7 Potential Dividers

7/28/2019 7 Potential Dividers

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Potential Dividers

You will be familiar with the use

of a variable resistor to varycurrent.



M

The variable resistor acts as a control overthe flow of current. It is being used as arheostat. The result is a control over the

speed of the motor.more resistance

less current

slower motor

The nextcircuit is verydifferent..



R 1

R 2

V1

V2

V

V does not change

V = V1 + V2

The two resistors aredividing up thepotential (or voltage) -(the Potential Divider)

21

1

1

RR

RVV

21

2

2

RR

RVV



What happens if:

R1 increases and R2 isunchanged?

R1 decreases and R2 remains the same?

The values of bothR1 and R2 aredoubled? The values of both R1

and R2 are halved?

The supply voltage V istrebled?

V1 V2 V

x 3 x 3 3V

V

V

V

V

V1

V1

V2

V2



Uses of the potential divider

• To supply a variable voltage

• To make an input sensor from othercomponents - many detectors will switch on as avoltage goes above or below a certain threshold(such as in a thermostat).



Consider the thermistor

-its resistance decreases as the temperature increases

R 1

R 2

V1

V2

V

R1 is a thermistor. Suppose a heater switcheson if V2 “went high”.

As it gets hot, R1 xxcreasesand so V1 xxxxs.

This means V2 mustxxxx. ie it “goes ?”

The thermostat turns on.

Falls

decreases

Rise

Could you explain what

would happen if you put thethermistor where R2 is?



Now using theoriginal circuit as inthe diagram, whatwould be the effect ofmaking R2 a variableresistor?

R 1

R 2

V1

V2

V

It would effectively

vary the switching

temperature.



An LDR (light dependent resistor) has a

resistance which decreases with increasedillumination. Sketch a circuit to show how you

could use a power supply, voltmeter, LDR and

a fixed resistor to measure light intensities.

What would be the point in replacing the fixed

resistor with a variable resistor?

How would you modify your circuit to measure

temperature instead of light intensity?



R 1

R 2

V1

V2

V

Replace with

an LDR



What would be thepoint in replacing thefixed resistor with avariable resistor?

How would youmodify your circuit to

measure temperatureinstead of lightintensity?

Change the lighting level at

which it switched

Use a thermistor



Measuring PD with a potential divider.

V

S

A B

2 volt

Moving the slider S from Ato B will make the

voltmeter read from zeroup to the supply voltage -in this case from 0V to 2V.

The resistance AB could be

1m of “resistance wire”.

If AS was 25cm, what

would the voltmeter read?2Vx(25cm/100cm) = 0.5V

If SB was 20cm, what would

the voltmeter read? 2V((100cm-20cm)/100cm) = 1.6V

We don’t really need thevoltmeter to know what that

voltage is!



We don’t really need the voltmeter to know what thatvoltage is!

This is only because the current through the wire AB isthe same along its length and we assume none flows

through the voltmeter.

S

A B

2 volt

Centre Zero

galvanometer - a very

sensitive ammeter.

V

For practical purposes a resistor is

frequently placed in series with the galvoto protect it from high currents.



Suppose the voltage from A to S is the same as the emf of

the second cell.

S

A B

2 volt

Centre Zero

galvanometer - a very

sensitive ammeter.

V

The voltage drop from A to S due to the 2 V cell will matchthe emf from the other cell so no current will flow throughthe galvo.

If we move S to the left, VAS falls, so V will drive currentthrough the galvo.

If we move S to the right, VAS rises and forces currentback through the galvo and cell, the other way.

We now know that V = VAS.



QuestionIf the wire AB is 1m long and a “balance point” is

reached when AS = 20cm, What is the value of V ?

V = 2volt x (20cm/100cm)V = 0.4volt



Why might you do the following?

S

A B

2 volt

V

Add this

resistor

here?

S

A B

2 volt

V Add this switch and

resistor here?

This type of circuit is frequentlyused to measure very small emfssuch as those generated bythermocouples - what are they?

It is called a null method as areading is made when the galvois reading zero.

It is more accurate to look for thebalance point when making theconnection at S produces nodeflection of the galvo - why?



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7 Potential Dividers