4.3 circuits Flashcards
kirchoff’s second law
the sum of the e.m.f is equal to the sum of the p.ds around a closed loop
resistors in series equation
- I is the same through each resistor
- RT = R1+ R2 + R3
- p.d is shared in proportion to resistances
resistors in parallel equation
- I is shared (Kirchoffs 1st law)
- p.d is the same across the resistors
- 1/RT = 1/R1 + 1/R2 + 1/R3
cells in series
- same direction = total emf = sum of emfs
- different direction = total emf = difference in emfs
cells in parallel
doesnt change the total emf provided to the circuit
what does a potential divider do
provides a p.d less than the terminal p.d
what does the ratio of the p.ds across the resistors in a potential divider equal?
the ratio of their resitances
formula for Vout in a potential divider
Vout = R2 = Vin
R1 + R2
potential divider and thermistor circuit
- as T increases
- R decreases
- Vout decreases
potential divider and LDR circuit
- light intensity increases
- R decreases
- Vout decreases
what is a potentiometer
variable resistor with a connection in the middle that allows Vout to be varied from 0V to the max source p.d between the middle and end connection
when are potentiometers useful
- in volume and light controls, where the range needs to be from 0 to a max
- finding the I-V characteristics of a compontent when you also need to vary the p.d across the component
what is the internal resistance of a power supply
the resistance to the flow of current inside the power supply itself
* so some of the power supply transfers to thermal energy
what are ‘lost volts’
energy transfer per unit charge of the energy transferring from electrical to thermal
terminal p.d equation:
= V = IR
