Physics • Electricity

What Is Ohm's Law?

Q: What is Ohm's Law?

Ohm's Law states that, at constant temperature and other physical conditions, the current through a conductor is directly proportional to the potential difference across it.

Q: Is Ohm's Law valid for every electrical device?

No. Ohm's Law is valid for ohmic conductors under constant physical conditions. Devices such as diodes do not show a simple constant resistance over all operating conditions.

Ohm's Law gives one of the most useful relationships in basic electricity: it connects potential difference, current and resistance. Here you will learn the law, its formula, circuit diagram, V-I graph, solved examples and practical applications in simple language.

In simple words: If the temperature and other physical conditions remain constant, increasing the voltage across an ohmic conductor increases the current through it in the same proportion.

1. What Is Ohm's Law?

Ohm's Law states that, at constant temperature and other physical conditions, the current flowing through a conductor is directly proportional to the potential difference applied across it.

In symbols, direct proportionality between potential difference V and current I can be written as:

V \propto I

Replacing the proportionality sign by a constant gives:

V = IR

The constant R is called the resistance of the conductor.

Condition of the law: The physical conditions, especially temperature, must remain constant. A change in temperature can change the resistance of many conductors.

2. The Three Quantities in Ohm's Law

Quantity	Symbol	Meaning	SI unit
Potential difference	V	Energy transferred per unit charge between two points.	volt (V)
Current	I	Rate of flow of electric charge through a conductor.	ampere (A)
Resistance	R	Opposition offered by a conductor to the flow of electric current.	ohm (Ω)

What does resistance mean?

Think of electric current like water flowing through a pipe. The potential difference is similar to the pressure that pushes the water, while resistance is like a restriction in the pipe. A larger resistance makes it harder for current to flow.

R = \frac{V}{I}

3. Ohm's Law Circuit Diagram

A simple Ohm's Law experiment uses a cell or battery, a key, a rheostat, an ammeter and a resistor. The ammeter is connected in series to measure current, while a voltmeter is connected in parallel across the resistor to measure potential difference.

A typical school-level circuit used to study the relationship between V and I.

4. V-I Graph for an Ohmic Conductor

For an ohmic conductor at constant physical conditions, current is directly proportional to potential difference. Therefore, a graph of V against I is a straight line passing through the origin.

For a constant resistance, the V-I graph is linear. Its slope depends on which quantity is plotted on the vertical axis.

Exam point: Be careful about the axes. For a graph of V against I, the slope is R. If the graph is of I against V, the slope is 1/R.

R = \frac{V}{I} = \text{slope of the } V\text{-}I\text{ graph}

5. Ohm's Law Formula Triangle

The relation V = IR can be rearranged to find any one of the three quantities.

To find	Formula
Potential difference	V = IR
Current	I = V/R
Resistance	R = V/I

A useful memory trick is to remember the three symbols as a relationship rather than memorising three unrelated formulas.

\boxed{V = IR}

6. Solved Examples on Ohm's Law

Example 1: Find the Current

A resistor of resistance 5 Ω is connected across a potential difference of 10 V. Find the current through the resistor.

Given: V = 10 V and R = 5 Ω

I = \frac{V}{R}

I = \frac{10}{5}

I = 2\,\mathrm{A}

Answer: The current through the resistor is 2 A.

Example 2: Find the Resistance

A current of 0.5 A flows through a conductor when the potential difference across it is 12 V. Find its resistance.

Given: V = 12 V and I = 0.5 A

R = \frac{V}{I}

R = \frac{12}{0.5}

R = 24\,\Omega

Answer: The resistance of the conductor is 24 Ω.

Example 3: Find the Potential Difference

A current of 3 A flows through a resistor of 4 Ω. Find the potential difference across it.

Given: I = 3 A and R = 4 Ω

V = IR

V = 3\times4

V = 12\,\mathrm{V}

Answer: The potential difference across the resistor is 12 V.

