# Resources

## Resource 1: Sources of difficulty in the electricity topic

This resource is used in Activity 1.

Table R1.1 What might your students find difficult in the electricity topic?
SectionActivityKey teaching points/What do I want students to learn from activity and related text?Sources of difficulty? Possible misunderstandings?
12.1

Current (measured in amperes) is the flow of charge (measured in coulombs) per second. Current measured by an ammeter. Conventional current flow is from + to –

Current and electron drift through a conductor. Current is instantaneous but drift speed is about 1mms–1

Charge not something that is visible

Confusion over electron flow direction and conventional current

Reconciling slow drift of electrons with instantaneous current

12.2

Potential difference across a conductor makes charge move through it

Potential difference = work done per unit charge

1 volt = 1 joule per coulomb measured using a voltmeter

Idea that a battery provides current rather than voltage
12.3Conventional symbols for commonly used components
12.412.1Voltage and current relationship for a conductor. Ohm’s Law derived from graph of V vs I for different numbers of cells

Residual confusion between voltage and current

Relating circuit diagram to real circuit construction

Voltmeter and ammeter connections

12.2Changing the component affects the current. Concept of resistance: increasing resistance gives lower current

Possible ‘current is used up by components’ misunderstanding

Mental model of electrons moving through a conductor used in text discussion

12.3

Factors affecting resistance of a conductor

The greater the resistivity or length of wire, the greater the resistance

The greater the cross-sectional area, the lower the resistance

Measuring current and inferring resistance – not measuring resistance directly.

To derive cross-sectional area rule, need to remind students that doubling the diameter quadruples the area

Remembering relationship

12.512.4For resistors in series: current the same anywhere in a series circuit; current depends on the total value of the resistance Relating circuit to circuit diagram – ‘current used up’ misunderstanding
12.6.112.5

For resistors in series, total potential difference is sum of potential differences across each resistor.

As V = IR, combined resistance of resistors in series = sum of individual resistances

Relating circuit to circuit diagram
12.6.212.6

For three resistors in parallel, pd across each resistor is the same as the pd across the combination

Current though undivided part of circuit = sum of currents through each resistor

Relating circuit to circuit diagram

Measurements could be confusing to follow

Derivation of total resistance that follows activity could be challenging; idea of reduced total resistance is at first counter-intuitive

12.7

Some energy is dissipated as heat when a current flows through a conductor

Power P = VI

Energy H = V I t

Energy

12.7.1Practical applications of heating effect: heaters, toasters, etc., filament lamps, fusesWill all students be familiar with all of these examples?
12.8

Electric power P = V I I

P = V/R

Power is measured in watts

Commercial unit of energy = kilowatt hour (kW h) = 3.6 × 106 joules.

Charge is not used up by electrical equipment. We pay for energy used, not charge

Confusion between energy and charge