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Charge and current

Charge and current. A challenge. In pairs: Using only the items provided, make the bulb light in as many different ways as you can. Sketch every configuration you try, whether it works or not. ( pictures, not circuit diagrams ). Getting started. Learning outcomes.

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Charge and current

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  1. Charge and current

  2. A challenge In pairs: Using only the items provided, make the bulb light in as many different ways as you can. Sketch every configuration you try, whether it works or not. (pictures, not circuit diagrams)

  3. Getting started

  4. Learning outcomes • recognise that charge is a fundamental property of matter • begin to use the field model of interactions to predict forces • explain the attraction between neutral and charged objects in terms of polarisation of molecules • infer the existence of an electric current from its effects • define electric current as a flow of charge • use rope loop(s) to model electric current in simple circuits • describe how currents behave in series and parallel circuits, relating this to ideas of resistance and charge conservation • draw and interpret relevant circuit diagrams and symbols • appreciate the distinction between modelling the behaviour of electric circuits and explaining it at a fundamental level • develop confidence in using an ammeter & troubleshooting

  5. Misconceptions Battery • stores electricity, sources current, or supplies charges but eventually runs out of them Current in a simple battery-lamp circuit • one thing happens after another, so current gets used up as it goes round a circuit. (sequential model) • Only one wire is ‘active’ (a single connection to lamp sufficient). • Current at lamp from both ends of battery (‘clashing currents’). Electricity = current = voltage = energy = power

  6. Teaching challenges • Few learners appreciate that charge is a fundamental property of matter because, in most everyday materials, positive and negative charges are balanced. • Few learners do know that charged objects of either sign will attract a neutral object. • Most learners are aware of the need for a ‘complete circuit’ but they have no insight into why. • Charges & energy, used to explain circuit behaviour, cannot be seen or experienced directly. • Many learners have difficulties drawing & interpreting circuit diagrams.

  7. Learners model a simple circuit

  8. Charge is a fundamental property Electric charge is present inside all ordinary matter. e = 1.6 x 10-19 coulombs, buttypically huge number of free charges Charge is a conserved quantity. Action-at-a-distance (concept of a force field)

  9. Static electricity • using an electroscope • charging by contact • earthing • charging by induction Early experiments with electricity:William Gilbert (~1600), Stephen Gray (~1729), du Fay (~1733), Benjamin Franklin (~1750). Nicola Kingsley (1989) Benjamin Franklin, ASE Nature of Science series.

  10. Charged balloon on a wall LINK to Phet animation

  11. Charged balloon on a wall • Electrons are mobile. • Charge is induced in the wall. • Electric force depends sensitively on distance. Another example: charged comb deflecting a water stream

  12. Current electricity Charges, whether static or moving, cannot be seen directly. In pairs: What different, perceptible effects might indicate an electric current?

  13. Moving charges constitute an electric current Current is the rate of flow of charge past a point. I = current an amps, Q = charge in coulombs.

  14. Experimenting with circuits series and parallel, batteries and bulbs, ammeter(s) Students learn • rules for current (current the same everywhere in series circuits, adds with parallel branches) • semi-quantitative idea of resistance Current in a simple circuit is larger if • voltage of the supply is larger • resistance in the circuit is smaller

  15. The rope model Energy is transferred from battery to lamp. What happens when there’s • More than one battery • More than one lamp … in series? • More than one lamp … in parallel? Strengths & weaknesses of this model?

  16. The BIG circuit Q: How do charges get to the lamp when a switch is closed? A: They are there already. They feel the effect of the power supply almost instantly. Q: How much energy is used up in the wires? A: Almost none. Why?

  17. An A-level explanation where n = charge density, A = cross-sectional area, v = drift speed, e = electron charge In copper wires:1 free electron per atom (8.5 x 1028 electrons per m3), in random thermal motion. Battery makes all of these electrons drift the same way, colliding with metal ions. Drift speed ~0.02 mm/s Lamp filamentmade of tungsten (3.4 x 1028 electrons per m3), with smaller cross-sectional area. Drift speed ~250 mm/s  more frequent collisions  heating effect Another battery:bigger push  greater drift speed  more electrons pass any point per second (bigger current)

  18. Drinking straws What’s inside a straw? What is it doing? (Air. The molecules move randomly, but there is no net movement!)

  19. Can you make the air flow? (A force is needed to make anything start moving and, if there are resistive forces, to keep it moving.) What if the pressure is bigger?

  20. What if there are two straws?

  21. Straws of different shapes?

  22. Current, voltage and resistance Current in a simple circuit will be larger if • voltage of the supply is larger • resistance in the circuit is smaller where R is resistance,  is resistivity of the material, l is its length and A is its cross-sectional area. The same relationships apply in networks of identical resistors.

  23. Conventional current Conventional current flows + to – (Benjamin Franklin) Charge carriers can be • ions (in electrolytes) • holes (in semiconductors) • electrons (in metals) Electrons (discovered 1897) flow in opposite direction to conventional current. Can we? Should we? … change every book and educated mind in the world? What’s meant by aconventionin science?

  24. Circuit diagrams Conventional symbols Standard procedure for building circuits • consider series circuit, starting at + terminal of supply and working around to – terminal. Some components require attention to polarity e.g. meters, diodes. • add parallel branches, again working from + to -.

  25. Fault-finding In pairs: • What problems can arise in simple circuits? • How can each of these problems be identified? (Give procedural order for checking.)

  26. Experiments • electrostatics: electroscope, charging, earthing • ERIC board • Practical Physics experiments • Using ammeters • Problem circuit • Electrolysis of copper sulfate solution • Series and branching circuits • From galvanometer to ammeter

  27. The Van de Graaff generator • how it works • how to use it effectively in teaching • good housekeeping and repairs

  28. Building ‘squishy circuits’ Conducting dough (recipe), LEDs, battery pack and imagination TRY • Single and series LEDs (use same colour) • Switches • Traffic lights

  29. Support, references www.talkphysics.org SPT 11-14 Electricity & Magnetism Ep1 Developing an electric circuit model Ep2 More about electric currents Ep3Adding elements to circuits David Sang (ed., 2011) Teaching secondary physics ASE / Hodder PhetsimulationsElectricity, Magnets & Circuits Practical Physics Guidance pages e.g.Models of electric circuits

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