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physics AS
Electricity AS
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There are three key quantities:
V
, I, and
R
Current
(
I
)
The
rate of flow of charge particles
Conventional current
Flows from positive to negative, but the
actual charge carriers
(
electrons
) move from negative to positive
Potential difference (V)
The energy transferred per unit charge
Resistance
(
R
)
The
ratio of potential difference across a component
to
the current
in that component
Investigating component characteristics
1. Set up circuit with
ammeter
and
voltmeter
2. Vary
current
/
potential difference
3. Measure
current
and
potential difference
IV characteristics
Relationship
between current (I) and
potential difference
(V)
For ohmic conductors, a
straight line
through the origin
For non-ohmic conductors, a
non-linear
relationship
Ohm's law states that
current
is proportional to
potential difference
, provided physical conditions are constant
Resistance
Not always equal to 1 over the
gradient
of the
IV
characteristic
IV characteristics
Resistor
: straight line through origin
Filament lamp
: non-linear
Diode
: allows current flow in one direction only
Resistivity
(ρ)
A material property that determines the
resistance
of a conductor based on its
length
and cross-sectional area
As temperature increases
Resistance
of most materials
increases
Semiconductors
As temperature
increases
, resistance
decreases
due to more charge carriers being liberated
Superconductors
Below a critical temperature, resistance drops to
zero
Kirchhoff's first law: the
sum
of currents into a junction
equals
the sum of currents out of the junction
EMF (ε)
The
energy
per unit
charge
transferred to the circuit by a source (e.g. battery)
Around any closed
loop
in a circuit, the
sum
of the EMFs equals the sum of the potential differences
EMF
Energy
transferred to the circuit by a
battery
Potential difference
Energy transferred within a component
Around any closed loop in the
circuit
The sum of the
EMFs
is
equal
to the sum of the potential differences
Series circuit
Current is the
same
everywhere in the circuit
Parallel
circuit
Current is
split
at a
junction
In a
parallel
circuit
The
potential difference
is the
same
across each component
Total resistance
in series
circuit
Sum of
individual resistors
Total resistance in parallel circuit
1
over
(1/r1 + 1/r2 + ...)
Power
Rate of energy
transfer
= IV =
I^2R
= V^2/R
Total energy transferred
Power x time =
IVt
Potential divider circuit
Splits
potential difference
between
two resistors
Potential divider components
Thermistor
Light dependent
resistor
Variable
resistor
Internal resistance
Resistance within a
cell
or
power supply
EMF
Equal to
terminal potential difference
+ current x
internal resistance
As current
increases
Terminal potential difference
decreases
Cells in series
Internal resistances
add up
Cells
in
parallel
Combined internal resistance decreases