Module 4 Fundamental electricity Flashcards
Definition of electric current
the rate of flow of (negative) charge
Unit for current?
Amperes, A
Unit for charge
Coulombs, C
Which letter is current in equations
I (i)
Definition of a coulomb
1 coulomb is the electric charge flowing past a point in 1 second when there is a current of 1 ampere
In terms of base units, what does 1C equal?
1As
that is 1 ampere second
What are charge carriers in a metal wire?
electrons
Formula for number of electrons using charge?
n=Q/e where Q is the charge and e is the elementary charge
In a metal where there is no current, describe the positioning of ions and electrons
Ions vibrate around their fixed positions - delocalised/free electrons move in random directions.
Describe the movement of electrons when a current flows in a metal?
electrons drift from negative to positive terminal (repelled by -ve, attracted by +ve)
with drift velocity
In an electrolyte, what are charge carriers?
positive and negative ions
What’s the direction of flow of conventional current?
from the positive to negative terminals
State Kirchhoff’s first law
the sum of currents entering a junction at any point in a circuit is equal to the sum of currents leaving the same point
What is conserved in Kirchhoff’s first law?
expression of conservation of charge
Define drift velocity
average displacement of electrons along a wire per unit of time towards the positive terminal of the power supply
What is number density?
number of conduction electrons per unit volume
(unit of number density m^-3)
What is an insulator?
materials which have very few conduction electrons that we say it’s negligible
Equation for current using number density?
I=Anev
I current
n number density of electrons
e elementary charge
v drift velocity
Effect of doubling radius on drift velocity in a wire?
A=I/nev, I n e are constant,
A proportional to 1 /v,
area is formed from πr^2 – 2r^2 = 4r so area is increased by 4
therefore 4A=1/0.25v
velocity is multiplied by 0.25
What is the unit of potential difference?
Volts V
Definition of potential difference
energy transferred per unit charge when electrical energy is converted to another form of energy
Equation for potential difference?
V=W/Q
V potential difference
W work done
Q charge
Electromotive force definition
Energy is transferred per unit charge when a type of energy is converted into electrical energy
Equation for EMF?
E=W/Q
E EMF
W work done
Q charge
Difference and similarities between EMF and potential difference?
Both are energy per unit of charge (both are measured in volts)
In EMF energy is transferred to electrical energy, in PD electrical energy is transferred into another form
Define what is a volt?
joule per coulomb
Why are both potential difference and EMF’s measured in volts?
both are energy transferred per unit of charge
What is electrical power defined as?
rate of energy transfer
Formula for power using time?
P=W/t
P Power
W work done or energy transferred
t time
How is the rate of energy transfer related to I, V and R of an electrical component?
P=IV or P = I^2 R or P=V^2/R
P Power or rate of energy transfer
I current
V potential difference across a component
R is resistance of the component
How is potential difference related to resistance and current?
V=IR
What is the unit for resistance?
Ohm’s - Ω
Derive the equation for energy transfer (W=IVt) using P=W/t
P=W/t
Substituting P=IV we get IV=W/t
Rearrange to get W=IVt
What effects the cost of running an electrical device?
cost is per unit of energy transferred,
power of the device and
how long it has been left running for
What is the unit used to measure electrical energy in homes?
kilowatt-hour (kWh)
Definition of the kilowatt-hour
Energy transferred by a device with a power of 1kW operating for a time of 1 hour
How many joules in 1kWh?
1kW=1000W=1000Js^-1 (W=J/t)
1hr = 3600s
1kWh = 1000x3600
=3.6MJ
State Kirchhoff’s Second Law
it states that in any circuit, the sum of electromotive forces is equal to the sum of the potential differences around a closed loop
Which quantity is conserved in Kirchhoff’s second law?
Energy is conserved
How can you distinguish between a conductor, a semiconductor and an insulator?
Refer to number density of charge carriers
conductor - high number of charge carriers per unit volume
Semiconductors - fewer
Insulators - negligible
Describe the direction of flow of electrons in a circuit
move from the negative terminal (repelled) to the positive side (attracted) of power supply
