IGCSE Physics (Extended) - Unit 4: Electricity (Full Cheat Sheet until Series and Parallel Resistance)

📑 TABLE OF CONTENTS

1. Key Definitions

2. Important Conversions

3. Electricity Formulas

4. Current, Voltage, Resistance

5. Series and Parallel Circuits

6. Electric Charge and Energy

7. Model Questions & Answers with Keywords

8. Tips to Gain Marks

📘 Key Definitions

Important Conversions

Electricity Formulas

Current, Voltage, Resistance

🔹 Current (I)

• Flows in a closed circuit.

• Measured using an ammeter in series.

• Units: Amperes (A)

🔹 Voltage (V)

• Energy transferred per unit charge.

• Measured using a voltmeter in parallel.

• Units: Volts (V)

🔹 Resistance (R)

• Slows down current.

• Higher resistance = smaller current (for same voltage).

💡 Ohm’s Law: V = I × R

Current ∝ Voltage if Resistance is constant.

Series and Parallel Circuits

Series Circuit

Parallel Circuit

Electric Charge and Energy

Charge (Q)

• Formula: Q = I × t

• Measured in Coulombs (C)

• 1 Coulomb = charge transferred by 1 A in 1 second

Energy (E)

• Formula: E = Q × V or E = I × V × t

• Measured in Joules (J)

📘 Model Questions & Answers with Keywords

🔸 Q1: Define electric current.

✅ Answer (with keywords):

Electric current is the rate of flow of charge in a circuit, measured in amperes (A).

🔸 Q2: A current of 2 A flows for 30 seconds. Calculate the charge.

Formula: Q = I × t

✅ Working: Q = 2 × 30 = 60 C

✅ Answer: 60 Coulombs

✅ Keywords: charge, current, time, Coulombs

🔸 Q3: Explain why a voltmeter must be connected in parallel.

✅ Answer:

A voltmeter must be connected in parallel so that it measures the potential difference across a component without changing the current in the circuit.

🔸 Q4: Two resistors of 4Ω and 6Ω are connected in series. Find total resistance.

✅ Working:

Rtotal = R₁ + R₂ = 4 + 6 = 10Ω

🔸 Q5: Two resistors of 6Ω and 3Ω are connected in parallel.

Formula: 1/R = 1/R₁ + 1/R₂

✅ Working:

1/R = 1/6 + 1/3

= (1 + 2)/6 = 3/6 = 1/2

R = 2Ω

🔸 Q6: A 12V battery supplies 2A of current. What is the power output?

Formula: P = V × I

✅ Working: P = 12 × 2 = 24W

✅ Answer: 24 watts

✅ Keywords: power, voltage, current

🔸 Q7: A device uses 5A for 60s at 6V. How much energy does it use?

Formula: E = I × V × t

✅ Working: 5 × 6 × 60 = 1800 J

✅ Answer: 1800 Joules

🎯 Tips to Gain Marks

• ALWAYS use correct units (A, V, Ω, J, C, W, s)

• Show formula first, then working, then answer with units

• Use correct circuit symbols if drawing:

  • Cell: |‾|

  • Resistor: ─///\─

  • Voltmeter: Ⓥ

  • Ammeter: Ⓐ

• Label circuit diagrams clearly

• Include direction of current flow

🔁 Visual Summary Table

✅ Final Things to Memorise

✅ All formulas in the table above

✅ Definitions with keywords

✅ Circuit rules for series and parallel

✅ Unit conversions (mA to A, etc.)

✅ Power and energy formulas

✅ How to lay out full working in 3 steps:

1. Formula

2. Substitution

3. Final answer with unit