Download 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
 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
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
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.
Learners model a simple circuit
Charge is a fundamental property
Electric charge is present inside all ordinary matter.
e = 1.6 x 10-19 coulombs, but typically huge number of free charges
Charge is a conserved quantity.
Action-at-a-distance (concept of a force field)
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.
Charged balloon on a wall
LINK to Phet animation
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
Current electricity
Charges, whether static or moving, cannot be seen
directly.
In pairs:
What different, perceptible effects might indicate an
electric current?
Moving charges constitute an
electric current
Q
I
t
Current is the rate of flow of charge past a point.
I = current an amps, Q = charge in coulombs.
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
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?
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?
An A-level explanation
I  nAve
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 filament made 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)
Drinking straws
What’s inside a straw? What is it doing?
(Air. The molecules move randomly,
but there is no net movement!)
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?
What if there are two straws?
Straws of different shapes?
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
R
l
A
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.
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 a convention in science?
Circuit diagrams
Conventional symbols
Standard procedure for building circuits
1. consider series circuit, starting at + terminal of supply and
working around to – terminal. Some components require
attention to polarity e.g. meters, diodes.
2. add parallel branches, again working from + to -.
Fault-finding
In pairs:
1. What problems can arise in simple circuits?
2. How can each of these problems be identified?
(Give procedural order for checking.)
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
The Van de Graaff generator
• how it works
• how to use it effectively in teaching
• good housekeeping and repairs
Building ‘squishy circuits’
Conducting dough (recipe), LEDs, battery pack and imagination
TRY
• Single and series LEDs (use same colour)
• Switches
• Traffic lights
Support, references
www.talkphysics.org
SPT 11-14 Electricity & Magnetism
Ep1 Developing an electric circuit model
Ep2 More about electric currents
Ep3 Adding elements to circuits
David Sang (ed., 2011) Teaching secondary physics ASE / Hodder
Phet simulations Electricity, Magnets & Circuits
Practical Physics Guidance pages e.g. Models of electric circuits