Physics - Electricity

⚡ Electricity Study Notes (with Practice + Answers)

🔌 Core Concepts

• Coulomb: A coulomb is a very large number of electrons.

• Voltage (V): The amount of energy each coulomb has.

  • 1 Volt = 1 Joule per Coulomb

  • e.g. V = 12.0V means 12.0 Joules of energy per coulomb.

• Current (I): The rate at which charge flows. Measured in Amps (A) using an ammeter.

  • 1 Amp = 1 Coulomb per second

  • e.g. I = 4.0A means 4.0 coulombs pass each second.

💧 Ohm’s Law - Water Pipe Analogy

Ohm’s Law can be understood using a water pipe analogy:

• Voltage is like the water pressure pushing the water through the pipe.

• Current is like the amount of water flowing through.

• Resistance is like anything that slows the flow, such as a narrow or blocked section.

This analogy helps explain the relationship between Voltage (V), Current (I), and Resistance (Ω).

💡 More Key Concepts

• Power measures how much energy is converted in the circuit each second.

• Unit of Power = Watt (W)

  • 1 Watt = 1 Joule per second

• Formula:

  Power=Voltage×Current(P=V×I)\text{Power} = \text{Voltage} × \text{Current} \quad (P = V × I)Power=Voltage×Current(P=V×I)

🧲 Conductors vs Insulators

• Conductors = materials that allow electricity to flow

• Insulators = materials that do not allow electricity to flow

Examples:

🧬 Atoms and Charges

• All matter is made of atoms

• Atoms contain:

  • Protons → Positive charge (+1)

  • Electrons → Negative charge (–1)

  • Neutrons → No charge (neutral) but have mass

• Similar charges repel:
  ➖ Repels ➖
  ➕ Repels ➕

• Opposite charges attract:
  ➕ Attracts ➖
  ➖ Attracts ➕

🔁 Law of Conservation of Energy

⚡ Energy in Electrical Systems

• Electrons (negative charge) carry energy through circuits.

• Examples of energy transformations:

  • A motor: electrical → kinetic (motion)

  • A buzzer: electrical → sound

  • A battery: stores and supplies electrical energy

🔢 Key Formulas

1. Power (W) = Voltage (V) × Current (A)

2. Voltage (V) = Current (A) × Resistance (Ω)

3. Current (A) = Voltage (V) ÷ Resistance (Ω)

4. Resistance (Ω) = Voltage (V) ÷ Current (A)

5. Energy (J) = Power (W) × Time (s)

🧠 Ohm’s Law Practice

Q1: Voltage = 6V, Current = 2A → Resistance?
A1: R = 6 ÷ 2 = 3Ω

Q2: Voltage = 5V, Resistance = 10Ω → Current?
A2: I = 5 ÷ 10 = 0.5A

Q3: Voltage = 3V, Current = 0.3A → Resistance?
A3: R = 3 ÷ 0.3 = 10Ω

Q4: Voltage = 10V, Resistance = 5Ω → Current?
A4: I = 10 ÷ 5 = 2A

🔋 Power Practice

Q5: V = 5V, I = 3A → P = ?
A5: P = 5 × 3 = 15W

Q6: V = 6V, I = 3A → P = ?
A6: P = 6 × 3 = 18W

Q7: V = 10V, I = 3A → P = ?
A7: P = 10 × 3 = 30W

Q8: V = 12.0V, I = 2.0A → P = ?
A8: P = 12.0 × 2.0 = 24W

Q9: V = 24.0V, I = 4.0A → P = ?
A9: P = 24.0 × 4.0 = 96W

Q10: V = 5.0V, I = 0.2A → P = ?
A10: P = 5.0 × 0.2 = 1.0W

⏱ Energy Practice

Use:

Energy (J)=Power (W)×Time (s)\text{Energy (J)} = \text{Power (W)} × \text{Time (s)}Energy (J)=Power (W)×Time (s)

Convert time:

• 1 min = 60 sec

• 1 hour = 3600 sec

Q11: Power = 25W, Time = 20 minutes
→ Time = 1200s
→ Energy = 25 × 1200 = 30,000J

Q12: Power = 100W, Time = 3 minutes
→ Time = 180s
→ Energy = 100 × 180 = 18,000J

Q13: Power = 200W, Time = 5 minutes
→ Time = 300s
→ Energy = 200 × 300 = 60,000J

💰 Cost Example: Electric Blanket

• Power: 12W

• Time used: 7:00am – 4:30pm = 9.5 hours

• Time in minutes: 9.5 × 60 = 570 min

• Time in seconds: 570 × 60 = 34,200s

• Energy used = 12 × 34,200 = 410,400 J = 410.4 kJ

• Cost = 410.4 × $0.06 = $24.62

📈 Graphing — Rise Over Run

On a Voltage vs Current graph:

Gradient (Slope)=Rise (Voltage)Run (Current)=Resistance (Ω)\text{Gradient (Slope)} = \frac{\text{Rise (Voltage)}}{\text{Run (Current)}} = \text{Resistance (Ω)}Gradient (Slope)=Run (Current)Rise (Voltage)​=Resistance (Ω)

Example: From (0,0) to (2,1)

• Rise = 1

• Run = 2
  → Gradient = 1 ÷ 2 = 0.5Ω

🧪 Bonus: Resistance Table

✅ Final Reminders

• Voltage = Independent variable

• Current = Dependent variable

• Resistance = Often constant (fixed resistors)

• Power = Energy per second (Watts)

• Energy = Work done (Joules)

• Electrons = Move through circuits, negatively charged

• Conductors = Allow electricity

• Insulators = Block electricity

• Atoms = Contain protons, electrons, neutrons

• Like charges repel, unlike charges attract