Fundamental Concepts of Electricity Flashcards
Electric Current
The rate of flow of (negative) charge
Measured in Amperes (SI Unit)
I = ΔQ / Δt
Δ: (delta) change
I: Current
Q: Charge
t: Time
Electric Charge
Coulomb, C
Defined Unit (In terms of SI units)
One Coulomb is Charge transferred in one second at one ampere
1C = 1As
Elementary Charge, e
Charge of one electron
e = -1.6 x 10^-19 C (Data book)
Charge of a proton: -e
n = Q/e
n: Number of Electrons
Q: Charge
e: Elementary Charge
Flow of Electric Charge
Current is due to flow of electrons in Metals
Free electrons are attracted to the +ve end and repelled by the -ve end
In Electrolytes - Liquids that carry electric charge - Is due to Ions
Conventional Current and Electron Flow
Since Electrons are -vely charge particles they flow around a circuit from the -ve to +ve terminals
Conventional current is +ve to -ve
It was discovered before the knowledge of -ve electron flow (Assumed to be +ve -> -ve)
Kirchhoff’s First Law
The sum of the currents entering a point in a circuit is equal to the sum of currents leaving the point
Conservation of Charge
Drift Velocity - v
Average displacement of electrons along the wire per unit time, towards the positive terminal of the power supply
Free electrons repeatedly collide with the positive metal ions as they drift towards the positive terminal
Number Density - n
Number of free conduction electrons per unit volume, that is in 1m^3 of the metal (Units: per meter cubed: m^-3)
Conductors -> High 10^28 - 10^29 m^-3
Semi-Conductors -> medium 10^23
Insulators -> Negligible
The higher the number density, the better the conductor
Equation for Current
I = Anev
I: Current (A)
A: Cross Sectional Area of the wire (m^2)
n: Number Density (m^-3)
e: Elementary Charge (C) - Data book
v: Average Drift Velocity (ms^-1)
Units cancel out to Cs^-1
Changes for the Equation for Current
If I x2 (n, A, e are constants) -> v x2
If A /2 (I, n, e are constants) -> v x2
If r x3 -> A x9 so v /9
Potential Difference (pd)
Energy transferred per unit charge when electrical energy is converted into another form of energy (usually in a component)
Equation for potential difference: V = W/Q
V: Potential Difference (Volts)
W: Energy Transferred (Joules)
Q: Charge (Coulombs)
Electromotive Force, EMF
Energy transferred per unit charge when one other type of energy is converted into electrical energy
Equation for EMF is Ε = W/Q
E: E.M.F (Volts)
W: Energy Transferred (Joules)
Q: Charge (Coulombs)
Volt (V)
Potential Difference across a component when one Joule of energy is transferred per unit charge passing through the component
Used for Potential Difference and EMF
Base Units: Volt
1V = 1JC^-1 (V = W/Q)
1J = 1Nm (E = Fd)
N = kgms^-2 (F = ma)
1J = 1kgm^2s^-2
1V = 1kgm^2s^-3A^-1
Base Units: Watt
1W = 1Js^-1 (P = E/t)
1J = 1Nm (E = fd)
1N = 1kgms^-2 (F = ma)
1J = 1kgm^2s^-2
1W = 1kgm^2s^-3
Electrical Power
Rate at which energy is transferred is called power and is measured in watts (W)
Power = Energy Transferred / Time Taken
(Watts) (Joules) (Seconds)
Power = Current x Potential Difference
P = IV
(Watts) (Amps)(Volts)
Power Equations
P = IV
V = IR
P = I x (IR)
P = I^2R
P = V x (V/R)
P = V^2/R
Equation for Energy Transfer
P = W/t
IV = W/t
W = IVt
W: Energy Transferred (Joules)
I: Current (Amperes)
V: P.D. (Volts)
t: Time (Seconds)
Cost of Electricity
Energy transferred to each device (Cost), depends on:
Power
How Long it is used for
Energy SI unit for energy is Joule, which is too small for energy usage in homes
Derived unit of kilowatt - hour (kWh)
Check for realistic values
kilowatt - hour: Energy transferred by a device operating at 1kW for a time of 1 hour
Energy transferred in kWh = Power in kW x time running in hrs
Calculating Cost
Cost = no.kWh x Cost per kWh (Around 15p)
Kirchhoff’s Second Law
The sum of electromotive force (emf, Ε) is equal to the sum of the potential differences around a closed loop in a circuit
Conservation of Energy
Series Circuit
Emf from the power supply is shared between components
Parallel Ciruit
P.D. across each branch is the same
Each closed loop Σpd = Σ emf
Shared between components in each closed loop
