# Electricity | Q & A

What does an electric circuit mean?

Ans: A closed conducting path through which electric current may flow is called an electric circuit.

Define the unit of current.

Ans: The SI unit of electric current is ampere (A). When 1 coulomb of electric charge flows through any cross-section of a conductor in 1 second, the electric current flowing through it is said to be 1 ampere.

$\dpi{120} 1 ampere (A)=\frac{1 coulomb (C))}{1 second(s)}$

Calculate the number of electrons constituting one coulomb of charge.

Ans: Charge on an electron = 1.6 × 10–19 C.

∵  If the value of charge is 1.6 × 10–19 C, then number of electron = 1

∴  If the value of charge is 1 C, then number of electrons

$\dpi{120} =\frac{1}{1.6\times 10^{19}}=\frac{1}{1.6}\times 10^{19}$

= 0.625 × 1019 = 6.25 × 1018

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

Ans: A cell or a battery.

What is meant by saying that the potential difference between two points is 1 V?

Ans: The potential difference between two points in a current carrying conductor is said to be 1 V when 1 joule of work is done to move a charge of 1 coulomb from one point to the other.

Therefore, 1V = 1J / 1C = 1 JC-1

How much energy is given to each coulomb of charge passing through a 6 V battery?

Ans: Charge, Q = 1 C

Potential difference, V = 6 V

Work done, W = VQ = 6 V × 1 C = 6 J

The work done on each coulomb = 6 J.

Therefore, the energy given to each coulomb of charge is also 6 J.

On what factors does the resistance of a conductor depend?

Ans: The resistance of a conductor depends:

(i) on its length,

(ii) on its area of cross-section,

(iii) on the nature of its material and

(iv) on temperature of the conductor.

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 resistance of a conductor is inversely proportional to its area of cross-section. A thick wire has a greater area of cross-section whereas a thin wire has a smaller area of cross section. Thus, a thick wire has less resistance and a thin wire has more resistance. Therefore, current will flow more easily through a thick wire.

Let the resistance of an electrical component remain 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: Since the resistance remains constant, so the current is directly proportional to potential difference. If the potential difference is halved, the current also gets halved.

Why are coils of electric toasters and electric irons made of an alloy rather than a pure metal?

Ans: Coils of electric 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 pure metal and

(ii) an alloy does not undergo oxidation easily even at high temperature.

Which among iron and mercury is a better conductor?

Ans: Among iron and mercury, iron is better conductor of electricity because resistivity of iron (10.0 × 10–8Ω m) is less than that of mercury (94.0 × 10–8 Ω m).

Which material is the best conductor?

Ans: We know that a good conductor of electricity should have a low resistivity and a poor conductor of electricity will have a high resistivity. Silver has the lowest resistivity of 1.60 × 10–8 Ω m, which means that silver offers the least resistance to the flow of current through it. So, silver is the best conductor of electricity.

Draw a schematic diagram of a circuit consisting of a battery of three cells of 2 V each, a 5 Ω resistor, an 8 Ω resistor, and a 12 Ω resistor and a plug key, all connected in series.

Ans:

The potential difference of each cell = 2 V

The total potential difference (or voltage) of 3 cells

= 3 × 2 V = 6 V.

Redraw the circuit of previous question, putting in an ammeter to measure the current through the resistors and a voltmeter to measure the potential difference across the 12 Ω resistor. What would be the readings in the ammeter and the voltmeter?

Ans:

It is noted that the ammeter has been put in series with the circuit and the voltmeter has been put in parallel with the 12 Ω resistor.

(i) Here, R1 = 5 Ω, R2 = 8 Ω and R3 = 12 Ω

These three resistors are connected in series.

∴    Total resistance, R = R1 + R2 + R3

= 5 + 8 + 12 = 25 Ω

Potential difference, V = 6 V

Current, I = ?

Applying Ohm’s law,

V = IR

I = V/R = 6/25 = 0.24A

Therefore, ammeter will show a reading of 0.24 A.

