Electrical Circuit
Electricity and Magnetism

Bulbs marked as being 2 ampere

Teaching Guidance for 14-16 Supporting Physics Teaching

Markings on bulbs serve a practical purpose

Wrong Track: This is a 2 ampere bulb: it says so on the box. So if it is used in a circuit there must be a current of 2 ampere flowing around the circuit and through the bulb.

Right Lines: The 2 ampere on the box refers to the current in the bulb when it is being used at its normal operating potential difference. A 12 volt and 2 ampere filament bulb has a current of 2 ampere in the filament when used with a 12 volt supply so that there is a 12 volt drop across the bulb (wire the bulb directly across the supply).

Teaching and learning foci

Thinking about the learning

The key point for the student to get hold of here is that, so long as the current is not large enough to make the bulb fail, the bulb will allow any current to pass: the size of the current depends on the cell voltage. For example, if a 6 volt supply is used, the current through the bulb will be rather less than the 2 ampere expected for a 12 volt supply.

Thinking about the teaching

As students first start to make sense of electric circuits, their understanding might be thrown by the teacher referring to a 2 ampere bulb, which seems to contradict the message that the electric current is a dependent variable (it depends on the cell potential difference and component resistance). Careless talk can therefore be confusing. We would suggest addressing the matter head-on and having the conversation with your students, following the leading question:

Teacher: How can we make sense of a 2 ampere marking on a bulb?

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