Elect!city
Conventional Current : Flow from + to - (opposite of the electron flow)
Electron Flow : the flow of electrons from - to +, making a current.
Conduction/free/delocalised electrons : the particles which allow a metal to
conduct an electric current.
Electrolyte : A solution which conducts electricity because it has charge carriers
Charge Carriers : Any charged particle which contribute to an electric current. Can
be electrons, protons, or ions.
1 Coulomb is the amount of charge which flows past a point in a circuit in a time of 1 s when the
current is 1 A.
Elementary Charge : the charge 1 electron has.
e = 1.6 × 10 −19 C
1C = 6.25 × 1018 electrons
Equation
Charge = Current x Time
Q = It
Kirchkoff’s
First
Law
The sum of the currents entering any point in a circuit is equal to the sum of the currents leaving
that same point.
Equation
I = Anev
I = Current
A = cross-sectional area
n = Number density
v = mean drift velocity
Number density : Number of conduction electrons per unit
volume (in m −3 )
Mean Drift Velocity : The average velocity at which the
electrons travel (m/s)
Electric force causes the electrons to move towards + end
of battery.
Electrons randomly collide with the metal ions.
Journey along metal is very haphazard, so its called drift
velocity.
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I = Anev
I = m 2 × m −3 × C × ms −1
m3 × C
I= 3
m ×s
I =C/s
I=
Q
,QED.
t
Resistance
Resistance : a material's opposition to the flow of electric current; measured in ohms.
Dependant on voltage and current.
V
R=
I
I-V Characteristics
In a metal resistance wire/resistor,
Graph is a straight line, passes through origin
I ∝V
Wire has a constant resistance
In a filament Lamp,
Graph is a curve.
Gradient increases as I increases
As current increases, metal filament gets hotter and R
increases
In a Diode,
There’s almost no current when V is is
applied in reverse.
Reverse diode have very high resistance
In the forward direction, current
increases very rapidly
Forward diode have very low resistance
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Ohm’s
Law
For a metallic conductor at constant temperature, the current in the conductor is directly
proportional to the potential difference across its ends.
Thermistors
-> Components which are designed to have resistance which rapidly changes with
temperature.
2 types :
Negative Temperature Coefficient (NTC)
!
The resistance decreases as temperature increases.
Positive Temperature Coefficient (PTC)
!
The resistance rises abruptly at a certain temperature
Resistance
and
Temperature
At low temperatures,
The particles vibrate in place
Electrons flow relatively freely.
At high temperatures,
Ions vibrate more
Electrons collide more with the ions, obstructing the flow
Resistance is higher.
Resistivity
Resistivity : A measure of how strongly a material opposes the flow of electric
current. Depends on the material, not affected by V or I.
resis tan ce =
R=
ρL
A
ρ=
RA
L
resistivity × length
cros − sectional _ Area
-> Resistivity increases with temperature.
Voltage
Potential Difference : when charges lose energy by transferring electrical energy to
other forms. (across a resistor, in external circuit)
Electromotive force : when charges gain electrical energy from a power supply.
(across battery)
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Energy
and
Power
Potential difference
Potential _ difference =
V=
Energy _ lost _ by _ ch arg e
ch arg e
Work
Q
1 volt = 1 joule/coulomb.
Power
Power =
P=
Energy _ transferred
time _ taken
W
t
Power and Current
So, put the 2 together:
W = VQ
VQ
P=
t
Q
P =V
t
P = VI
Power = p.d. × current
Power and Resistance
P = VI,V = IR
P = I 2R
V2
P=
R
Calculating Energy
Since _ P = VI,W = Pt
W = IVt
Energy Transferred = Current x Voltage x Time
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EQUATIONS
USED
e = 1.6 × 10 −19 C
1C = 6.25 × 1018 electrons
Q = It
I = Anev
V
R=
I
RA
ρL
R
=
L or
A
Work
V=
Q
W
P=
t
ρ=
P = VI
P = I 2R
V2
P=
R
W = IVt
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