Why Ohm's Law matters
Ever wondered why a tiny battery can light up a big bulb while another one can’t? The answer lies in Ohm's Law – the secret rule that ties voltage, current and resistance together.
Ohm's Law tells us that the voltage across a conductor is directly proportional to the current flowing through it. In simple words, push harder (higher voltage) or make the path easier (lower resistance) and more electricity will flow.
What is Ohm's Law?
George Ohm, a 19th‑century German scientist, discovered a neat relationship while experimenting with wires. He wrote it as:
V = I × R
- V stands for voltage (the electrical “pressure” that pushes charges), measured in volts (V).
- I stands for current (the flow of electric charge), measured in amperes (A).
- R stands for resistance (how much the material resists the flow), measured in ohms (Ω).
Think of water in a pipe: voltage is the water pressure, current is the amount of water moving, and resistance is the pipe’s width. Narrower pipe = more resistance, so less water gets through unless you increase the pressure.
How to use the formula
Because the equation is simple multiplication, you can rearrange it to find any one of the three quantities:
- Find voltage: V = I × R
- Find current: I = V ÷ R
- Find resistance: R = V ÷ I
Just plug in the numbers you know, do the arithmetic, and you’ve solved the problem.
Worked example 1 – Finding the current
A 9 V battery is connected to a resistor of 3 Ω. What current flows?
We have V = 9 V and R = 3 Ω. Using I = V ÷ R:
I = 9 ÷ 3 = 3 A. So three amperes of current travel through the circuit.
Worked example 2 – Finding the resistance
A lamp glows with a current of 2 A when connected to a 12 V supply. What is the lamp’s resistance?
Here V = 12 V and I = 2 A. Using R = V ÷ I:
R = 12 ÷ 2 = 6 Ω. The lamp offers six ohms of resistance.
Simple circuit basics
In an ICSE exam you’ll often see a single‑loop circuit: a battery, a resistor (or bulb), and connecting wires. The loop is closed, meaning the path for the current is complete.
Key points to remember:
- The same current flows through every component in a series loop.
- The total voltage supplied equals the sum of voltage drops across each component.
- If you add another resistor in series, the total resistance just adds up: R_total = R1 + R2 + …
Quick reference table
| What you know | What you need | Formula to use |
|---|---|---|
| V and R | I | I = V ÷ R |
| V and I | R | R = V ÷ I |
| I and R | V | V = I × R |
Common pitfalls and how to avoid them
- Mixing up units: Always write volts, amperes and ohms with their symbols. Forgetting a unit can lead to a wrong answer.
- Using the wrong formula: Look at the question – which two quantities are given? Choose the formula that solves for the missing one.
- Assuming all circuits are series: If the problem mentions parallel branches, the total resistance is not a simple sum. For now, stick to series as the chapter focuses on that.
Tips for the ICSE exam
- Write the formula on the first line of your answer. It earns you marks even if the arithmetic is off.
- Show each step clearly – examiners love neat work.
- Round off only at the final step unless the question tells you otherwise.
📝 Likely Exam Questions
- Question: A 6 V cell is connected to a resistor of 2 Ω. Find the current and the power dissipated in the resistor.
Answer: I = V ÷ R = 6 ÷ 2 = 3 A. Power P = V × I = 6 × 3 = 18 W. - Question: In a series circuit, two resistors of 4 Ω and 6 Ω are connected to a 12 V battery. Calculate the current flowing through the circuit.
Answer: R_total = 4 + 6 = 10 Ω. I = V ÷ R_total = 12 ÷ 10 = 1.2 A. - Question: A bulb draws a current of 0.5 A when supplied with 120 V. Determine the resistance of the bulb.
Answer: R = V ÷ I = 120 ÷ 0.5 = 240 Ω. - Question: If the resistance in a circuit is doubled while the voltage remains the same, what happens to the current?
Answer: Current halves, because I = V ÷ R; doubling R makes I half of its original value. - Question: Sketch a simple series circuit with a battery and a resistor, label V, I, and R, and write the Ohm’s Law equation for it.
Answer: (Student draws a loop: battery → resistor → back to battery). Labels: V across battery, I through the loop, R of resistor. Equation: V = I × R.