P10 Flashcards
Kirchoffs first law and related property
sum of current in junction = sum of current out
relating to conservation of charge
Kirchoffs second law and related property
sum of pd = sum of emf around a closed loop
Equation for resistors in series
total resistance = r1 + r2…..
equation for resistors in parallel
total R = (R1^-1 + R2^-1….)^-1
most common sf when working with circuits
2sf
If you need a small pd for a certain task but power source has much more pd how do you get only the pd you need
use a potential divider
how is voltage shared across a potential divider circuit
ratio of resistance = ratio of voltage share i.e
V1/V2 = R1/R2
potential divider equation (for v out)
V out = R2/(R1 + R2) x V in
what is loading in a potential divider circuit
adding another component parallel to R2 or V out - a large load i.e high resistance reduces V out slightly, a small resistance load reduces significantly - or similarly connect load parallel to R1 to increase V out later on
How do sensing circuits work
Same as potential divider but change R2 to a variable resistor/ thermistor/ LDR so resistance and V out can vary manually/ with temperature/ with light intensity
what is internal resistance
the resistance within the power source e.g battery
why do batteries have internal resistance
work done by charges - chemical cell is reactions between chemicals, solar cell due to resistance of materials
energy lost i.e lost volts
emf equation with internal resistance
and how it changes with current
emf = terminal pd + lost volts
or emf = IR + Ir
current increase mean more charge flow through battery so more work done so more lost volts so less terminal pd, emf stays same
in what condition is lost volts lowest
when current is negligible so negligible work done inside battery
experiment for internal resistance
connect battery ammeter and variable resistor in series and voltage parallel to battery
y = mx + c and emf = V + Ir
so V = -rI + emf
so a graph of v against I, the negative gradient = internal resistance and the y intercept is emf
uses of high and low internal resistance
low internal resistance useful for e.g car batteries and rechargeables to allow high recharge currents with minimal energy waste - so they recharge quickly
high internal resistance useful for safety features so power supply esp with higher voltages doesnt give fatal current e.g in schools
