1.4.34- Ohm’s law - simple electrical circuits, problems & terms
Ohm’s law
In 1826 George Simon Ohm
discovered that for metallic conductor, there is a substantially constant ratio
of the potential difference between the ends of the conductor Ohm’s law gives
the relation between the voltage, current and resistance of a circuit.
Ohm's law states
that in any electrical closed circuit, the current (I) is directly proportional
to the voltage (V), and it is inversely proportional to the resistance 'R' at
constant temperature.
I
α V
I α 1/R
I α V/R
I = V/R
Ans:- I =V/R
I =10/5 = 2 amp
Example:- Find the value of voltage across a 10 Ohms resistor in the circuit. When the current of 2 Amps flows through the 10 Ohm resistor.
Ans:- Voltage across 10 Ohm
V= I x R = 2 x 10 = 20 Volt
Example:- A heater is
connected across 220 v supply and the current flowing through it is 4 amp.
Calculate the resistance?
Ans:- R = V / I= 220/4=55Ω
Work, Power and Energy
Work: Work is said to
be done, when a force (F) displaces a body from one distance (s) to another
(or) Work done = Force x distance moved
W.D = F x S
It is generally denoted as “W”
The smallest unit of work done is
“Erg”
The bigger unit is ‘Joule’
Power (P): The rate of
doing work is called as Power (P).
Power (P) = work done
/ time taken
P = W / T = F x S / T
It is in two types:-
1. 1. Electrical
Power
2. 2. Mechanical power
Electrical
Power: The
product of voltage (V) and current (I) is called electrical power.
Electrical power (P) = Voltage x Current P=V x
I
The unit of
Electrical power is ‘Watt’ It is denoted by the letter ‘P’ It is measured by
Watt meter. The following
Formulae can also be
derived from formula of power (P) as
(i) P = V X I
= IR x I
P = I² R
(ii) P = V X I
= V x V / R = V²/R
Mechanical Power:
The unit of Mechanical power is
“Horse Power” (H.P)
Horse Power (HP) further
classified into two:
They are:-
1. Indicated Horse
Power - (IHP)
2. Brake Horse
Power - (BHP)
Indicated Horse
Power (IHP)
The power developed inside the
engine (or) pump (or) motor is called Indicated Horse Power (IHP)
Brake Horse
Power (BHP)
The useful Horse Power which is
available at the shaft of the engine/motor/pump is called Brake Horse Power
(BHP)
So, IHP is always greater than
BHP due to friction losses
IHP > BHP
The relation between Mechanical
and Electrical Power
(i.e) 1 HP (British) = 746 Watt
1 HP (Metric) = 735.5 Watt
Energy: The
capacity for doing work is called as electrical Energy
(or)
The product of power and time is known as Electrical energy (i.e)
Energy = Power x time=VI x t
S.I unit of energy is
“Joule”
The energy can be divided into
two main categories (ie)
(i) Potential Energy (eg. Loaded
gun, energy (stored in spring etc)
(ii) Kinetic Energy (eg. Moving
of car, raining etc)
The unit of electrical energy is
“Watt hour” (Wh)
The commercial unit of Electrical energy is “Kilo watt hour” (KWH) or unit
B.O.T (Board of Trade) unit / KWH/Unit
One B.O.T (Board of Trade) unit
is defined as that one thousand watt lamp is used for one hour time, it
consumes energy of one kilowatt hour (1kWH). It is also called as“unit”
Energy = 1000W x 1Hr = 1000WH
(or) 1kWH
Example:- How much electrical energy is consumed in an electric iron rated as 750W/250V used for 90 Minutes
Solution:
Electrical Energy (E) = P x t
=750 w x 1.5Hr
= 1125 WH (or)
E = 1.125 kWH
Example:- Calculate the power of a lamp, which takes a current of 0.42 Amp at 240 V supply
Solution:
P = V X I
= 240 x 0.42
= 100.8W
Hence, Power (P) = 100 W (approx)
Example:- Calculate the hot resistance (R) of the 200W/250V rated bulb?
Solution:
R= V²/P= 250 X 250
/200
(R) Resistance =
312.5 Ohm
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