Example 4: What Happens if Resistance Is Doubled?

A fixed voltage of 20 V is applied to a resistor. Its resistance is increased from 5 Ω to 10 Ω. Find the current before and after the change.

Before the change:

I_1 = \frac{20}{5} = 4\,\mathrm{A}

After the resistance is doubled:

I_2 = \frac{20}{10} = 2\,\mathrm{A}

Answer: The current falls from 4 A to 2 A. At constant voltage, doubling the resistance halves the current.

7. How Is Ohm's Law Verified Experimentally?

In a school laboratory, the potential difference across a resistor is varied using a rheostat. For each setting, the voltmeter reading and ammeter reading are recorded.

Connect the circuit correctly with the ammeter in series and voltmeter in parallel.
Close the key and set a small current using the rheostat.
Record the corresponding current I and potential difference V.
Change the rheostat setting and take several pairs of readings.
Calculate V/I for each reading.
Plot a graph of V against I.

V (V)	I (A)	V/I (Ω)
2	0.2	10
4	0.4	10
6	0.6	10
8	0.8	10

Since the ratio V/I remains constant in these readings, the resistance is constant and the relationship is consistent with Ohm's Law.

8. Ohmic and Non-Ohmic Conductors

Not every electrical component has a constant resistance over its entire operating range. This is why the condition attached to Ohm's Law is important.

Ohmic behaviour	Non-ohmic behaviour
V is directly proportional to I when physical conditions remain constant.	V and I do not maintain a simple direct proportionality over the operating range.
V-I graph is a straight line through the origin under the stated conditions.	V-I graph is generally curved or otherwise non-linear.
Resistance remains constant for the stated conditions.	Resistance changes with operating conditions or current/voltage.

Examples of components that can show non-ohmic behaviour include a semiconductor diode and an incandescent lamp filament, because their electrical behaviour changes with operating conditions.

9. Applications of Ohm's Law

Finding the current in a circuit when voltage and resistance are known.
Choosing a suitable resistance for a required current.
Estimating voltage drops across resistive components.
Analysing simple electrical circuits in school laboratory experiments.
Checking measured circuit values for consistency with an expected resistance.

Daily-life connection: Electrical engineers and technicians use voltage, current and resistance relationships when selecting components and checking circuit behaviour. The same three quantities appear repeatedly when analysing simple electrical networks.

10. Common Mistakes Students Make

Writing I = VR instead of I = V/R.
Forgetting that resistance is measured in ohms (Ω), not volts or amperes.
Putting the ammeter in parallel instead of series in the experimental circuit.
Putting the voltmeter in series instead of parallel across the component.
Ignoring the constant-temperature or constant-conditions requirement of Ohm's Law.
Calling every electrical device an ohmic conductor.
Forgetting to check units before substituting values into the formula.

11. Quick Revision

Ohm's Law: At constant physical conditions, current is directly proportional to potential difference.

Formula:

Current:

Resistance:

SI units: V in volt (V), I in ampere (A), R in ohm (Ω).

V against I graph: A straight line through the origin for an ohmic conductor under constant conditions.

Experiment: Ammeter in series; voltmeter in parallel across the resistor.

12. Frequently Asked Questions

What is Ohm's Law?

At constant temperature and other physical conditions, the current through a conductor is directly proportional to the potential difference across it.

What is the formula of Ohm's Law?

The formula is V = IR, where V is potential difference, I is current and R is resistance.

What is the SI unit of resistance?

The SI unit of resistance is ohm (Ω).

What happens to current if resistance is doubled at constant voltage?

From I = V/R, doubling resistance halves the current, provided the voltage remains constant.

Is Ohm's Law valid for every electrical device?

No. It applies to ohmic conductors under constant physical conditions. Some devices, such as diodes, show non-linear V-I behaviour.

What is an ohmic conductor?

An ohmic conductor is a conductor for which current is directly proportional to potential difference when the relevant physical conditions remain constant.

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