When a potential difference is applied between the two terminals
of a conductor, an electric
current is set up. Due to this, work is done
and the electrons acquire energy. This electrical energy may be transformed
into different forms of energy (e.g. -
heat, light, mechanical
energy etc.) according to the nature of the circuit.
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Electric power
|
Say, AB is a conductor of resistance
R and Q amount of charge is flowing through it. The potential
difference between the points A and B is V. We know if the potential difference
between the two terminals of a conductor is 1 volt and 1 coulomb charge flows
through it, then the amount of work done or energy spent is
1joule. Now, if Q coulomb charge flows through the conductor, the amount
of work done = VQ joule. Therefore, energy spent or the amount of energy converted
is
W =VQ
Again, electric current, I =
Q ÷ t
Or, Q= It
∴ W =VIt - - - - (i)
Using ohm’s law this relationship
can be expressed as below,
∴ W = VIt = I2Rt =
(V2 ÷ R) t Joule
Electric power
The electric appliances which we use in houses or in offices are
generally marked with the voltage by which it runs and the electric power in
watt. We know the rate of work done or the rate of energy
conversion is called power.
Therefore, the rate at which energy is
converted into other forms in an electric device is its power.
Therefore, Power = Work done
÷ time
= energy
converted ÷ time
∴ P = W ÷ t
Substituting the value of W from equation (i) we get,
P =VI
Applying Ohm’s law P can be expressed in terms of V, I and
R as below-
P = VI = I2 R = V2 ÷ R
We know the unit of power is watt
(W). In the calculations of electric energy generally kW, MW etc. are used
instead of watt. 1 kW=103 W and 1
MW=106 W.
The power of some of the electric appliances which we use in our
houses is mentioned below. The power of an electric bulb is generally 40, 60
and 100 W. The power of an electric fan is found to 65-75 W commonly. Power of
a television is generally 60-70 W. The energy saving bulbs which we use
now-a-days has power of 11-30 W.
Besides this, we use refrigerator, heater, iron etc. in houses-
their power is more. So, it is commended not to use these appliances during
peak hour.
Calculation of electrical energy spent
We have to pay for the electrical energy we utilize in our houses,
shops, mills and factories. There is an electricity meter in the houses those use
electricity, which maintain the accounts of spent electrical energy. Throughout
the world, the electricity supply authority measures the amount of electrical
energy consumed in units of kilowatt-hour (kWh). We call this kilowatt-hour
unit as ‘board of trade’ unit or in brief ‘unit’. From the difference of the
readings of two times in the electricity meter, we get the amount of consumed
electric power during this period.
Since power, P = Work done ÷
time
=
Converted energy ÷ time
= W ÷ t
∴ W = Pt
If P=1 kW and t =1h, then W=1 kW×1h=1
kWh.
Therefore, the amount of electrical
energy converted or spent when an electric device of
1 kilowatt power works for 1 hour is
called 1 kilowatt-hour or 1 unit.
System loss and load shedding
We know, electrical energy is produced in power stations situated
at different places. The electricity thus generated has to be transmitted at different
places according to the demand. The electrical energy produced is transmitted
to the different substations situated at different places by the electricity
transmission system. Then from different substations this electrical energy is
distributed to the consumers by electricity distribution system again.
In the power station the electrical energy is generated at low
voltage. Then this low voltage is transformed into high voltage by the step-up
transformer. The conducting wires which are used for electricity transmission
have a definite amount of resistance.
As a result, to overcome this resistance, part of the electrical energy is
converted to heat. That is, a loss or decay of energy occurs. This loss of
energy is termed as system loss. Due to the transmission of electricity at high
voltage, the loss that occurs due to the power grid or of conductor is
decreased to a great extent. For a definite amount of electrical energy, the
value of the electric current becomes lower due to high voltage transmission.
As an example- if the transmission line voltage is increased by ten times, then
the electric current becomes one tenth. As a result, the i2R loss
of the power grid becomes one hundredth. Therefore, by increasing the
transmission line voltage we can lower the system loss.
Load shedding
Each of the power stations generates a definite amount of electric
power. The electricity generated by all the power stations is added in the
national power grid. According to the demand of different locality power
sub-stations collect electricity from the national grid. Then the power
sub-station delivers or distributes the electricity to the consumer level.
When in a particular area, the demand of electricity exceeds the
supply or generation; the power sub-station can no longer fulfill the demand of
electricity. Then the sub-station authority is forced to switch off or to
disconnect the power distribution for a while in some parts of the distribution
network. This is called load shedding. When the substation gets the supply
according to its demand, then it distributes electricity in that region again.
If the load shedding takes place for a couple of hours continuously,
authority load sheds circularly in different area to make load shedding
tolerable at the consumer level.
