Electric Energy, Power and Household Circuits Goyal Brothers ICSE Physics

Solutions Ch-9

Page-150

Question 1.
(a) What do you understand by the term electric work ?
Answer:
Electric work : “Electric work is said to be done when an electric charge flows
through a conductor, at some potential difference.”

(b) State and define SI unit of electric work.

Answer:
S.I. unit of electric work → Joule (J)
1J = 1 volt × 1 coulomb
1 Joule : “Is the amount of work done when a charge of one coulomb flows
through a conductor at a pot. diff. of 1 volt.”

(c) Name two bigger units of electric work. How are they related to SI unit ?
Answer:
Two bigger units of electric work :

1. Kilo – Joule (kJ) = 103 J

2. Mega-joule (Mj) = 106 J
Question 2.
Derive an expression for electric work connecting :

(a) Current, resistance and time

(b) Current, potential difference and time.
(c) Potential difference, resistance and time.

Answer:

Question 3.
State three factors which determine the quantity of heat produced in a
conductor.
Answer:
Three factors which determine heat produced in a conductor :
Heat produced = Energy Work done
W=H=I2Rt
∴ Three factors are I — current
R — Resistance of conductor
t — time for which current flows.
Question 4.

(a) What do you understand by the term electric power ?

(b) State SI unit of electric power and define it
(c) Name two bigger units of electric power and their relation with SI unit.

Answer:

Question 5.
Derive an expression for electric power connecting :

(a) Current and resistance.

(b) Current and potential difference.
(c) Potential difference and resistance.

Answer:

Question 6.

(a) What do you understand by term electric energy ?

(b) Name and define the smallest commercial unit of electric energy.
(c) Name and define the standard commercial unit of electric energy.

Answer:
(a) Electric energy : E = P × t = VIt
“Electric energy is the product of power and time.” i.e. electric energy
consumed by an electrical appliance is produced of ‘power rating’ and ‘time’
for which it is used.
(b) Unit of electric energy → Joule (J)
S.I. unit of electric work → Joule (J)
1J = 1 volt × 1 coulomb
1 Joule : “Is the amount of work done when a charge of one coulomb flows
through a conductor at a pot. diff. of 1 volt.”
(c) Standard Commercial unit of Electric Energy : Kilo-watt-Hour 1 kwH :
“Is the amount of electrical energy consumed when an
electrical appliance having a power rating of 1 kilowatt is used for 1 hour.”
Question 7.
With respect to electricity, define :

1. watt hour
2. watt
3. kilowatt
4. kilowatt hour. Amongst the above units, which are the units of : (i)
electric energy (ii) electric power.
Answer:
Question 8.
How many joules of energy is equal to one kilowatt hour?
Answer:

Question 9.
Distinguish between kilo-watt and kilowatt hour.
Answer:

Question 10.
How many kilowatt is equal to one hour power ?
Answer:

Multiple Choice Questions-1

Electric Energy, Power and Household Circuits Class-10 Goyal Brothers

ICSE Physics Solutions

1. A bulb has a resistance of 20 Ω and the p.d across its terminals is V.

If the bulb is usedfor t seconds then energy consumed by the bulb is :

Answer:

2. A current Iflows through a resistance R for the time ‘t’, the electric
energy consumed by the resistance is :

Answer:

3. The unit for electric work in SI system :

(a) Joule
(b) watt
(c) watt second
(d) watt hour
Answer:
(a) Joule

4. An electric appliance has a rating of 1000 W – 200 V. The resistance

of the element of electric appliance is :
(a) 200 Ω
(b) 400 Ω
(c) 40 Ω
(d) 4000 Ω
Answer:
(c) 40 Ω
5. Kilowatt hour is commercial unit of :
(a) power
(b) electric energy
(c) heat energy
(d) mechanical energy
Answer:
(b) electric energy

6. Kilowatt hour and kilowatt are :

(a) SI units of power and electric energy
(b) commercial units of power and electric energy
(c) SI units of-electric energy and power
(d) commercial units of electric energy and power
Answer:
(d) commercial units of electric energy and power

