Lesson Notes By Weeks and Term v3 - Senior Secondary 1
Ohms law
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Ohms law
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Subject: Basic Electricity
Class: Senior Secondary 1
Term: 3rd Term
Week: 1
Theme: Basic Electron Theory
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Students should beable to:State OHM's law:' State the relationshipbetween current,voltage and resistance Perform anexperiment to determine OHM'slaw Calculate current,voltage and resistance givenany two of the three quantities
Ohm's Law Ohm's Law states that the current flowing through a metallic conductor is directly proportional to the potential difference (voltage) across its ends, provided that the temperature and other physical conditions of the conductor remain constant.
Mathematical Representation: Ohm's Law is mathematically expressed as: V = I * R Where: V represents the Voltage (or Potential Difference), measured in Volts (V). Voltage is the "push" or electrical pressure that drives current through a circuit. It is the energy per unit charge. I represents the Current, measured in Amperes (A). Current is the rate of flow of electric charge (electrons) through a conductor. R represents the Resistance, measured in Ohms (Ω). Resistance is the opposition to the flow of electric current. Rearranging the Formula (Ohm's Law Triangle/Mnemonic): The formula V = IR can be rearranged to find any of the three quantities if the other two are known: To find Current (I): I = V / R To find Resistance (R): R = V / I A helpful mnemonic for students is the "Ohm's Law Triangle": ``` V I | R ``` To find any quantity, cover it up in the triangle, and the remaining letters show the formula.
Analogy for Understanding: Imagine water flowing through a pipe: Voltage (V) is like the water pressure pushing the water. Higher pressure means a stronger push. Current (I) is like the amount of water flowing through the pipe per second. More water flowing means higher current. Resistance (R) is like the narrowness or friction within the pipe. A narrower or rougher pipe offers more resistance to water flow.
Conditions for Ohm's Law: Ohm's Law is strictly applicable only when the temperature and other physical conditions (e.g., mechanical strain) of the conductor remain constant. For most metallic conductors, heating occurs as current flows, which can change resistance. Devices that obey Ohm's Law are called ohmic conductors (e.g., resistors, most metals at constant temperature). Devices that do not obey Ohm's Law are called non-ohmic conductors (e.g., semiconductor diodes, transistor junctions, filament lamps where resistance increases with temperature). Worked
Examples: Example 1: Calculating Current A car headlight in a typical Nigerian vehicle operates at 12 Volts. If the resistance of the headlight filament is 4 Ohms, what is the current flowing through it?
Given: Voltage (V) = 12 V Resistance (R) = 4 Ω Required: Current (I)
Formula: I = V / R Calculation: I = 12 V / 4 Ω I = 3 A Answer: The current flowing through the car headlight is 3 Amperes.
Example 2: Calculating Voltage A small fan in a Nigerian home, when connected to a solar power system, draws a current of 0.5 Amperes. If the fan's motor has an internal resistance of 24 Ohms, what voltage is supplied to the fan?
Given: Current (I) = 0.5 A Resistance (R) = 24 Ω Required: Voltage (V)
Formula: V = I R Calculation: V = 0.5 A * 24 Ω V = 12 V Answer: The voltage supplied to the fan is 12 Volts.
Example 3: Calculating Resistance An electric pressing iron, commonly used in Nigeria, is connected to a 240 Volt mains supply and draws a current of 5 Amperes. Calculate the resistance of the heating element in the iron.
Given: Voltage (V) = 240 V Current (I) = 5 A Required: Resistance (R)
Formula: R = V / I Calculation: R = 240 V / 5 A R = 48 Ω Answer: The resistance of the pressing iron's heating element is 48 Ohms. of proportionality.)
Phase 4: Problem Solving and Application (20 minutes)
Teacher Activity: Work through the examples provided in the "Key Concepts and Explanations" section on the board, explaining each step. Provide guided practice questions to students. Circulate around the classroom, offering support and checking students' work.
Student Activity: Pay attention to the worked examples. Attempt the guided practice questions individually or in pairs. Ask questions if they encounter difficulties.
Phase 5: Consolidation and Conclusion (5 minutes)
Teacher Activity: Summarize the key takeaways of the lesson: Definition of Ohm's Law, the formula V=IR and its variations, and the relationship between V, I, and
R. Assign independent practice questions as homework.
Student Activity: Participate in the summary. * Copy down homework questions.
