Textbook: Effects of Electric Current | Science and Technology Class 10 (Maharashtra SSC Board) PDF Download

 Page 1


47
  We have learnt in earlier standards about static electricity. We performed various 
experiments regarding negatively and positively charged objects. The reason behind the 
object becoming positively and negatively charged is the transfer of negatively charged 
particles from one object to another object. In previous standard, we also studied about 
electric current. 
In this chapter, we will study about an electric current flowing through a conducting 
wire, an electric current flowing through a resistor, electromagnetic induction, electric 
motor and generator.
4. Effects of electric current
Ø 	Energy transfer in electric circuit.
Ø 	Heating effects of electric current.
Ø 	Magnetic effects of electric current.
Observe and Discuss
What do you observe in the following pictures? 
 Which effects of electric current do you find?  
1. How do we decide that a given material is a good conductor 
    of electricity  or is an insulator?
2. Iron is a conductor of electricity, but when we pick up a piece 
of iron resting on the ground, why don’t we  get electric shock?
a b
Energy transfer in an electric circuit 
Procedure: Connect the circuit as shown in the accompanying figure 4.2 after taking 
the components with proper values. Measure the current (I). Also measure the potential 
difference (V
AB
) between the two ends (A and B) of the resistance. 
The potential at A is higher than the potential at B as the point A is connected to the 
positive electrode of the cell and the point B to the negative electrode of the cell. 
4.1 Effects of electric current
c
Materials: connecting wires, electric cells, electrical resistance, 
voltmeter, ammeter, plug key.
Can you recall?
Try this
Page 2


47
  We have learnt in earlier standards about static electricity. We performed various 
experiments regarding negatively and positively charged objects. The reason behind the 
object becoming positively and negatively charged is the transfer of negatively charged 
particles from one object to another object. In previous standard, we also studied about 
electric current. 
In this chapter, we will study about an electric current flowing through a conducting 
wire, an electric current flowing through a resistor, electromagnetic induction, electric 
motor and generator.
4. Effects of electric current
Ø 	Energy transfer in electric circuit.
Ø 	Heating effects of electric current.
Ø 	Magnetic effects of electric current.
Observe and Discuss
What do you observe in the following pictures? 
 Which effects of electric current do you find?  
1. How do we decide that a given material is a good conductor 
    of electricity  or is an insulator?
2. Iron is a conductor of electricity, but when we pick up a piece 
of iron resting on the ground, why don’t we  get electric shock?
a b
Energy transfer in an electric circuit 
Procedure: Connect the circuit as shown in the accompanying figure 4.2 after taking 
the components with proper values. Measure the current (I). Also measure the potential 
difference (V
AB
) between the two ends (A and B) of the resistance. 
The potential at A is higher than the potential at B as the point A is connected to the 
positive electrode of the cell and the point B to the negative electrode of the cell. 
4.1 Effects of electric current
c
Materials: connecting wires, electric cells, electrical resistance, 
voltmeter, ammeter, plug key.
Can you recall?
Try this
48
 If a charge Q flows from A to B,  work V
AB
 Q, 
has been done on Q while going from A to B (Refer 
to chapter 3 of std 9). From where does the energy 
come to do this work? The source of energy is the 
cell. The cell gives this energy through the charge 
Q to the resistance where work V
AB
 Q is performed. 
If the charge Q flows from A to B in time t, i.e. the 
work is performed in time t, then during that time 
the energy  V
AB
 Q is given to the resistor.  
 What happens to this energy? This energy is 
received by the resistor and is converted into heat 
energy, the temperature of the resistor is increased.
 If in the circuit, the resistor is replaced by a motor, 
in which form will the energy given by the cell get 
transformed into?  
The source of energy, the cell, gives in time t, the energy P x t to the resistor. If I is the 
current flowing continuously through the circuit, the heat produced in the resistor in time t 
will be
H = P x t = V
AB
 x I x t  ................................ (2)                 
 
