10 To 11 Flashcards
Internal reisstance in batteries, leading to volte being lost
However why do the volts get lose : what even is the resistance
Whenever there is a current in a power source, work HAS to be done by the charges as they move through the poeer dourcr
So ina chemical cell this is due to reactions thst take place
And solar cells because if resistance in the materials,,
As a result the actual emf and energy avialbek in the circuit is not the same, some nergy is lose as heat, and this is due to the internal resistance
Hat are you left with
That the emf = terminak of + lost volts
This is due to Kirchhoff second law, emf in a closed loop = pd in a closed loop
So emf = terminak of = lost volts
Emf= IR + Ir E= I(R+ r)
Current if the circuit constant for both resistances
Essentially if the current increases, then the lost volts all increase too , so terminak of decreases
So if you want the emf to be the same as terminak of what must happen
The lower the current, the lower the lost volts, so a current of 0 would mean no lost volts, and here isnthe only exception where the emf and the terminal of would be the same
This is why a voltmeter with infinite resistance and almost no current gives the value of the emf
If something needs high currents, you want the internal resistance ti be low
This is because I = V/R, and so a lower r gives a higher I
So graph for E = V + Ir , what shows what etc
We know that emf = lost volts + terminal PD
We set up a practical where by you measure the terminal PD of a cell varies with a change of current which is done by a variable resistor
Thus rearranging equation
V= -RI+ E
Y is V, x is I
So gradient is -r (internal resistance)
And the y intercept is the EMF
Here we can see this occurs when the current is 0
And as current increases the value for terminal OD decreases
How should a graoh of v to air look like
A graoh if twice the internal resistance will be twice the gradient, and twice emf twice the y intercept!
This is actually how a graph looks like
So again if you want high currents what do you use low and why
Low currents for using appliances ina. Safe setting like school, use higher internal resistances
For high currents like. Car energised yiu need low internal resistances which means less r and more current
Thus ti get same voltage you can just connect 20 Aa batteries, because of their internal resistances.
Speaking of, how does internal resistances etc affected ins eriedparallel
How does adding cells in series and parallel differe for internal resistance and why choose what
If you want more emf, yiuadid. in series. However this means you add the internal resistances too.
- By adding in series yiu increases emf
- but decreases current available
- which is good for safety
In parallel , connecting two cells
- the emf is the SAME
- however the resistances are split wlsrt likemparallel ways
+ this reduces emf, but increases the current available
- so if you cleverly balance enough emf with current, then you cns do thongs like start a car!!:
Also epanitherfewtyre if high currents snd low itnerlwnreeietwnde
Also recharged quickly !
Pot etila divider equation
Total voltage x selected / total resistance = volt
What is a potentiometer?
Is a variable resistor but with THREE TERMINALS and a sliding fintsftm
When you slide intact PD VARIES from 0 when it hasn’t even moved, to the full range of vin when it has moved the whole way. As a result a potentiometer can use the full range of voltage so good in experiments l advantage . This happens only through full swing of resistance
Sdvtbages of using portniometer over a potential divider
- it can be made very compact
- it can offer full range of voltage (via full swing of resistance) + done without having to change the variable resistors each time to a. Dw value
- increase of redtsinace can be linear or logarithmic too
Allows voltsge to be changed at doefici temperatures too without having to change resistances
Essentially
Compact
Full swing
Don’t have to change
And at tempertaured too
When doing graph experiment for finding internal reditsnce, what to do about current
Using potentiometer try not to make current to crazy because a high current will increase temp thus increasing internal resistance
This current thing happens for everything except semiconductors!
Why should you connect variable resistor with thermistor at sending circuit
Allows bout to be varied at a SPECIFIC TEMOERTAURE TOO
In General for a potential divider ti give different resistances what do you want ideally
So variabel residtor and fixed residotndreting resistances to be similar so that the swing idngrewyedtb, but starting to be half way tonget maximum range
Also to be not too small and so poeer is loem
