Module 4 - Section 1 - The Potential Divider Flashcards
What is a potential divider ?
A potential divider is a simple circuit that uses resistors (or thermistors / LDR’s) to supply a variable output potential difference.
So basically if you had an emf of 12V and 2 resistors - 1ohm and 2ohm - then the resistor of 1ohm will get 1/3 of the 12V emf - so 4V and the 2ohm resistor would get 2/3 of the 12V emf which is 8V
Whats the equation for the potential divider ?
Vout = R2 / R1 + R2 x Vin
This equation basically tells you the share of pd that the resistor gets from the source pd
How can you tell which resistor is R2 ?
The R2 resistor is the one which you have Vout over
Whats an example of a source pd - aka voltage source ?
A battery, cell, power supply
What does it mean when you load your potential divider ?
It is when you place a component where the Vout is - so placing a component parallel with Vout
This lowers the resistance of this part of the circuit and so lowers the Vout
However if Rload has a bigger resistance than R2 then it has little effect on the Vout
However if the Rload has a smaller resistance than R2 then Vout is significantly reduced as 1 / Rt = 1/R1 + 1/R2 and Rt is always smaller than the smallest R
How can you make a potential divider with an LDR or a thermistor
LDR has a very high resistance in the dark but a little resistance in the light
Therefore when its dark the Vout will be far greater then when there’s light as when there is light that means there will be less resistance and therefore a smaller share of pd to the Vout so a smaller Vout
So it can be used as a light sensor or heat sensor ( when using a thermistor ) as when its dark the LDR will sense this and have a high resistance and so a higher pd and a bigger Vout
What is a potentiometer ?
The potentiometer has a variable resistor replacing R1 and R2 of the potential divider
You can move a slider or turn a knob to adjust the relative sizes of R1 and R2 - this means that you can vary the Vout from 0V up to the input voltage - Vin
This is because Kirchhoff’s second law - the emf of the series circuit = the sum of the pds across each component - therefore if you move the slider and make the R1 have a higher resistance then R1 will have a greater share of the source pd and therefore Vout will be smaller and so technically the Vout could be 0V because you made the R2 have no resistance as you gave all the pd to R1 by moving the slider - or it can have the full source pd if you make R2 have a lot of resistance
Whats the point of a potential divider ?
The point of it is that you can choose the resistances to get the voltage you want across one of them
How are potentiometers used ?
Potentiometers are very useful in things that you want to change the voltage continuously - like for example the volume control of a stereo - because if you want to get max volume then you would move a slider and give R2 the most voltage and therefore Vout the source pd and therefore with more pd there is a greater current and therefore the speaker will be louder
How to find the pd across a resistor if you only get the resistance of one of the resistors and no input pd and you also get the 2 resistances of the resistors .
V1 / V2 = R1 / R2
