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Ohm's law — KCSE Physics

KCSE Physics · 107 practice questions · 9 syllabus objectives · 9 revision lessons

32 easy38 medium37 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Define EMF, distinguish it from terminal voltage, calculate internal resistance, and solve Wheatstone bridge problems

State Ohm's law and write the formula V = IR; identify ohmic and non-ohmic conductors

Calculate current, voltage or resistance using V = IR and electrical power using P = IV = I²R = V²/R

Plot and interpret an I-V graph; determine resistance from the gradient; identify non-linear behaviour

State Ohm’s law, define resistance, and distinguish between ohmic and non-ohmic conductors

State factors affecting resistance, explain the effect of temperature, and solve resistivity problems

Draw circuit diagrams, describe experiments to verify Ohm’s law, and analyse V-I graphs

Calculate effective resistance for series and parallel combinations and solve circuit problems for current, voltage and power

Ohm's law

Revision Notes

Concise lesson notes for Ohm's law, written to the KCSE Physics marking standard. Read the first lesson free below.

Understanding EMF and Terminal Voltage

Electromotive force (EMF) is defined as the energy provided by a cell or battery per unit charge. It is measured in volts (V). EMF represents the maximum potential difference when no current flows.

Terminal voltage, on the other hand, is the voltage output of a battery when it is supplying current. It is always less than the EMF due to internal resistance.

To calculate internal resistance (r), use the formula:

[ r = \frac{EMF - V}{I} ]

where:

  • V is the terminal voltage,
  • I is the current flowing through the circuit.

For example, if a battery has an EMF of 12V, a terminal voltage of 10V, and a current of 2A, the internal resistance can be calculated as follows:

[ r = \frac{12V - 10V}{2A} = 1 \Omega ]

In solving Wheatstone bridge problems, remember that the bridge is balanced when:

[ \frac{R_1}{R_2} = \frac{R_3}{R_4} ]

This relationship allows you to find unknown resistances in the circuit.

Key points to remember

  • EMF is the energy per unit charge from a source.
  • Terminal voltage is the voltage when current flows.
  • Internal resistance affects the terminal voltage.
  • Use the formula to calculate internal resistance.
  • Wheatstone bridge balance condition involves resistance ratios.

Worked example

A battery has an EMF of 9V and a terminal voltage of 7V when a current of 1.5A flows. Calculate the internal resistance. \n * Internal resistance, r = (9V - 7V) / 1.5A = 1.33Ω.

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More lessons in this topic

Lesson 2: Understanding Ohm's Law and Conductors

Objective: State Ohm's law and write the formula V = IR; identify ohmic and non-ohmic conductors

Ohm's Law is a fundamental principle in physics that relates voltage, current, and resistance in an electrical circuit. It states that: The current flowing through a conductor between two points is directly proportional to the voltage across the two points, provided the temperature remains constant.

The formula representing Ohm's Law is:
V = IR
Where:

  • V is the voltage in volts (V)
  • I is the current in amperes (A)
  • R is the resistance in ohms (Ω)

Conductors can be classified as ohmic or non-ohmic:

  • Ohmic conductors obey Ohm's Law, meaning their resistance remains constant regardless of the voltage and current. Examples include metals like copper and aluminum.
  • Non-ohmic conductors do not follow Ohm's Law; their resistance changes with voltage or current. Examples include diodes and thermistors.

Understanding these concepts is crucial for analyzing electrical circuits effectively.

  • Ohm's Law states V = IR for electrical circuits.
  • Ohmic conductors have constant resistance.
  • Non-ohmic conductors have variable resistance.
  • Voltage is directly proportional to current in ohmic conductors.
  • Examples include metals for ohmic and diodes for non-ohmic.

State Ohm's Law and identify an ohmic conductor.
Ohm's Law states that the current through a conductor is directly proportional to the voltage across it. An example of an ohmic conductor is copper wire.

Lesson 3: Understanding Ohm's Law and Power Calculations

Objective: Calculate current, voltage or resistance using V = IR and electrical power using P = IV = I²R = V²/R

Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points and inversely proportional to the resistance (R) of the conductor. The relationship is given by the formula:

V = IR
Where:

  • V = voltage in volts (V)
  • I = current in amperes (A)
  • R = resistance in ohms (Ω)

You can also calculate electrical power (P) using the following formulas:

  • P = IV
  • P = I²R
  • P = V²/R

To calculate any of these variables, rearrange the formulas accordingly. For instance, to find current (I), you can rearrange Ohm's law as:

I = V/R.

