Current Equation
I = Q / T
Voltage Equation
Voltage = E / Q
Voltage, Resistance and Current Equation
V = I * R
Voltage
Work done per unit charge
Current
Rate of flow of charge
Ohms Law
Current and voltage are directly proportional when at a constant temperature
Drift Speed Equation
Electron Drift
At normal temperatures, with no current flowing, electrons hurdle around continuously. They collide with ions but because their movement is random, there is no net energy transfer.
However when a voltage is applied, electrons are accelerated from negative to positive. This movement is superimposed on top of random velocities and is responsible for electrical effects.
Series Circuit Rules
Current is the same at any point
Voltage is split up on each component
Parallel Circuit Rules
Current splits up over each strand
Voltage is the same at any point
Non - Ohmic
As bulb filament gets hotter, resistance increases so rate of current decreases or vice versa
Diode
Allows convential current to pass in the direction of the arrow
Resistors in Series
Resistors in Parallel
Internal Resistance
V = E - Ir
Internal resistance causes lost volts. The higher the internal resistance the more lost volts there are. Electrical energy converted to heat energy and power supply heats up.
Electromotive Force (EMF)
Energy transferred per unit of charge
E = W / Q
Electrical energy
The amount of energy transferred by a chatge travellling through a potential difference.
W = QV
W = IVt
Q = It & W = QV so W = IVt
Power
The rate that energy is transferred.
P = W / t
P = I^2 * R or P = V^2 / R
P = IV & V = IR so P = I^2 * R or P = V^2 / R
Efficiency
( Useful / Total ) * 100
Kirchoff’s 1st Law
Current In = Current Out
Based on the assumption charge is conserved at each junction within a circuit.
Kirchoff’s 2nd Law
Sum of emf in a closed loop is equal to the sum of P.D in that loop
CPAC 3 - Find EMF and Internal Resistance
Set up equipment as shown
Change current by adjusting R and measure A and V.
Plot V on the y axis and I on the x axis. E is y intercept and gradient is -r
Potential Divider vs Variable Resistor
Both suitable because allow readings of P.D and current.
For potential divider, the minimum P.D across the lamp is 0V whilst for variable resistor it is greater than 0V.
Potential dividers max P.D is that of the supply P.D.
Variable resistor changes resistance to change current so minimum P.D relies on its resistance.
Hence Potential Divider is better because it can use P.D down to 0V.
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