EMF vs Terminal Voltage. What's the difference?

#physics #electricity #batteries


This video was built as part of the learning resources provided by the Western Canadian Learning Network (a non-profit collaboration).

0:00to this point we've treated any battery in our circuits
0:04as a perfect Supplier occurrence that is
0:08it doesn't have any internal resistance it just supplies current to the rest to
0:13the circuit
0:14Matt say now this isn't
0:17ever quite true all batteries have some
0:21internal resistance and internal resistance does effectively
0:25adding that the resistor to any circuit in most cases
0:31the resistance is very small and we choose to ignore it
0:35nevertheless we need to acknowledge
0:38that it's there in order to better understand how batteries for
0:42also there are times
0:45when the batteries alder effective and the internal resistance becomes quite
0:50high
0:50in these cases someone who doesn't understand
0:55internal resistance might be totally confused when they take the battery
0:59editor
0:59faulty device measured with a voltmeter battery would look perfect on the
1:04voltmeter
1:04but plug it back into the device it's still doesn't work
1:08so let's take a look at how this works
1:12here's a battery but this time
1:16will include other a resistor are to represent
1:21the internal resistance up the battery if we measure the voltage the battery
1:27well it's not in a circuit here where really you
1:31measuring its EMF its
1:34a lecture motive force sense there's
1:39no current running through the battery there's no
1:42altijd rock across this little internal resistance
1:45are and we get the full twelve holes
1:49the EMF our battery here
1:52his 12 balls now
1:56if we clear battery in toward device
1:59effectively facing the battery into a circuit
2:03bed current begins to flow now
2:07when the current close to the battery we have a little
2:10voltage shop here voltage equals
2:13I R the current times a resistor R
2:18now when we measure the voltage across the battery
2:23we're measuring the EMF minus
2:27the voltage drop due to this internal resistance
2:30that s EMF minus
2:35are I are we call this
2:40the terminal voltage the voltage across the terminals and a battery
2:46when it's in a circuit in a battery that is working well
2:51the internal resistance is really small so when we calculate the voltage drop
2:56I times are it's really small and we may measure
3:0111.94 storm or
3:04as a terminal voltage very little difference
3:09now if the internal resistance is large
3:13the voltage again tight ends are can become
3:16very significant it may drop down
3:1926 votes are even lower your device wouldn't work
3:23even though the EMS was
3:27and still missed 12 holes now and knowledgeable
3:32car mechanic re Electronix repair person well
3:36not only check a question of batteries EMF
3:39but they'll also check the battery under
3:42load that is the terminal voltage
3:45when the batteries in a circuit they know that
3:49EMF may not tell the whole story
3:52if the EMF and the terminal voltage
3:56are significantly different the battery may have an internal problem
4:01causing a large internal resistance many people who don't understand this simply
4:06check the EM bath and assume that the battery spy unfortunately
4:11this often results in moving on to replacing other expensive are treated
4:16throwing away perfectly good devices by the time you're done this unit
4:21you recognize the truth to this air and be better
4:24at problem solving

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