Download File - The Physics Doctor

A –Level Physics:
Electrical Quantities:
Potential Dividers
Objectives:
42. understand how the potential along a uniform currentcarrying wire varies with the distance along it
43. understand the principles of a potential divider circuit and
understand how to calculate potential differences and resistances
in such a circuit
44. be able to analyse potential divider circuits where one
resistance
Additional skills gained:
• Practical Planning
• Integrating GCSE content
Starter:
Design a circuit with two resistors, that gives a
relative potential across R1 as 2V and R2 as 6V
Draw resistance-light intensity and resistancetemperature graphs for LDRs and thermistors
respectably.
Explain a) why resistance increases with
voltage for a filament lamp and b) why
thermistors work differently to normal
wires/filament lamps
Potential Dividers
At its simplest, a potential divider is a power source and some
resistors in series. You’re used to this set up!
I.e. by altering one resistor in the circuit, you can change the
division of the potential used by the resistors
Without working out
the current (which we
can do!) how could we
calculate the potential
difference around R2?
Ratios
HINT
Potential Dividers
So…what
happens if I
decrease the
resistance of
R1?
Total series
resistance
Examples:
Calculate p.d around R2
a) V(in)= 12V, R1=10Ω,
Ratio= 0.75, so R2=9V
R2=30 Ω
b) V(in)= 120V, R1=1kΩ,
Ratio= 0.71, so R2=86V
R2= 2.5kΩ
c) V(in)= 50V, R1=20Ω,
Ratio= R2=
0.60,30Ω
so R2=30V
d) ε=50V, R1=2Ω, R2=
Ratio= 0.86,
12Ωso R2=43V
d) ε=12MV,
R1=50kΩ,
Ratio= 0.19,
so
R2=
12kΩ
R2=2.3MV
Uses of potential dividers
We commonly replace one of the resistors with a semi conductor
which is a material that changes resistance depending on a certain
factor (e.g. LDR/thermistor/diode)
Remember: semiconductors usually have a low density of free
electrons but this increases when exposed to heat or light (thus
allowing current to flow easier!)
The ratio for R1 is
So for this circuit,
10,000/10100=
calculate the p.d
99.0
across the top
resistor with the
So p.d= 4.95V
thermistor set at
100Ω
Potentiometer
(Variable Resistor)
Instead of using two different resistors we can also just use a
single rheostat/potentiometer (variable resistor)
Explain
The longer
how the
the distance
distance of
along
a wire,
a wire
the affects
higher the
resistance
resistance
So by moving the contact, we can
alter the p.d across the terminals
(Vout)
Just think of the remaining part
of the variable resistor as R1.
Volume
Suggest
control/Dimmer
a real-life
lights function
(anythingfor
you
a want
to
potentiometer
alter continuously)
circuit
Exam Practice
Complete the exam questions provided on
the worksheet. Subsequently use the mark
scheme to mark and correct your answers
Independent Study