1. A –Level Physics: Electrical Quantities: Potential Dividers

2. Objectives: Additional skills gained: Practical Planning
42. understand how the potential along a uniform current-carrying 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

3. 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 resistance-temperature 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
21978

4. Potential Dividers Ratios
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?
Delta
HINT
Ratios

5. So…what happens if I decrease the resistance of R1?
Potential Dividers
So…what happens if I decrease the resistance of R1?
Delta
Total series resistance

6. Examples: Calculate p.d around R2
a) V(in)= 12V, R1=10Ω, R2=30 Ω
Ratio= 0.75, so R2=9V
b) V(in)= 120V, R1=1kΩ, R2= 2.5kΩ
Ratio= 0.71, so R2=86V
c) V(in)= 50V, R1=20Ω, R2= 30Ω
Ratio= 0.60, so R2=30V
d) ε=50V, R1=2Ω, R2= 12Ω
Ratio= 0.86, so R2=43V
d) ε=12MV, R1=50kΩ, R2= 12kΩ
Ratio= 0.19, so R2=2.3MV

7. 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!)
So for this circuit, calculate the p.d across the top resistor with the thermistor set at 100Ω
The ratio for R1 is 10,000/10100= 99.0
So p.d= 4.95V
Delta

8. Potentiometer (Variable Resistor)
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!)
Potentiometer (Variable Resistor)
Instead of using two different resistors we can also just use a single rheostat/potentiometer (variable resistor)
Explain how the distance along a wire affects resistance
The longer the distance of a wire, the higher the 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.
Suggest a real-life function for a potentiometer circuit
Volume control/Dimmer lights (anything you want to alter continuously)

9. Exam Practice
Complete the exam questions provided on the worksheet. Subsequently use the mark scheme to mark and correct your answers
Delta

10. Independent Study