Download Electric

CURRENT ELECTRICITY
Q.1
A.
Define Electric current and its unit.
Electric Current
It can be defined as “the time rate of flow of charge in a conductor is called
Electric Current.“
The amount of flow of charge “Q“ per unit time is called Electric Current. It
is a scalar quantity. It is denoted by “ I “. Mathematically
Ch arg e
time
Q
I 
t
Electric Current 
The SI unit of Electric Current is Ampere (A).
Ampere
It can be defined as “if one coulomb of charge flows in a conductor in one
second time, then the current is said to be one Ampere.“
Q.2
A.
Write a note on conventional current?
Conventional Current
It can be defined as “ the current due to the flow of positive charge is
called conventional current.“
Before the discovery of electron the scientists thought that current is
causes due to the flow of positive charge proton. But later on the scientists
stated that current is causes due to the flow of positive as well as negative
charge.
Q.3
A.
State and explain Ohm's Law?
Statement
A German scientist George Simon Ohm in 1826 state that “the current
flows in a conductor is directly proportional to the voltage supplied across
the ends of the conductor provided the temperature and Resistance of the
conductor is kept constant. “ Mathematically
I V
V I
V  Cons tan t ( I )
V  IR
Where R is constant of
proportionality and are known as
resistance of the wire.
Experimental verification
Take a thin wire of about one
meter length. Connect both ends of
the
wire to the positive and negative plate of the battery. The voltage across
the wire is measured by voltmeter and the current in the wire is measured
by ammeter. By drawing a graph between current and voltage it will give a
straight line.
V
I
Validity of Ohm's Law
Ohm's Law is not valid for those conductors whose temperature does not
remain constant. For examples
Electric tubes, Transistors and Ionic plasma.
Q.4
A.
Define Electrical Resistance and its unit?
Electrical resistance
It is the property of a conductor by which it opposes the flow of current in
the conductor. It is denoted by "R". Mathematically
V
R 
I
The SI unit of electrical resistance is ohm. It is denoted by omega "  ".
Definition of ohm
The electrical Resistance of a conductor is said to be one Ohm, if one
ampere of current passes through it by having a potential difference of one
volt across its ends. Mathematically
1V
1 
1A
Q.5
A.
What is meant by specific resistance?
Specific Resistance
The Resistance of a conductor is said to be Specific Resistance, if the
conductor have a unit cross section area and unit length.
The Resistance of a conducting wire is directly proportional to the length
and inversely proportional to the cross section area. Mathematically
R L
----------------- (1)
1
R
----------------- (2)
A
Combining equation (1) and (2), we get
L
R 
A
L
R  Constant
A
L
A
A
  R
L
Where  is the constant of proportionality and it is called specific
Resistance. The SI unit of specific Resistance is Ohm-meter (   m )
R 
Q.6
A.

What is the effect of temperature on electrical resistance?
Effect of temperature on Resistance
The Resistance of a conductor directly depends on the temperature and
initial Resistance of the conductor.
Explaination
Consider a conductor has an initial Resistance " R " and temperature" T ".
After a temperature change "  T "the Resistance of conductor
becomes" R ". Mathematically
R  R
R  T
Combining both equations, we get
R  R T
R  Constant R T
R  R T
R  R
R


R T
RT
Rt  R 1  T 
Q.7
A.
Define Electric circuit and its types?
Electric circuit
The path of the flow of Electric current is known as Electric circuit.
There are two types of an Electric circuit. i-e
Closed circuit
A complete circuit for the flow of Electric current is called closed circuit.
current can easily flow in this circuit.
Open circuit
An incomplete circuit for the flow of Electric current is called open circuit.
current can not flow in this circuit due to a break in the circuit.
Q.8
A.
Write a note on series combination of Resistors?
Series combination of Resistors
The end to end connection of two or more than two Resistors with each
other is called Series combination of the Resistors. As shown in Fig
(i)
(ii)
Series combination has the following characteristics.
There is a single path for the flow of current it means that current is
constant in the circuit. Mathematically
I  I1  I 2  I 3
Voltage across each Resistor is drop. It means that the sum of the
individual voltage is equal to the total voltage. Mathematically
----------------- (A)
Veq  V1  V2  V3
But according to Ohm’s law;
V3  IR3
V1  IR1 ,
V2  IR2 ,
Veq  IReq ,
Put these values in equation (A)
IR eq  IR1  IR 2  IR3
Req  R1  R2  R3
For "n" numbers of Resistors, we have
Req  R1  R2  R3         Rn
Q.9
A.
(i)
Write a note on parallel combination of Resistors?
Parallel combination of Resistors
The connection of two or more than two Resistors in which each and
every Resistor is parallel to each other is called parallel combination of
Resistors. As shown in Fig below.
Parallel combination has the following characteristics.
Voltage across each Resistor is constant. Mathematically
V  V1  V2  V3
(ii)
The current flows in each Resistor is different. The sum of the individual
current across the Resistors is equal to the equivalent current.
Mathematically
I  I 1  I 2  I 3 --------- (A)
But according to Ohm’s law;
V  I 3 R3
V  I 1 R1 ,
V  I 2 R2 ,
V  IReq ,
V
V
V
,
,
,
I1 
I2 
Req
R1
R2
Put these values in equation (A), we get
V
V
V
V



