Resistance and Resistivity

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Subject: Physics

Semester: 1

Period: 2

Week: 8

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School Name:

Teacher’s Name:

Subject: Physics

Grade Level: Grade 12

Week & Period: Week 8, Period II

Date:

Topic: Resistance and Resistivity
Sub-topics:

• Ohm’s Law
• Factors affecting the resistance of a conductor

Learning Objectives:

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

1. Define resistance and state its unit.
2. State and apply Ohm’s Law in circuit problems.
3. Identify and explain factors that affect the resistance of a conductor.
4. Perform and interpret experiments to verify Ohm’s Law.
5. Solve problems on resistance and resistivity.

 

Instructional Materials:

• Dry cells or battery packs
• Resistors of known values
• Connecting wires
• Switch
• Rheostat
• Ammeter
• Voltmeter
• Multimeter
• Ruler, copper wire of known length and thickness
• Calculator

 

Anticipation (Warm-Up):

Ask: “Why does a longer wire make a bulb dimmer?”
Demonstrate connecting a short and long wire between a bulb and battery, then observe brightness.

 

Building Knowledge (Main Lesson):

1. Resistance

• Opposition to the flow of electric current.
• Symbol: R, Unit: Ohm (Ω)

 

2. Ohm’s Law

• States that:
  V=IR
  Where:
  V = Voltage (Volts),
  I = Current (Amperes),
  R = Resistance (Ohms)
• Valid for conductors where current and voltage are directly proportional.

 

3. Factors Affecting Resistance

• Length (L): Resistance increases with length.
• Area (A): Wider wires have less resistance.
• Material: Copper has less resistance than iron.
• Temperature: Higher temperatures increase resistance in metals.
• Formula:
  

• Where ρ is the resistivity of the material.

 

Experiment: Verifying Ohm’s Law

Title: Relationship Between Voltage and Current in a Resistor
Materials: Resistor, dry cell, voltmeter, ammeter, switch, wires
Procedure:

1. Connect circuit as per standard Ohm’s Law setup.
2. Gradually increase voltage and record corresponding current.
3. Plot V against I.
   Observation: The graph is a straight line, proving Ohm’s Law.
   Conclusion: Resistance remains constant; V∝I

 

Assessment Questions:

1. Objective Questions

• Ohm’s Law relates:
  A. Voltage, current, resistance
  B. Current, power, energy
  C. Charge, mass, time
  D. Voltage, frequency, period
• Which of these will decrease resistance in a wire?
  A. Increase length
  B. Use thinner wire
  C. Use copper instead of iron
  D. Heat the wire

2. Short Answer Questions

• Define resistance and give its unit.
• Explain why resistance increases with temperature.

3. Problem Solving
   A wire has a resistance of 5Ω and a current of 2A flows through it.
   i. Find the voltage across the wire.
   ii. What energy is dissipated in 10 seconds?

 

Homework:

• Measure the resistance of various materials at home (e.g., pencil lead, foil) using a battery and bulb.
• Draw a labeled diagram showing an Ohm’s Law experiment.
• Use Ohm’s Law to calculate current if 12V is applied across a 4Ω resistor.

 

Expanded Notes:

• Materials with low resistivity are good conductors (e.g., copper).
• Resistance leads to heat generation in electric cookers and irons.
• Carbon resistors are commonly used in electronics.
• Multimeters help measure resistance directly.

 

Differentiation:

• Diagrams for visual learners
• Hands-on circuit assembly for kinesthetic learners
• Group calculations for peer learning

 

Teacher’s Reflection:

• Were learners able to grasp the concept of resistance practically?
• Was the Ohm’s Law graph interpreted correctly?
• Did all groups complete the calculation and experiment?