Class: Senior Secondary School 2 (SS2 / SSS 2)
Term: 2nd Term
Week: 7
Age: 16 years
Duration: 45 minutes
Subject: Basic Electricity
Curriculum Theme: Electricity and Magnetism
Previous Lesson: Measuring Instruments: Identification of Electrical Measuring Instruments and Operational Range.
Topic: MEASURING INSTRUMENTS
Subject Matter: Meaning of moving coil instrument (definition), conversion of moving coil instrument into ammeter (using shunt), conversion of moving coil instrument into voltmeter (using multiplier)

Specific Objectives

By the end of the lesson, pupils should be able to:

Cognitive Domain:

Define a moving coil instrument.
Explain the principle of converting a moving coil instrument into an ammeter.
Explain the principle of converting a moving coil instrument into a voltmeter.
State the formulas for calculating shunt and multiplier resistances.

Affective Domain:

Appreciate the importance of measuring instruments in electrical circuits.
Show interest in the practical applications of electrical principles.

Psychomotor Domain:

Calculate the required shunt resistance for an ammeter conversion.
Calculate the required multiplier resistance for a voltmeter conversion.
Solve problems involving the conversion of moving coil instruments.

Collaborate with peers to solve problems related to instrument conversion.
Apply knowledge of electrical measurements in real-world contexts.

Reference Materials

The following resources were used in planning this lesson:

Senior Secondary Schools Education Curriculum
State Unified Scheme of Work
NELSON WAEC Basic Electricity for Senior Secondary Schools.

Instructional Materials

The teacher will teach this lesson with the aid of:

Moving coil instrument (galvanometer)
Resistors (various values to represent shunt and multiplier)
Diagrams illustrating ammeter and voltmeter conversions.

Rationale for the Lesson

This lesson helps pupils understand how basic electrical components like galvanometers are adapted to measure current and voltage. This knowledge is important for practical work in electricity and for solving related examination questions.

Prerequisite/Previous Knowledge

Pupils should have basic knowledge of electric current, voltage, resistance, Ohm’s Law, and how a galvanometer works.

Lesson Content/Board Summary

MEASURING INSTRUMENTS

Meaning of Moving Coil Instrument

A moving coil instrument, often referred to as a galvanometer, is a sensitive device used to detect and measure small electric currents. It works on the principle that a current-carrying coil placed in a magnetic field experiences a torque, causing it to deflect. The deflection is proportional to the current flowing through the coil.

Conversion of Moving Coil Instrument into Ammeter

An ammeter is an instrument used to measure electric current in a circuit. To convert a moving coil galvanometer into an ammeter, a low resistance called a shunt resistor (R_s) is connected in parallel with the galvanometer. This shunt resistor allows most of the current to bypass the galvanometer, protecting it from damage and extending its measurement range.

The formula for calculating the shunt resistance is:

R_s = (I_g * R_g) / (I - I_g)

Where:

R_s = Shunt resistance (in Ohms, Ω)
I_g = Full-scale deflection current of the galvanometer (in Amperes, A)
R_g = Resistance of the galvanometer (in Ohms, Ω)
I = Total current range to be measured by the ammeter (in Amperes, A)

Worked Examples:

Example 1: A galvanometer has a resistance of 50 Ω and gives a full-scale deflection for a current of 5 mA. How can it be converted into an ammeter to read up to 5 A?

Step 1: Identify given values and units.
R_g = 50 Ω
I_g = 5 mA = 5 × 10^-3 A
I = 5 A

Step 2: State the formula for shunt resistance.
R_s = (I_g * R_g) / (I – I_g)

Step 3: Substitute the values into the formula and calculate.
R_s = (5 × 10^-3 A * 50 Ω) / (5 A – 5 × 10^-3 A)
R_s = (0.25) / (4.995)
R_s ≈ 0.05005 Ω

Step 4: State the answer.
A shunt resistance of approximately 0.05005 Ω should be connected in parallel with the galvanometer.

Example 2: A galvanometer with a resistance of 20 Ω shows full-scale deflection with a current of 100 µA. Calculate the shunt resistance required to convert it into an ammeter capable of measuring 1 A.

Step 1: Identify given values and units.
R_g = 20 Ω
I_g = 100 µA = 100 × 10^-6 A = 1 × 10^-4 A
I = 1 A

Step 2: State the formula for shunt resistance.
R_s = (I_g * R_g) / (I – I_g)

Step 3: Substitute the values into the formula and calculate.
R_s = (1 × 10^-4 A * 20 Ω) / (1 A – 1 × 10^-4 A)
R_s = (0.002) / (0.9999)
R_s ≈ 0.00200 Ω

Step 4: State the answer.
A shunt resistance of approximately 0.00200 Ω should be connected in parallel with the galvanometer.

Conversion of Moving Coil Instrument into Voltmeter

A voltmeter is an instrument used to measure potential difference (voltage) across two points in a circuit. To convert a moving coil galvanometer into a voltmeter, a high resistance called a multiplier resistor (R_m) is connected in series with the galvanometer. This multiplier resistor limits the current flowing through the galvanometer and allows the instrument to measure a higher voltage range.

The formula for calculating the multiplier resistance is:

R_m = (V / I_g) - R_g

Where:

R_m = Multiplier resistance (in Ohms, Ω)
V = Total voltage range to be measured by the voltmeter (in Volts, V)
I_g = Full-scale deflection current of the galvanometer (in Amperes, A)
R_g = Resistance of the galvanometer (in Ohms, Ω)

Worked Examples:

Example 1: A galvanometer has a resistance of 100 Ω and gives a full-scale deflection for a current of 1 mA. How can it be converted into a voltmeter to read up to 10 V?