 According to Ohm’s law, 
 V
AB
 = I x R  ................................................ (3)
H = V
2
AB
 ×  
t
R
.......................................... (4)
Similarly,  H = I x I x R x t = I
2
 x Rt  ......... (5)
H = I
2
 x R x t is called Joules law of heating
Unit of electrical power
P = V
AB
 x I = Volt x Amp ........................... (6)
4.2 Electric circuit
1 Volt x 1 Amp =        x
............... (7)
1J       1C 
1C      1s
   1J
    1s 
= W (watt) ...................................... (8)
  How can we write mechan-
ical power in a manner similar to 
the electrical power?
Heating effect of electric current
When a resistor is connected in an electrical circuit, heat is produced in it due to the 
current. This is known as the heating effect of current.
V
AB
V
A
A
B
+
-
R
+
-
I
(  )
.
+
-
Use your brain power !
Thus the unit of electrical power is 1W (watt).
Think about it
 P = Electrical power = 
Energy 
Time required
  =  
V
AB 
Q
t
= V 
AB
 I...................(1)  
\
Q
 t
= I ,                      
Page 3


47
  We have learnt in earlier standards about static electricity. We performed various 
experiments regarding negatively and positively charged objects. The reason behind the 
object becoming positively and negatively charged is the transfer of negatively charged 
particles from one object to another object. In previous standard, we also studied about 
electric current. 
In this chapter, we will study about an electric current flowing through a conducting 
wire, an electric current flowing through a resistor, electromagnetic induction, electric 
motor and generator.
4. Effects of electric current
Ø 	Energy transfer in electric circuit.
Ø 	Heating effects of electric current.
Ø 	Magnetic effects of electric current.
Observe and Discuss
What do you observe in the following pictures? 
 Which effects of electric current do you find?  
1. How do we decide that a given material is a good conductor 
    of electricity  or is an insulator?
2. Iron is a conductor of electricity, but when we pick up a piece 
of iron resting on the ground, why don’t we  get electric shock?
a b
Energy transfer in an electric circuit 
Procedure: Connect the circuit as shown in the accompanying figure 4.2 after taking 
the components with proper values. Measure the current (I). Also measure the potential 
difference (V
AB
) between the two ends (A and B) of the resistance. 
The potential at A is higher than the potential at B as the point A is connected to the 
positive electrode of the cell and the point B to the negative electrode of the cell. 
4.1 Effects of electric current
c
Materials: connecting wires, electric cells, electrical resistance, 
voltmeter, ammeter, plug key.
Can you recall?
Try this
48
 If a charge Q flows from A to B,  work V
AB
 Q, 
has been done on Q while going from A to B (Refer 
to chapter 3 of std 9). From where does the energy 
come to do this work? The source of energy is the 
cell. The cell gives this energy through the charge 
Q to the resistance where work V
AB
 Q is performed. 
If the charge Q flows from A to B in time t, i.e. the 
work is performed in time t, then during that time 
the energy  V
AB
 Q is given to the resistor.  
 What happens to this energy? This energy is 
received by the resistor and is converted into heat 
energy, the temperature of the resistor is increased.
 If in the circuit, the resistor is replaced by a motor, 
in which form will the energy given by the cell get 
transformed into?  
The source of energy, the cell, gives in time t, the energy P x t to the resistor. If I is the 
current flowing continuously through the circuit, the heat produced in the resistor in time t 
will be
H = P x t = V
AB
 x I x t  ................................ (2)                 
 
 According to Ohm’s law, 
 V
AB
 = I x R  ................................................ (3)
H = V
2
AB
 ×  
t
R
.......................................... (4)
Similarly,  H = I x I x R x t = I
2
 x Rt  ......... (5)
H = I
2
 x R x t is called Joules law of heating
Unit of electrical power
P = V
AB
 x I = Volt x Amp ........................... (6)
4.2 Electric circuit
1 Volt x 1 Amp =        x
............... (7)
1J       1C 
1C      1s
   1J
    1s 
= W (watt) ...................................... (8)
  How can we write mechan-
ical power in a manner similar to 
the electrical power?
Heating effect of electric current
When a resistor is connected in an electrical circuit, heat is produced in it due to the 
current. This is known as the heating effect of current.
V
AB
V
A
A
B
+
-
R
+
-
I
(  )
.
+
-
Use your brain power !
Thus the unit of electrical power is 1W (watt).
Think about it
 P = Electrical power = 
Energy 
Time required
  =  
V
AB 
Q
t
= V 
AB
 I...................(1)  
\
Q
 t
= I ,                      49
 Equipment such as water boiler, electric cooker, 
electric bulb make use of the heating effect of 
electric current. Electrical conductors having higher 
resistivity  are used here. For example, a coil made 
up of an alloy Nichrome is used in the electric 
heater-cooker as a resistor, while a tungsten wire is 
used in an electric bulb. Because of the current, this 
wire gets heated (to nearly 3400 
0
C) and emits light. 
The hot wire also radiates heat to a certain extent.
Coil of cooker
Coil of Heater
 The unit of electric power 1W is a very small 
unit, hence 1000 W or 1 kW is used as a unit to 
measure electric power, in practice. If 1 kW 
power is used for 1 hour, it will mean 1kW ´ 1 hr 
of electrical energy is used (see equation 1) 
1kWh  =1 kilowatt hour = 1000 W × 3600 s  
= 3.6 × 10
6
 Ws  = 3.6 x 10
6
 J
 Several times we hear or read about a 
building catching fire due to short circuit. 
Sometimes, if we switch on an equipment in our 
house, the electrical fuse wire melts and the 
electric supply shuts down. Let us discuss about 
the cause briefly. The home electrical connection 
consists of ‘live’, ‘neutral’ and ‘earth’ wires. 
The ‘live’ and the ‘neutral’ wires have potential 
difference of 220V. The ‘earth’ is connected to 
ground. Due to a fault in the equipment or if the 
plastic coating on the ‘live’ and the ‘neutral’ 
wires gives way, the two wires come in contact 
with each other and a large current flows through 
it producing heat. If any inflammable material 
(such as wood, cloth, plastic etc.) exists around 
that place it can catch fire. Therefore, a fuse wire 
is used as  a precautionary measure. We have 
learnt about fuse wire in the previous standard. 
As soon as high current flows in a circuit, the 
fuse wire melts and breaks the circuit and any 
mishap is avoided. 
 Check monthly electricity 
bill received from the electricity 
distribution Co. Ltd. Observe 
various details and get 
information about them. The 
electricity bill specifies the 
usage in ‘Units’. What is this 
unit? When 1 kWh electrical 
energy is used, it is termed as 1 
unit of energy.
4.3  Uses of coil
Solenoid Type
coil
Glass Bulb  
Glass Support
Screw cap
Vacuum/ 
nitrogen gas
Coiled coil Coil (Solenoid type)
Find out
Always remember
Page 4


47
  We have learnt in earlier standards about static electricity. We performed various 
experiments regarding negatively and positively charged objects. The reason behind the 
object becoming positively and negatively charged is the transfer of negatively charged 
particles from one object to another object. In previous standard, we also studied about 
electric current. 
In this chapter, we will study about an electric current flowing through a conducting 
wire, an electric current flowing through a resistor, electromagnetic induction, electric 
motor and generator.
4. Effects of electric current
Ø 	Energy transfer in electric circuit.
Ø 	Heating effects of electric current.
Ø 	Magnetic effects of electric current.
Observe and Discuss
What do you observe in the following pictures? 
 Which effects of electric current do you find?  
1. How do we decide that a given material is a good conductor 
    of electricity  or is an insulator?
2. Iron is a conductor of electricity, but when we pick up a piece 
of iron resting on the ground, why don’t we  get electric shock?
a b
Energy transfer in an electric circuit 
Procedure: Connect the circuit as shown in the accompanying figure 4.2 after taking 
the components with proper values. Measure the current (I). Also measure the potential 
difference (V
AB
) between the two ends (A and B) of the resistance. 
The potential at A is higher than the potential at B as the point A is connected to the 
positive electrode of the cell and the point B to the negative electrode of the cell. 
4.1 Effects of electric current
c
Materials: connecting wires, electric cells, electrical resistance, 
voltmeter, ammeter, plug key.
Can you recall?
Try this
48
 If a charge Q flows from A to B,  work V
AB
 Q, 
has been done on Q while going from A to B (Refer 
to chapter 3 of std 9). From where does the energy 
come to do this work? The source of energy is the 
cell. The cell gives this energy through the charge 
Q to the resistance where work V
AB
 Q is performed. 
If the charge Q flows from A to B in time t, i.e. the 
work is performed in time t, then during that time 
the energy  V
AB
 Q is given to the resistor.  
 What happens to this energy? This energy is 
received by the resistor and is converted into heat 
energy, the temperature of the resistor is increased.
 If in the circuit, the resistor is replaced by a motor, 
in which form will the energy given by the cell get 
transformed into?  
The source of energy, the cell, gives in time t, the energy P x t to the resistor. If I is the 
current flowing continuously through the circuit, the heat produced in the resistor in time t 
will be
H = P x t = V
AB
 x I x t  ................................ (2)                 
 
