conductor,
(ii) area of cross-section of the conductor (or thickness of the conductor),
(iii) nature of the material of the conductor, and
(iv) temperature of the conductor.
8. Will current flow more easily through a thick wire or a thin wire of the same material, when
connected to the same source? Why?
Ans. The current will flow more easily through a thick wire than through a thin wire of the same
material when connected to the same source (like a battery). This is due to the fact that the resistance
of a wire is inversely proportional to the square of its diameter. A thick wire has greater diameter and
hence lesser resistance making the current to flow through it more easily. On the other hand, a thin
wire has smaller diameter and hence greater resistance to the flow of current through it
9. Let the resistance of an electrical component remains constant while the potential difference
across the two ends of the component decreases to half of its former value. What change will
occur in the current through it?
Ans. The current through the component will also decrease to half of its former value
10. Judge the equivalent resistance when the following are connected in parallel – (a) 1 Ω and 106 Ω,
(b) 1 Ω and 103 Ω, and 106 Ω.
Ans: (a) The equivalent resistance of two resistances 1 Ω and 10 6 Ω connected in parallel will be less
than 1 Ω. This is because when a number of resistances are connected in parallel, then their
equivalent resistance is less than the smallest individual resistance (which is 1 Ω in this case).
(b) The equivalent resistance of three resistances 1Ω , 103 Ω and 106 Ω connected in parallel will be
less than 1 Ω.
11. An electric lamp of 100 Ω, a toaster of resistance 50 Ω, and a water filter of resistance 500 Ω
are connected in parallel to a 220 V source. What is the resistance of an electric iron connected
to the same source that takes as much current as all three appliances, and what is the current
through it?
12. What are the advantages of connecting electrical devices in parallel with the battery instead of
connecting them in series?
Ans: The advantages of connecting electrical devices in parallel are the following :
(i) In parallel circuits, if one electrical appliance stops working due to some defect, then all other
appliances keep working normally. On the other hand, in series circuit, if one electrical appliance
stops working due to some defect, then all other appliances also stop working (because the whole
circuit is broken).
(ii) In parallel circuits, each electrical appliance has its own switch due to which it can be turned on or
turned off independently, without affecting other appliances. In series circuit, all the electrical
appliances have only one switch due to which they cannot be turned on or turned off independently.
(iii) In parallel circuits, each electrical appliance gets the same voltage as that of the battery due to
which all the appliances work properly. In series circuit, the appliances do not get the same voltage as
that of the battery because the voltage is shared by all the appliances.
(iv) In the parallel connection of electrical appliances, the overall resistance of the circuit is reduced
due to which the current from the battery is high and hence each electrical appliance can draw the
required amount of current. In the series connection of electrical appliances, the overall resistance of
the circuit increases too much due to which the current from the battery is low and hence all the
electrical appliances cannot draw sufficient current for their proper working.

13. How can three resistors of resistances 2Ω, 3Ω, and 6Ω be connected to give a total resistance of
(a) 4 Ω, (b) 1 Ω?
14. What is (a) the highest, (b) the lowest total resistance that can be secured by combinations of
four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?

15. Why does the cord of an electric heater not glow while the heating element does?
 The heating element of an electric heater is made of an alloy (such as nichrome) which has
high resistance whereas the cord is made of copper metal which has very, very low resistance.
 Now, the heating element of an electric heater made of nichrome glows because it becomes red-
hot due to the large amount of heat produced on passing current (because of its high
resistance).
 On the other hand, the connecting cord of the electric heater made of copper does not glow
because negligible heat is produced in it by passing the same current (due to its extremely low
resistance).
16. Compute the heat generated while transferring 96000 coulomb of charge in one hour through a
potential difference of 50 V.

17. An electric iron of resistance 20 Ω takes a current of 5 A. Calculate the heat developed in 30 s.

18. What determines the rate at which energy is delivered by a current?

Ans: Electric power of the appliance
19. An electric motor takes 5A from a 220V line. Determine the power of the motor and the energy
consumed in 2h.
 Exercise
1. A piece of wire of resistance R is cut into five equal parts. These parts are then connected in
parallel. If the equivalent resistance of this combination is R′, then the ratio R/R′ is –
(a) 1/25 (b) 1/5 (c) 5 (d) 25
Soln: The resistance of wire is R. This wire is cut into five equal pieces, so the resistance of each piece
of the wire will be R/5 .
Now, five pieces of wire, each piece of wire having resistance of R/5 are connected in parallel to give
the equivalent resistance R′
𝑅
𝑅’ = 5
5
𝑅’ 1 1
= =
𝑅 5 × 5 25
𝑅
= 25𝛺
𝑅′
2. Which of the following terms does not represent electrical power in a circuit?
(a) I2R (b) IR2 (c) VI (d) V 2/R
3. An electric bulb is rated 220 V and 100 W. When it is operated on 110 V, the power consumed
will be –
(a) 100 W (b) 75 W (c) 50 W (d) 25 W
Soln:
𝑉2 220×220
The resistance of the electric bulb = 𝑃 = = 484𝛺
100
Now the 484Ω bulb connected to 110 V
𝑉2 110×110
Power P= 𝑅 = = 25𝑊
484
4. Two conducting wires of the same material and of equal lengths and equal diameters are first
connected in series and then parallel in a circuit across the same potential difference. The ratio
of heat produced in series and parallel combinations would be –
(a) 1:2 (b) 2:1 (c) 1:4 (d) 4:1
Soln:
Since both wires has same resistivity, length and diameter(area) then it means both have same
resistance
Let the resistance of each be x
(i) When connected in series R1=2x
𝑉
Current passing through the circuit is 𝐼1 = 2𝑥
Heat Produced by the series combination is