Numerical Problem on Electric Energy

Electric Energy, Power and Household Circuits Class-10 Goyal Brothers ICSE
Physics Solutions

Page-152

Practice Problems 1

Question 1.
Calculate the energy released by a heater, which draws a current of 5A at 220
V for 1 min.
Answer:

Question 2.
An electric device consumes 8640 J of energy in 30 min. while operating at 24
V. Calculate the current drawn by the device.
Answer:

Practice Problems 2
Question 1.
An ekctric kettle draws a current of4A for 2.5 min. if the resistance of its
element is 100 Ω , calculate the electric energy drawn by kettle in kilojoules.
Answer:

Question 2.
A soldering iron draws an energy of 43200 J in 4 min, when the current flowing
through its element is 6 A, calculate the resistance of its heating element.
Answer:

Practice Problems 3

Question 1.
Calculate the heat energy given out by the filament of an electric bulb in 20 s,
when its resistance is 4 Ω and p.d. across its ends in 12 V.
Answer:

Question 2.
An electric device gives out 5760 J of heat energy in 1 min, when current flows
through it at a p.d. of 24 V. Find the resistance of the device.
Answer:

Practice Problems 4

Question 1.
An electric heater draws a current of 3.5 A at a p.d. of 250 V. Calculate the
power of 4 such heaters.
Answer:

Question 2.
An electric bulb is rated 500 W – 200 V. Calculate the magnitude of current.
Answer:

Question 3.
An electric heater of power 1000 W, draws a current of 5.0 A. Calculate the line
voltage.
Answer:

Practice Problems 5

Question 1.
An electric heater has a resistance of 40 Ω and draws a current of 4 A.
Calculate :
 1. its power
2. p.d. at its ends
Answer:

Question 2.
An electric heater of power 900 W, has a resistance of 36 Ω Calculate the
magnitude of current and the p.d. at its ends.
Answer:

Practice Problems 6

Question 1.
An electric motor of power 1000 W, operates at 250 V. Calculate the inductive
resistance of motor and current flowing through it.
Answer:

Question 2.
An electric device operates at 24 V and has a resistance of 8 Ω Calculate the
power of the device and current flowing through it
Answer:

Practice Problems 7

Question 1.
An electric bulb is rated 200 W – 200 V. It is immersed in 200 g of oil (SHC 0.8
Jg-10 C-1) at 10°C. The bulb is switched on for 2 minutes. If all the electric energy
is absorbed in the form of heat energy by the oil, calculate :

(a) Resistance of the filament of the bulb.

(b) Current flowing through the bulb.
(c) Final temperature of the oil.

Answer:
Question 2.
An electric kettle is rated 1000 W – 250 V. It is used to bring water at 20°C to
its boiling point. If the kettle is used for 11 minutes and 12
seconds, calculate :

(a) Resistance of the element of the kettle.

(b) Current flowing through the element.
(c) Mass of water in the kettle [SHC of water = 4.2 Jg -10 C-1]

Answer:
Practice Problems 8

Question 1.
Calculate the resistance of nichrome wire, which will bring 200 g of water at
20° C to its boiling points in 7 minutes, when current flowing through wire is 4A.
Answer:

Question 2.
Calculate p.d. at the ends of a power source which, supplies current to a 4 ohm
resistance wire for 20 minutes and raises temperature of 400 g of water
through 20 .
Answer:

Question 3. Class-10 Goyal Electric Energy

Calculate the current flowing through an electric drill, connected to 200 V
supply, if it drills a hole in a metal plate of mass 500 g, such that its
temperature rises from
10 to 60 in 5 minutes, assuming all the work done is converted into heat
energy.
[S.H.C. of metal 0.6 Jg-1 -1]
Answer:

Practice Problems 9

Question 1.
Circuit diagram shows four dry cells of e.m.f. 1.5 V and internal resistance 0.25
Ω connected to an external circuit A 3 Ω wire is immersed in 20 g of water at
20°C. The current switched on for 6 minutes and 36 seconds. Calculate :

(a) Reading shown by the ammeter

(b) Current in 1.5 Ω wire
(c) Final temperature of water

Answer:

Question 2.
A battery of 12 V and negligible internal resistance is connected to an external
circuit consisting of three resistors of 6 Ω, 3 Ω and 2 Ω in parallel, which further
connected to a resistance of 3 Ω in series to the battery. The 3 Ω resistance is
immersed in 50 g oil of sp. heat capacity 0.8 Jg-10 C -1, when the temperature of
the oil rises by 54 0C.
(a) Draw the labelled circuit diagram.
(b) Calculate the value of current in the tnain circuit.
(c) Calculate the current following in 2 Ω resistance in parallel.
(d) Calculate the time for which current is switched on.

Answer:

Practice Problems 10

Question 1.
An electric kettle rated 250 V can bring a certain amount of water to its boiling
point in 8 min. If it is connected to 200 V mains, calculate the time in which
water comes to its boiling point.
Answer:

Question 2.
An immersion heating rod is rated 220 V and can bring certain amount of water
to its boiling point in 15 min. When this immersion rod is actually connected to
an electric circuit, it brings the water to boil in 18.15 min. Calculate the line
voltage.
Answer:

Practice Problems 11

Question 1.
An electric oven is marked 1000 W – 200 V. Calculate :

(a) Resistance of its element

(b) Energy consumed by the oven in 1/2 hour in joules.
(c) Time, in which ii will consume 15 kWh of energy.

Answer:

Question 2.
An electric motor is rated 2 HP – 250 V. Calculate :

(a) Current flowing through it

(b) Energy consumed by it in one second
(c) Time in which it will consume 90 kWh of energy. [1 HP = 750 W]
Answer:

Practice Problems 12

Question 1.
A geyser is rated 2000 W and operates 2 hours a day on 200 V mains. Calculate
the monthly bill for running the geyser when energy costs ₹ 1.90 per kWh.
Answer:

Question 2.
An electric oven of resistance 20 Ω draws a current of 10 A. It works 3 hours
daily. Calculate the weekly bill when enregy costs ₹ 1.50 per kWh.
Answer:

Question 3. Class-10 Goyal Electric Energy

An electric bulb draws a current of 0.8 A and works on 250 V on an average 8
hrs a day. If energy costs ₹ 1.50 per board of trade unit, calculate the monthly
bill
Answer:

Practice Problems 13

Question 1.
4 tube lights of 40 W each and 2 fans of 100 W each are connected to 200 V
mains and operate on an average 8 hours a day. If energy costs ₹ 1.50 kWh,
calculate

(a) monthly bill

(b) minimum fuse rating.

Answer:

Question 2.
An electric motor of 2 H.P. and two coolers of 500 W each operate on 250 V
mains for 4 hours a day. If the energy costs ₹ 1.80 per kWh, calculate

(a) weekly bill

(b) minimum fuse rating. [Take 1 HP = 750 W]
Answer:

Practice Problems 14

Question 1.
A boys hostel has following appliances when energy is supplied at 200 V and
costs ₹ 5.25 per kWh.
(a) 40 bulbs of 100 W each, working 8 hours a day.
(b) 20 fans each drawing a current 9.8 A and working 15 hours a day.
(c) Two T.V. sets, each offering a resistance of 200 Ω and working 4 hours a
day.
(d) Two electric motors of 1.5 H.P. each and working 4 hours a day
1. Calculate the monthly bill
2. Amongst the fuse of 48 A and 50 A which one you will use and why ?
Answer:

Question 2. Class-10 Goyal Electric Energy

An establishment receives electric energy at a rate of ₹ 4.50 per kWh at a p.d.
of 240 V. It uses following appliances.
(a) 20 tube lights of 40 W each working 10 h a day.
(b) Two stero systems, each drawing a current of 2A and working 4h a day.
(c) Four ovens, each of resistance 24 Q working 6 h a day.
(d) Two cooling machines of 4 H.P. each working 15 hours a day.
1. calculate monthly bill
2. minimum fuse rating of circuit
Answer:

Exe-2 Class-10

Electric Energy, Power and Household Circuits Goyal Brothers ICSE

Physics Solutions Ch-9

Page-165,166

Question 1.
(a) What do you understand by the term electric fuse ?
Answer:
Electric fuse : “Is a safety device which is used to limit the current in an
electric circuit. The use of a fuse safe guards the circuit and
the appliances connected in that circuit from being damaged.”
(b) Name a material from which an electric fuse is made.
Answer:
Material used for fuse is ‘Alloy of lead and Tin’ having low melting point and
high resistance.”

(c) State two properties of a material which makes it suitable for an electric
fuse.
Answer:
Two properties of material are :

1. Low melting point

2. High resistance.
(d) Draw a diagram of a fuse wire, connected in a fuse socket.
Answer:

Question 2.
(a) Why is a fuse wire always placed in a live wire ?
Answer:
So that ‘The fuse may melt first, before current reaches the appliance”.

(b) How does fuse wire protect an electric circuit ?

Answer:
When current in the circuit exceeds the specified value (due to any reason such
as high voltage, short circuiting etc.) the fuse wire gets heated up to the extent
that it melts. As a result a gap is produced and the circuit beaks. The current
does not flow through the live wire and appliance is saved.
(c) Two fuse wires of the same length are rated 15A and 5A. Which of the two
is thicker and why ?
Answer:
5A wire is thicker as resistance is α 1/a Thicker the wire less is resistance and
electrons can run freely with less collisions.

(d) Why is it dangerous to replace a fuse wire with a copper wire?

Answer:
“Copper has low resistivity and high melting point and does not serve the
purpose of fuse.”

Question 3.
(a) What do you understand by the term earthing ?
Answer:
Earthing : or Grounding : “Bringing the appliance to zero potential”.
“Earthing means to connect the metal case of electrical appliance to the earth
(at zero potential) by means of a metal wire is called “Earth wire”.
(b) How does earthing protect a user from receiving an electric shock?
Answer:
When we connect the earth wire to the metal case of the electrical appliance by
using three pinplug.
The metal casing of the appliance will now always remain at zero potential of
the earth, this will avoid the risk of shock if we touch the metal body as the
current passes directly to earth through the earth wire.

(c) How is a household circuit earthed ?

Answer:
The neutral and earth wires on supply end (power station) are connected
potential together, so that both of them are at zone potential and inside the
house live and neutral wires are connected to the input terminals of kWh
meter, where as earth wire is connected to the body of kWh meter. The live
wire coming out from the out put terminals of the kWh meter has another fuse
in it, which is commonly called main fuse. The neutral wire and earthwire is
common to all circuits.

(d) Explain how the fuse melts when a short circuit appliance gets earthed.
Answer:
When an earthed appliance gets short circuited, then current from its metal
body flows into the earth. Since earth does not offer any resistance, therefore
magnitude of the current in the circuit of a short-circuited appliance suddenly
rises to a very high value.
This rise in magnitude of current in turn overloads the circuit, and hence. The
fuse in that circuit melts.
Question 4. Class-10 Goyal Electric Energy
(a) What is the function of a switch in an electric circuit?
Answer:
Function of a switch :
Function of a switch is to connect or disconnect an electric appliance in the
electric circuit.
(b) Why is switched placed in a live wire ?
Answer:
Let the appliance (heater) when connected in a neutral wire, when the switch
is in the off position and even if no current is flowing through the heater and it
is not operating, how ever the heater is in contact with live wire and heating
element is at the same potential as the live wire. Thus if we touch the heating
coil of the heater, current will flow through our body giving us a severe shock
and may prove fatal, this proves that switch should always be placed in the live
wire.

