A Level Physics Electrical Quantities Potential Dividers Objectives

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 R 1 as 2 V and R 2 as 6 V 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 R 2? Ratios HINT

Potential Dividers So…what happens if I decrease the resistance of R 1? Total series resistance

Examples: Calculate p. d around R 2 a) V(in)= 12 V, R 1=10Ω, Ratio= 0. 75, so R 2=9 V R 2=30 Ω b) V(in)= 120 V, R 1=1 kΩ, Ratio= 0. 71, so R 2=86 V R 2= 2. 5 kΩ c) V(in)= 50 V, R 1=20Ω, Ratio= R 2= 0. 60, 30Ω so R 2=30 V d) ε=50 V, R 1=2Ω, R 2= Ratio= 0. 86, 12Ωso R 2=43 V d) ε=12 MV, R 1=50 kΩ, Ratio= 0. 19, so R 2= 12 kΩ R 2=2. 3 MV

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 R 1 is So for this circuit, 10, 000/10100= calculate the p. d 99. 0 across the top resistor with the So p. d= 4. 95 V 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 distance of along a wire, a wire the affects 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 R 1. Volume control/Dimmer Suggest a real-life function lights (anything you want for a potentiometer circuit to alter continuously)

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

Independent Study