electric charge (q)
current (I) x time (t)
measured in Coulombs (C)
1 Coulomb is the amount of charge carried by 1 Ampere in 1 second (derived unit)
current
measured in amperes, fundamental unit
potential difference (V)
work per unit charge, V=W/q
measured in Volts (V)
1 Volt is 1 Joule of work done on or by 1 Coulomb of charge
Coulombâs law
The forces on two charges are equal in magnitude and opposite in direction
F âq1q2 â also Fâq1 and Fâq2
Fâr^-2 â Fâq1q2/r²
constant â F=k q1q2/r²
k = 1/4ĎÎľ0 = permittivity of free space
the elementary charge (e)
every electron and proton has a charge of the same magnitude â 1.6Ă10^-19 C
positive for protons, negative for electrons
drawing electric field lines
lines donât cross
begin on positive charges, end on negative charges
direction of the field is the direction of the force on a positive charge
more lines = stronger field
perpendicular to the surface of conductors
no field inside a conductor
uniform electric field
same strength and direction throughout the field
parallel plates and even distance between field lines
electric fields = vectors
electric field strength
the force per unit charge exerted on a stationary positive charge at a point
E=Fe/q so measured in NC^-1
volts
work done per unit charge
JC^-1
show that 1 NC^-1 = 1 Vm^-1
Joule = Nm, so N=J/m
electric field strength measured in N/C
âŚ
behaviour of a charge in a uniform electric field
mathematically same equations for Fg and Fe so charge is also subject to projectile motion following a parabolic path
Finding E ne = mg
electric potential difference (V)
also called voltage
work is the transfer of energy
eg when a charge passes through a light bulb electric potential energy becomes thermal and light energy
in this process the electric potential energy of the electron changes
this difference divided by the charge (V=W/Q) is the electric potential difference
electromotive force (EMF, Îľ)
work done per unit charge by the power supply
not actually a force
Îľ = W/Q
difference between electromotive force and electrical potential difference
whether the charges are doing work (V) or work is being done on the charges (EMF)
energy is transferred from electrical to another form (V)
energy is transferred from another form to electrical (EMF)
Finding electrical power and showing the two meanings of watts
P=W/t
W=VQ
P=VQ/t
I=Q/t
P=VI (so watts are Js^-1 and VA)
[conventional] current (I)
flow of positive charges, I=Q/t
measured in amperes (A), fundamental unit
current is in the direction of positive charges, same direction as the electric field
drift speed
the average movement of all charge carriers
current moves in the opposite direction
earth/ground
where V=0
Ohmâs law
IâV
R=V/I
ohmic materials
follow Ohmâs law that IâV
linear graph
non-ohmic materials
do not follow Ohmâs law that IâV
a lot more non-ohmic materials than ohmic materials in the universe
non-linear graph
resistance
symbol is capital omega
measured in Ohms
R=V/I
how do the flow of positive and negative charges equal each other?
negative charge flowing to the left = positive charge flowing to the right
equation for drift velocity
I=nAvq
I = current
n = number density
A = cross sectional area
v = drift velocity
q = charge on each carrier
number density
number of charge carriers per unit volume
depends on the material
units of m^-3
why do electrons start drifting almost instantly after a system is turned on?
an electromagnetic wave propagates through the cable close to the speed of light
what happens to the drift speed if current is doubled?
it will also double (directly proportional)
what happens to the drift speed is the area is halved?
drift speed doubles (inversely proportional)
what happens to the drift speed if number density is halved?
drift speed doubles (inversely proportional)
thermistor IV graph
lightbulb IV graph
diode, current gate IV graph
line is 0 up to a point (resistance is infinite) then current kicks in
series circuit
current can only follow one specific path
parallel circuit
current can follow multiple different paths
resistors in series
the same current passes through each resistor
the total potential difference is the sum if the individual potential differences
effect of temperature on drifting electrons
increases resistance
Kirchhoffâs first law
current in = current out
current cannot be created or destroyed
which path will more current in a series circuit take?
the one of least resistance
total resistance in parallel vs series
parallel: 1/R = 1/R1 + 1/R2
series: R = R1 + R2
relationship between Rt and Ri in parallel
Rt < Ri
how does current split in a parallel circuit?
according to the ratio of the resistance
what is constant in a series circuit?
what is used up?
current is constant
potential difference is used up
what is constant in a parallel circuit?
what splits up?
potential difference is constant
current splits up
devices used to measure current and potential difference
potential difference â voltmeter
current â ammeter
for a voltmeter to not affect the current going to the rest of the circuit, it needs to have infinite resistance
connected in parallel
measuring devices should not be part of the experiment
an ammeter should have no resistance because all the current needs to pass through it
connected in series
potential divider
a way to regulate the potential difference supplied to components
knowing the ratio of V total to R total, what is the ratio of V1 to R1 and V2 to R2?
the ratio for all three situations is the same
V1 = R1/(R1+R2) V total
Voltmeter | Ammeter | |
Ideal resistance | ||
How it is connected |
Voltmeter | Ammeter | |
Ideal resistance | infinite | zero |
How it is connected | in parallel | in series |
When is P=I²R and P=V²/R applicable?
if and only if dealing with non-ohmic materials