Phase 1: Introduction and Review (10 minutes)
Teacher Activity: Begin by reviewing the basic concepts of current, voltage (potential difference), and resistance, including their definitions, units, and measurement instruments (ammeter, voltmeter, ohmmeter/multimeter). Ask questions to gauge students' prior understanding, e.g., "What pushes electrons in a circuit?", "What opposes the flow of electrons?", "How do we measure current?".
Student Activity: Respond to teacher's questions, recalling previously learned concepts. Take brief notes on the review points.
Phase 2: Introduction to Ohm's Law (20 minutes)
Teacher Activity: State Ohm's Law clearly and write its mathematical form (V=IR) on the board.
Explain the proportionality aspect: "Current is directly proportional to voltage, inversely proportional to resistance." Introduce the Ohm's Law triangle mnemonic to help students remember the formulas. Use the water pipe analogy (pressure, flow rate, pipe size) to illustrate the relationships between V, I, and
R. Discuss the conditions under which Ohm's Law holds true (constant temperature).
Student Activity: Listen attentively and participate in discussions. Copy down the definition of Ohm's Law, the formulas, and the Ohm's Law triangle. Ask clarifying questions regarding the concept and analogy.
Phase 3: Practical Demonstration/Experiment to Verify Ohm's Law (40 minutes)
Teacher Activity: If resources (DC power supply/batteries, ammeter, voltmeter, connecting wires, resistors, rheostat/variable resistor, switch) are available, guide students through an experiment to verify Ohm's Law. Demonstrate the proper connection of components in a series-parallel circuit (ammeter in series, voltmeter in parallel across the resistor). Emphasize safety precautions (e.g., connecting power supply last, checking for loose connections, not exceeding voltage limits). Instruct students to set up their circuits (if working in groups), collect data, and plot a V-I graph.
Procedure Outline:
1. Connect the components as shown in a standard circuit diagram for verifying Ohm's Law: power supply, switch, ammeter (in series with the resistor), fixed resistor, and voltmeter (in parallel across the resistor).
2. Switch on the power supply to a low voltage.
3. Record the readings from the ammeter (Current, I) and voltmeter (Voltage, V) in a table.
4. Increase the voltage from the power supply in steps (or adjust a rheostat to vary resistance if using a fixed voltage source) and record new pairs of V and I values. Repeat 5-6 times.
5. Switch off the circuit.
6. For each pair of V and I, calculate the resistance R = V/I. Observe if R remains approximately constant.
7. Plot a graph of Voltage (V) on the y-axis against Current (I) on the x-axis. Supervise groups, providing assistance and ensuring safety.
Student Activity: Observe the teacher's demonstration carefully. In groups (or individually), set up the circuit using the provided components. Carefully take readings of voltage and current for various settings.
Record data in a table format: | Reading No. | Voltage (V) | Current (I) | Resistance R = V/I (Ω) | | :---------- | :---------- | :---------- | :--------------------- | | 1 | | | | | 2 | | | | | ... | | | | Calculate the resistance for each reading. Plot a V-I graph on graph paper. Conclude that a straight line passing through the origin demonstrates that V is directly proportional to I, thus verifying Ohm's Law for the resistor used. The slope of the graph (V/I) represents the resistance. (If practical equipment is unavailable, the teacher can draw a circuit diagram and provide sample data, guiding students to plot the graph and calculate R, emphasizing observation of proportionality.)
Phase 4: Problem Solving and Application (20 minutes)
Teacher Activity: Work through the examples provided in the "Key Concepts and Explanations" section on the board, explaining each step. Provide guided practice questions to students. Circulate around the classroom, offering support and checking students' work.
Student Activity: Pay attention to the worked examples. Attempt the guided practice questions individually or in pairs. Ask questions if they encounter difficulties.
Phase 5: Consolidation and Conclusion (5 minutes)
Teacher Activity: Summarize the key takeaways The teacher should present these questions one by one, allowing students to attempt them before revealing the solution.
Question 1: A torchlight bulb typically runs on 3 Volts. If the bulb's filament has a resistance of 6 Ohms, what current does it draw from the battery?
Solution: Identify Given: Voltage (V) = 3 V Resistance (R) = 6 Ω Identify Required: Current (I)
Apply Formula: I = V / R Substitute Values: I = 3 V / 6 Ω Calculate: I = 0.5 A Answer: The torchlight bulb draws a current of 0.5 Amperes.
Question 2: A car battery is being tested. When a resistor of 0.15 Ohms is connected across its terminals, a current of 80 Amperes flows through the resistor. What is the voltage of the car battery under this load?
