Ohm’s Law – CBSE Class 10 Science (Physics – Electricity)

1. Statement of Ohm’s Law

Ohm’s Law states that the current ( $I$ ) flowing through a conductor is directly proportional to the potential difference ( $V$ ) across its ends, when temperature and physical conditions remain constant.
If you increase the voltage, the current also increases in the same ratio, as long as the conductor is not heated or changed in any way.
This law is a fundamental rule for simple electric circuits.

Examples:

If you have a copper wire and you double the voltage across it without heating it up, the current through the wire will also double.
For a steel rod at room temperature, increasing the applied voltage from 2 V to 4 V will increase the current from 0.1 A to 0.2 A.
In a simple torch with a metallic filament, inserting two cells (instead of one) doubles the voltage — which doubles the current, making the bulb burn brighter.

2. Mathematical Expression of Ohm’s Law

The law is mathematically expressed as:
$V = IR$
Where:
- $V$ is the potential difference (in volts, V)
- $I$ is the current (in amperes, A)
- $R$ is the resistance (in ohms, $\Omega$ )
Rearranging the formula helps you solve various questions:
- To find current: $I = \frac{V}{R}$
- To find resistance: $R = \frac{V}{I}$

Examples:

Suppose you have a resistor of $R = 20\; \Omega$ , and a battery of $V = 10\; V$ is connected: $I = \frac{V}{R} = \frac{10}{20} = 0.5\,A$
If a bulb draws $0.3$  A when a $6$  V battery is connected, $R = \frac{6}{0.3} = 20\, \Omega$
An unknown wire draws $2$  A at $4$  V: $R = \frac{4}{2} = 2\, \Omega$

3. Ohm’s Law – Activity (Activity-Based Understanding)

Let’s make this fun and easy!

Activity: Verifying Ohm’s Law

Objective:
To show that current is directly proportional to the applied voltage for a conductor at constant temperature.

Materials Required:

Battery or power supply
Connecting wires
Ammeter (to measure current)
Voltmeter (to measure voltage)
Resistor (a simple wire or coil)
Rheostat (to vary resistance, optional)
Switch

Step By Step Instructions:

Arrange the Circuit:
- Connect the battery, resistor, ammeter, and switch in series.
- Connect a voltmeter in parallel across the resistor.
Close the Switch:
- Let the current flow.
Record Readings:
- Note the current shown by the ammeter ( $I_1$ ) and the voltage across resistor ( $V_1$ ).
Change Voltage:
- Change the battery voltage or move the rheostat to increase the voltage.
Read Again:
- Note new ammeter ( $I_2$ ) and voltmeter ( $V_2$ ) readings.
Repeat:
- Take at least three sets of readings for different voltages.

Observations to Make:

Prepare a table:

Serial No. Potential Difference (V) Current (I) (A) V/I
1 2 0.1 20
2 4 0.2 20
3 6 0.3 20
You will notice $V/I$ (the resistance) remains nearly constant.
When you plot voltage against current, you get a straight line.

Serial No.	Potential Difference (V)	Current (I) (A)	V/I
1	2	0.1	20
2	4	0.2	20
3	6	0.3	20

What’s Happening?

This shows the proportionality described by Ohm’s law.
Resistance of the wire remains constant as long as it doesn't heat up.

Fun Angle:

You can use colored water for current and draw a "water circuit" with pipes and syringes, where more pressure (voltage) pushes more water (current) through the same narrow pipe (resistance).

4. V-I Graph and Resistance

For ohmic conductors, plotting potential difference ( $V$ ) on the y-axis and current ( $I$ ) on the x-axis produces a straight line through the origin.
The slope (V/I) of the line gives the resistance of the conductor.
Steepness:
- A flatter line means higher resistance (needs more voltage to get same current).
- A steep line means lower resistance (gets more current for same voltage).

Examples:

A copper wire and a nichrome wire are tested. The copper wire's graph is steeper than nichrome because copper has less resistance.
For a device like a bulb, the graph is not a straight line. As bulb heats up, resistance changes, so it is non-ohmic.
A diode’s graph allows current only after a certain minimum voltage, not a straight line.

5. Practical Applications of Ohm’s Law

Designing Circuits:
Engineers use Ohm’s law to decide what size resistor to use with LEDs. Too much current can burn out components.
Home Electric Wiring:
Wires are chosen based on the expected current and Ohm’s law to avoid overheating and fire.
Measuring Resistance:
Simple multimeters use Ohm’s law to calculate the resistance of any object.

Examples:

You want to reduce current in a heater for safety, so you use a resistor based on Ohm’s law.
Making a phone charger, manufacturers calculate resistance needed for safe charging.
When replacing a fuse in your home, you choose its rating based on the normal current and Ohm's law.

Scenario Based Questions

Scenario: You increase the voltage across a metallic wire from 3 V to 6 V and notice the current doubles.
- Question: Is the wire obeying Ohm’s law? Explain.
- Answer: Yes, because the current increases in direct proportion to voltage, showing constant resistance.
Scenario: You replace a resistor with a glowing bulb in a circuit and notice the V-I graph is curved.
- Question: What could be the reason for this?
- Answer: The bulb is heating up; its resistance changes with temperature, so it does not strictly follow Ohm’s law.
Scenario: In a lab, you plot current (I) on the x-axis and voltage (V) on the y-axis for an unknown wire and get a straight line through the origin.
- Question: What does the slope of this line represent?
- Answer: The slope ( $V/I$ ) represents the resistance of the wire.
Scenario: A student adds a resistor in series with a lamp, and the lamp becomes dimmer.
- Question: Why does this happen?
- Answer: Adding resistance reduces the current according to Ohm’s law, so less current flows through the lamp, making it dimmer.
Scenario: You are told a component does not obey Ohm’s law.
- Question: What kind of graph would you expect for its V-I characteristics?
- Answer: You would see a curve or nonlinear relationship, not a straight line.

Remember, Ohm’s law is simple and magical! It’s like a relationship between pressure (voltage), flow (current), and roadblock (resistance) in an electric circuit. Play with safe circuits and see it yourself!