(ii) When 2 resistors are connected in parallel

𝑥
𝑅2 =
2
2𝑉
Current passing through the circuit is 𝐼2 = 𝑥
Heat Produced by the parallel combination is
5. How is a voltmeter connected in the circuit to measure the potential difference between two
points?
Ans. The voltmeter is always connected in parallel across the two points (in the circuit) where the
potential difference is to be measured
6. A copper wire has diameter 0.5 mm and resistivity of 1.6 × 10–8 Ω m. What will be the length of
this wire to make its resistance 10 Ω? How much does the resistance change if the diameter is
doubled?

7. When a 12 V battery is connected across an unknown resistor, there is a current of 2.5 mA in

the circuit. Find the value of the resistance of the resistor.

8. A battery of 9 V is connected in series with resistors of 0.2 Ω, 0.3 Ω, 0.4 Ω , 0.5 Ω and 12 Ω,
respectively. How much current would flow through the 12 Ω resistor?
9. How many 176 Ω resistors (in parallel) are required to carry 5 A on a 220 V line?

10. Show how you would connect three resistors, each of resistance 6 Ω, so that the combination
has a resistance of (i) 9 Ω, (ii) 4 Ω.

11. Several electric bulbs designed to be used on a 220 V electric supply line, are rated 10 W. How
many lamps can be connected in parallel with each other across the two wires of 220 V line if
the maximum allowable current is 5 A?
12. A hot plate of an electric oven connected to a 220 V line has two resistance coils A and B, each
of 24 Ω resistance, which may be used separately, in series, or in parallel. What are the currents
in the three cases?

13. Compare the power used in the 2 Ω resistor in each of the following circuits: (i) a 6 V battery in series
with 1 Ω and 2 Ω resistors, and (ii) a 4 V battery in parallel with 12 Ω and 2 Ω resistors.
14. Two lamps, one rated 100 W at 220 V, and the other 60 W at 220 V, are connected in parallel to
electric mains supply. What current is drawn from the line if the supply voltage is 220 V?
15. Which uses more energy, a 250 W TV set in 1 hr, or a 1200 W toaster in 10 minutes? An electric
heater of resistance 8 Ω draws 15 A from the service mains 2 hours. Calculate the rate at which
heat is developed in the heater.
16. Explain (a) Why is the tungsten used almost exclusively for filament of electric lamps?
Ans: Tungsten is used almost exclusively for making the filaments of electric lamps (or electric bulbs)
because it has very high melting point (of 3380°C) due to which the tungsten filament can be kept
white-hot without melting away. Moreover, tungsten has high flexibility and low rate of evaporation at
high temperature
(b) Why are the conductors of electric heating devices, such as bread-toasters and electric irons,
made of an alloy rather than a pure metal?
Ans: The coils (or heating elements) of toasters and electric irons are made of an alloy rather than a
pure metal because : (i) the resistivity of an alloy is much higher than that of a pure metal, and (ii) an
alloy does not undergo oxidation (or burn) easily even at high temperature, when it is red hot.
(c) Why is the series arrangement not used for domestic circuits?
Ans: The series arrangement is not used for domestic circuits because of the following disadvantages :
(i) In series arrangement, if one electrical appliance stops working due to some defect, then all other
appliances also stop working (because the whole circuit is broken).
(ii) In series arrangement, all the electrical appliances can have only one switch due to which they
cannot be turned ‘on’ or ‘off’ independently.
(iii) In series arrangement, all the appliances do not get the same voltage (220 V) as that of the power
supply line (because the line voltage is shared by all the appliances). Due to this, the appliances do not
work properly.
(iv) In series arrangement of electrical appliances, the overall resistance of the circuit increases too
much due to which the current from power supply is low. Because of this, all the appliances of
different power ratings cannot draw sufficient current for their proper working
(d) How does the resistance of a wire vary with its area of cross-section?
1
The resistance of a wire is inversely proportional to its area of cross-section. That is : R ∝ 𝐴
. So, when
the area of cross-section of wire increases (or thickness of wire increases), then its resistance
decreases. And when the area of cross-section of wire decreases (or thickness of wire decreases), then
its resistance increases
(e) Why are copper and aluminium wires usually employed for electricity transmission?
Copper and aluminium wires are usually employed for transmission of electricity because copper and
aluminium have low electrical resistivity (due to which they are very good conductors of electricity).