Why Ohm's Law matters in everyday life
Ever wondered why a flashlight dims when you add more bulbs? That’s Ohm's Law at work.
Ohm's Law tells us how voltage, current and resistance are linked. If you know any two, you can find the third – just like solving a puzzle.
What is Ohm's Law?
Ohm's Law is a rule that says the voltage across a conductor equals the current flowing through it multiplied by its resistance.
Key symbols
- V – voltage (measured in volts, V)
- I – current (measured in amperes, A)
- R – resistance (measured in ohms, Ω)
Ohm's Law formula
The compact form is V = I R. It reads “voltage equals current times resistance”.
How to rearrange the formula
- To find current: I = V / R
- To find resistance: R = V / I
Simple circuit problems – step‑by‑step
Step 1: Read the question carefully
Spot what’s given – voltage, current or resistance – and what the question asks for.
Step 2: Write down the known values
Put them in the same units. If a resistance is given in kilo‑ohms, turn it into ohms by multiplying by 1,000.
Step 3: Pick the right version of Ohm's Law
Use V = I R, I = V / R or R = V / I depending on the unknown.
Step 4: Solve and keep track of units
Do the arithmetic, then attach the correct unit (V, A or Ω).
Worked example 1
Question: A 9 V battery is connected to a resistor of 3 Ω. Find the current flowing through the circuit.
Solution: We know V = 9 V and R = 3 Ω. Use I = V / R.
I = 9 V / 3 Ω = 3 A. So the current is 3 amperes.
Worked example 2 – a series circuit
Question: Two resistors, 2 Ω and 4 Ω, are placed in series across a 12 V supply. What is the total resistance and the current?
Solution: In series, resistances add up: R_total = 2 Ω + 4 Ω = 6 Ω.
Now use I = V / R_total = 12 V / 6 Ω = 2 A.
Each resistor gets the same 2 A, but the voltage across each differs (V = I R).
Quick comparison 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 |
Bullet summary
- Voltage pushes electrons, like water pressure in a pipe.
- Current is the flow of electrons, similar to water flow.
- Resistance opposes flow, like a narrow section of pipe.
- Ohm's Law ties them together: V = I R.
- Remember to keep units consistent.
📝 Likely Exam Questions
- State Ohm's Law and write its formula.
Answer: Voltage across a conductor equals the product of current and resistance; V = I R. - A 6 V battery is connected to a resistor of 12 Ω. Calculate the current.
Answer: I = V / R = 6 V / 12 Ω = 0.5 A. - Two resistors of 5 Ω and 10 Ω are in series across a 30 V source. Find the total resistance and the current.
Answer: R_total = 5 Ω + 10 Ω = 15 Ω; I = 30 V / 15 Ω = 2 A. - Explain how you would solve a problem where the voltage and current are given, but resistance is unknown.
Answer: Use R = V / I, substitute the known values, compute, and write the result in ohms.