Solution: Identify Given: Current (I) = 80 A Resistance (R) = 0.15 Ω Identify Required: Voltage (V)
Apply Formula: V = I R Substitute Values: V = 80 A 0.15 Ω Calculate: V = 12 V Answer: The voltage of the car battery under this load is 12 Volts.
Question 3: A locally made fan uses 240 Volts from the mains supply and draws a current of 1.2 Amperes. Determine the resistance of the fan's motor winding.
Solution: Identify Given: Voltage (V) = 240 V Current (I) = 1.2 A Identify Required: Resistance (R)
Apply Formula: R = V / I Substitute Values: R = 240 V / 1.2 A Calculate: R = 200 Ω Answer: The resistance of the fan's motor winding is 200 Ohms.
Question 4: A technician tests a faulty electrical cooker element. He measures a voltage of 220 V across the element, and his ammeter shows a current of 11 A passing through it. What resistance should the element have according to these readings?
Solution: Identify Given: Voltage (V) = 220 V Current (I) = 11 A Identify Required: Resistance (R)
Apply Formula: R = V / I Substitute Values: R = 220 V / 11 A Calculate: R = 20 Ω Answer: The cooker element should have a resistance of 20 Ohms.
Example 1: Calculating Current
A car headlight in a typical Nigerian vehicle operates at 12 Volts. If the resistance of the headlight filament is 4 Ohms, what is the current flowing through it?
Given:
Voltage (V) = 12 V
Resistance (R) = 4 Ω
Required: Current (I)
Formula: I = V / R
Calculation:
I = 12 V / 4 Ω
I = 3 A
Answer: The current flowing through the car headlight is 3 Amperes.
Example 2: Calculating Voltage
A small fan in a Nigerian home, when connected to a solar power system, draws a current of 0.5 Amperes. If the fan's motor has an internal resistance of 24 Ohms, what voltage is supplied to the fan?
Given:
Current (I) = 0.5 A
Resistance (R) = 24 Ω
Required: Voltage (V)
Formula: V = I R
Calculation:
V = 0.5 A * 24 Ω
V = 12 V
Answer: The voltage supplied to the fan is 12 Volts.
Example 3: Calculating Resistance
An electric pressing iron, commonly used in Nigeria, is connected to a 240 Volt mains supply and draws a current of 5 Amperes. Calculate the resistance of the heating element in the iron.
Given:
Voltage (V) = 240 V
Current (I) = 5 A
Required: Resistance (R)
Formula: R = V / I
Calculation:
R = 240 V / 5 A
R = 48 Ω
Answer: The resistance of the pressing iron's heating element is 48 Ohms.
Teaching and Learning Activities
Phase 1: Introduction and Review (10 minutes)
Household Appliances and Electrical Safety (Nigerian Homes): Ohm's Law helps understand why different appliances (e.g., electric kettle, fan, light bulb) draw varying amounts of current from the same 220V/230V mains supply. An electric kettle with low resistance will draw a high current, leading to rapid heating. A fan with higher resistance draws less current. This knowledge is critical for understanding circuit breakers and fuses. When too many low-resistance appliances are connected, the total resistance decreases, leading to a dangerously high current (I = V/R). This "overload" can trip the circuit breaker or blow a fuse, preventing damage or fire, a common occurrence in many Nigerian homes experiencing power fluctuations or use of multiple high-power appliances. Understanding the relationship helps identify why touching a live wire is dangerous – very low resistance of the human body leads to very high current flow at mains voltage, causing severe shock. Generator Sizing and Use (Nigerian Context): Many Nigerian homes and businesses rely on generators during power outages. Ohm's Law is crucial for sizing a generator correctly and avoiding overloading. Generators have a specified voltage output (e.g., 220V) and maximum current capacity (e.g., 10A, 20A). Users can apply Ohm's Law (P=VI, which links to V=IR) to estimate the total current drawn by all connected appliances. If the total current exceeds the generator's capacity, it can damage the generator or cause it to shut down. This knowledge helps individuals calculate resistance needed for specific loads and choose appropriate generators. Electrical Maintenance and Repairs (Artisans and Technicians): Local electricians and technicians (artisans) in Nigeria use Ohm's Law daily to diagnose faults in electrical circuits and appliances. By measuring voltage across a component and the current flowing through it, they can determine if the component's resistance is within its expected range. For example, if a pressing iron is not heating, a technician might measure its resistance. If the resistance is infinite (open circuit), it indicates a broken heating element. If the resistance is very low, it could indicate a short circuit. This direct application of V=IR is fundamental to their troubleshooting process.