Lesson Notes By Weeks and Term v5 - Grade 10

Lesson Video

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Performance objectives

• Define electric current (I), potential difference (V), and resistance (R), and state their SI units.
• Draw circuit diagrams using standard symbols.
• Apply Ohm's Law (V = IR) to solve problems.
• Calculate total resistance for series and parallel circuits.
• Explain the difference between series and parallel circuits.

Lesson summary

Electricity is the lifeblood of modern society, and understanding electric circuits is fundamental to grasping how the devices we use every day actually work. From the lights in our homes to the smartphones in our pockets, electric circuits are essential. In South Africa, access to reliable electricity remains a challenge for many communities, making a thorough understanding of basic circuits even more crucial for potential future electricians, engineers, and informed citizens. Knowing how circuits work allows us to troubleshoot problems, conserve energy, and even develop new technologies to improve the quality of life for everyone.

Lesson notes

2.1 Electric Current (I) Electric current is the rate of flow of electric charge through a conductor. Think of it like water flowing through a pipe. The more water that flows per second, the higher the water current. Similarly, the more charge that flows per second, the higher the electric current.

Definition: The amount of electric charge (Q) flowing past a point in a circuit per unit time (t).

Formula: I = Q/t SI Unit: Ampere (A), where 1 Ampere = 1 Coulomb per second (1 A = 1 C/s)

Direction of Current: Conventional current is defined as the direction in which positive charge would flow. In reality, it is electrons (negatively charged) that move in a conductor, so the electron flow is actually opposite to the conventional current direction. We will primarily focus on conventional current. 2.2 Potential Difference (V) or Voltage Potential difference, often called voltage, is the amount of energy required to move one coulomb of charge between two points in a circuit. Think of it as the "push" that makes the electric current flow.

Definition: The work done (W) per unit charge (Q) to move the charge between two points.

Formula: V = W/Q SI Unit: Volt (V), where 1 Volt = 1 Joule per Coulomb (1 V = 1 J/C) 2.3 Resistance (R) Resistance is the opposition to the flow of electric current in a circuit. Think of it as a narrow section of pipe that restricts the flow of water.

Definition: The ratio of potential difference (V) across a component to the current (I) flowing through it.

Formula: R = V/I SI Unit: Ohm (Ω) 2.4 Ohm's Law Ohm's Law states that the potential difference (V) across a conductor is directly proportional to the current (I) flowing through it, provided the temperature remains constant.

Formula: V = IR Important

Note: Ohm's Law is not a fundamental law of nature, but it holds true for many materials, especially at constant temperature. 2.5 Series Circuits In a series circuit, components are connected one after the other along a single path for the current to flow.

Key Characteristics: The current is the same at all points in the circuit. (I total = I 1 = I 2 = I 3 ...) The total potential difference (voltage) is the sum of the potential differences across each component. (V total = V 1 + V 2 + V 3 ...) The total resistance is the sum of the individual resistances. (R total = R 1 + R 2 + R 3 ...) 2.6 Parallel Circuits In a parallel circuit, components are connected along multiple paths, allowing the current to split.

Key Characteristics: The potential difference (voltage) is the same across all branches of the circuit. (V total = V 1 = V 2 = V 3 ...) The total current is the sum of the currents in each branch. (I total = I 1 + I 2 + I 3 ...) The reciprocal of the total resistance is the sum of the reciprocals of the individual resistances. (1/R total = 1/R 1 + 1/R 2 + 1/R 3 ...) 2.7 Circuit Diagrams It's crucial to be able to represent circuits using standard symbols.

Here are some common symbols: Cell: | | - Battery: | | - | | - (series of cells)

Resistor: -/\/\/\- Switch (Open): - -/ - Switch (Closed): - - - Ammeter: -(A)- Voltmeter: -(V)-

Worked example

Example 1: Series Circuit

Three resistors with resistances of 2 Ω, 3 Ω, and 5 Ω are connected in series to a 12 V battery.

Calculate:

(a) The total resistance of the circuit.

(b) The current flowing through the circuit.

(c) The potential difference across the 3 Ω resistor.

Solution: