H214 Current of Electricity Flashcards
Define electric curent
Electric current is the rate of flow of charge
What is the formula of electric current?
I = Q/T
How do you find the rate of flow of electrons in a circuit?
I = (n/t)e
What is charge?
Charge is the property of a particle that causes an interaction of the particle with other charged particles and material of electrical nature.
In a current over time graph, what does the area under the graph represent?
Area under I-t graph is the total charge passing through the point during the time interval.
For a current I in a wire that causes the charge carriers to have a drift velocity v, derive an equation relating current to the number density of charge carriers in the wire n, the cross-sectional area of the wire A and the charge on each charge carrier q.
For a wire segment of length L, total charge in this volume is:
Q = (number of charge carriers in volume)(q)
= n(volume)q
=nALq
Time taken, t, for charge carriers to have net displacement of L:
L = v/t
Current is the rate of flow of charge:
I = Q/t
I = nALq / L/v
= nAvq
What is the formula for the drift velocity of a current-carrying conductor?
I = nAvq
I: current (A)
n: number density of charge carriers
A: cross-sectional area
V: drift velocity
q: charge
State the difference between drift velocity and thermal agitation.
- Drift velocity is zero when there is no potential difference across a conductor while thermal agitation is zero only at a temperature of 0K
- Drift velocity is associated with the net flow of charge carriers while thermal agitation is not associated with current
- Drift velocity speed = 10^-4
Thermal Agitation speed = 10^5
Define electromotive force.
Electromotive force (emf) the energy transformed from chemical to electrical per unit charge when charge is driven round a complete circuit.
Define potential difference.
Potential difference is the energy transformed from electrical to other forms per unit charge when charge passes through an electrical component.
What is the formula for potential difference?
V = W/Q
State the formulas relating power output of electrical component.
P = IV
= I^2R
= V^2/R
State the formulas relating power output of electrical component.
P = Iemf
= I^2R
= emf^2/R
Define resistance.
Resistance is the ratio of potential difference across a component to the current passing through it.
What does the resistance of a component depend on?
a) The number density of charge carriers
b) the extent of vibration of the atomic lattice
Using these components, explain why the resistance of metals increases with increasing temperatures.
- the valence electrons of metal atoms make up the seat of delocalised electrons responsible for conduction - the number density is therefore fixed
- however higher temperatures result in greater vibration of the giant metallic lattice, and more frequent collisions with delocalised electrons disrupt charge flow.
Using these components, explain why the resistance of thermistors decreases with increasing temperatures
- at higher temperatures, more electrons are set free to carry charge. the freed electrons also create holes that can act as mobile positive charge carriers.
- the increased number density of charge carriers is greater in effect than the increased vibration of lattice causing more frequent collisions with charge carriers
What is the formula relating geometry and resistance?
R = pL/A
What is the formula for terminal pd?
Vt = emf - Ir
How do you maximise efficiency in a circuit?
To increase efficiency, minimise internal resistance of source emf or maximise resistance of a component.
Define the coulomb
The coulomb is an ampere second
