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, and 4 cells.
| Number of cells used | Current through the nichrome wire, I (ampere) | Potential difference across nichrome wire, V (volt) | V/I (volt/ ampere) |
|---|---|---|---|
| 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.
A straight-line plot shows that as the current through a wire increases,
potential difference across the wire increases linearly-this is Ohm's law.
- In 1827, Georg Simon Ohm (Germany, 1787–1854) found out the 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
V/I = constant = R
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 the ammeter reading.
- Replace the nichrome wire with the torch bulb in the circuit and note down the ammeter reading.
- Repeat this with the 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 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.