With internal resistance experiment, why does I increase when V decreases?

Watch
blobbybill
Badges: 11
Rep:
?
#1
Report Thread starter 3 years ago
#1
For a normal wire at a constant temperature, I know that the graph would have a positive correlation, as when V increases, I increases too because V=IR.

However, I see that for internal resistance experiments, they vary the value of R with an external variable resistor, and plot a graph of V against I, where as V increases, I decreases.

Why is that? Can you explain that to me please? I have a feeling it is very simple but i cannot understand why when you vary the resistance, the current would increase when the voltage decreases.

Thanks

Name:  IMG_20170128_094016.jpg
Views: 180
Size:  515.5 KB
0
reply
Darth_Narwhale
Badges: 8
Rep:
?
#2
Report 3 years ago
#2
So the cell produces a fixed e.m.f right? So e.m.f = V(across internal resistant) + V(across variable resistor)

therefore e.m.f. = IR(internal) + IR(variable)
e.m.f. is fixed so as you increase R(variable), I has to fall to maintain constant e.m.f, but by a smaller amount (because I is the same for both terms, so as I drops, so does the value of both terms.
Therefore, looking at just the variable resistor now, V = IR. R goes up, I goes down, but by less, therefore V goes up.
Therefore there is a negative correlation between I and V on the variable resistor.

I may not have explained this very well, so if you still have any questions, just say and I'll draw some graphs and send some pics
0
reply
JickDee
Badges: 13
Rep:
?
#3
Report 3 years ago
#3
as you decrease resistance, current increases, which increases lost volts (as lost volts = IR) and decreases terminal p.d. (which is what you measure). The emf doesn't change and is equal to terminal p.d. + lost volts.
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

With HE fairs postponed, would a virtual HE fair be useful?

Yes (103)
61.31%
No (65)
38.69%

Watched Threads

View All