3 Determine the EMF and Internal Resistance of an Electrical Cell Flashcards
what is a sources internal resistance
resistance of the materials inside the source. equal to lost volts per unit current in the source
what is meant by lost volts
difference between the emf and the terminal pd
what is the emf of a power source
a power supplies emf is the work done by the source per unit charge. equal to the potential difference across the source when no current flows
how ca you find the emf of a battery from a graph of V against I
emf is the y intercept
why should the battery be disconnected between readings
temperature in this experiment should remain constant so as to not affect resistance
why should the use of a rechargeable battery cell be avoided in this experiment
rechargeable power sources have a very low internal resistance which would be hard to measure in this experiment
why should a new cell/battery be used in this experiment
run down cells will have an internal resistance that may fluctuate throughout the experiment. a new cell or batter would lead to a more constant value
what safety precautions should be taken when carrying out this experiment
if carried pit for a long time the battery and circuitry will become hot. avoid touching bare metal and unplug the battery when not in use
what device should be used to find a value of the batteries internal resistance
an ohmmeter
why may the value of emf determined by the experiment be slightly different then the true value
ideal voltmeters are assumed to have infinite resistance. in reality a small current will flow through resulting in there being additional pd in parallel with the supply
why are high voltage supplies safe for use in a lab
they have a very high internal resistance. terminal pd is therefore very low, so little current flows through the circuit