 According to Ohm’s law, 
 V
AB
 = I x R  ................................................ (3)
H = V
2
AB
 ×  
t
R
.......................................... (4)
Similarly,  H = I x I x R x t = I
2
 x Rt  ......... (5)
H = I
2
 x R x t is called Joules law of heating
Unit of electrical power
P = V
AB
 x I = Volt x Amp ........................... (6)
4.2 Electric circuit
1 Volt x 1 Amp =        x
............... (7)
1J       1C 
1C      1s
   1J
    1s 
= W (watt) ...................................... (8)
  How can we write mechan-
ical power in a manner similar to 
the electrical power?
Heating effect of electric current
When a resistor is connected in an electrical circuit, heat is produced in it due to the 
current. This is known as the heating effect of current.
V
AB
V
A
A
B
+
-
R
+
-
I
(  )
.
+
-
Use your brain power !
Thus the unit of electrical power is 1W (watt).
Think about it
 P = Electrical power = 
Energy 
Time required
  =  
V
AB 
Q
t
= V 
AB
 I...................(1)  
\
Q
 t
= I ,                      49
 Equipment such as water boiler, electric cooker, 
electric bulb make use of the heating effect of 
electric current. Electrical conductors having higher 
resistivity  are used here. For example, a coil made 
up of an alloy Nichrome is used in the electric 
heater-cooker as a resistor, while a tungsten wire is 
used in an electric bulb. Because of the current, this 
wire gets heated (to nearly 3400 
0
C) and emits light. 
The hot wire also radiates heat to a certain extent.
Coil of cooker
Coil of Heater
 The unit of electric power 1W is a very small 
unit, hence 1000 W or 1 kW is used as a unit to 
measure electric power, in practice. If 1 kW 
power is used for 1 hour, it will mean 1kW ´ 1 hr 
of electrical energy is used (see equation 1) 
1kWh  =1 kilowatt hour = 1000 W × 3600 s  
= 3.6 × 10
6
 Ws  = 3.6 x 10
6
 J
 Several times we hear or read about a 
building catching fire due to short circuit. 
Sometimes, if we switch on an equipment in our 
house, the electrical fuse wire melts and the 
electric supply shuts down. Let us discuss about 
the cause briefly. The home electrical connection 
consists of ‘live’, ‘neutral’ and ‘earth’ wires. 
The ‘live’ and the ‘neutral’ wires have potential 
difference of 220V. The ‘earth’ is connected to 
ground. Due to a fault in the equipment or if the 
plastic coating on the ‘live’ and the ‘neutral’ 
wires gives way, the two wires come in contact 
with each other and a large current flows through 
it producing heat. If any inflammable material 
(such as wood, cloth, plastic etc.) exists around 
that place it can catch fire. Therefore, a fuse wire 
is used as  a precautionary measure. We have 
learnt about fuse wire in the previous standard. 
As soon as high current flows in a circuit, the 
fuse wire melts and breaks the circuit and any 
mishap is avoided. 
 Check monthly electricity 
bill received from the electricity 
distribution Co. Ltd. Observe 
various details and get 
information about them. The 
electricity bill specifies the 
usage in ‘Units’. What is this 
unit? When 1 kWh electrical 
energy is used, it is termed as 1 
unit of energy.
4.3  Uses of coil
Solenoid Type
coil
Glass Bulb  
Glass Support
Screw cap
Vacuum/ 
nitrogen gas
Coiled coil Coil (Solenoid type)
Find out
Always remember
50
 These days miniature circuit 
breakers (MCB) switches are used in 
homes. When the current in the circuit 
suddenly increases this switch opens 
and current stops. Different types of 
MCBs are in use. For the entire house, 
however the usual fuse wire is used.
4.4  Different types of fuses in use
 Many times particularly in the summer season, huge electrical power is used in the 
evenings due to home lighting, fans, air conditioners, use of electricity in shops etc.  As a 
result, excessive current is drawn from the transformer supplying the electricity, and if the 
capacity of the transformer is insufficient, its fuse wire melts and the supply gets shut 
down. Such events occur due to overloading.  
Example 1. A 6 m long wire made from an 
alloy, nichrome, is shaped into a coil and 
given for producing heat. It has a resistance 
of 22 ohms. Can we get more heat if the 
wire is cut into half of its original length 
and shaped into a coil? For getting energy, 
the two ends of the wire are connected to a 
source with a potential
difference of 220V.
Given : Resistance 22 ohm, potential 
difference = 220 V
A. Coil of whole wire.    
B. Coil of half-length wire
  V
2
R
 P =             =             = 2200 watts
(220)
2
 22
  V
2
R
 P =              =              = 4400 watts
(220)
2
 11
This means that more heat will be obtained 
after cutting the wire into half.
Example 2.  A cell is connected to a 9 ohm 
resistance, because of which heat of 400 J 
is produced per second due to current 
flowing through it. Obtain the potential 
difference applied across the resistance.
Given:
Heat at 400 J per second means 
P = 
400 J
  1 s
        V
2
  R               
P =
400 x 9  = V
2
         V
2
  9               
400 =
\ V =      (400 x 9)  = 20 x 3 = 60 V
 Solved examples
Do you know ?
Page 5


