Lower 6 Cameroon A-Level Physics: Electricity (Extremely Detailed Notes)

1. Introduction

Electricity is the branch of physics concerned with electric charges, their behavior, and energy transfer in circuits. It forms a major part of the Cameroon Advanced Level Physics syllabus (Lower 6).

Main areas include:

2. Electric Charge

Electric charge is a fundamental property of matter. Charges can be:

Unit of charge: Coulomb (C)

Q = n × e
where Q = charge (C), n = number of electrons, e = elementary charge (1.6 × 10⁻¹⁹ C)

Key points:

3. Electric Current

Current is the rate of flow of charge through a conductor.

I = Q / t
I = current (A), Q = charge (C), t = time (s)

Types of current:

Drift velocity:

I = n × A × v_d × e
where n = number density of electrons, A = cross-sectional area, v_d = drift velocity, e = electron charge

4. Potential Difference (Voltage) and EMF

Potential difference (V) is the work done per unit charge to move a charge between two points:

V = W / Q
W = work done (J), Q = charge (C)

Electromotive force (EMF, E) is the maximum potential difference of a source when no current flows.

E = W / Q

Terminal voltage (V_terminal) of a cell taking internal resistance (r) into account:

V_terminal = E - I × r

5. Resistance

Resistance is the opposition to current flow in a conductor.

R = V / I

Factors affecting resistance:

R = ρ × (L / A)
ρ = resistivity (Ω·m), L = length (m), A = cross-section (m²)

6. Ohm's Law

Ohm’s law: The current through a conductor is directly proportional to the potential difference across it, provided temperature is constant.

V = I × R

Graphical representation: Linear V-I graph for ohmic conductors; nonlinear for non-ohmic.

7. Series and Parallel Circuits

7.1 Series Circuits

7.2 Parallel Circuits

8. Kirchhoff's Laws

9. Electrical Power and Energy

P = V × I = I² × R = V² / R
E = P × t = V × I × t

Units: Power in watts (W), Energy in joules (J)

10. Internal Resistance of a Cell

No real cell is ideal. The internal resistance reduces the voltage available to the circuit.

V_terminal = E - I × r

Key points:

11. Capacitors

Capacitance is the ability to store charge:

C = Q / V

Energy stored in capacitor:

E = 1/2 × C × V²

Series and parallel combination:

12. Detailed Example Problems

Example 1: A 12 V battery supplies a current of 3 A to a resistor of 4 Ω. Find the power dissipated.

Solution: P = I² × R = 3² × 4 = 36 W

Example 2: A cell of EMF 1.5 V and internal resistance 0.5 Ω is connected to a resistor of 4.5 Ω. Find current and terminal voltage.

Solution:

I = E / (R + r) = 1.5 / (4.5 + 0.5) = 0.3 A

V_terminal = E - I × r = 1.5 - 0.3 × 0.5 = 1.35 V

13. Diagram Placeholders

14. Important Tips for Exams