ELECTRICITY
OHM’S LAW
Set up a circuit consisting of a nichrome wire XY (0.5 m length), an ammeter, a voltmeter and four cells of 1.5 V each. (Nichrome = an alloy of Ni, Cr, Mn & Fe.)
Using one cell, note the ammeter reading I for the current and voltmeter reading V for potential difference across the nichrome wire. Repeat this using 2, 3 & 4 cells.
|
Number of
cells used |
Current
through the nichrome wire, I (ampere) |
Potential
difference |
V/I |
|
1 |
0.5 |
1.5 |
3 |
|
2 |
1.0 |
3.0 |
3 |
|
3 |
1.5 |
4.5 |
3 |
|
4 |
2.0 |
6.0 |
3 |
In each case, V/I
value is approximately the same.
V–I graph is a straight line. Thus, V/I is a constant
ratio.
In 1827, Georg Simon Ohm (Germany, 1787–1854) found out
relationship between current and the potential difference. Potential
difference (V) across the ends of a metallic wire in an electric
circuit is directly proportional to the current I flowing through
it at constant temperature. It is called Ohm’s law.
V ∝ I
or V/I = constant = R
or V = IR or I = V/R
R
is a constant for the given metallic wire at a given temperature and is called
its resistance. It is the property of a conductor to resist the flow of
charges. Its SI unit is ohm (Ω).
According to Ohm’s law, R = V/I
If the potential difference is 1
V and the current is 1 A, then the resistance (R) of the conductor
is 1 Ω.
A conductor having appreciable
resistance is called a resistor. It is used to control electric current.
The current through a resistor is
inversely proportional to its resistance. If the resistance is doubled the
current gets halved. It has many practical applications.
A component used
to regulate current without changing the voltage source is called variable
resistance. Rheostat is a variable resistor used to change resistance.
Electrical resistance of a conductor:
Take a nichrome wire, a torch
bulb, a 10 W bulb, an ammeter (0 – 5 A range), a plug key and some wires.
Set up a circuit by connecting
four dry cells of 1.5 V each in series with the ammeter leaving a gap XY in the
circuit.
Complete the circuit by
connecting the nichrome wire in the gap XY. Plug the key. Note down ammeter
reading.
Replace the nichrome wire with
the torch bulb in the circuit and note down the ammeter reading.
Repeat this with 10 W bulb and any
material component.
The current is different for
different components. In certain components, there is easy flow of electric
current while the others resist the flow.
Within a conductor, electrons are
not completely free to move due to the attraction by atoms. So, resistance is increased
and motion of electrons is retarded.
A component that conducts
electricity and has a low resistance is called a good conductor.
A component with a high
resistance is a poor conductor.
A component with very high resistance is called an insulator. It does not conduct electricity.
