Lesson Notes By Weeks and Term v5 - Grade 9

Lesson Video

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Performance objectives

• Define electric current (Amperes) and resistance (Ohms).
• Explain the relationship between current and resistance using Ohm's Law.
• Calculate current and resistance in a simple circuit.
• Identify factors affecting resistance in a conductor.
• Distinguish between conductors, insulators, and semiconductors.

Lesson summary

This week, we will be diving into the exciting world of electric circuits! Understanding how electricity flows and behaves in circuits is crucial because electricity powers almost every aspect of our modern lives, from the lights in our homes to the cellphones we use to communicate. In South Africa, access to reliable electricity is a significant issue, and understanding how circuits work can help us appreciate the importance of efficient energy use and the potential for alternative energy solutions. We will be specifically focusing on two key concepts: resistance and current, and how they relate to each other.

Lesson notes

What is an Electric Circuit? An electric circuit is a complete, closed pathway through which electric current can flow. Think of it like a highway for electrons. For a circuit to work, it needs: A source of energy: This is usually a battery or a generator that provides the electrical potential difference (voltage) to push the electrons around the circuit.

A conductor: This is a material, usually a metal wire, that allows electrons to flow easily through it.

A load: This is a device that uses the electrical energy, such as a light bulb, a resistor, or a motor.

A switch (optional): This is used to open or close the circuit, controlling the flow of current. When the switch is closed, the circuit is complete, and current can flow. When the switch is open, the circuit is broken, and current stops flowing.

Electric Current (I): Electric current is the rate of flow of electric charge through a conductor. It's essentially the number of electrons passing a point in the circuit per unit of time.

Unit: Ampere (A). One Ampere is equal to one Coulomb of charge passing a point per second (1 A = 1 C/s).

Conventional Current: We conventionally say that current flows from the positive terminal of the battery to the negative terminal. This is opposite to the actual flow of electrons (which flow from negative to positive) because the convention was established before the discovery of the electron. It's important to remember and use conventional current direction.

Measuring Current: We use an ammeter to measure current. An ammeter must be connected in series with the circuit element whose current you want to measure. This means the current must flow through the ammeter.

Resistance (R): Resistance is the opposition to the flow of electric current in a circuit. All materials resist the flow of current to some extent.

Unit: Ohm (Ω).

How it works: Electrons flowing through a conductor collide with the atoms of the conductor. These collisions impede the flow of electrons, creating resistance.

Factors Affecting Resistance: Material: Different materials have different inherent resistance. Copper, for example, has low resistance, while rubber has very high resistance.

Length: A longer conductor has higher resistance. Imagine trying to squeeze through a long, crowded hallway – it's harder than getting through a short one!

Cross-sectional Area (Thickness): A thicker conductor has lower resistance. A wider hallway is easier to navigate than a narrow one.

Temperature: For most conductors, resistance increases with temperature. As temperature increases, the atoms in the conductor vibrate more, increasing the likelihood of collisions with electrons. Conductors, Insulators, and Semiconductors: Conductors: Materials with low resistance that allow electric current to flow easily. Examples include copper, aluminum, and silver. We use these for wires in our homes.

Insulators: Materials with very high resistance that prevent electric current from flowing. Examples include rubber, plastic, and glass. We use these to coat wires and protect us from electric shock.

Semiconductors: Materials with resistance between that of conductors and insulators. Their resistance can be controlled by adding impurities or changing the temperature. Examples include silicon and germanium. These are essential components in electronic devices like computers and cell phones.

Ohm's Law: Ohm's Law describes the relationship between voltage (V), current (I), and resistance (R) in a circuit: V = I * R Where: V is the voltage in volts (V) I is the current in amperes (A) R is the resistance in ohms (Ω)

Ohm's Law tells us that: The current through a conductor is directly proportional to the voltage across it (if resistance is constant). If you increase the voltage, the current increases proportionally. The current through a conductor is inversely proportional to the resistance (if voltage is constant). If you increase the resistance, the current decreases proportionally.

Worked example

Example 1: A light bulb in a Soweto home has a resistance of 240 ohms and is connected to a 220V electrical outlet. What is the current flowing through the light bulb?

Given: R = 240 Ω, V = 220 V

Find: I = ?

Formula: V = I R => I = V / R

Solution: I = 220 V / 240 Ω = 0.92 A (approximately)

Answer: The current flowing through the light bulb is approximately 0.92 Amperes.

Example 2: A heating element in a kettle draws a current of 10 A when connected to a 220 V power supply. What is the resistance of the heating element?

Given: I = 10 A, V = 220 V

Find: R = ?

Formula: V = I R => R = V / I

Solution: R = 220 V / 10 A = 22 Ω

Answer: The resistance of the heating element is 22 Ohms.

Example 3: A copper wire has a resistance of 5 ohms. If the length of the wire is doubled, what happens to the resistance?

Explanation: Resistance is directly proportional to length. Doubling the length doubles the resistance.

Answer: The resistance will double to 10 ohms.

Guided Practice (With Solutions)

Question 1: A cellphone charger has an internal resistance of 5 ohms and draws a current of 1 A. What voltage is required to operate the charger?

Solution:

Given: R = 5 Ω, I = 1 A

Find: V = ?

Formula: V = I R

V = 1 A 5 Ω = 5 V

Answer: The voltage required is 5

V. Commentary: This is a direct application of Ohm's Law. We simply plug the given values into the formula to find the unknown voltage.