47
  We have learnt in earlier standards about static electricity. We performed various 
experiments regarding negatively and positively charged objects. The reason behind the 
object becoming positively and negatively charged is the transfer of negatively charged 
particles from one object to another object. In previous standard, we also studied about 
electric current. 
In this chapter, we will study about an electric current flowing through a conducting 
wire, an electric current flowing through a resistor, electromagnetic induction, electric 
motor and generator.
4. Effects of electric current
Ø 	Energy transfer in electric circuit.
Ø 	Heating effects of electric current.
Ø 	Magnetic effects of electric current.
Observe and Discuss
What do you observe in the following pictures? 
 Which effects of electric current do you find?  
1. How do we decide that a given material is a good conductor 
    of electricity  or is an insulator?
2. Iron is a conductor of electricity, but when we pick up a piece 
of iron resting on the ground, why don’t we  get electric shock?
a b
Energy transfer in an electric circuit 
Procedure: Connect the circuit as shown in the accompanying figure 4.2 after taking 
the components with proper values. Measure the current (I). Also measure the potential 
difference (V
AB
) between the two ends (A and B) of the resistance. 
The potential at A is higher than the potential at B as the point A is connected to the 
positive electrode of the cell and the point B to the negative electrode of the cell. 
4.1 Effects of electric current
c
Materials: connecting wires, electric cells, electrical resistance, 
voltmeter, ammeter, plug key.
Can you recall?
Try this
48
 If a charge Q flows from A to B,  work V
AB
 Q, 
has been done on Q while going from A to B (Refer 
to chapter 3 of std 9). From where does the energy 
come to do this work? The source of energy is the 
cell. The cell gives this energy through the charge 
Q to the resistance where work V
AB
 Q is performed. 
If the charge Q flows from A to B in time t, i.e. the 
work is performed in time t, then during that time 
the energy  V
AB
 Q is given to the resistor.  
 What happens to this energy? This energy is 
received by the resistor and is converted into heat 
energy, the temperature of the resistor is increased.
 If in the circuit, the resistor is replaced by a motor, 
in which form will the energy given by the cell get 
transformed into?  
The source of energy, the cell, gives in time t, the energy P x t to the resistor. If I is the 
current flowing continuously through the circuit, the heat produced in the resistor in time t 
will be
H = P x t = V
AB
 x I x t  ................................ (2)                 
 
