When someone grips the rope more tightly, they feel their hands getting warmer, and the rope gets harder to pull. The extra grip is increased resistance. The person’s hands being warmed is like energy being transferred out of the circuit. The person gripping the rope is like a bulb or a resistor. Strengths This model shows that all the charge is moving around the circuit at the same time, and it makes a link between resistance and energy transfer. Limitations If the person pulling the rope starts to pull harder when someone grips the rope more tightly, it could suggest that the battery works harder to keep the current the same when the resistance increases. Resource 4: Using models and analogies to teach electricity This resource is used in Activity 2. Table R3.1 identifies models and analogies being used and provides some suggestions for other models that might be helpful. Table R4.1 Using models and analogies to teach electricity. Section Activity Key teaching points/what do I want students to learn from activity and related text? Sources of difficulty? What models or analogies are being used or might help here? 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 1 mm s–1 Charge not something that is visible Confusion over electron flow direction and conventional current Reconciling slow drift of electrons with instantaneous current Being used: Electric current as a flow. Circuit is a continuous closed path – any break stops the flow Might also help: Rope model Section Activity Key teaching points/what do I want students to learn from activity and related text? Sources of difficulty? What models or analogies are used or might help here?
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 Being used: Gravitational potential difference needed for water to flow downhill. Electrical potential difference needed for flow of charge Might also help: Rope model 12.3 – Conventional symbols for commonly used components. – 12.4 12.1 Voltage 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 Being used: Circuit diagram as representation of circuit (used throughout activities) 12.2 Changing 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 Might help: Rope model, sweet model Section Activity Key teaching points/what do I want students to learn from activity and related text? Sources of difficulty? What models or analogies are being used or might help here?
12.4 12.3 Factors affecting resistance of a conductor The greater the resistivity or length of wire, the greater the resistance The greater the area, the lower the resistance Measuring current and inferring resistance – not measuring resistance directly To derive area rule, need to remind students that doubling the diameter quadruples the area Remembering relationship Might help: (Something that could be acted out?) Carrying stacks of parcels in a crowded corridor. Things get knocked off the stacks by collisions More gets knocked off the longer the corridor (length), and the narrower the corridor, the higher the frequency of collisions 12.5 12.4 For 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 Might help: Sweet model 12.6.1 12.5 For resistors in series: potential difference is sum of potential differences across each resistor As V = IR, the combined resistance of resistors in series = sum of individual resistances Relating circuit to circuit diagram – Section Activity Key teaching points/what do I want students to learn from activity and related text? Sources of difficulty? What models or analogies are being used or might help here?
12.6.2 12.6 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 resistance that follows activity could be challenging; idea of reduced total resistance is at first counter-intuitive Might help: A slope model for pd. Allow three ball-bearings to roll simultaneously down from the lip of a wide plastic funnel (held over a bowl). Each has its own route but the drop distance is the same each time. help: Current model of group splitting up to go along three routes then rejoining again. Reduced resistance effect like using three delivery vans at the same time rather than one 12.7 - Some energy is dissipated as heat when a current through a conductor Power P = VI Energy H = V I t Energy H = I2 R Might help: Rope model 12.7.1 - Practical applications of heating effect: heaters, toasters, etc., filament lamps, fuses Will all students be familiar with all of these examples? – Section Activity Key teaching points/what do I want students to learn from activity and related text? Sources of difficulty? What models or analogies are used or might help here?