Electricity Flashcards
Current is…
…rate of flow of charge.
Current is measured in what?
Amperes.
Equation linking current, charge and time:
I=ΔQ/Δt
Kirchhoff’s current law:
The total current flowing into a point is equal to the current flowing out of a point.
What is Kirchhoff’s current law based on?
Conservation of charge principle. Charge is conserved in a circuit meaning current is also conserved.
Potential difference is the energy per…
…coulomb of charge.
Potential difference is a measure of what?
The amount of energy per unit of charge transferred between two points in a circuit.
Equation linking potential difference, energy and charge:
V = W/Q
In series potential difference across the components…
…adds to the potential difference across the circuit.
In parallel potential difference…
…remains the same for every branch across the circuit (it is equal throughout).
Kirchhoff’s potential difference law:
States that around any closed loop in a circuit the sum of the potential difference across all components equals the potential difference across the supply.
What is Kirchhoff’s potential difference law based on?
Conservation of energy principle.
Ohm’s Law is stated as:
The voltage drop across an ohmic conductor is proportional to the current flowing through it provided temperature remains constant.
Formula for resistance linking potential difference and current:
R = V/I
Power is the rate of…
…transferring energy.
Equation for power linking energy and time:
P = W/t
Equation for power linking current and potential difference:
P = IV
Equation for energy linking current, potential difference and time:
W = IVt
Equation for power linking current and resistance:
P = I^2R
Factors that affect resistivity:
Material
Cross-sectional area
Length
Temperature
As the length of material increases, what happens to resistivity?
It also increases.
As cross-sectional area increases what happens to resistivity?
Resistivity decreases.
Equation for resistance linking length, cross-sectional area and resistivity:
R = ρl/A
A potentiometer is a device that does what?
It is a device that provides a pd ranging from 0V up to the voltage of the supply.
Equation for potentiometers.
Vout/Vs=x/l
Equation for drift velocity.
I=nAvq I is in A v in ms^-1 A in m^2 n in m^-3 q in C
What is resistivity?
The resistance of a 1m^2, 1m long piece of material.
EMF stands for…
…electromotive force.
A cell has an internal what?
Resistance.
Equation for emf?
ε=V+Ir Where ε is emf in V. V is terminal pd in V I is current in A. r is internal resistance in Ω.
If compared to y=mx+c, equation for internal resistance can be used to find r and ε. How?
y is V.
x is I.
m is -r.
c is emf.
What is Ir sometimes described as (emf equation)?
The ‘lost volts’.
A thermistor is a resistor that varies with what?
Temperature.
LDR stands for what?
Light Dependent Resistor.
What are the two types of thermistors?
NTC and PTC
What does NTC stand for and what does it do?
NTC stands for negative temperature coefficient so as temperature increases resistance decreases.
What does PTC stand for and what does it do?
PTC stands for positive temperature coefficient and this means as temperature increases so too does the the resistance.
2 factors to consider when talking about resistance in LDRs and thermistors.
1) Increased light/heat energy means increases lattice vibrations which leads to more electron collisions. This reduces the drift velocity (I=nAqv), reduces the current which means increased resistance. (PTC answer and half NTC/LDR answer).
2) Energy absorbed as light/heat results in the release of more conduction electrons. This increases the density of charge carriers (I=nAqv) so increases the current, thus reducing the resistance.
In NTC/LDR factor 2 has a bigger effect than factor 1 resulting in reduced resistance.