 According to Ohm’s law, 
 V
AB
 = I x R  ................................................ (3)
H = V
2
AB
 ×  
t
R
.......................................... (4)
Similarly,  H = I x I x R x t = I
2
 x Rt  ......... (5)
H = I
2
 x R x t is called Joules law of heating
Unit of electrical power
P = V
AB
 x I = Volt x Amp ........................... (6)
4.2 Electric circuit
1 Volt x 1 Amp =        x
............... (7)
1J       1C 
1C      1s
   1J
    1s 
= W (watt) ...................................... (8)
  How can we write mechan-
ical power in a manner similar to 
the electrical power?
Heating effect of electric current
When a resistor is connected in an electrical circuit, heat is produced in it due to the 
current. This is known as the heating effect of current.
V
AB
V
A
A
B
+
-
R
+
-
I
(  )
.
+
-
Use your brain power !
Thus the unit of electrical power is 1W (watt).
Think about it
 P = Electrical power = 
Energy 
Time required
  =  
V
AB 
Q
t
= V 
AB
 I...................(1)  
\
Q
 t
= I ,                      49
 Equipment such as water boiler, electric cooker, 
electric bulb make use of the heating effect of 
electric current. Electrical conductors having higher 
resistivity  are used here. For example, a coil made 
up of an alloy Nichrome is used in the electric 
heater-cooker as a resistor, while a tungsten wire is 
used in an electric bulb. Because of the current, this 
wire gets heated (to nearly 3400 
0
C) and emits light. 
The hot wire also radiates heat to a certain extent.
Coil of cooker
Coil of Heater
 The unit of electric power 1W is a very small 
unit, hence 1000 W or 1 kW is used as a unit to 
measure electric power, in practice. If 1 kW 
power is used for 1 hour, it will mean 1kW ´ 1 hr 
of electrical energy is used (see equation 1) 
1kWh  =1 kilowatt hour = 1000 W × 3600 s  
= 3.6 × 10
6
 Ws  = 3.6 x 10
6
 J
 Several times we hear or read about a 
building catching fire due to short circuit. 
Sometimes, if we switch on an equipment in our 
house, the electrical fuse wire melts and the 
electric supply shuts down. Let us discuss about 
the cause briefly. The home electrical connection 
consists of ‘live’, ‘neutral’ and ‘earth’ wires. 
The ‘live’ and the ‘neutral’ wires have potential 
difference of 220V. The ‘earth’ is connected to 
ground. Due to a fault in the equipment or if the 
plastic coating on the ‘live’ and the ‘neutral’ 
wires gives way, the two wires come in contact 
with each other and a large current flows through 
it producing heat. If any inflammable material 
(such as wood, cloth, plastic etc.) exists around 
that place it can catch fire. Therefore, a fuse wire 
is used as  a precautionary measure. We have 
learnt about fuse wire in the previous standard. 
As soon as high current flows in a circuit, the 
fuse wire melts and breaks the circuit and any 
mishap is avoided. 
 Check monthly electricity 
bill received from the electricity 
distribution Co. Ltd. Observe 
various details and get 
information about them. The 
electricity bill specifies the 
usage in ‘Units’. What is this 
unit? When 1 kWh electrical 
energy is used, it is termed as 1 
unit of energy.
4.3  Uses of coil
Solenoid Type
coil
Glass Bulb  
Glass Support
Screw cap
Vacuum/ 
nitrogen gas
Coiled coil Coil (Solenoid type)
Find out
Always remember
50
 These days miniature circuit 
breakers (MCB) switches are used in 
homes. When the current in the circuit 
suddenly increases this switch opens 
and current stops. Different types of 
MCBs are in use. For the entire house, 
however the usual fuse wire is used.
4.4  Different types of fuses in use
