PAG 3.3 - Determining Internal Resistance And Maximum Power Of A Cell Flashcards
What is a source’s internal resistance
Resistance of the materials inside the source
What is the EMF of a power source
Work done of the source per unit charge, potential when no current flows
State the equation used to calculate EMF from load resistance and internal resistance
EMF = I (R + r)
Lost volts
Difference between the battery’s EMF and terminal PD
How do you calculate the battery’s EMF from terminal PD, current and internal resistance
EMF = V +Ir
Demonstrate how the equation of a V-I graph, for a source with internal resistance r can be obtained
V = -rI + EMF
Y = mx + c
How can you find the internal resistance and EMF of battery from a graph of V against I
V = -rI + EMF
R = (-)gradient
EMF is the y intercept
Why should the battery be disconnected between readings?
Temperature of circuit should remain constant throughout the experiment, so does not affect resistance.
Why should you avoid using a rechargeable battery/cell when carrying out this experiment?
Rechargeable power sources have a very low internal resistance, which is hard to measure
Why should a new cell/battery be used when carrying out this experiment?
Run-down cells and batteries have fluctuations in internal resistance, new sources will give a more constant value
What safety precautions should be taken when carrying out this experiment
Disconnect battery when not in use
Avoid touching bare metal contacts
What device could be used to check value for internal resistance
Ohmmeter
Suggest why your value for the cell’s EMF may be slightly different to the true value
Ideal voltmeter is assumed to have an infinite resistance. In reality, a small current may still flow through the voltmeter, resulting in there being a pd across the internal resistance