(ii) Calculation of potential difference reading across 12 Ω resistor.

Current, I = 0⋅24 A, Resistance, R = 12 Ω, Potential difference, V = ?

Applying Ohm’s law

V = IR = 0⋅24 × 12 = 2.88 V

Therefore, the voltmeter reading is 2.88 V.

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?

Ans: Resistance of the electric iron = 31.25Ω

Current through the electric iron = 7.04 A

What are the advantages of connecting electrical devices in parallel with the battery instead of connecting them in series?

Ans: (i) In parallel circuit, if one electrical appliance stops working due to some defect, then all other appliances keep working normally. In series circuit, if one electrical appliance stops working due to some defect, then all other appliances also stop working.

(ii) In parallel circuits, each electrical appliance gets the same voltage as that of the power supply line. In series circuit, appliances do not get the same voltage as that of the power supply line.

(iii) In the parallel connection of electrical appliances, the overall resistance of the household circuit is reduced due to which the current from the power supply is high. In the series connection, the overall resistance of the circuit increases too much due to which the current from the power supply is low.

How can three resistors of resistance 2 Ω, 3 Ω and 6 Ω be connected to give a total resistance of (i) 4 Ω, (ii) 1 Ω?

Ans: (i)

(ii)

What is (i) the highest, (ii) the lowest total resistance that can be secured by combinations of four coils of resistance 4 Ω, 8 Ω, 12 Ω, 24 Ω?

Ans: (i) The highest can be secured by series combination and is equal to

R = 4 Ω + 8 Ω + 12 Ω + 24 Ω = 48 Ω

(ii) The lowest total resistance can be secured by parallel combination, which is given by

$\dpi{120} \frac{1}{R}=\frac{1}{4}+\frac{1}{8}+\frac{1}{12}+\frac{1}{24}\Rightarrow \frac{1}{R}=\frac{12}{24}=\frac{1}{2}$

$\dpi{120} \therefore R=2\Omega$

Why does the cord of an electric heater not glow while the heating element does?

Ans: The cord of the electric heater is made of copper. It does not glow because negligible heat is produced in it by the passing current due to its extremely low resistance.

The heating element of an electric heater is made of nichrome. It glows because large amount of heat is produced in it by the passing electric current due to its high resistance.

Compute the heat generated while transferring 96,000 coulombs of charge in one hour through a potential difference of 50 V.

Ans: 4800kJ

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

Ans: 15 kJ

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

Ans: Electric power.

An electric motor takes 5 A from a 220 V line. Determine the power of the motor and the energy consumed in 2 h.

Ans: 2.2 kWh

How is a voltmeter connected in the circuit to measure the potential difference between two points?

Ans: A voltmeter is always connected in parallel in the circuit to measure the potential difference between two points.

Why are copper and aluminium wires usually employed for electricity transmission?

Ans: Copper and aluminium wires are usually employed for electricity transmission because copper and aluminium have very low resistivities.

How does the resistance of a wire vary with its area of cross-section?

Ans: The resistance of a wire is inversely proportional to its cross-sectional area. Thus, a thick wire has less resistance, and a thin wire has more resistance.

Why is the series arrangement not used for domestic circuits?

Ans: The series arrangement is not used for domestic circuits because:

(a) if connected in series total resistances will increase. Therefore, current flowing through the circuit will be low.

(b) if one appliance is switched off or gets damaged than all other appliances will also stop working because their electricity supply will be cut off.

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 resistivity of an alloy is generally higher than that of its constituent metals. Alloys do not oxidise (burn) readily at higher temperatures. Therefore, conductors of electric heating devices, such as toasters and electric irons, are made of an alloy rather than pure metal.

Why is the tungsten used almost exclusively for filament of electric lamp?

Ans: Pure tungsten has a high resistivity and a high melting point (nearly 3000°C). When an electric current is passed through the filament, the electric energy is converted to heat and light energy due to the heating of the filament to a very high temperature. Due to the high melting point of tungsten, the filament does not melt.