Lesson Note on Electric Fields: E.M.F, Internal Resistance and Potentiometer Use for SSS 2

Class: Senior Secondary School 2 (SS2 / SSS2)
Term: Second Term
Week: 9
Age: 16 years
Duration: 45 minutes
Subject: Physics
Curriculum Theme: Electricity and Magnetism
Previous Lesson: Electric Fields: Meaning, Cells and Series-Parallel Circuits.
Topic: ELECTRIC FIELDS
Subject Matter: E.m.f of a cell (meaning and measurement), internal resistance of a cell (effect on current), standard resistors and rheostats (uses), resistivity and conductivity (material properties), potentiometer wire use (measure e.m.f, compare two e.m.f values)

Specific Objectives

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

Cognitive Domain:

• Define electromotive force (e.m.f) and internal resistance of a cell.
• Explain the effect of internal resistance on current in a circuit.
• Define resistivity and conductivity and state their units.
• State the uses of standard resistors and rheostats.
• Explain the principle of a potentiometer.

Affective Domain:

• Appreciate the importance of accurate measurement in electrical experiments.
• Demonstrate carefulness in handling electrical components and setting up circuits.

Psychomotor Domain:

• Set up a simple potentiometer circuit.
• Measure the e.m.f of a cell using a potentiometer.
• Compare the e.m.f values of two cells using a potentiometer.

Social Domain:

• Collaborate effectively in group activities during circuit setup.
• Communicate their observations and findings clearly to their peers.

Reference Materials

The following resources were used in planning this lesson:

• Senior Secondary Schools Education Curriculum
• State Unified Scheme of Work
• Anyakoha, M.W. (2012). New School Physics for Senior Secondary Schools. Africana First Publishers Plc.
• https://www.physicsclassroom.com/class/circuits/Lesson-2/Electric-Field-Calculations
• https://www.khanacademy.org/science/physics/electricity-magnetism/electric-potential/a/electric-field-lines

Instructional Materials

The teacher will teach this lesson with the aid of:

• Potentiometer wire
• Jockey
• Cells (e.g., Leclanche cell, Daniell cell)
• Galvanometer
• Connecting wires
• Standard resistors
• Rheostat
• Ammeter
• Voltmeter
• Diagrams of potentiometer circuits

Rationale for the Lesson

This lesson helps pupils understand fundamental concepts of electrical circuits beyond simple Ohm’s Law, such as the true potential of a cell and how materials conduct electricity. Understanding these concepts is important for practical applications in electronics and advanced physics studies.

Prerequisite/Previous Knowledge

Pupils have prior knowledge of basic electrical concepts, including current, voltage, resistance, and Ohm’s Law.

Lesson Content/Board Summary

ELECTRIC FIELDS: E.m.f, Internal Resistance, Resistivity, Conductivity and Potentiometer

Electromotive Force (e.m.f) of a Cell

The electromotive force (e.m.f) of a cell is defined as the total work done per unit charge in moving a charge around a complete circuit. It is the potential difference across the terminals of a cell when no current is flowing through the external circuit (open circuit). The unit of e.m.f is the Volt (V).

Measurement: e.m.f can be measured by connecting a high resistance voltmeter across the terminals of the cell when no external load is connected.

Internal Resistance of a Cell

Internal resistance (r) is the opposition to the flow of current within the cell itself, caused by the electrolyte and electrodes. When a current (I) flows through a cell, a potential drop (Ir) occurs across its internal resistance. The terminal potential difference (V) across the cell is then given by:

V = E – Ir

Where E is the e.m.f of the cell.

Effect on Current: Internal resistance reduces the effective voltage supplied to the external circuit, thereby reducing the current that can be drawn from the cell.

Resistivity and Conductivity

Resistivity (ρ): This is an intrinsic property of a material that quantifies how strongly it resists the flow of electric current. It is defined as the resistance of a material of unit length and unit cross-sectional area. Its unit is Ohm-metre (Ωm).

Formula: ρ = RA/L

Where R is resistance, A is cross-sectional area, and L is length.

Conductivity (σ): This is the reciprocal of resistivity and measures a material’s ability to conduct electric current. Its unit is Siemens per metre (S/m) or (Ωm)^-1.

Formula: σ = 1/ρ

The following are examples of materials based on their properties:

• Good Conductors: Low resistivity, high conductivity (e.g., copper, silver, aluminium).
• Insulators: High resistivity, low conductivity (e.g., rubber, glass, plastic).
• Semiconductors: Intermediate resistivity (e.g., silicon, germanium).

Standard Resistors and Rheostats

The following are uses of standard resistors and rheostats:

• Standard Resistors: Used to provide a precise, fixed value of resistance in a circuit, often for calibration or specific voltage division.
• Rheostats: Variable resistors used to control current in a circuit by changing the resistance. They can also be used as potential dividers.

Potentiometer

A potentiometer is an instrument used for measuring the e.m.f of a cell or comparing the e.m.f values of two cells, without drawing any current from the cell being measured at the point of balance.

Principle: The potential drop across any length of a uniform wire carrying a constant current is directly proportional to its length. i.e., V ∝ L.

Circuit Setup: A potentiometer consists of a long, uniform resistance wire (AB) connected across a driver cell (E_D) and a rheostat (Rh) to maintain a constant current. The cell whose e.m.f (E) is to be measured is connected in series with a galvanometer (G) and a jockey (J) across a part of the potentiometer wire.

Measurement of e.m.f:

Step 1: Set up the primary circuit with the driver cell and potentiometer wire.

