Lesson Notes By Weeks and Term v5 - Grade 8

Lesson Video

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Performance objectives

• Draw circuit diagrams using standard symbols.
• Build and describe series and parallel circuits.
• Predict the effect of changing components in circuits.
• Define and calculate resistance using Ohm's Law.
• Explain the difference between AC and DC.

Lesson summary

Electricity is a fundamental part of our modern lives. From the lights that allow us to study at night to the cellphones that keep us connected, electricity powers our world. In South Africa, access to reliable electricity is crucial for economic growth, education, and improving the quality of life for all citizens.

However, many communities still lack consistent access to electricity. Understanding how electricity works, how circuits are built, and how to use electricity safely is therefore not just a scientific concept; it's a practical skill that empowers us to understand and potentially contribute to solving real-world problems in our communities.

Lesson notes

2.1 Circuit Diagrams and Symbols: A circuit diagram is a simplified, visual representation of an electrical circuit. It uses standard symbols to represent different components, making it easier to understand and analyze the circuit.

Here are some key symbols: Cell: A long line and a short line. The long line represents the positive (+) terminal, and the short line represents the negative (-) terminal. A battery is simply multiple cells connected together.

Bulb: A circle with a cross inside.

Resistor: A zig-zag line.

Switch (Open): A line with a break in it. This means the circuit is incomplete, and current cannot flow.

Switch (Closed): A continuous line. This means the circuit is complete, and current can flow.

Ammeter: A circle with an "A" inside. An ammeter measures the current (in Amperes) flowing through a specific point in the circuit. It must be connected in series.

Voltmeter: A circle with a "V" inside. A voltmeter measures the potential difference (voltage, in Volts) between two points in the circuit. It must be connected in parallel.

Connecting Wire: A straight line. 2.2 Series Circuits: In a series circuit, components are connected one after another along a single path. There is only one route for the current to flow.

Current: The current is the same at all points in a series circuit. Think of it like water flowing through a single pipe – the same amount of water passes each point in the pipe per unit of time.

Voltage: The total voltage supplied by the battery is divided among the components in the series circuit. The voltage drop across each component depends on its resistance.

Resistance: The total resistance in a series circuit is the sum of the individual resistances of all the components. R total = R 1 + R 2 + R 3 + ...

Example: Imagine a series circuit with a 6V battery and two bulbs. If one bulb blows, the entire circuit is broken, and both bulbs will go out. This is because the circuit path is broken. 2.3 Parallel Circuits: In a parallel circuit, components are connected along multiple pathways. The current has more than one route to flow.

Current: The total current flowing from the battery is divided among the different branches of the parallel circuit. The current through each branch depends on the resistance of that branch.

Voltage: The voltage across each component in a parallel circuit is the same and equal to the voltage supplied by the battery.

Resistance: The total resistance in a parallel circuit is less than the resistance of the smallest resistor. This is because the parallel pathways provide more routes for the current to flow. The formula for calculating the total resistance (R total ) in a parallel circuit with two resistors (R 1 and R 2 ) is: 1/R total = 1/R 1 + 1/R 2 . To find R total you must take the reciprocal of the answer.

Example: Consider a parallel circuit with a 6V battery and two bulbs. If one bulb blows, the other bulb will continue to shine because the circuit path for that bulb remains complete. Many household circuits are wired in parallel for this very reason. 2.4 Ohm's Law: Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in a circuit.

It states: V = IR Where: V is the voltage (potential difference) in Volts (V) I is the current in Amperes (A) R is the resistance in Ohms (Ω) Ohm's Law can be rearranged to solve for current (I = V/R) or resistance (R = V/I).

Example 1: A resistor has a resistance of 10 Ω and a current of 0.5 A flowing through it. What is the voltage across the resistor?

Solution: V = IR = (0.5 A)(10 Ω) = 5 V Example 2: A bulb is connected to a 1.5 V battery, and a current of 0.2 A flows through it. What is the resistance of the bulb?

Solution: R = V/I = (1.5 V)/(0.2 A) = 7.5 Ω Example 3: A 12V car battery is connected to a headlight with a resistance of 3 ohms. How much current flows through the headlight?

Solution: I = V/R = 12V / 3 ohms = 4A 2.5 Alternating Current (AC) vs.

Direct Current (DC): Direct Current (DC): DC flows in one direction only. Batteries and solar panels are common sources of D

C. Many electronic devices operate on D

C. Alternating Current (AC): AC changes direction periodically. The electricity that comes from the wall sockets in our homes and schools is AC. Eskom generates and distributes AC power across South Africa. AC is easier to transmit over long distances. Why AC in South Africa? Eskom generates electricity as AC because AC voltage can be easily increased or decreased using transformers. Increasing the voltage allows electricity to be transmitted over long distances with less energy loss. Before it enters our homes, the voltage is reduced to a safe level using transformers. Guided Practice (With Solutions)

Question 1: Draw a circuit diagram of a series circuit containing a 9V battery, a switch, and two resistors (R 1 = 10 Ω and R 2 = 20 Ω). The switch is closed.

Solution: [Imagine a drawing here.