Physics Lesson 2b

Voltage, Current & Resistance

Ohm's Law in Action — PhET Practical 🔬

🔋 ⚡ 💡

Today's Focus (Spec 2.13 & 2.14):

✓

Apply V = I × R to real circuits — by building them.

✓

Measure voltage and current using ammeters & voltmeters.

✓

Calculate resistance from your own readings.

📌 Why this lesson? Last week we got almost to the practice — today we finish it. Same spec points, but you'll be building the circuits yourself using a free online lab.

🖨️ Teacher Print Tools — Pupil Packs

⏱️ 3 min

Arrival Task

Quick Recall — From Last Week

Instructions: 3 quick questions. One word or one number is fine. Answer in your books. Go!

1. Units 🟢

What unit is voltage measured in?

Volts (V).

2. Formula 🟢

Fill the gap: V = I × ___

R (resistance).

3. Equipment 🟢

What does an ammeter measure?

Current (in amps).

1. Units 🟠

Match: V → ?, I → ?, R → ?

V → volts, I → amps, R → ohms (Ω).

2. Quick Calc 🟠

I = 2 A, R = 3 Ω. What is V?

V = I × R = 2 × 3 = 6 V

3. Equipment 🟠

Where in a circuit do you put a voltmeter?

In parallel — across the component.

1. Definition 🔵

Current is the rate of flow of ___?

Charge (carried by electrons).

2. Quick Calc 🔵

V = 12 V, I = 4 A. Find R.

R = V ÷ I = 12 ÷ 4 = 3 Ω

3. Reasoning 🔵

If R doubles but V stays the same, what happens to I?

I halves (because I = V ÷ R, larger R = smaller I).

⏱️ 3 min

Starter

Starter: The VIR Triangle — All 3 Formulas

Instructions: Look at the triangle. Write down the three formulas it gives you. One minute. Go!

The VIR Triangle

Cover what you want to find — the rest is your formula.

📝 In your books, write:

1. Cover V → V = ___

2. Cover I → I = ___

3. Cover R → R = ___

If they're side-by-side, you multiply. If one is on top, you divide.

Quick Check

If you wrote V = I × R, I = V ÷ R, R = V ÷ I — give yourself a tick. ✅

V = I × R | I = V ÷ R | R = V ÷ I

⏱️ 3 min

I Do

I Do: The Quick Recap (3 minutes — eyes up!)

Just listening: No need to write. We're recapping last week so you can apply it today.

⚡ The Three Quantities

V — Volts the "push" — energy per coulomb of charge.

I — Amps the rate of flow of charge.

R — Ohms (Ω) what slows the flow.

V = I × R

✏️ Worked Example (1 min)

Q: A bulb has I = 0.5 A and R = 12 Ω. Find V.

1. Formula: V = I × R

2. Substitute: V = 0.5 × 12

3. Answer: V = 6 V ✅

🔑 Three Things You'll Need Today

✓ Read voltmeter (V across component)

✓ Read ammeter (I in series)

✓ Use V = I × R (or rearrange) to find what's missing

📋 Always Show:

Formula → Substitute → Answer + Unit. Even if you make a calc mistake, you still get marks for the formula and substitution.

🌟 Today's twist: You won't be given imaginary numbers — you'll read your own from a circuit you build.

⏱️ 3 min

We Do

We Do: How to Build a Circuit in PhET

Together: 3 steps to build your first circuit. Open the sim on your device. Pairs only — share a screen.

🔧 Build Steps

1. Drag a battery onto the workspace.

2. Drag a light bulb next to it.

3. Drag wires to connect them in a loop. Bulb should light up! 💡

4. Drag an ammeter — break the wire and put it in the loop (in series).

5. Drag a voltmeter — place its two leads on either side of the bulb (in parallel).

🚀 Open PhET Sim in New Tab

⚠️ Sim won't load? Click the launch button above to open in a new tab. You can also use a phone — works on any device.

🎯 Your First Reading (try now)

Set battery to 10 V. Add one bulb. Read your meters and type the values:

Voltmeter (across bulb): V

Ammeter (in series): A

Voltmeter has two probes — one each side of the bulb. Ammeter breaks the wire (sits in line with it).

🔮 Predict

If you doubled the battery to 20 V, the current would:

Where does the ammeter go?

⏱️ 20 min — INDEPENDENT

Independent Practical

Independent Practical: Build, Read, Calculate

Instructions: Pick your level. Build the circuit in PhET. Read the meters. Type your numbers and calculations into the boxes below. When you're done, hit Submit for marking + a personalised report.

