CP 3 - Investigation of the e.m.f and Internal Resistance for a Cell Flashcards
what would your circuit look like when trying to do this practical
- you would have a zinc-carbon cell in series with an ammeter and a variable resistor
- with a voltmeter in parallel with the cell
what value should the rheostat initially be at and why
- it should be on its highest value
- to minimise any heating effects which would occur if you were gradually increasing the resistance instead
what would you do after setting the rheostat up
- record the values of V, across its terminals, and I
- repeat for settings of the rheostat
what equation would you rearrange to plot a graph for V against I
- V = e - Ir
- rearranged to V = -Ir + e
if a graph for V against I (y axis to x axis) was plotted, what would you expect the gradient to look like and represent
- it should be a straight line with a negative gradient
- and the gradient should represent -r from the rearranged equation
what would the intercept on the y axis be and why
- it would be the emf, e
- because when the current is 0, that is the emf that is being produced
- which should be equal to V on the y axis
- as no internal resistance is currently in effect
why may the line not be perfectly straight in practice
- because the internal resistance may not be constant
- especially for large currents
what is an example of the internal resistance of a battery or cell being crucial for the correct functioning of a power supply
- a car battery must have a very small internal resistance
- usually as low as 0.01 ohms
- this is because the starter motor takes a very large current when the engine is started (200A)
why would a laboratory high voltage (E.H.T.) supply have a very large built in internal resistance and what would its usual value be
- for safety reasons
- usually 50M-ohms
what does a large internal resistance do
- it limits the current to a fraction of a milliamp
- which is still enough to give you a bad shock
- so EHT supplies must be treated with care
