Physics · Year 11 · Electricity and Circuitry · Term 3

Electric Current and Resistance

Defining electric current as the flow of charge and resistance as the opposition to that flow.

• ACARA Content Descriptions
• AC9SPU14

About This Topic

Electric current describes the flow of electric charge through a conductor, while resistance measures the opposition to that flow. For a continuous current to exist, students learn that a complete circuit with a potential difference, such as from a battery, is essential. Charge carriers, usually electrons, move under this electric field, and the rate of flow defines current in amperes.

Ohmic conductors follow Ohm's law, where current is directly proportional to voltage at constant temperature, producing a straight-line graph. Non-ohmic conductors, like diodes or lamps, deviate from this due to changing resistance. Physical properties of wires influence resistance: longer wires or thinner cross-sections increase it, as does higher resistivity of the material. Temperature effects appear more in non-ohmic cases.

This topic aligns with AC9SPU14 by building quantitative skills in analysing circuits. Active learning shines here because students construct and measure real circuits, directly observing how changes in components alter current and voltage. Hands-on adjustments reveal relationships that equations alone cannot convey, fostering deeper understanding and problem-solving confidence.

Key Questions

1. Explain the conditions necessary for a continuous electric current to flow.
2. Differentiate between ohmic and non-ohmic conductors.
3. Analyze how the physical properties of a wire affect its resistance.

Learning Objectives

• Calculate the electric current in amperes given the rate of charge flow in coulombs per second.
• Analyze the relationship between voltage, current, and resistance for ohmic conductors using experimental data.
• Compare the resistance of two wires with identical material and length but different cross-sectional areas.
• Explain the conditions required for a sustained electric current in a simple circuit.
• Classify conductors as ohmic or non-ohmic based on their current-voltage characteristics.

Before You Start

Electric Charge and Forces

Why: Students need to understand the concept of electric charge and the forces it experiences to comprehend the flow of charge in a current.

Energy and Work

Why: Understanding potential energy and work done is foundational to grasping potential difference (voltage) as the energy per unit charge.

Key Vocabulary

Electric Current	The rate of flow of electric charge, measured in amperes (A). It is defined as the amount of charge passing a point per unit time.
Resistance	The opposition to the flow of electric current in a material, measured in ohms (Ω). It quantifies how much a material impedes the movement of charge.
Ohm's Law	A physical law stating that the current through a conductor between two points is directly proportional to the voltage across the two points and inversely proportional to the resistance between them (V=IR).
Potential Difference (Voltage)	The difference in electric potential between two points in a circuit, which drives the flow of electric charge. Measured in volts (V).
Resistivity	An intrinsic property of a material that quantifies how strongly it resists electric current. It is independent of the object's shape or size.

Watch Out for These Misconceptions

Common Misconception

Electric current is used up by resistors in a circuit.

What to Teach Instead

Current remains the same throughout a series circuit; resistors cause voltage drops instead. Building circuits with ammeters at multiple points lets students measure and see constant current, correcting this through direct evidence.

Common Misconception

Thicker wires have higher resistance.

What to Teach Instead

Resistance decreases with larger cross-sectional area because more charge carriers flow easily. Experiments varying wire thickness and measuring resistance help students plot inverse relationships, building accurate mental models.

Common Misconception

All conductors obey Ohm's law at all times.

What to Teach Instead

Only ohmic conductors do so under constant temperature; non-ohmic show curved graphs. Graphing activities with lamps reveal temperature effects, as peer analysis clarifies distinctions.

Active Learning Ideas

Circuit Building: Series Circuits

Provide batteries, resistors, wires, and ammeters. Students connect components in series, measure current and voltage across each resistor, then record data in tables. Discuss why current remains constant while voltage divides.

45 min·Pairs

Progettazione (Reggio Investigation): Wire Resistance Factors

Supply wires of varying lengths, diameters, and materials. Students build simple circuits, measure resistance with multimeters, and plot graphs of resistance against length or area. Calculate resistivity from data.

50 min·Small Groups

Comparison: Ohmic vs Non-Ohmic

Set up circuits with resistors and filament lamps. Students vary voltage, plot V-I graphs using data loggers, and identify linear vs nonlinear trends. Label graphs and explain differences.

40 min·Pairs

Whole Class: Resistance Demo Relay

Demonstrate circuits on projector; pairs predict outcomes for changes like adding resistors, then test predictions on mini-circuits. Share results in class discussion.

30 min·Whole Class

Real-World Connections

• Electrical engineers designing power grids must account for the resistance of transmission lines to minimize energy loss due to heat. They select materials and wire gauges to optimize efficiency over long distances.
• Consumer electronics manufacturers, such as those making smartphones, use resistors in circuit boards to control the flow of current to sensitive components like LEDs and microprocessors, preventing damage.
• Automotive technicians diagnose electrical faults by measuring resistance in components like spark plug wires and sensors. Deviations from expected resistance values indicate wear or damage.

Assessment Ideas

Exit Ticket

Provide students with a simple circuit diagram containing a battery and a resistor. Ask them to calculate the current flowing through the circuit if the voltage is 12V and the resistance is 4Ω. Then, ask them to state one condition necessary for this current to flow.

Quick Check

Present students with a graph showing current versus voltage for two different conductors. Ask them to identify which conductor is ohmic and which is non-ohmic, and to justify their answer by referring to the shape of the graph and Ohm's Law.

Discussion Prompt

Pose the question: 'Imagine you need to reduce the resistance in a circuit. What are three specific changes you could make to the wire or components?' Facilitate a class discussion where students share their ideas and explain the underlying physics.

Frequently Asked Questions

How do physical properties affect wire resistance?

Resistance increases with length, decreases with cross-sectional area, and depends on material resistivity. Temperature rises resistance in metals. Students quantify this via experiments plotting resistance against variables, applying R = ρL/A formula to real data for practical insight.

What are the conditions for continuous electric current?

A complete closed circuit and potential difference source are required. Charge flows from high to low potential. Circuit-building tasks confirm this: open circuits show zero current, reinforcing the need for unbroken paths.

How can active learning help teach electric current and resistance?

Active approaches like constructing circuits and measuring live data make abstract flow concepts concrete. Students adjust variables, observe effects immediately, and collaborate on graphs, which strengthens retention over lectures. This builds skills in prediction, testing, and analysis central to AC9SPU14.

How to differentiate ohmic and non-ohmic conductors?

Ohmic show straight V-I lines; non-ohmic curve due to resistance changes. Use data loggers for precise plots in labs. Comparing resistor and lamp graphs helps students identify patterns and link to real devices like semiconductors.