Step 2: Connect the cell (E) to be measured, a galvanometer, and a jockey to the secondary circuit.

Step 3: Slide the jockey along the wire until the galvanometer shows no deflection (null point). This is the balancing length (L).

Step 4: At the null point, the e.m.f of the cell (E) is equal to the potential drop across the balancing length L. So, E = kL, where k is the potential gradient of the wire.

Comparison of e.m.f of two cells (E₁ and E₂):

Step 1: Find the balancing length (L₁) for the first cell (E₁).

Step 2: Find the balancing length (L₂) for the second cell (E₂).

Step 3: The ratio of their e.m.f values is directly proportional to their balancing lengths:

E₁/E₂ = L₁/L₂

Teaching Methods/Instructional Techniques

Discussion, Lecture, Demonstration, Question and Answer, Visual Aids, Practical/Experimental

Instructional Procedures

Step 1: Introduction

Time: 5 minutes
Teaching Skill: Set Induction
Teacher’s Activity: The teacher introduces the topic “Electric Fields” and reviews previous knowledge on basic electricity, current, voltage, and resistance.
Pupils’ Activity: Pupils recall and share their knowledge about basic electrical concepts.
Learning Point: Pupils connect the new topic to their existing knowledge base.

Step 2: Electromotive Force (e.m.f) and Internal Resistance

Time: 10 minutes
Teaching Skill: Explanation/Definition
Teacher’s Activity: The teacher defines e.m.f and internal resistance of a cell, explaining the formula V = E – Ir and its effect on current.
Pupils’ Activity: Pupils listen attentively, ask questions for clarification, and take notes.
Learning Point: Pupils understand the meaning and effect of e.m.f and internal resistance.

Step 3: Resistivity and Conductivity

Time: 10 minutes
Teaching Skill: Explanation/Examples
Teacher’s Activity: The teacher defines resistivity and conductivity, states their formulas and units, and provides examples of materials based on these properties.
Pupils’ Activity: Pupils define the terms, note the formulas and units, and discuss examples of conductors and insulators.
Learning Point: Pupils grasp the concepts of resistivity and conductivity and their relevance to materials.

Step 4: Potentiometer Principle and Uses

Time: 5 minutes
Teaching Skill: Explanation/Illustration
Teacher’s Activity: The teacher introduces the potentiometer, explains its principle (V ∝ L), and states its uses for measuring and comparing e.m.f.
Pupils’ Activity: Pupils pay attention to the explanation of the potentiometer’s principle and its applications.
Learning Point: Pupils understand the basic principle and purpose of a potentiometer.

Step 5: Potentiometer Circuit Setup

Time: 5 minutes
Teaching Skill: Demonstration/Guidance
Teacher’s Activity: The teacher demonstrates how to set up the primary and secondary circuits of a potentiometer, emphasizing proper connections and safety.
Pupils’ Activity: Pupils observe the teacher’s demonstration and ask questions about the circuit setup.
Learning Point: Pupils learn the correct procedure for setting up a potentiometer circuit.

Step 6: Measurement and Comparison of e.m.f using Potentiometer

Time: 5 minutes
Teaching Skill: Practical Application/Guidance
Teacher’s Activity: The teacher guides pupils on how to find the balancing length to measure the e.m.f of a cell and compare the e.m.f values of two cells using the formula E₁/E₂ = L₁/L₂.
Pupils’ Activity: Pupils follow the instructions, attempting to find balancing points and perform calculations under supervision.
Learning Point: Pupils gain practical experience in using a potentiometer for e.m.f measurements and comparisons.

Step 7: Evaluation/Review

Time: 5 minutes
Teaching Skill: Questioning/Assessment
Teacher’s Activity: The teacher evaluates the learning by asking the following questions:

1. Define electromotive force (e.m.f) of a cell.
2. Explain the term “internal resistance” of a cell.
3. State two uses of a potentiometer.
4. How does the internal resistance of a cell affect the current flowing in a circuit?
5. Distinguish between resistivity and conductivity, stating their S.I. units.

Pupils’ Activity: Pupils answer orally and in writing.

Learning Point: Pupils demonstrate understanding of the lesson.

Step 8: Conclusion

Time: 0 minutes
Teaching Skill: Summarization
Teacher’s Activity: The teacher summarizes the key points of the lesson, reiterating the importance of e.m.f, internal resistance, resistivity, conductivity, and the practical application of the potentiometer.
Pupils’ Activity: Pupils listen to the summary and ask any final questions.
Learning Point: Pupils reinforce their understanding of the lesson’s main concepts.

Lesson Keywords

• e.m.f – Electromotive force, the potential difference across a cell in an open circuit.
• Internal Resistance – Opposition to current flow within a cell.
• Resistivity – A material’s intrinsic opposition to electric current flow.
• Conductivity – A material’s ability to conduct electric current.
• Potentiometer – An instrument for measuring and comparing e.m.f without drawing current.
• Balancing Point – The point on a potentiometer wire where the galvanometer shows zero deflection.

Differentiation

For struggling learners, the teacher will provide simplified circuit diagrams and closer supervision during practical activities. Advanced learners will be challenged with additional problems involving calculations of internal resistance or potential gradient, and encouraged to research more complex applications of the potentiometer.

Note for teachers using this lesson plan

Ensure all electrical components are in good working condition. Emphasize safety precautions when dealing with electrical circuits. Clear diagrams of the potentiometer setup should be drawn on the board or projected. Encourage hands-on participation from pupils during the demonstration of the potentiometer to enhance understanding of the balancing point concept.