V = I Γ R & Circuit Rules
Intervention Β· Edexcel IGCSE Single Science (FoundationβHigher) Β· Electricity
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V=IROHM'S LAW
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π― Learning Objectives
- Use V = I Γ R to find voltage, current or resistance.
- Describe the current & voltage rules for series and parallel circuits.
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Success Criteria
- I can use the triangle to rearrange V = I Γ R.
- I can calculate a missing value (with units!).
- I can state the current & voltage rule for series and parallel.
Match the symbol to its unit
| Quantity | Correct unit |
|---|---|
| V β Voltage | |
| I β Current | |
| R β Resistance |
πͺ Stuck? "Victor has Volts." Current = amps. Resistance = ohms (Ξ©).
Pick the correct unit, then answer the extra question.
V β Voltage β I β Current β R β Resistance β
The letter I stands for:
Match the unit and the measuring instrument.
Current is measured in using a(n) .
Voltage is measured in using a(n) .
What do you already know?
1. What does the battery provide?
the resistancethe voltage (the push)current made by the wire
2. A higher resistance meansβ¦
current flows more easilyit is harder for current to flowthe voltage disappears
1. The battery provides the , which pushes the current round.
2. If resistance increases, the current .
1. A circuit keeps voltage the same but the resistance doubles. The current will .
2. Best description of resistance: .
π Aspire: "Voltage pushes the current, and resistance slows it down."
The equation: V = I Γ R
We use a triangle to rearrange it. Tap a letter to cover it.
Cover the one you want to findβ¦
π Watch me β worked example
A bulb has I = 2 A and R = 3 Ξ©. Find the voltage V.
Step 1 β Formula. Cover V β V = I Γ R
Step 2 β Numbers in. V = 2 Γ 3
Step 3 β Calculate. V = 6 V (don't forget the unit!)
π Exam Tip: write the formula first, then the numbers, then the unit.
Now we try together
βοΈ Find the current
βοΈ Find the resistance
Heater: V = 12 V, R = 4 Ξ©. Cover I β I = V Γ· R
I = 12 Γ· 4 = A
12 shared into 4 equal parts.
βοΈ Find the resistance
Lamp: V = 9 V, I = 3 A. Cover R β R = V Γ· I
R = 9 Γ· 3 = Ξ©
π£οΈ Turn & Talk: how did you know whether to multiply or divide?
Circuit rules: series vs parallel
Watch the orange flow β that's the current moving round.
π Series (one loop)
Current: the SAME everywhere π
Voltage: SHARED across components (adds up)
Resistance: ADD them β R = Rβ + Rβ
πͺ’ Parallel (branches)
Current: SHARED between branches (splits)
Voltage: the SAME across each branch π
Resistance: total goes DOWN (more paths)
π« Common misconception
"Current gets used up as it goes round the circuit."
Apply the rules together
βοΈ Series β total resistance, then current
Step 1: series β add resistances. R = 2 + 4 = Ξ©
Step 2: I = V Γ· R = 12 Γ· 6 = A
Use your Step 1 answer (6 Ξ©) as R.
π£οΈ Turn & Talk: in parallel, what stays the same across each branch?
Independent task
Questions completed: 0
Calculate1. Complete the table (V = I Γ R)
| V | I | R |
|---|---|---|
| 2 | 5 | |
| 20 | 4 | |
| 18 | 3 |
Cover the missing letter on the triangle.
Diagram2. Label the circuit
A: B: C:
Calculate3. Series circuit 3 Ξ© + 5 Ξ© + 2 Ξ©. Total R = Ξ©
Apply4. (a) Series, ammeter reads 2 A at battery β further round it reads
(b) Parallel, 6 V battery β each branch is
Calculate1. Complete the table (V = I Γ R)
| V | I | R |
|---|---|---|
| 3 | 6 | |
| 24 | 4 | |
| 30 | 5 |
Calculate2. Series 5 Ξ© + 5 Ξ© + 2 Ξ© with a 24 V supply. Total R = Ξ©, so current I = A
Add the resistances first, then I = V Γ· R.
Apply3. In a parallel circuit the current from the battery is
Calculate1. A 230 V supply drives a current of 5 A. The resistance is Ξ©
R = V Γ· I = 230 Γ· 5.
Calculate2. Two 6 Ξ© resistors in series share a 24 V supply. Total R = Ξ©, current = A, and the voltage across ONE resistor = V
Equal resistors share the voltage equally β 24 Γ· 2.
β Reason3. Why does adding a second bulb in parallel (not series) keep both bulbs bright?
Show me what stuck
1. V = 10 V, R = 2 Ξ©. Find I. I = V Γ· R = 10 Γ· 2 = 5 A
2. In a series circuit, the current is⦠the same everywhere
3. In a parallel circuit, the voltage across each branch is⦠the same across each branch
1. V = 18 V, I = 3 A. Find R. R = V Γ· I = 18 Γ· 3 = 6 Ξ©
2. In series, two equal bulbs share a 12 V supply. Each gets⦠6 V (the voltage is shared equally)
3. Total resistance of 4 Ξ© + 6 Ξ© in series? 10 Ξ© (resistances add)
1. A 12 V supply, total resistance 8 Ξ©. Find the current. I = V Γ· R = 12 Γ· 8 = 1.5 A
2. Explain why current is the same everywhere in a series circuit. There is only one path, so the same charge flows through every component each second β nothing is used up.
3. Two resistors in parallel across 6 V. What is the voltage across each? 6 V β each branch gets the full supply voltage.