📝 Your name:

🚀 Launch Sim

Sim by PhET Interactive Simulations, University of Colorado Boulder. Free to use under CC-BY licence.

🟢 Foundation Task

Build: 1 battery + 1 bulb + ammeter (in series) + voltmeter (across the bulb).

Step 1: Set battery to 9 V. Read meters.

Step 2: Set battery to 4.5 V. Read meters again.

Step 3: Use R = V ÷ I to calculate the resistance.

Battery	V (volts)	I (amps)	R = V÷I (Ω)
9 V
4.5 V

🌟 Quick Q: Did the resistance stay roughly the same? (It should — the bulb's R doesn't change much.)

⚡ Calculations (use your readings)

Q1. If V = 6 V and I = 2 A, what is R? Ω

Q2. If I = 3 A and R = 4 Ω, what is V? V

Q3. If V = 10 V and R = 5 Ω, what is I? A

✅ When you finish: Put your hand up. Mr Roper will check your readings and you'll do the exit ticket.

⏱️ 3 min

Exit Ticket

Exit Ticket — Show What You Know

Instructions: No notes. No sim. Just you. Answer in your books or on the printed exit slip.

Tap how confident you feel — this prints onto your evidence report.

1) What is current the rate of flow of?

Charge.

2) Write the formula linking voltage, current and resistance.

V = I × R

3) A 4 A current flows through a 3 Ω resistor. Calculate V.

V = I × R = 4 × 3 = 12 V

⚡ Next lesson: Voltage across components in parallel circuits — and why your house wiring is never in series.

Knowledge Organiser: Ohm's Law (Recap)

Use this: Read it before starting. Refer back during the practical. Cover it for the exit ticket.

Concept	Details
Voltage (V)	The "push" — energy per coulomb. Volts (V). Voltmeter — in parallel.
Current (I)	Rate of flow of charge. Amps (A). Ammeter — in series.
Resistance (R)	Slows the flow. Ohms (Ω).
Ohm's Law	V = I × R
Rearranged	I = V ÷ R R = V ÷ I
VIR Triangle	V on top, I × R on bottom. Cover what you need.
1 Amp	1 coulomb of charge per second.
UK Mains	230 V.

🧠 Memory Tricks

Finding I or R? You DIVIDE.

Finding V? You MULTIPLY.

Always: Formula → Substitute → Answer + UNIT.

Arrival – Foundation 🟢

1. What unit is voltage measured in?

2. Fill the gap: V = I × ___

3. What does an ammeter measure?

Arrival – Middle 🟠

1. Match: V → ?, I → ?, R → ?

2. I = 2 A, R = 3 Ω. What is V?

3. Where in a circuit do you put a voltmeter?

Arrival – Higher 🔵

1. Current is the rate of flow of ___?

2. V = 12 V, I = 4 A. Find R.

3. If R doubles but V stays the same, what happens to I?

Starter – VIR Triangle

Look at the VIR triangle. Write the three formulas.

1. Cover V → V = ____________

2. Cover I → I = ____________

3. Cover R → R = ____________

Independent Practical – Foundation 🟢

Aim: Build a circuit. Read the meters. Calculate the resistance.

Setup

Build: 1 battery + 1 light bulb + 1 ammeter (in series) + 1 voltmeter (across the bulb).

Section A: Take Readings

Battery setting	V (volts)	I (amps)	R = V ÷ I (Ω)
9 V
4.5 V

Did the resistance stay roughly the same?

Section B: Calculations (show your working)

1. If V = 6 V and I = 2 A, what is R?

2. If I = 3 A and R = 4 Ω, what is V?

3. If V = 10 V and R = 5 Ω, what is I?

4. (Stretch) A bulb with R = 6 Ω is in a 12 V circuit. What is the current?

Section C: Quick Check

5. What does an ammeter measure, and where do you put it in a circuit?

Independent Practical – Middle 🟠

Aim: Investigate how current changes when you change the voltage.

Setup

Build: 1 battery (variable) + 1 resistor (set to ~10 Ω) + ammeter (in series) + voltmeter (across the resistor).

Section A: Investigation

V (set on battery)	I (read meter, A)	R = V ÷ I (Ω)
3 V
6 V
9 V
12 V

1. What happens to I when you double V?

2. Did your calculated R stay roughly the same? Why does that make sense?

Section B: Calculations

3. A 9 V battery is connected to a 3 Ω resistor. Calculate I.

4. V = 12 V, I = 0.5 A. Find R.

5. A lamp has R = 15 Ω and I = 0.4 A. Find V.

6. (Grade 3 stretch) Same lamp, V doubles to 12 V. New current?

Independent Practical – Higher 🔵

Aim: Investigate what happens to current and voltage when you add resistors in series.

