Episode 100: Preparation for electric current topic

Advance warning

The equipment used in this topic is relatively basic. Look out for display meters, excellent for demonstrations, and which are usually provided with shunts to allow you to change the range of the meter. Find out what ammeters and voltmeters are available for class use; review the note on multimeters in:

Introduction to circuits

Lamps (light bulbs) are often useful in place of resistors, since they light up to show when a current is flowing. However, two lamps may look identical but their resistances may not be equal – check them out before you start a demonstration. Also, the resistance of a lamp increases as it gets hotter, so double the voltage may not give double the current.

To show an electron beam, you will need to use a vacuum tube (Teltron tube). These are very useful in later work on electron beams, so it is worth learning how to set one up, and to use it without damaging it.

Prepare yourself to be careful in the language you use. Current flows through a component, and around a circuit. A potential difference is between two points, or across a component. Link this to the way you connect meters in a circuit. The circuit must be broken to insert an ammeter; a voltmeter is connected between two points.

If you discuss energy, it is most helpful to say that, whilst the lamp was lilt, the amount of energy stored chemically (by the battery) has gone down and the amount of energy stored thermally (by the surroundings) has gone up. Try to avoid referring to 'electrical energy' or 'light energy'.

Main aims of this topic

Electric Current

Students will:

• describe current as a rate of flow of electric charge:
I =  Δ Q Δ t
• give examples of charge carriers in different conductors
• use flow analogies for current: e.g. water circuit, traffic etc.
• relate current to microscopic charge flow: I = nAqv
• use ammeters correctly
• describe cells, batteries etc. as electrical pumps
• use analogies – e.g. relating potential difference to changes in height
• give examples of energy transfers (e.g. in a filament lamp or kettle, electrical working in a filament to raise its temperature and heating by radiation or coduction as it raises the temperature of the surroundings)
• define the volt using V =  Δ E Δ Q, 1 V = 1 J1 C
• use voltmeters correctly
• calculate power in electric circuits P = I × V and electrical work

Prior knowledge

Most students will be familiar with concepts of charge, current and voltage from their previous work at pre-16 level. However, these ideas are often muddled and this can be a real obstacle to progress so it is well worth reinforcing simple ideas and providing basic training in the use of ammeters, voltmeters (and/or multimeters) fairly early on. It is particularly important to separate the idea of an electric current as a flow of real stuff (i.e. charge carriers) from the more abstract idea of potential difference which does not flow and which is related to potentials around the circuit.

Where this leads

Note that the idea of resistance is not included here, but will be dealt with in the next topic. The ideas of charge and energy conservation are implicit in this topic; they will be formalised in Kirchhoff’s laws.