Electric Charges and Current Electricity — Lesson Notes + JAMB CBT Practice (40 Questions) | Edwin Ogie Library

Physics • Lesson Notes + CBT Practice

Electric Charges and Current Electricity

A complete, exam-focused lesson on electric charge, conductors and insulators, circuits, Ohm's law, electrical power, and resistor arrangements — with worked examples and a 40-question JAMB CBT-format exam (15 minutes).

1. Electric Charges

Electric charge is a fundamental property of matter. A body becomes charged when electrons move between it and another body — a charged object gains or loses electrons, and this imbalance is what we call static electricity.

Types of Electric Charges

Positive charge (+): the body has lost electrons.
Negative charge (−): the body has gained electrons.

Like charges repel each other; unlike charges attract.

How Charges Are Produced

Friction: rubbing two different insulators together transfers electrons from one to the other (e.g., a plastic comb rubbed through dry hair).
Induction: charging a neutral object without touching it, by bringing a charged object close and then earthing the object.
Contact (conduction): charge flows directly when a charged body touches a neutral one.

2. Conductors and Insulators

Conductors (allow charge to flow)	Insulators (block charge flow)
Metals (copper, aluminium, iron)	Plastic and rubber
Graphite	Glass and dry wood
Salt solutions (electrolytes)	Silk and dry air
The human body	Ceramics and paper

3. The Electroscope

An electroscope detects and tests small electric charges. Its main parts are:

a thin gold or aluminium leaf attached to a metal rod;
a brass cap or disc at the top;
an insulated case that shields the leaf from air currents and external charge.

When a charged object touches the cap, charge flows down the rod and the leaf diverges — the greater the charge, the wider the divergence.

4. Lightning and Lightning Conductors

Lightning is a massive discharge of static electricity between a cloud and the ground (or between clouds). A lightning conductor is a metal rod mounted on a building, connected by a thick cable to a metal plate buried in the ground. It provides a safe, low-resistance path for the discharge, protecting the structure from damage.

5. Electric Circuits

An electric circuit is a closed path along which current flows. Every complete circuit needs:

a source of electrical energy (battery, cell, generator);
conducting wires;
a load that converts electrical energy (bulb, resistor, motor);
a switch to open or close the path.

Current flows only when the circuit is closed; an open switch stops the flow.

6. Producing Electric Current

Electric current is produced by converting other forms of energy:

Chemical energy — cells and batteries.
Heat energy — thermocouples.
Mechanical energy — generators and dynamos.
Solar (light) energy — solar panels/photocells.

Defects of a Simple Cell

Polarization: hydrogen bubbles collect on the copper plate, opposing the current and weakening it.
Local action: impurities in the zinc plate react with the acid, wasting the zinc even when no current flows.

7. Ohm's Law

Ohm's Law states that the current I through a metallic conductor is directly proportional to the potential difference V across its ends, provided temperature and other physical conditions remain constant.

V = I × R

V = voltage, in volts (V)
I = current, in amperes (A)
R = resistance, in ohms (Ω)

8. Electrical Power and Energy

Electrical energy (work done)

W = I × V × t = I²Rt = (V²/R) × t

W = energy/work done, in joules (J)
t = time, in seconds (s)

Electrical power

P = I × V = I²R = V²/R

Power is measured in watts (W), where 1 W = 1 J/s. In the home, energy is billed in kilowatt-hours (kWh): 1 kWh = 3.6 × 10⁶ J.

9. Arrangement of Resistors

Series

R_total = R₁ + R₂ + R₃ + … + Rₙ

The same current flows through every resistor; the voltage is shared among them.

Parallel

1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + … + 1/Rₙ

The same voltage appears across every resistor; the current divides among the branches. For two resistors: R_total = (R₁R₂)/(R₁ + R₂).

10. Worked Examples

Example 1 — Current from Ohm's Law. A 12 V battery is connected across a 6 Ω resistor. Find the current.
I = V/R = 12/6 = 2 A

Example 2 — Energy consumed. An 11 Ω heater is connected to a 220 V supply for 2 s. Find the energy consumed.
W = (V²/R) × t = (220²/11) × 2 = 8 800 J = 8.8 kJ

Example 3 — Resistors in series. Resistors of 4 Ω, 6 Ω and 10 Ω are in series. Find the total resistance.
R_total = 4 + 6 + 10 = 20 Ω

Example 4 — Resistors in parallel. Resistors of 8 Ω and 12 Ω are in parallel. Find the total resistance.
1/R_total = 1/8 + 1/12 = 5/24 → R_total = 24/5 = 4.8 Ω

Example 5 — Power consumed. A device draws 4 A from a 240 V supply. Find its power.
P = IV = 4 × 240 = 960 W

JAMB CBT Practice Exam — Electric Charges & Current Electricity

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40 questions 15 minutes JAMB CBT format

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