 Many times particularly in the summer season, huge electrical power is used in the 
evenings due to home lighting, fans, air conditioners, use of electricity in shops etc.  As a 
result, excessive current is drawn from the transformer supplying the electricity, and if the 
capacity of the transformer is insufficient, its fuse wire melts and the supply gets shut 
down. Such events occur due to overloading.  
Example 1. A 6 m long wire made from an 
alloy, nichrome, is shaped into a coil and 
given for producing heat. It has a resistance 
of 22 ohms. Can we get more heat if the 
wire is cut into half of its original length 
and shaped into a coil? For getting energy, 
the two ends of the wire are connected to a 
source with a potential
difference of 220V.
Given : Resistance 22 ohm, potential 
difference = 220 V
A. Coil of whole wire.    
B. Coil of half-length wire
  V
2
R
 P =             =             = 2200 watts
(220)
2
 22
  V
2
R
 P =              =              = 4400 watts
(220)
2
 11
This means that more heat will be obtained 
after cutting the wire into half.
Example 2.  A cell is connected to a 9 ohm 
resistance, because of which heat of 400 J 
is produced per second due to current 
flowing through it. Obtain the potential 
difference applied across the resistance.
Given:
Heat at 400 J per second means 
P = 
400 J
  1 s
        V
2
  R               
P =
400 x 9  = V
2
         V
2
  9               
400 =
\ V =      (400 x 9)  = 20 x 3 = 60 V
 Solved examples
Do you know ?
51
Example 3. An electrical iron uses a power 
of 1100 W when set to higher temperature. 
If set to lower temperature, it uses 330 W 
power. Find out the electric current and the 
respective resistances for the two settings. 
The iron is connected to a potential 
difference of 220 V.
Given: potential difference = 220 V.
 Power P = (A) 1100 W, (B) = 330 W.
A. Power = 1100 W.
P
V
 I
1  
= 
1100
 220
  =            =  5 A             
P
V
 I
2
 = 
330
220
  =              = 1.5  A       
Example 4. An electric tungsten bulb is 
connected into a home circuit. The home 
electric supply runs at 220 V potential dif-
ference. When switched on, a current of 
0.45 A flows through the bulb. What must 
be power (wattage) of the bulb? If it is kept 
on for 10 hours, how many units of elec-
tricity will be consumed?
Given : Potential difference = 220 V.
Current = 0.45 A.
Power (W) = Potential difference(V) 
            x Current (A)
                    = 220 x 0.45 W 
                    = 99 W.
The bulb must be of power 99 W.
In 10 hrs, 
              99 W x 10 h =990 Wh.
            = 0.99 kWh.
Resistance R
1
 = 
V
I
1 
 
= 44 W = 
220
 5
Resistance R
2
 = 
V
I
2
 
= 146 W = 
220
1.5
  Connect the circuit as 
shown in figure 4.5. Connect 
a copper wire, thicker and 
straight as compared to the 
connecting wires, between A 
and B. Keep a magnetic 
needle adjacent to the wire. 
Keep the plug key open in 
the circuit and observe the 
direction of the needle. Close 
the plug key and observe the 
direction of the needle. What 
do you notice? Now 
interchange the connecting 
wires connected to the cell 
and observe the direction of 
the magnetic needle. Do you 
notice any relation between 
the direction of current and 
position of the needle?
4.5  Magnetic effects of a current
Plug 
key
Plug 
key
Magnetic needle
B
A
B
Magnetic needle
  
A
Try this
B. Power  = 330 W
Magnetic effect of electric current
We have learnt about heating effect of electric current. In previous standards, we have 
studied about magnets and magnetic lines of force. However, it will be interesting to see 
if an electric current and magnetic field are related to each other.