Req R1 R2 R3
V
1 1 1
V(   )
Req
R R R
I
I3 
V
R3
1
1
1
1



Req R1 R2 R3
For "n" numbers of Resistors
1
1
1
1
1




Req R1 R2 R3
Rn
For only two Resistors "R1" and "R2"
1
1
1


Req
R1
R2
R2  R1
1

Req
R1 R2
Req 
R1 R2
R1  R2
Q.10 What is meant by Electric Power and Electric Energy? Explain
A.
Electric Power
The time rate of doing work is called Power. Mathematically
W
P
----------- (1)
t
The work done in electric field on a unit charge is represented by "V". So
the work done on charge "Q", will be
W  QV
Put the value of "W" in equation (1)
QV
P
t
We know that;
I
Q
t
So
P  IV
But we also know that from Ohm’s law;
V  IR
P  I ( IR )  I 2 R
Unit of Power
The SI unit of Power is watt. The other units of Power are Kilowatt, Horse
Power. Mathematically
1KW  1000W
1 Horse Power  746watt
Electric Energy
The energy produce by generators and consumed by different appliances
such as Heater, fans, bulb, etc is called Electric Energy. It is denoted by
"H". Mathematically
Energy  Power  time
H  P  t --------- (2)
As we know that
P  I 2R
Put this value of "P" in equation (2)
H  I 2 R  t  I 2 Rt
Unit of Energy
The different units of Energy are joule, kilowatt hour, electron volt, etc.
1KWh  3  6  106 J
1eV  1  6  1019 J
Q.11 Differentiate between DC and AC?
A.
Direct Current
D.C stands for direct current. This type of current always flow in one
direction. This current flows from positive to negative terminal of the
battery.
Alternating current
A.C stands for alternating current. This type of current changes its
direction many times in a second. Alternating current changes its direction
alternatively from the positive half to the negative half.
Q.12 What are the main components of a circuit?
A.
The basic components of a circuit are as under;
 Switches
It is an electric circuit control element with two discrete states ON and
OFF. Current flows in a circuit if switch is ON, otherwise current can not
flow. The symbol for a switch is “
“.
 Resistors
The devices connected in a circuit to reduce current is called resistor.
They are made from wire coil of special alloys or from carbon. The symbol
for a resistor is “
“.
 Batteries
The combination of many cells is called a battery. Battery is often made
from chemical substances. The symbol for a battery is “
“.
Q.13 Write note on the following instruments.
i. Galvanometer ii. Ammeter
iii. Voltmeter
A.
Galvanometer
It is a sensitive device used to detect small amount of current in a circuit. It
consist of a dial on which a needle can rotate. In order to detect current in
the circuit galvanometer is connected in series with the circuit. The
deflection of the needle indicates the presence of current in the circuit.
Ammeter
The device which measure current in a circuit is called Ammeter. It is a
modified galvanometer for measuring large currents. This modification is
accomplished by connecting a low resistance in parallel with galvanometer.
This resistance is known as shunt resistance which provides an alternate
path for the flow of current. To measure current Ammeter is connected in
series with the circuit.
Voltmeter
The device which measure Voltage in a circuit is called Voltmeter. It is
also a modified galvanometer in which a large resistance is connecting in
series. To measure voltage across the terminals Voltmeter is connected in
parallel with the circuit.
Q.14 Differentiate between Series and Parallel circuits.
A.
Series circuit
The circuit in which the current in different paths flow one after the other is
called Series circuit.
In case of Series circuit the voltage drops across each resistor. The total
voltage is equal to the sum of individual voltage across each resistor.
Mathematically
V = V1 + V 2
Parallel circuit
The circuit in which current flows on different path is called Parallel circuit.
In case of Parallel circuit the current drop across each resistor. The total
current is equal to the sum of individual current across each resistor.
Mathematically
I = I1 + I2
Q.15 What is electricity Hazards?
A.
Hazards
The electricity comes to our houses by means of cable. These cables
consist of two wire live wire and neutral wire. The live wire and any
terminal connected to it are very dangerous. Anyone who touches a live
wire would suffer a fatal shock, because human body conducts electricity.
The electric shock also occurs due to damage insulation, overheating of
cables and damped condition. We should not walk upon wires bare foot.
Q.16 How electricity is used safely in homes?
A.
The electricity supplied to our homes is alternating current (A-C). It