(c) what consequences will follow, if a switch is placed in the neutral wire ?
Answer:
Even if switch is in off position the appliance will be at the potential of live wire
and current will flow through the appliance.
Question 5.
(a) Why is household wiring done in parallel ? Give at least two reasons ?
Answer:
Two advantages of parallel wiring :

1. Electrical appliances appliances get same voltage (220 V) as that of

the power supply line.
2. If one electrical appliance stops working due to some defect, then all
other appliances keep working normally.
(b) What are the disadvantages of wiring in series in a house?
Answer:
Disadvantages in series circuit :

1. Over all resistance increases and current from the power supply is
low.
2. If one electrical appliance stops working, due to some defect, then
all other appliances also stop working.
Question 6.
Draw a circuit diagram for distribution of power from pole to the main switch
and label it
Answer:
Circuit Diagram For Distribution Of Power From Pole To Main Switch :

Question 7.
Name two systems of distribution of power in a house.Give the advantages and
disadvantages of each system.
Answer:
Two systems of power distribution.
(i) Tree system Advantages

1. All appliances are connected parallel and can be independently

switched on and switched off.
2. The neutral wire and earth wire is common to all circuits.
3. From the main distribution board, circuits are taken out for different
rooms.
Disadvantages :

1. It is expensive and takes a long time to install.

2. It requires plugs and sockets of different current value for
different appliances depending upon their power rating.
3. When a fuse in one particular line melts. It disconnects all
appliances in that circuit.
(ii) Ring system :
Advantages :

1. Every appliance has its own fuse and can be handled without
disturbing other appliances.
2. Length of wire used is very small.
3. It is easier to install and maintain.
Question 8. Class-10 Goyal Electric Energy
(a) State the colour of (i) live wire, (ii) neutral wire, (iii) earth wire according to
international convention
(b) State the position of (i) earth pin, (ii) live pin and (iii) neutral pin in an
electric plug.
(c) Why is the earth terminal of a plug made (i) thicker, (ii) longer
Answer:
(a) The colour of
 1. Live wire —– Brown
2. Neutral wire —– Light Blue .
3. Earth wire —– geen or Yellow
(b) Position of

1. Earth pin —– is Long and Thicker so that it should not enter live or
neutral cylinder of socket.
2. Live pin —– L is on the right when earth pin goes into E, live pingoes
into L.
3. Neutral-pin —– N is on the left.When earth pin goes into E, live wire
in L automatically neutral goes into N.
(c) Earth-pin is made thicker, so that it should not enter other cylinder of
socket and helps in putting right pin in right place.
Longer —– So that earth connection is done first. This ensure the user will not
get a shock.

Multiple Choice Questions-2

Electric Energy, Power and Household Circuits Class-10 Goyal Brothers ICSE
Physics Solutions

1. A fuse wire is connected in before the switch.

(a) neutral wire
(b) earth wire
(c) live wire
(d) either (a) or (c)
Answer:
(c) live wire

2. A switch in a circuit is always connected in the :

(a) live wire
(b) earth wire
(c) neutral wire
(d) either (a) or (b)
Answer:
(a) live wire

3. According to old convention, the colour of neutral wire is :

(a) red
(b) green
(c) black
(d) none of these
Answer:
(c) black
4. According to new convention, the colour of live wire is :
(a) light blue
(b) yellow
(c) green
(d) brown
Answer:
(d) brown

5. Which is not the characteristic of a fuse wire ?

(a) It has high resistance
(b) It has low melting point
(c) It has low resistance
(d) It is an alloy of lead and tin
Answer:
(c) It has low resistance

6. In a three pin plug the live pin in :

(a) thinner and is toward left
(b) thicker and is towards left
(c) thinner and is towards right
(d) thicker and is towards right
Answer:
(c) thinner and is towards right

7. In a household electric circuit all appliances are connected in :

(a) parallel circuit
(b) series circuit
(c) mixed circuit
(d) any of these
Answer:
(a) parallel circuit
8. An average lighting circuit of a cpoor family has a fuse rating of
(a) 10 A
(b) 15 A
(c) 5 A
(d) 2 A
Answer:
(c) 5 A