Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define electric current (I) and resistance (R), including their units.
• Use Ohm's Law (V=IR) to solve circuit problems.
• Explain the factors affecting resistance in a wire.
• Understand how to measure current in a series circuit.
• Recognise the importance of fuses and circuit breakers.

Lesson summary

Electric circuits are fundamental to how almost all our modern technology works. From the lights in our homes during load shedding to the cell phones we use to communicate, understanding how electricity flows and behaves in a circuit is crucial. Resistance and current are two key properties that govern how electric circuits function. Understanding these concepts will empower you to understand, troubleshoot, and even design simple electrical systems. It’s directly relevant to understanding energy usage, electrical safety, and even renewable energy solutions which are increasingly important in South Africa.

Lesson notes

Electric Current (I): Electric current is the flow of electric charge through a circuit. Imagine a pipe filled with water. The current is like the amount of water flowing through the pipe per second. In an electric circuit, this charge is carried by electrons, which are negatively charged particles.

Definition: The rate of flow of electric charge.

Symbol: I Unit: Ampere (A)

Measurement: Ammeters are used to measure current. Ammeters are always connected in series within the circuit.

Resistance (R): Resistance is the opposition to the flow of electric current. Think of it as a narrowing in the water pipe; the narrower the pipe, the harder it is for the water to flow. Similarly, a component with high resistance makes it difficult for electric current to pass through.

Definition: The opposition to the flow of electric current.

Symbol: R Unit: Ohm (Ω)

Measurement: Ohmmeters are used to measure resistance. Ohmmeters are connected across (in parallel) a component.

Voltage (V): Voltage is the electrical potential difference or "push" that drives the current around a circuit. It's like the water pressure in the pipe, which causes the water to flow.

Definition: The potential difference that drives the current.

Symbol: V Unit: Volt (V)

Measurement: Voltmeters are used to measure voltage. Voltmeters are connected across (in parallel) a component.

Ohm's Law: Ohm's Law is the fundamental relationship between voltage (V), current (I), and resistance (R). It states that the voltage across a conductor is directly proportional to the current flowing through it, provided the temperature remains constant.

Formula: V = I R This means: Current (I) = Voltage (V) / Resistance (R) ( I = V/R ) Resistance (R) = Voltage (V) / Current (I) ( R = V/I )

Factors Affecting Resistance: The resistance of a wire depends on several factors: Length (L): Longer wires have higher resistance. It's like a longer pipe offering more friction to the water flow. The resistance is directly proportional to the length. (R ∝ L)

Cross-sectional Area (A): Thicker wires have lower resistance. A wider pipe allows water to flow more easily. The resistance is inversely proportional to the cross-sectional area. (R ∝ 1/A)

Material: Different materials have different inherent abilities to conduct electricity. Copper is a good conductor (low resistance), while rubber is a poor conductor (high resistance).

Temperature: For most materials, resistance increases with temperature. The increased heat causes the atoms to vibrate more, hindering the flow of electrons.

Worked example

Example 1:

A light bulb connected to a 220V mains supply draws a current of 0.5A. Calculate the resistance of the light bulb.

Given: V = 220V, I = 0.5A

Formula: R = V / I

Solution: R = 220V / 0.5A = 440 Ω

Answer: The resistance of the light bulb is 440 Ohms.

Example 2:

A heater element has a resistance of 20 Ohms. If a current of 10A flows through it, what is the voltage across the element?

Given: R = 20 Ω, I = 10A

Formula: V = I R

Solution: V = 10A 20 Ω = 200V

Answer: The voltage across the heater element is 200 Volts.