changes its direction 50 times in a second. So its frequency is 50Hz and
voltage is 220V. For the safe use of electricity the following devices should
be used in the house circuit.
 Fuse
It consists of a thin piece of metal wire having low melting point. When too
much current flows through the fuse wire. It melts before the copper wire
in the circuit get hot. As a result the circuit breaks and no damage occur.
 Circuit breaker
It consists of an electromagnet, when current exceeds the rated value of
the circuit breaker. It automatically separates the contact of the circuit
wires and breaks the circuit. Now a day it is used instead of fuse, because
it works quickly than fuse.
 Earth wire
A wire which goes to the top connection on all power sockets and is
connected with the earth. This wire is a safety precaution to prevent
electric shock when an appliance develops a fault.
SHORT QUESTIONS
1.
A.
2.
A.
3.
A.
4.
A.
5.
A.
Electric current is a flow of charge, why two wires rather than a
single wire is used to carry current.
The electric current flow just like water in a pipe. The water flows from a
higher concentration end of the pipe to the lower concentration end.
Similarly electric current also flow from a higher concentration end to the
lower concentration end. Therefore two wires are used rather than a single
wire for the flow of current.
Which solids are good electrical conductors and which are good
insulators? How well do these substance conduct heat.
Metallic solids are good conductor of electricity. For example copper,
silver, iron etc.
Non metallic solids are good insulator of electricity. For example wood,
plastic, rubber etc.
Basically good conductors of electricity are good conductor of heat.
Therefore metallic solids conduct heat. While non metallic solids can not
conduct heat.
State how the resistance of a wire would change (i) if its length is
increased (ii) if its diameter is decreased.
The resistance of a wire depends upon the length and area of the wire.
Mathematically
L
R
A
d
A  r 2   ( ) 2
2
4L
R 2
d
This equation shows that if we increase the length of the wire the
resistance increases. Also if we decrease the diameter of the wire the
resistance will also be increases.
To reduce the brightness of a light bulb an auxiliary resistance be
connected in series with it or in parallel? Why
To reduce the brightness of the bulb auxiliary resistance must be
connected in series. Because in series combination potential difference
across the resistors decreases. As a result the brightness of bulb will be
reduced.
A number of light bulbs are to be connected to a single power outlet.
Will they provide more illumination if connected in series or in
parallel? Why
To get more illumination the bulb must be connected in parallel. Because
in parallel combination of the resistors voltage remain constant. As voltage
reached equally to each and every bulb. Therefore the illumination will be
greater.
6.
What is meant by the statement that an electric kettle has power of
2KW?
A.
Electric kettle has a power of 2KW means that its power is 2000W. so if an
operating voltage of (220 to 240volts) is applied across its heating element,
then it will convert electrical energy into heat energy at the rate of
2000 j/sec. mathematically
E
P
t
E  Pt
E  2000W  1Sec  2000 j
7.
A.
8.
A.
9.
A.
When incandescent lamps burn out, they usually do so just after
they are switched on. Why?
Incandescent lamps are generally used to get heat energy using electrical
energy. That is why they may burn out when switched on. Most of the
electrical energy is converted into thermal energy and some produce light.
Why is it possible for a bird to perch on a high voltage wire without
being electrocuted?
it is possible for a bird to perch on a high voltage wire without being
electrocuted. Because the potential difference between the wire and the
feet of the bird is zero.
Why is it dangerous to turn on a light when you are in the bath tub?
The human body is a good conductor of electricity. Therefore when a
person in a bath tub tries to turn on a bulb, he may get on an electric
shock.
NUMERICAL PROBLEMS
1.
A steady current of 0.5A flows through wire. How much charge
passes through the wire in one minute?
Given data
I  0  5A
t  1 min  60 sec
Q?
We know that
Q
t
Q  It
Q  0  5  60  30C
I
2.
A wire carries a current of 1A. How many electrons pass at any point
in the wire each second?
Given data
I  1A
t  1sec
e  1  6  10 19 C
n?
We know that
Q  ne      (1)
Q  It      (2)
Comparing equation (1) and (2) we get
ne  It
It
n
e
11
1