Setup

Build: battery (12 V) + 2 resistors in series (R₁ ≈ 10 Ω, R₂ ≈ 20 Ω) + ammeter + voltmeter (move it to read across each resistor in turn).

Section A: Investigation

Setup	V total	V across R₁	V across R₂	I (anywhere)
R₁ alone (12 V battery)			—
R₁ + R₂ in series (12 V)

1. What happened to the current when you added R₂?

2. Do V₁ + V₂ add up to V total? (They should.)

3. Explain in terms of charge flow why current is the same everywhere in a series circuit. (Grade 5)

Section B: Exam-Style Calculations

4. (3 marks) A hairdryer runs on 230 V mains and draws 10 A. Calculate its resistance.

5. (3 marks) Two resistors (10 Ω + 20 Ω) in series with a 6 V battery. Calculate the current.

6. (Grade 5 — 4 marks) A resistor: V = 8 V, I = 0.4 A. The voltage is increased to 12 V (resistor unchanged). Calculate the new current. Show your reasoning.

Exit Ticket – Foundation

Name: _________________________ Date: _____________

1) What is current the rate of flow of?

2) Write the formula linking voltage, current and resistance.

3) A 4 A current flows through a 3 Ω resistor. Calculate V.

Confidence: 🟢 / 🟡 / 🔴 (circle one)

Exit Ticket – Middle

Name: _________________________ Date: _____________

1) A 9 V battery is connected to a 3 Ω resistor. Calculate I.

2) What does "1 amp" mean in terms of charge?

3) A lamp: R = 15 Ω, I = 0.4 A. Find V.

Confidence: 🟢 / 🟡 / 🔴

Exit Ticket – Higher

Name: _________________________ Date: _____________

1) A hairdryer on 230 V mains draws 10 A. Find R.

2) Explain why increasing voltage across a fixed resistor increases the current. Use "charge" or "electrons".

3) Two resistors (10 Ω + 20 Ω) in series with a 6 V battery. Find I.

Confidence: 🟢 / 🟡 / 🔴

⚡ Evidence Report — Ohm's Law Practical

Progress Schools • Lesson 2b • IGCSE Physics Spec 2.13 & 2.14 (V = I × R)

Pupil: —

Tier: —

Date: —

Teacher: Mr Roper

Final score: —

Question-by-question breakdown

Question	Pupil's answer	Correct ans.	Mark	Diagnosis

👁️ TA Brief — This Slide

🎯 Random Pupil Picker

💡 Use the Triangle

📊 Marking Complete!

✅ Question Breakdown

🎯 Things to Watch For

🚀 Your Next Step

Lesson Complete!

Voltage, Current & Resistance

🖨️ Teacher Print Tools — Pupil Packs

Quick Recall — From Last Week

1. Units 🟢

2. Formula 🟢

3. Equipment 🟢

1. Units 🟠

2. Quick Calc 🟠

3. Equipment 🟠

1. Definition 🔵

2. Quick Calc 🔵

3. Reasoning 🔵

Starter: The VIR Triangle — All 3 Formulas

The VIR Triangle

📝 In your books, write:

Quick Check

I Do: The Quick Recap (3 minutes — eyes up!)

⚡ The Three Quantities

✏️ Worked Example (1 min)

🔑 Three Things You'll Need Today

📋 Always Show:

We Do: How to Build a Circuit in PhET

🔧 Build Steps

🎯 Your First Reading (try now)

🔮 Predict

Independent Practical: Build, Read, Calculate

🟢 Foundation Task

⚡ Calculations (use your readings)

🟠 Middle Task — Investigate V vs I

Pattern Check

⚡ Calculations

🔵 Higher Task — Series Resistors

Reasoning

⚡ Exam-Style Calculations

Exit Ticket — Show What You Know

Knowledge Organiser: Ohm's Law (Recap)

🧠 Memory Tricks

Arrival – Foundation 🟢

Arrival – Middle 🟠

Arrival – Higher 🔵

Starter – VIR Triangle

Independent Practical – Foundation 🟢

Setup

Section A: Take Readings

Section B: Calculations (show your working)

Section C: Quick Check

Independent Practical – Middle 🟠

Setup

Section A: Investigation

Section B: Calculations

Independent Practical – Higher 🔵

Setup

Section A: Investigation

Section B: Exam-Style Calculations

Exit Ticket – Foundation

Exit Ticket – Middle

Exit Ticket – Higher

⚡ Evidence Report — Ohm's Law Practical

Question-by-question breakdown

🎯 Misconceptions identified

🚀 Your most important next step

👨‍🏫 Teacher comment