Physics · Year 12 · Charge and Current · Autumn Term

Ohm's Law and I-V Characteristics

Students will define electric potential and electric potential energy, calculating work done in electric fields.

• National Curriculum Attainment Targets
• A-Level: Physics - Electricity
• A-Level: Physics - Electric Fields

About This Topic

Ohm's Law relates current, voltage, and resistance in conductors: V = IR, where current is proportional to potential difference at constant temperature. Year 12 students graph I-V characteristics for components such as resistors, filament lamps, diodes, and thermistors. They define electric potential as work done per unit charge and calculate potential energy changes in fields, linking these to circuit behavior.

This unit from the Charge and Current module meets A-Level standards in electricity and electric fields. Students explain ohmic conditions, like steady temperature in resistors yielding linear graphs, versus non-ohmic cases: lamps heat up to increase resistance, diodes allow current one direction due to charge carrier asymmetry, thermistors vary resistance with temperature for sensors. Key skills include experiment design, data analysis, and linking curves to physics.

Active learning suits this topic well. Students gain insight by building circuits, measuring live data, and plotting graphs collaboratively. These methods reveal patterns firsthand, correct faulty intuitions, and foster skills in prediction, testing, and refinement essential for A-Level practicals.

Key Questions

1. Explain the conditions under which a component obeys Ohm's law and identify the physical reasons why certain components exhibit non-ohmic behaviour.
2. Analyse the I-V characteristics of a resistor, filament bulb, and semiconductor diode, linking each curve to the underlying physical processes.
3. Design an experiment to obtain the I-V characteristic of a thermistor and justify how its resistance-temperature relationship makes it suitable for use in sensing circuits.

Learning Objectives

• Analyze the graphical relationship between current and voltage for ohmic and non-ohmic components.
• Explain the microscopic origins of resistance in conductors and semiconductors.
• Calculate the work done when moving charge between points in an electric field.
• Design an experiment to determine the I-V characteristic curve of a thermistor.
• Compare the I-V characteristics of a resistor, filament bulb, and diode, justifying differences based on physical properties.

Before You Start

Basic Circuit Concepts: Voltage, Current, and Resistance

Why: Students need a foundational understanding of these three quantities and their basic relationships before exploring Ohm's Law and I-V characteristics.

Electric Fields and Forces

Why: Understanding how charges interact in electric fields is necessary to define electric potential and work done.

Key Vocabulary

Electric Potential Difference (Voltage)	The work done per unit positive charge in moving the charge between two points in an electric field. It is measured in volts (V).
Electric Current	The rate of flow of electric charge. It is measured in amperes (A).
Resistance	A measure of how difficult it is for current to flow through a material. It is measured in ohms (Ω).
Ohmic Conductor	A conductor for which the current is directly proportional to the potential difference across it, provided the temperature remains constant.
Semiconductor Diode	An electronic component that allows current to flow predominantly in one direction, exhibiting a non-linear I-V characteristic.
Thermistor	A type of resistor whose resistance varies significantly with temperature, often used in temperature sensing.

Watch Out for These Misconceptions

Common Misconception

All components obey Ohm's Law at all times.

What to Teach Instead

Resistors do under constant temperature, but lamps heat to raise resistance, diodes conduct asymmetrically. Hands-on plotting of real curves lets students see deviations, discuss causes like thermal effects, and refine models through group analysis.

Common Misconception

The slope of an I-V graph is voltage or current.

What to Teach Instead

Slope equals 1/R for ohmic devices. Active graphing activities help students derive this from data points, calculate R values, and connect to V=IR, building proportional reasoning via peer teaching.

Common Misconception

Non-ohmic behaviour means no resistance relationship exists.

What to Teach Instead

Curves still show V-I links, just nonlinear. Circuit investigations reveal patterns like diode thresholds, encouraging students to predict and test, which clarifies physics without rote memorization.

Active Learning Ideas

Circuit Build: Resistor I-V Graph

Provide components: power supply, resistor, ammeter, voltmeter, leads. Pairs connect in series, vary voltage from 0-12V in 1V steps, record I and V. Plot graph on paper or software, calculate resistance from slope. Discuss linearity.

30 min·Pairs

Stations Rotation: Non-Ohmic Components

Set three stations with lamp, diode, variable supply. Small groups measure I-V data at each, plot curves. Rotate every 10 minutes, compare shapes. Whole class shares predictions versus results.

45 min·Small Groups

Thermistor Experiment Design

Groups design circuit with thermistor, heater, data logger for resistance vs temperature. Test from 20-80°C, plot graph. Justify setup choices, link to sensor use. Peer review designs first.

50 min·Small Groups

Data Logger Challenge: Compare Curves

Use sensors for automated I-V sweeps on resistor and lamp. Individuals or pairs analyze digital graphs, identify ohmic/non-ohmic features. Export data for class comparison.

35 min·Pairs

Real-World Connections

• Electrical engineers use I-V characteristics to select appropriate components for circuits, such as designing power supplies where a diode's forward bias is crucial for rectification.
• Automotive technicians diagnose faults in vehicle lighting systems by measuring the current-voltage relationship of bulbs and wiring, identifying potential shorts or open circuits.
• Thermistors are integral to home heating systems and thermostats, where their changing resistance with temperature allows for precise control of room temperature.

Assessment Ideas

Quick Check

Present students with three unlabeled I-V characteristic graphs. Ask them to identify which graph corresponds to a metallic conductor at constant temperature, a filament lamp, and a diode, providing a brief justification for each choice.

Discussion Prompt

Pose the question: 'Why does the resistance of a filament lamp increase as it heats up, while the resistance of a semiconductor diode decreases when it conducts?' Facilitate a class discussion where students explain the underlying physical mechanisms.

Exit Ticket

On a slip of paper, ask students to define electric potential difference and then calculate the work done in moving 5 Coulombs of charge across a potential difference of 12 Volts.

Frequently Asked Questions

How can active learning help students understand Ohm's Law and I-V characteristics?

Active approaches like circuit building and live graphing engage students directly with concepts. They measure voltage-current pairs, plot curves, and spot ohmic linearity versus non-ohmic curves from lamps or diodes. Group discussions of anomalies, such as heating effects, build explanatory models. Data logging adds precision, while design tasks for thermistors develop practical skills. These methods make abstract relations tangible and memorable, boosting retention for exams.

What causes a filament lamp to show non-ohmic behaviour?

As current flows, the lamp filament heats, raising its temperature and resistance. This produces a curved I-V graph, steeper at low voltages. Students confirm this by comparing room-temperature resistance to hot-filament values in experiments, linking atomic vibrations to increased electron scattering.

Why do diodes have asymmetric I-V characteristics?

Diodes conduct when forward-biased, allowing majority carriers to flow easily past the depletion layer. Reverse bias widens this layer, blocking current until breakdown. Practical circuits with LEDs demonstrate the threshold voltage around 0.7V for silicon, helping students visualize p-n junction physics.

How to design an experiment for a thermistor's I-V characteristic?

Use a circuit with thermistor, variable heater or water bath for temperatures 20-80°C, ammeter, voltmeter. Measure V and I at each temperature, calculate R = V/I, plot R-T graph. Control variables like power supply stability. Justify with error analysis and repeats for reliability, showing NTC/PTC behaviour for sensors.