DC Circuits
Symbols
Electromotive Force (e.m.f)
Energy transferred through a power supply per unit coulomb in a circuit. It is the conversion of chemical energy into electrical energy
Potential Difference
Energy transferred through a circuit per unit coulomb. It is the conversion of electrical into other energy
E.m.f or p.d. (V) V = W Energy transferred/Work done (J)
Q Charge (C)
Q Charge (C)
Kirchhoff's Second Law
The total voltage across a circuit loop is equal to the sum of the voltage drops across the components in that loop.
Types of Circuit
Series
How to work out the overall resistance
Use Kirchhoff's second law
So:
VTot = V1 + V2 + V3 etc.
VTot = IRTot
IRTot = I(R1 + R2 + R3 etc.)
'I' cancels out giving
RTot = R1 + R2 + R3 etc.
Use Kirchhoff's second law
So:
VTot = V1 + V2 + V3 etc.
VTot = IRTot
IRTot = I(R1 + R2 + R3 etc.)
'I' cancels out giving
RTot = R1 + R2 + R3 etc.
Parallel
How to work out the overall resistance
Use Kirchhoff's first law
So:
ITot = I1 + I2 + I3 etc.
ITot = V/RTot
V/RTot = V(1/R1 + 1/R2 + 1/R3 etc.)
'V' cancels out
1/RTot = 1/R1 + 1/R2 + 1/R3 etc.
The total resistance is lower than all of the other resistance values
Use Kirchhoff's first law
So:
ITot = I1 + I2 + I3 etc.
ITot = V/RTot
V/RTot = V(1/R1 + 1/R2 + 1/R3 etc.)
'V' cancels out
1/RTot = 1/R1 + 1/R2 + 1/R3 etc.
The total resistance is lower than all of the other resistance values
Potential Divider
Used to divide the voltage between resistors. This is done through a circuit with resistors in series.
To work out the Voltage over a resistor you can use the equation.
V1 = R1 x Vin
R1 + R2
V1 = R1 x Vin
R1 + R2
Power
The rate of energy transfer in a unit time. Measured in Watts
Power (W or J/s) P = W Work Done (J)
t Time (s)
t Time (s)
Large appliances are usually measured in kilowatts
Companies keep track of how much electricity is being supplied in kilowatt hours (kWh)
Kilowatthour - the amount of energy used by a 1 kW appliance every hour
This is worked out by
Power in kW multiplied by time in hours = kWh
Companies keep track of how much electricity is being supplied in kilowatt hours (kWh)
Kilowatthour - the amount of energy used by a 1 kW appliance every hour
This is worked out by
Power in kW multiplied by time in hours = kWh