Step 1: Identify given values and units.
R_g = 100 Ω
I_g = 1 mA = 1 × 10^-3 A
V = 10 V

Step 2: State the formula for multiplier resistance.
R_m = (V / I_g) – R_g

Step 3: Substitute the values into the formula and calculate.
R_m = (10 V / (1 × 10^-3 A)) – 100 Ω
R_m = (10000) – 100 Ω
R_m = 9900 Ω

Step 4: State the answer.
A multiplier resistance of 9900 Ω should be connected in series with the galvanometer.

Example 2: A moving coil meter has a resistance of 200 Ω and a full-scale deflection current of 500 µA. Calculate the series resistance needed to enable it to measure voltages up to 50 V.

Step 1: Identify given values and units.
R_g = 200 Ω
I_g = 500 µA = 500 × 10^-6 A = 5 × 10^-4 A
V = 50 V

Step 2: State the formula for multiplier resistance.
R_m = (V / I_g) – R_g

Step 3: Substitute the values into the formula and calculate.
R_m = (50 V / (5 × 10^-4 A)) – 200 Ω
R_m = (100000) – 200 Ω
R_m = 99800 Ω

Step 4: State the answer.
A multiplier resistance of 99800 Ω should be connected in series with the galvanometer.

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher asks pupils about common electrical measurements and the instruments used, such as ammeters and voltmeters.
Pupils’ Activity: Pupils respond by naming instruments like ammeters and voltmeters.
Learning Point: Pupils recall prior knowledge about electrical measurements.

Step 2: Explanation of Moving Coil Instrument

Time: 7 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines a moving coil instrument (galvanometer) and explains its basic working principle, using a diagram or a physical instrument if available.
Pupils’ Activity: Pupils listen attentively and observe the instrument.
Learning Point: Pupils understand the definition and function of a moving coil instrument.

Step 3: Conversion to Ammeter

Time: 10 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher explains how a galvanometer is converted into an ammeter by connecting a low resistance (shunt) in parallel. The teacher writes the formula for shunt resistance and works through Example 1 from the board summary. The teacher demonstrates this with the instructional materials.
Pupils’ Activity: Pupils observe the demonstration, copy the formula and the worked example into their notebooks.
Learning Point: Pupils learn the method and formula for converting a galvanometer into an ammeter.

Step 4: Practice Ammeter Conversion

Time: 5 minutes
Teaching Skill: Problem Solving
Teacher’s Activity: The teacher guides pupils to solve Example 2 on ammeter conversion.
Pupils’ Activity: Pupils attempt to solve Example 2 in their notebooks, with teacher’s guidance.
Learning Point: Pupils practice calculating shunt resistance.

Step 5: Conversion to Voltmeter

Time: 10 minutes
Teaching Skill: Demonstration/Explanation
Teacher’s Activity: The teacher explains how a galvanometer is converted into a voltmeter by connecting a high resistance (multiplier) in series. The teacher writes the formula for multiplier resistance and works through Example 1 from the board summary. The teacher demonstrates this with the instructional materials.
Pupils’ Activity: Pupils observe the demonstration, copy the formula and the worked example into their notebooks.
Learning Point: Pupils learn the method and formula for converting a galvanometer into a voltmeter.

Step 6: Practice Voltmeter Conversion

Time: 5 minutes
Teaching Skill: Problem Solving
Teacher’s Activity: The teacher guides pupils to solve Example 2 on voltmeter conversion.
Pupils’ Activity: Pupils attempt to solve Example 2 in their notebooks, with teacher’s guidance.
Learning Point: Pupils practice calculating multiplier resistance.

Step 7: Evaluation/Review

Time: 5 minutes

Teaching Skill: Questioning/Assessment

Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

Define a moving coil instrument.
Explain how a moving coil instrument is converted into an ammeter.
State the formula for calculating shunt resistance.
A galvanometer with a resistance of 40 Ω gives a full-scale deflection for a current of 2 mA. Calculate the shunt resistance required to convert it into an ammeter to read up to 2 A.
Explain how a moving coil instrument is converted into a voltmeter.
State the formula for calculating multiplier resistance.
A galvanometer has a resistance of 80 Ω and a full-scale deflection current of 0.5 mA. Calculate the series resistance needed to convert it into a voltmeter capable of measuring 20 V.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 3 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson, emphasizing the formulas and the practical applications of converting galvanometers. The teacher assigns homework.
Pupils’ Activity: Pupils listen to the summary and copy down the homework.
Learning Point: Pupils consolidate their understanding and prepare for further practice.

Lesson Keywords

Moving coil instrument – A sensitive device that detects and measures small electric currents.
Ammeter – An instrument used to measure electric current in a circuit.
Voltmeter – An instrument used to measure potential difference (voltage) across two points in a circuit.
Shunt resistor – A low resistance connected in parallel with a galvanometer to convert it into an ammeter.
Multiplier resistor – A high resistance connected in series with a galvanometer to convert it into a voltmeter.

Differentiation

For pupils who grasp concepts quickly, the teacher can provide more complex problems involving internal resistance or multiple ranges. For pupils who need more support, the teacher can provide simplified examples and one-on-one guidance during problem-solving sessions.

Note for teachers using this lesson plan

Ensure that pupils understand the distinction between series and parallel connections for the shunt and multiplier resistors. Emphasize the importance of correct unit conversion (e.g., mA to A, µA to A) when solving problems. Practical demonstration with actual components, if available, will enhance understanding.