Why Ohm's Law matters in everyday life
Ever wondered why a flashlight dims when you squeeze the battery harder? That tiny change is all about Ohm's Law, the rule that ties voltage, current, and resistance together—just like water pressure, flow, and pipe width.
💡 In Simple Words: Ohm's Law says the electric push (voltage) equals how fast the charge moves (current) times how much the path resists (resistance). If you know any two, you can find the third—just like figuring out water flow if you know pressure and pipe size.
Key ideas you need to remember
Voltage (V)
Think of voltage as the pressure that makes electric charge move, similar to water pressure in a hose. It’s measured in volts (V).
Current (I)
Current is the flow of electric charge, like the amount of water flowing per second. It’s measured in amperes (A), often just called amps.
Resistance (R)
Resistance is anything that slows down the flow—like a narrow pipe or a kink in a hose. Measured in ohms (Ω).
Ohm's Law
The relationship is simple: V = I × R. Rearrange it any way you like:
- Find voltage: V = I × R
- Find current: I = V ÷ R
- Find resistance: R = V ÷ I
Remember: the formula works only for ohmic conductors (materials that keep the same resistance no matter the voltage), which most classroom resistors are.
Solving simple circuit problems
Most exam questions give you a small circuit—maybe a battery, a few resistors, and a switch. Follow these steps:
Series circuit basics
In a series circuit, components are linked end‑to‑end, so the same current flows through each. The total resistance is just the sum of each resistor:
R_total = R₁ + R₂ + … + R_n
Parallel circuit basics
In a parallel circuit, each component gets its own branch across the same voltage. The total resistance drops because the current has multiple paths:
1/R_total = 1/R₁ + 1/R₂ + … + 1/R_n
Worked example 1 – Finding current
Problem: A 12 V battery is connected to a single resistor of 4 Ω. What is the current?
Using I = V ÷ R:
I = 12 V ÷ 4 Ω = 3 A.
So the circuit carries 3 amperes of charge each second.
Worked example 2 – Series resistors
Problem: A 9 V battery powers two resistors, 2 Ω and 3 Ω, in series. Find the current and the voltage across each resistor.
Step 1: Total resistance R_total = 2 Ω + 3 Ω = 5 Ω.
Step 2: Current I = V ÷ R_total = 9 V ÷ 5 Ω = 1.8 A.
Step 3: Voltage across R₁ = I × R₁ = 1.8 A × 2 Ω = 3.6 V.
Step 4: Voltage across R₂ = I × R₂ = 1.8 A × 3 Ω = 5.4 V.
Check: 3.6 V + 5.4 V = 9 V, which matches the battery.
Worked example 3 – Parallel resistors
Problem: A 6 V source is connected to two parallel resistors, 6 Ω and 3 Ω. Find the total current drawn from the battery.
Step 1: Find the equivalent resistance:
1/R_total = 1/6 + 1/3 = 1/6 + 2/6 = 3/6 → R_total = 2 Ω.
Step 2: Total current I_total = V ÷ R_total = 6 V ÷ 2 Ω = 3 A.
Step 3: Current through each branch (optional): I₁ = V ÷ R₁ = 6 V ÷ 6 Ω = 1 A, I₂ = 6 V ÷ 3 Ω = 2 A. They add up to 3 A, confirming the answer.
Series vs Parallel – Quick Comparison
| Feature | Series | Parallel |
|---|---|---|
| Current | Same through all components | Splits among branches |
| Voltage | Divides according to resistance | Same across each branch |
| Total Resistance | Additive (R₁+R₂+…) | Reciprocal sum (1/R_total = 1/R₁+1/R₂+…) |
| Effect of a broken link | Whole circuit stops | Only the broken branch stops |
📝 Likely Exam Questions
- Q1: A 15 V battery is connected to three resistors in series: 2 Ω, 3 Ω, and 5 Ω. Find the current flowing through the circuit.
Answer: R_total = 10 Ω, I = 15 V ÷ 10 Ω = 1.5 A. - Q2: Two resistors, 4 Ω and 6 Ω, are connected in parallel across a 12 V source. Calculate the total current supplied by the battery.
Answer: 1/R_total = 1/4 + 1/6 = 5/12 → R_total = 12/5 Ω = 2.4 Ω. I_total = 12 V ÷ 2.4 Ω = 5 A. - Q3: In a series circuit, the current is 2 A and the total resistance is 8 Ω. What is the battery voltage?
Answer: V = I × R = 2 A × 8 Ω = 16 V. - Q4: A resistor of 10 Ω carries a current of 0.5 A. Determine the voltage across it and state whether it follows Ohm's Law.
Answer: V = I × R = 0.5 A × 10 Ω = 5 V. Since V, I, and R satisfy V = I R, it obeys Ohm's Law. - Q5: Explain why the total resistance of parallel resistors is always less than the smallest individual resistance.
Answer: Adding another path lets more charge flow for the same voltage, so the equivalent resistance drops. Mathematically, 1/R_total = sum of reciprocals, making R_total smaller than any single R.