Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define voltage, current, and resistance and state their SI units.
• State Ohm's Law in words and as a formula (V=IR).
• Apply Ohm's Law to calculate V, I, or R in simple circuits.
• Explain the relationship between voltage, current, and resistance.
• Solve practical problems using Ohm's Law.

Lesson summary

Welcome, Grade 10 Electrical Technology learners! This week, we delve into the fundamental building blocks of electricity: basic electrical quantities and Ohm's Law. Understanding these concepts is crucial, not just for acing your exams, but for understanding how the electrical devices that power our homes, schools, and businesses in South Africa actually work. Imagine being able to troubleshoot a faulty appliance, understand your electricity bill better, or even design simple circuits. This knowledge empowers you to be a more informed and responsible citizen in our electrically-driven world.

Lesson notes

2.1 Basic Electrical Quantities Voltage (Potential Difference): Voltage, measured in Volts (V), is the electrical potential difference between two points in a circuit. Think of it as the "electrical pressure" that drives the current. It’s the force that pushes electrons through a circuit. In South Africa, our standard household voltage is 230V AC, but we're focusing on DC circuits for simplicity now.

Analogy: Voltage is like the height of a waterfall; the higher the waterfall, the more force the water (electrons) have when they hit the bottom.

Current: Current, measured in Amperes (Amps or A), is the rate of flow of electric charge (electrons) through a conductor. It’s literally the number of electrons passing a point in the circuit per second.

Analogy: Current is like the amount of water flowing down the waterfall; more water equals more current.

Resistance: Resistance, measured in Ohms (Ω), is the opposition to the flow of electric current. It restricts the flow of electrons. Every material has some resistance. A good conductor, like copper, has low resistance, while an insulator, like rubber, has high resistance.

Analogy: Resistance is like rocks in the riverbed; the more rocks, the harder it is for the water to flow. 2.2 Ohm's Law Ohm's Law states the relationship between voltage (V), current (I), and resistance (R) in a circuit. It's a fundamental law in electrical engineering.

Statement: The current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them.

Mathematical Formula: V = IR Where: V = Voltage (in Volts) I = Current (in Amperes) R = Resistance (in Ohms)

Rearranging the Formula: We can rearrange the formula to solve for current (I) or resistance (R): I = V / R R = V / I Why Ohm's Law Matters: Ohm's Law is essential for: Calculating the current flowing through a circuit with a known voltage and resistance. Determining the resistance of a component if you know the voltage across it and the current flowing through it. Designing and troubleshooting electrical circuits. 2.3 Worked Examples Example 1: Calculating Voltage A light bulb with a resistance of 100 Ohms has a current of 0.5 Amps flowing through it. What is the voltage across the light bulb?

Solution: Given: R = 100 Ω, I = 0.5 A Formula: V = IR Calculation: V = 0.5 A 100 Ω = 50 V Answer: The voltage across the light bulb is 50 Volts.

Example 2: Calculating Current A heating element in a kettle has a resistance of 25 Ohms and is connected to a 230V power supply. What current flows through the heating element?

Solution: Given: R = 25 Ω, V = 230 V Formula: I = V / R Calculation: I = 230 V / 25 Ω = 9.2 A Answer: The current flowing through the heating element is 9.2 Amps.

Example 3: Calculating Resistance A small electric motor operates at 12V and draws a current of 2 Amps. What is the resistance of the motor's windings?

Solution: Given: V = 12 V, I = 2 A Formula: R = V / I Calculation: R = 12 V / 2 A = 6 Ω Answer: The resistance of the motor's windings is 6 Ohms.

Example 4: Application in Load Shedding (Hypothetical - Simple Circuit) Imagine a small solar panel system designed to power a single LED light during load shedding. The LED requires 3V to operate and has a current draw of 0.1A. What resistance needs to be added in series with the LED to operate it correctly from a 12V solar panel?

First find the resistance of the LED: R_LED = V/I = 3V / 0.1A = 30 Ohms. Find the total resistance needed in the circuit with the 12V solar panel: R_Total = V/I = 12V/0.1A = 120 Ohms. Then, find the additional resistor needed: R_additional = R_Total - R_LED = 120 Ohms - 30 Ohms = 90 Ohms.

Answer: A 90 Ohm resistor needs to be added in series with the LE

D. Guided Practice (With Solutions)

Question 1: A cellphone charger provides an output voltage of 5V and the cellphone draws a current of 1A while charging. What is the resistance of the cellphone's charging circuit?

Solution: Given: V = 5 V, I = 1 A Formula: R = V / I Calculation: R = 5 V / 1 A = 5 Ω Answer: The resistance of the cellphone's charging circuit is 5 Ohms.

Question 2: A resistor of 47 Ohms is connected to a 9V battery. Calculate the current flowing through the resistor.

Solution: Given: R = 47 Ω, V = 9 V Formula: I = V / R Calculation: I = 9 V / 47 Ω = 0.19 A (approximately)

Answer: The current flowing through the resistor is approximately 0.19 Amps.

Question 3: An electric fence requires a voltage of 7500V to be effective. If the current passing through the fence wire is 0.01A (10mA), what is the effective resistance of the fence circuit?

Solution: Given: V = 7500 V, I = 0.01 A Formula: R = V / I Calculation: R = 7500 V / 0.01 A = 750000 Ω Answer: The effective resistance of the fence circuit is 750000 Ohms or 750 kΩ (kilo Ohms). Independent Practice (Questions Only) A toaster has a heating element with a resistance of 15 Ohms. If it operates on a 230V power supply, what current does it draw?