Lesson Notes By Weeks and Term v5 - Grade 10
Basic electrical quantities and Ohm's law – Week 10 focus
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Basic electrical quantities and Ohm's law – Week 10 focus
Download the Lessonotes Mobile South Africa app for faster lesson access on Android and iPhone.
Subject: Electrical Technology
Class: Grade 10
Term: 1st Term
Week: 10
Theme: General lesson support
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Welcome, Grade 10 Electrical Technology learners! This week, we're diving into the fundamental building blocks of electricity: basic electrical quantities (voltage, current, resistance, and power) and how they relate to each other through Ohm's Law. Understanding these concepts is absolutely crucial, not just for this course but for grasping how almost all electronic devices work, from your cell phone to the lighting in your home.
Let’s start by defining our key players: Voltage (V): Voltage, also known as potential difference, is the electrical "pressure" that drives current through a circuit. It's the difference in electrical potential energy between two points in a circuit. Think of it like the pressure in a water pipe – higher pressure pushes more water. The unit of voltage is the Volt (V). In South Africa, our standard household voltage is typically 230V A
C. Current (I): Current is the flow of electrical charge (electrons) through a circuit. It's the rate at which charge flows. Think of it like the amount of water flowing through a pipe per second. The unit of current is the Ampere (A), often shortened to "amps".
Resistance (R): Resistance is the opposition to the flow of current in a circuit. It's like a constriction in a water pipe that reduces the flow of water. The unit of resistance is the Ohm (Ω). Resistors are components designed to provide a specific amount of resistance in a circuit.
Power (P): Power is the rate at which electrical energy is converted into other forms of energy, such as heat or light. It represents how much work is being done by the electrical circuit per unit time. Think of it as how much energy is being used by an appliance. The unit of power is the Watt (W).
Ohm's Law: Ohm's Law is a fundamental relationship between voltage, current, and resistance.
It states: Voltage (V) = Current (I) x Resistance (R) or V = IR This means: If you increase the voltage across a resistor, the current through it will increase proportionally. If you increase the resistance in a circuit with a constant voltage, the current will decrease. We can rearrange the formula to solve for current or resistance: Current (I) = Voltage (V) / Resistance (R) or I = V/R Resistance (R) = Voltage (V) / Current (I) or R = V/I Example 1: Calculating Current A cellphone charger connected to a 230V outlet has an internal resistance of 4600 ohms. Calculate the current flowing through the charger.
Given: V = 230V, R = 4600 Ω Formula: I = V/R Solution: I = 230V / 4600 Ω = 0.05A = 50mA (milliamps)
Example 2: Calculating Voltage A light bulb has a resistance of 200 ohms and a current of 0.5 amps flows through it. Calculate the voltage across the light bulb.
Given: R = 200 Ω, I = 0.5A Formula: V = IR Solution: V = 0.5A 200 Ω = 100V Example 3: Calculating Resistance A heater operating on 230V draws a current of 10 amps. What is the resistance of the heater?
Given: V = 230V, I = 10A Formula: R = V/I Solution: R = 230V / 10A = 23 Ω Electrical Power: Electrical power is the rate at which electrical energy is converted into other forms of energy. It can be calculated using the following formulas: P = VI (Power = Voltage x Current) P = I²R (Power = Current squared x Resistance) P = V²/R (Power = Voltage squared / Resistance) These formulas are all derived from Ohm's Law and the definition of power. Choose the formula that is most convenient based on the information you have.
Example 4: Calculating Power (P=VI) A radio operates on 12V and draws a current of 0.25
A. Calculate the power consumed by the radio.
Given: V = 12V, I = 0.25A Formula: P = VI Solution: P = 12V 0.25A = 3W Example 5: Calculating Power (P=I²R) A resistor of 100 ohms has a current of 0.1A flowing through it. Calculate the power dissipated by the resistor (as heat).
Given: R = 100 Ω, I = 0.1A Formula: P = I²R Solution: P = (0.1A)² 100 Ω = 0.01A² * 100 Ω = 1W Example 6: Calculating Power (P=V²/R) An electric iron connected to a 230V outlet has a resistance of 20 ohms. Calculate the power consumed by the iron.
Given: V = 230V, R = 20 Ω Formula: P = V²/R Solution: P = (230V)² / 20 Ω = 52900V² / 20 Ω = 2645W Guided Practice (With Solutions)
Question 1: A cellphone charger outputs 5V and delivers a current of 2
A. What is the power output of the charger?
Solution: Given: V = 5V, I = 2A Formula: P = VI Solution: P = 5V 2A = 10W
Commentary: This is a direct application of the P = VI formula. We simply plug in the given values for voltage and current to calculate the power.
Question 2: A 100-ohm resistor is connected to a 12V battery. What is the current flowing through the resistor?
Solution: Given: R = 100 Ω, V = 12V Formula: I = V/R Solution: I = 12V / 100 Ω = 0.12A
Commentary: This uses Ohm's Law (I = V/R) to find the current. Remember to use the correct units (Volts and Ohms) to get the current in Amps.
Question 3: An electric kettle draws 10 amps from a 230V supply. Calculate the power consumed by the kettle and its resistance.
Solution: Given: I = 10A, V = 230V Power Calculation: Formula: P = VI Solution: P = 230V 10A = 2300W Resistance Calculation: Formula: R = V/I Solution: R = 230V / 10A = 23 Ω
Commentary: This problem combines both power and resistance calculations. First, we calculate power using P = VI. Then, we use Ohm's Law (R = V/I) to calculate the resistance. Independent Practice (Questions Only) A flashlight bulb operates on 3V and has a resistance of 6 ohms.