 1019  0  625  1019
19
1 6
1  6  10
n  6  25  1018 electrons
n
3.
A 120V toaster has a resistance of 12ῼ. What must be the minimum
rating of the fuse in electrical circuit to which toaster is connected?
Given data
V  120V
R  12
I ?
We know that
V  IR
I 
4.
V
120

 10 A
R
12
An electric heater uses 5.0A when connected across 110V.
Determine its resistance.
Given data
I  5 0A
V  110V
R?
We know that
V  IR
V 110
R 
 22
I 50
5.
A metal rod is 2m long and 8mm diameter. Compute its resistance if
the resistivity of the metal is 1.76 x 10-7ῼm.
Given data
L  2m
d  8mm  8  103 m
d 8  103
r 
 4  103 m
2
2
  1  76 108 m
R?
We know that
R
L
A
A  r 2
R
L
r 2
R  1  76  10 8 
2
(3.14)(4  10 3 ) 2
3  52  10 8
R
3  14  16  10 6
3  52
R
 10 8  10 6
50  24
R  0  070  10 8 6
R  0  070  10  2
R  7  10  4 
6.
A platinum wire 80cm long is to have a resistance of 0.1ῼ. What
should its diameter be? The resistivity of platinum is 1.1 x 10-7ῼm.
Given data
L  80cm  0  8m
R  0  1
  1  1  10  7 m
d ?
We know that
R
L
A
L
d
 ( )2
2
4L
R
d 2
4L
d2  
R
R
d 2  1  1  107 
4(0  8)
3.14  0  1
3  52
 107
0  314
2
d  11 21 107
d2 
d 2  1  12  106
d  1  06  103 m  1  06mm
7.
A copper wire has a resistance of 10ῼ at 20˚C. What will be the
resistance at 0˚C? The temperature coefficient of resistance of
copper is 0.004C-1.
Given data
R  10
T1  20 C
T2  0  C
T  T2  T1  0  20  20 C
  0  004C 1
R?
We know that
R  R (1  T )
R  10(1  0  004  20)
R  10(1  0  08)
R  10(0  92)
R  9  2
8.
What is the equivalent resistance of three 5ῼ resistors connected in
series? If a potential difference of 60V is applied across the
combination, what is the current in each resistor?
Given data
R1  R2  R3  5
V  60V
Req  ?
I1 , I 2 , I 3  ?
We know that
Req  R1  R2  R3
Req  5  5  5 
Req  5
We also know that
V  IR
V 60
I 
 4A
R 15
In series combination of resistors current remain constant. So
I1  I 2  I 3  4 A
9.
What is the equivalent resistance of three 5ῼ resistors connected in
parallel? If a potential difference of 60V is applied across the
combination, what is the current in each resistor?
Given data
R1  R2  R3  5
V  60V
Req  ?
I1 , I 2 , I 3  ?
We know that
1
1
1
1



Req
R1
R2
R3
1
1 1 1
  
Req
5 5 5
1
111

Req
5
1
3

Req
5
Req 
5
 1  67
3
We also know that
V  IR
V
I
R
V
60
I1 

 12 A
R1
5
10.
I2 
V
60

 12 A
R2
5
I3 
V
60

 12 A
R3
5
An electric iron of resistance 20ῼ takes a current of 5.0A. Calculate
the thermal energy, in joules, developed in 30sec.
given data
R  20
I  5 0A
t  30Sec
H ?
We know that
H
H
H
H
H
 I 2 Rt
 (5)(20)(30)
 25  20  30
 15000J
 15KJ