This allows you to find the current if you know the voltage and resistance. Similarly, you can derive power using the appropriate formula based on the known variables.

  • Ohm's Law relates voltage, current, and resistance.
  • Use V = IR to calculate voltage, current, or resistance.
  • Power can be calculated using P = IV, P = I²R, or P = V²/R.
  • Rearranging formulas allows for solving for unknowns.

Calculate the current flowing through a resistor of 10 Ω when a voltage of 20 V is applied.

Answer:

  • Use Ohm's Law: I = V/R
  • I = 20V / 10Ω
  • I = 2A
Lesson 4: Understanding I-V Graphs and Ohm's Law

Objective: Plot and interpret an I-V graph; determine resistance from the gradient; identify non-linear behaviour

Ohm's Law states that the current (I) flowing through a conductor between two points is directly proportional to the voltage (V) across the two points, provided the temperature remains constant. This relationship can be represented graphically using an I-V graph.

To plot an I-V graph:

  • Label the axes: Voltage (V) on the x-axis and Current (I) on the y-axis.
  • Plot the points: Measure voltage and current, then plot the corresponding points.
  • Draw the line: For linear resistors, this will be a straight line through the origin.

Interpreting the graph:

  • The gradient of the line represents the resistance (R), calculated by the formula R = V/I.
  • For non-linear behavior, the graph will not be a straight line. This indicates that the resistance changes with voltage or current.

Example: If the voltage is 10V and the current is 2A, the resistance is:

  • R = V/I = 10V/2A = 5Ω.

In cases where the graph curves, such as with diodes, the resistance varies, showing non-linear behavior.

  • Ohm's Law relates voltage, current, and resistance.
  • An I-V graph shows the relationship between voltage and current.
  • The gradient of the line indicates resistance.
  • Non-linear graphs indicate changing resistance.
  • Linear resistors show a straight line through the origin.

Given a voltage of 12V and current of 3A, find resistance:

  • R = V/I = 12V/3A = 4Ω.

Sample Questions

Read 3 questions and answers free. Sign up to access all 107 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

A circuit consists of a 12 V battery connected in series with two resistors, R1 = 4 Ω and R2 = 6 Ω. (a) Calculate the total resistance of the circuit. (2 marks) (b) Determine the current flowing through the circuit. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
Total resistance in series, R_total = R1 + R2 = 4 Ω + 6 Ω (1 mk)
R_total = 10 Ω (1 mk)
Part (b) — 2 marks
Using Ohm's law, I = V/R_total = 12 V / 10 Ω (1 mk)
I = 1.2 A (1 mk)
2
easySHORT ANSWER2 marks

Name two factors that determine whether a material obeys Ohm's law. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
Temperature of the material must remain constant (1 mk)
Material must have a constant resistivity under the conditions applied (1 mk)
3
easySHORT ANSWER4 marks

A circuit contains a 12 V battery connected in series with two resistors of 4 Ω and 6 Ω. (a) Calculate the total resistance in the circuit. (2 marks) (b) Determine the current flowing through the circuit. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
Total resistance, R_total = R1 + R2 = 4 Ω + 6 Ω (1 mk)
Total resistance, R_total = 10 Ω (1 mk)
Part (b) — 2 marks
Current, I = V/R_total = 12 V / 10 Ω (1 mk)
Current, I = 1.2 A (1 mk)
4

List three factors that affect the resistance of a conductor. (3 marks)

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Frequently asked questions

What does the KCSE Physics topic "Ohm's law" cover?

Ohm's law covers Define EMF, distinguish it from terminal voltage, calculate internal resistance, and solve Wheatstone bridge problems; State Ohm's law and write the formula V = IR; identify ohmic and non-ohmic conductors; Calculate current, voltage or resistance using V = IR and electrical power using P = IV = I²R = V²/R, and more, all aligned to the official KNEC KCSE Physics syllabus.

How many practice questions are available for Ohm's law?

HighMarks has 107 Ohm's law practice questions for KCSE Physics, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Physics syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Ohm's law for the KCSE exam?

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