Dangers of electricity
Electricity plays a very important role in our daily lives. Though
electricity is very useful to us, it can also be very dangerous in the careless
uses. Any type of faults in electrical appliances or circuits can cause fires
and electric shocks. Due to the passage of electricity through the body there
is a risk of death of people.
Uses of electricity can be dangerous due to three reasons
described below.
1. Damaged insulation
2. Overheating of the cables
3. Damped conditions
Damaged insulation
The electrical appliances work when these are connected to the
voltage source by two conducting wires to complete the circuit. These two wires
are called live and neutral wire. These conducting wires are usually insulated
with rubber. Then they are wound together to form a cable and enclosed by PVC
or rubber.
These insulating materials become worn with time and use. For
example- the electrical cables of the electric iron which we use in home get
bent and twisted because of the way they are used. This might cause the
electrical insulation to crack and break. As a result, the conducting wires
inside is exposed. If by any means a person comes into contact with the exposed
live wire, it may cause severe electric shock to the user. If the live and neural
wires come into mutual contact due to the damaged insulation, a short circuit
will happen and may cause a fire.
Overheating of the cables
Overheating of cables occur when unusually large current flows
through the electric cables or conducting wires. For example- an unusual large
current flow, when an electric fan motor overheats and melts, as a result the
live and neutral wire is fused together. Besides this, we make connection of
too many electrical devices in a wall socket by using a multi-plug. Due to
this, the conducting wire connected to the socket draws more current from the main
line than the current which the conducting wire can draw safely. As a result,
the cable wire is overheated, insulation is melted and causes fire.
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Damped
conditions
|
Damped conditions
Many electrical accidents may occur in damped conditions. We know,
electricity can pass through water. The parts of an electric appliance which
are not insulated must be kept dry. Otherwise, there is a risk of short
circuits and electric shocks. As an example, leaving a hair dryer on a wet sink
is very dangerous. The person using the sink could be electrocuted if the wires
were exposed or the insulation had damaged. Besides this, switching on or off
of an electric switch by wet hand is risky.
Safe use of electricity
In the previous section you were informed about the dangers of
electricity. In the present section, we will learn about the safe uses of electricity.
When using electricity at home, the safety measures that are
needed:
1. Circuit breaker
2. Fuses
3. Correct connection of switch
4. Earth wire
Circuit breaker
Circuit breakers are used as safety devices. Generally this is
placed near the front door of a house. Circuit breaker switches off the
electrical supply in a circuit when there is an overflow of current.
Circuit breaker disconnects the electrical supply in a definite part of the
house. Without circuit breaker in a circuit, this excessive current can cause
damage of home appliances or even start a fire.
Fuses
A fuse is a safety device. A fuse is included in an electrical
circuit to prevent excessive current flow. The fuse is always connected to the live
wire of electrical cables. A short thin piece of wire is used as a fuse. The
fuse becomes hot and melts when the electric
current flowing through it is greater than a definite value. As a result,
the circuit is disconnected and the electrical appliances will be safe. The
fuse is marked with definite amount of current on its body. Fuses will be such
that it can bring slightly higher current than the maximum current an
electrical device or appliance can tolerate safely. If the fuse burns, the
appliance will not be electrified any more. Before changing fuse, you have to
switch off the mains of electricity supply.
Correct connection of switch
A switch breaks or completes an electrical circuit. During switch
connection in a circuit, an important precaution is that the switch must be fitted
onto the live wire. For this, switching off will disconnect the high voltage
source from the appliance instantly [Figure 1].
 |
Figure
1
|
If the switch is fitted onto the neutral wire wrongly, the
electric appliance will be ‘live’ even if the switch is ‘off’ [Figure 2] and
increase the risk of electric shock.
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Figure
2
|
Earth wire
All electrical appliances or devices need at least two wires to
form a complete circuit. These are the live (L) and neutral (N) wire. The live
wire delivers the electrical energy to the appliance. On the contrary, the current
returns back to the supply through the neutral wire and complete the circuit. The
potential of neutral wire is zero. The earth wire is a low-resistance
wire. It is usually connected to the metallic casing of the appliance. The
circuit may be faulty from different reasons. If the live wire is not properly
connected and it touches the metal casing of the appliance- the user may be electrocuted
from an electric shock. Earthling of the casing prevents this from happening. In
this case, the large current will flow from the live wire to the earth through
the metal casing. As a result, the fuse will blow out and cut off the electric
supply to the appliance. It is strongly recommended to provide an earthling to
the refrigerator in houses for safe use of it. (Figure 1) demonstrates how a
washing machine without earthling may be risky.
 |
Figure
1
|
How earth wire works as a safety precaution is demonstrated in
figure 2.
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| Figure 2 |
In addition, three-pin plugs are used in many portable
devices. Fuses are connected in these plugs as safety measures. Fuse keeps the
device safe.
End
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