Electricity (M4) Flashcards
Potential divider equation
V out = R2/(R1+R2) X V in
emf equation
E = V + Ir
E = I(R+r)
Power equations
P=VI
P=IIR
P=VV/R
Potential difference
The energy transferred by charge carriers from electrical energy to other forms (eg heat or light) per unit charge
Volt
Unit of potential difference and emf.
The energy transferred per unit charge, whether energy Is being transferred to or from the charge
V=JC^-1
emf
When energy is transferred from chemical energy (or other forms) to electrical energy per unit charge.
Internal resistance
The resistance of a source of emf which causes a loss in energy/voltage as charge passes through the source.
Symbol: r
Unit: ohm
Terminal potential difference
The p.d. across a power source. When there is no current, this is equal to the emf, but if there is a current in the source, it is equal to the emf minus the lost volts.
Lost volts
When the terminal p.d. is less than the actual emf due to some energy being ‘lost’ (transferred to heat) when there is a current in the power source and not all the energy transferred tonthe charge is available to the circuit.
Kilowatt-hour
A derived unit of energy, most often associated with paying for electrical energy.
Calculated by multiplying the power in kW by time in hours.
Symbol:kWh
1kWh = 3.6MJ
Kirchhoff’s second law
In any circuit, the sum of electromotive forces is equal to the sum of the p.d.s around a closed loop.
Resistors in series
R = R1 + R2 + …
Resistors in parallel
1/R = 1/R1 + 1/R2 + …
Ohm’s law
For a conductor at a constant temperature, the current is directly proportional to the potential difference across it.
Electrical resistance
A property calculated by dividing the potential difference across it by the current in it.
Symbol: R
Unit: ohm
V = IR
Resistivity
A property of a material defined as the product of the resistance of a component made of the material and its cross sectional-area divided by its length.
Symbol: rho
Unit: ohm meter
Electric current
The rate of flow of charge, measured in amperes (A). Normally a flow of electrons in metals or a flow of ions in electrolytes.
Symbol: I
Q = It
Coulomb
Unit of electrical charge. 1 C passes through a point in 1 second when there is an electric current of 1 A.
C = As
Ampère
The base SI unit of electric current. It is defined as the current flowing in 2 parallel wires in a vacuum 1m apart such that there is an attractive force of 2x10^-17 N per metre length of wire between them.
Charge of an electron
Negative elementary charge
-1.6x10^-19
Kirchhoff’s first law
At any point in an electrical circuit, the sum of currents into that point is equal to the sum of electrical currents out of that point.
Electrical charge is conserved.
Drift velocity equation
I = Anev
Conductor
Material with very high number density
Order of 10^28 per m^3
Eg metals
Number density = number of free electrons per cubic metre
Semiconductor
Material with number density around 10^17.
Eg silicon and germanium.
Insulator
Material with very low number density.
Eg wax and rubber
