You are Here: Home >< Physics

# AS Electricity Question watch

1. A 6.0V torch battery consists of four 1.5V cells connected in series. It is connected to a bulb rated at 6.0V, 0.5A

a) calculate the resistance and the power generated in the bulb when it is in normal operation

b) After a time, each cell develops an internal resistance of 0.15Ω. Show that the p.d. across the bulb falls to about 5.7V, assuming that the resistance remains the same.

c) To what extent is the assumption in part b reasonable?

I'm having difficulty with part c of the question, the rest I think I got the right answers to.

2. (Original post by KK.Violinist)

c) To what extent is the assumption in part b reasonable?

I'm having difficulty with part c of the question, the rest I think I got the right answers to.
This is concerned with tacit assumptions made vs the practical limits of reality.

1) the manufacturer specification for the batteries and lamp, state 'nominal' values which vary in practice. The question assumption is that all the batteries and lamps are identical - they are not. This means the internal resistance and actual terminal p.d. can and will differ between batteries, as will the resistance and hence current drawn by the lamp.

2) the lamp element has a thermal coefficient and its' resistance will therefore change dependent on the supply voltage.

Measured results and actual performance will differ from nominal calculations.

3) the technology of the batteries and torch bulb can be different with implications for both above. e.g. battery composition, incandescent vs semiconductor lamp etc.

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 30, 2017
Today on TSR

### Exam Jam 2018

All day biology and life sciences revision thread

### Solo - A Star Wars Story

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE