Ever wondered why a flashlight gets dimmer when you add another bulb?
💡 In Simple Words: Ohm's Law says that the electric push (voltage) across a component is directly linked to how many electrons flow (current) and how much the component resists that flow (resistance). Double the voltage, double the current – unless you change the resistance.
What is Ohm's Law?
Ohm's Law is a rule that connects three basic ideas in electricity:
- Voltage (V) – the electric push, like water pressure in a hose.
- Current (I) – the flow of electrons, similar to the amount of water moving through the hose.
- Resistance (R) – how much a material fights the flow, comparable to a narrow section of the hose.
The law is written as V = I × R. That means if you know any two of the numbers, you can find the third.
How to Use Ohm's Law in Simple Circuits
Most exam questions give you two of the three values and ask for the missing one. Follow these quick steps:
- Identify what you have (V, I, or R).
- Pick the right form of the formula:
- V = I R
- I = V / R
- R = V / I
- Insert the numbers, do the arithmetic, and keep an eye on units (volts, amperes, ohms).
Example 1: Find the current
Question: A 9 V battery is connected to a resistor of 3 Ω. What is the current?
Solution: Use I = V / R → I = 9 V ÷ 3 Ω = 3 A. So the current is 3 amperes.
Example 2: Find the voltage
Question: A circuit draws 0.5 A through a 20 Ω resistor. What voltage is applied?
Solution: V = I R → V = 0.5 A × 20 Ω = 10 V.
Example 3: Series circuit with two resistors
Question: Two resistors, 4 Ω and 6 Ω, are placed in series across a 12 V source. Find the total current.
In a series connection, resistances add up: R_total = 4 Ω + 6 Ω = 10 Ω.
Now use I = V / R_total → I = 12 V ÷ 10 Ω = 1.2 A.
Quick Reference Table
| What you need | Formula to use | Result |
|---|---|---|
| V and R | I = V / R | Current (A) |
| V and I | R = V / I | Resistance (Ω) |
| I and R | V = I R | Voltage (V) |
📝 Likely Exam Questions
- Q1: A 5 Ω resistor is connected to a 15 V supply. Calculate the current.
Ans: I = V / R = 15 V ÷ 5 Ω = 3 A. - Q2: A lamp draws 2 A from a 240 V mains. What is its resistance?
Ans: R = V / I = 240 V ÷ 2 A = 120 Ω. - Q3: Two resistors of 10 Ω and 15 Ω are placed in series across a 30 V battery. Find the voltage across each resistor.
Ans: Total R = 25 Ω, I = 30 V ÷ 25 Ω = 1.2 A. V₁ = I R₁ = 1.2 A × 10 Ω = 12 V, V₂ = 1.2 A × 15 Ω = 18 V. - Q4: In a circuit, the current is 0.25 A and the resistance is 40 Ω. Find the voltage.
Ans: V = I R = 0.25 A × 40 Ω = 10 V. - Q5: Explain why adding a resistor in series reduces the current in a simple circuit.
Ans: Series adds resistance, so total R increases. Since V is fixed, I = V / R_total gets smaller.