Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Define electric current, voltage, and resistance.
• Explain the relationship between them using Ohm's Law.
• Construct simple series and parallel circuits.
• Identify and draw common circuit components.
• Understand and apply electrical safety rules.

Lesson summary

Electricity is a fundamental part of modern life, especially in South Africa. From keeping our homes lit and our cell phones charged to powering our industries and hospitals, electricity is essential for our daily activities. Understanding how electricity works, how circuits are built, and how to use electricity safely is crucial for every South African. Unfortunately, many communities in South Africa still lack reliable access to electricity, highlighting the importance of understanding and potentially contributing to solutions for sustainable and accessible power in the future.

Lesson notes

2.1 Electric Charge: All matter is made up of atoms, which contain positively charged protons, negatively charged electrons, and neutral neutrons. Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electromagnetic field. We usually deal with the movement of electrons because they are more mobile in most materials than protons. 2.2 Electric Current (I): Electric current is the rate of flow of electric charge. It is the amount of charge that passes a given point in a circuit per unit of time.

Definition: The amount of charge (Q) flowing past a point per second.

Formula: I = Q / t (where I is current, Q is charge, and t is time)

Unit: Ampere (A). 1 Ampere = 1 Coulomb / 1 second (1 A = 1 C/s)

Conventional Current: We use "conventional current" which describes the flow of positive charge, moving from the positive terminal to the negative terminal of the battery, even though it is actually electrons (negative charge) that are moving.

Example: If 60 Coulombs of charge flow past a point in a wire in 2 seconds, what is the current?

Solution: I = Q / t = 60 C / 2 s = 30

A. The current is 30 Amperes. 2.3 Voltage (V) or Potential Difference: Voltage, also known as potential difference, is the electrical potential energy difference between two points in a circuit. It's the "push" that drives the electric current through the circuit. Think of it like the pressure in a pipe that pushes water through.

Definition: The energy required to move a unit charge between two points.

Unit: Volt (V)

Analogy: Like water pressure in a pipe – higher voltage means a stronger "push" on the electrons. 2.4 Resistance (R): Resistance is the opposition to the flow of electric current. It is a property of a material that hinders the movement of electric charges. Different materials have different resistances. A high resistance means it's harder for current to flow.

Definition: A measure of how difficult it is for current to flow through a material.

Unit: Ohm (Ω)

Analogy: Like a narrow section in a pipe that restricts the flow of water – higher resistance means less current can flow. 2.5 Ohm's Law: Ohm's Law describes the relationship between voltage, current, and resistance in a circuit: Formula: V = I R (Voltage = Current * Resistance)

Explanation: This means that: If you increase the voltage, the current will increase (if resistance stays the same). If you increase the resistance, the current will decrease (if voltage stays the same).

Example 1: A resistor has a resistance of 10 Ohms and a current of 2 Amperes is flowing through it. What is the voltage across the resistor?

Solution: V = I R = 2 A 10 Ω = 20

V. The voltage across the resistor is 20 Volts.

Example 2: A light bulb has a resistance of 4 Ohms, and is connected to a 1.5 V battery. What is the current flowing through the light bulb?

Solution: Rearrange Ohm's Law to solve for I: I = V / R = 1.5 V / 4 Ω = 0.375

A. The current flowing through the light bulb is 0.375 Amperes. 2.6 Series Circuits: In a series circuit, components are connected one after the other along a single path. The current is the same at all points in a series circuit.

Characteristics: Only one path for current to flow. If one component breaks, the entire circuit stops working. The total resistance is the sum of individual resistances: R total = R 1 + R 2 + R 3 + ... The voltage of the battery is divided across each component. V total = V 1 + V 2 + V 3 +...

Example: Three resistors of 2 Ohms, 3 Ohms, and 5 Ohms are connected in series to a 10 V battery. What is the total resistance and the current flowing through the circuit?

Solution: Total Resistance: R total = 2 Ω + 3 Ω + 5 Ω = 10 Ω Current: I = V / R = 10 V / 10 Ω = 1 A 2.7 Parallel Circuits: In a parallel circuit, components are connected along multiple paths. The voltage is the same across all components in a parallel circuit.

Characteristics: Multiple paths for current to flow. If one component breaks, the other components still work.

The total resistance is calculated as: 1/R total = 1/R 1 + 1/R 2 + 1/R 3 + ... The current from the battery is divided between each component. I total = I 1 + I 2 + I 3 +...

Example: Two resistors of 4 Ohms and 6 Ohms are connected in parallel to a 12V battery. What is the total resistance and the total current flowing from the battery?

Solution: Total Resistance: 1/R total = 1/4 Ω + 1/6 Ω = 3/12 + 2/12 = 5/12 R total = 12/5 = 2.4 Ω Total Current: I = V / R = 12 V / 2.4 Ω = 5 A 2.8 Circuit Diagrams: Circuit diagrams use symbols to represent different electrical components. It is important to be able to identify each of the symbols: Battery: A long line and a short line, often repeated in pairs.

Resistor: A zig-zag line.

Bulb: A circle with a cross in it.

Switch: A broken line that can be connected or disconnected.

Ammeter: A circle with an "A" in it (used to measure current). Always connected in series.

Voltmeter: A circle with a "V" in it (used to measure voltage).