Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define electric current and resistance.
• Explain and apply Ohm's Law (V=IR).
• Identify factors that affect resistance.
• Differentiate between conductors and insulators.
• Calculate resistance using Ohm's Law.

Lesson summary

Electric circuits are the backbone of modern life, powering everything from the lights in our homes and schools to the cell phones we use every day. Understanding how electricity flows within a circuit is crucial for developing technological literacy and problem-solving skills relevant to our modern world. In South Africa, where access to reliable electricity can be a challenge in some communities, understanding electric circuits allows us to appreciate the technology we have and perhaps even contribute to finding innovative solutions for energy efficiency and accessibility. This week, we will focus on two fundamental concepts: current and resistance.

Lesson notes

Electric Current Electric current is the rate of flow of electric charge through a circuit. This charge is carried by electrons, which are negatively charged particles that move through a conductor. Think of it like water flowing through a pipe – the current is the amount of water passing a point in the pipe per second.

Definition: The electric current (I) is the amount of charge (Q) that flows past a point in a circuit per unit of time (t).

Formula: I = Q / t Where: I is the current, measured in Amperes (A) Q is the charge, measured in Coulombs (C) t is the time, measured in seconds (s)

Units: Amperes (A). 1 Ampere is equal to 1 Coulomb per second (1 A = 1 C/s).

Conventional Current: Although electrons flow from negative to positive, conventional current is defined as flowing from positive to negative. This is a historical convention that we still use today.

Example 1: A cellphone charger passes a charge of 120 Coulombs in 2 minutes. What is the current flowing through the charger?

Solution: Identify the knowns: Q = 120 C t = 2 minutes = 2 60 seconds = 120 s Apply the formula: I = Q / t Substitute the values: I = 120 C / 120 s Calculate: I = 1 A Therefore, the current flowing through the cellphone charger is 1 Ampere. Resistance Resistance is the opposition to the flow of electric current in a circuit. Think of it like a narrower pipe in our water analogy – it's harder for water to flow through a narrow pipe, just as it's harder for current to flow through a component with high resistance.

Definition: Resistance (R) is the property of a material that opposes the flow of electric current.

Units: Ohms (Ω).

Factors Affecting Resistance: Material: Different materials have different abilities to conduct electricity. Conductors (like copper and silver) have low resistance, while insulators (like rubber and plastic) have very high resistance. Semiconductors (like silicon and germanium) have resistance values between conductors and insulators, and their resistance can be controlled.

Length: The longer the wire, the greater the resistance. Imagine trying to run through a long, crowded hallway versus a short one.

Cross-sectional Area: The thicker the wire, the lower the resistance. A thicker pipe allows more water to flow, and a thicker wire allows more current to flow.

Temperature: For most conductors, resistance increases with temperature. The increased thermal energy causes atoms to vibrate more, making it harder for electrons to flow. Ohm's Law Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in a circuit.

Formula: V = I R Where: V is the voltage, measured in Volts (V) I is the current, measured in Amperes (A) R is the resistance, measured in Ohms (Ω)

Explanation: Ohm's Law states that the voltage across a conductor is directly proportional to the current flowing through it, provided the temperature remains constant. This means that if you increase the voltage, the current will increase proportionally, assuming the resistance stays the same. Conversely, if you increase the resistance, the current will decrease proportionally, assuming the voltage stays the same.

Example 2: A light bulb in a shack in Khayelitsha has a resistance of 220 Ohms and is connected to a 220 V power supply. Calculate the current flowing through the light bulb.

Solution: Identify the knowns: R = 220 Ω V = 220 V Apply Ohm's Law: V = I * R Rearrange the formula to solve for I: I = V / R Substitute the values: I = 220 V / 220 Ω Calculate: I = 1 A Therefore, the current flowing through the light bulb is 1 Ampere.

Example 3: A heater in a classroom has a current of 5 A flowing through it when connected to a 220 V power supply. What is the resistance of the heater?

Solution: Identify the knowns: I = 5 A V = 220 V Apply Ohm's Law: V = I * R Rearrange the formula to solve for R: R = V / I Substitute the values: R = 220 V / 5 A Calculate: R = 44 Ω Therefore, the resistance of the heater is 44 Ohms. Guided Practice (With Solutions)

Question 1: A battery provides a voltage of 9 V to a circuit containing a resistor. If the current flowing through the resistor is 0.5 A, what is the resistance of the resistor?

Solution: Identify the knowns: V = 9 V, I = 0.5 A Apply Ohm's Law: V = I * R Rearrange the formula to solve for R: R = V / I Substitute the values: R = 9 V / 0.5 A Calculate: R = 18 Ω Answer: The resistance of the resistor is 18 Ohms.

Question 2: A cellphone charges through a cable with a resistance of 2 Ohms. If the voltage across the cable is 5 V, what is the current flowing through the cable?

Solution: Identify the knowns: R = 2 Ω, V = 5 V Apply Ohm's Law: V = I * R Rearrange the formula to solve for I: I = V / R Substitute the values: I = 5 V / 2 Ω Calculate: I = 2.5 A Answer: The current flowing through the cable is 2.5 Amperes.

Question 3: A long copper wire used to connect a generator to a home has a resistance of 10 Ohms.