Please enable JavaScript.
Coggle requires JavaScript to display documents.
H556/02 - Coggle Diagram
H556/02
Module 6
Capacitors
Electric Fields
Electromagnetism
Nuclear and Particle Physics
Medical Imaging
Module 4
Electricity
Current
is the
Rate
of
Flow
of
Charge
The
current
in a
wire
is like
water
flowing in a
pipe
. The
amount
of water that flows depends on the
flow rate
and the
time
. It's the same with electricity -
current is the rate of flow of charge
I = Q/t
Remember that conventional current flowsa from + to -, the opposite way from electron flow
The
Coulomb
is the
unit of charge
One coulomb
(C) is defined as the
amount of charge
that apsses in
1 second
when the
current
is
1 ampere*
You can measure the current flowing througfht part cirremt f;pwomg thrpight [art pf a corciot using an
ammeter
. This is the circuit synmbol for an ammeter:
Attach an ammeter in series with the component you're investigating
The charge in an
Electron
is the
Smallest Unit
of Charge
In electrical circuits, charge is usually carried by
electrons
(or sometimes by ions). Electronsall carry the
same charge
,
-e
where
e
is
elecmentary charge
e = 1.60 x 10^-19C
Protons carry an
opposite cahrge
of the
same magnitude
,
+e
The elementary charge is the
smallest unit
that cahrge comes in - the net charge on any particle or object will alays be a
nultiple of e
. We say that charge is
quantised
Potential Difference
is the
Work Done
per
Unit Charge
To make elctric charge flow through a conductor, you need to do
work
on it.
Potential difference
(p.d.), or
voltage
, is defined as the
work done per unit charge moved
W = VQ
or
V = W/Q
The
potential difference across a component is
1 volt
(V) when you do *1 joule
of work moving
1 coulomb
of charge through the component. This
defines
the volt. 1
V
= 1
J C^-1*
Back to the 'water analogy' again. The p.d. is like the pressure that's forcing water along the pipe.
You can measure the potential difference across a component using a
voltmeter
. This is the circuit symbol for a voltmeter
The maxmum value that a voltmeter or ammeter can measure is called trhe full scale deflection
Remember, the potentail differencea crros components in parallel is
the same
, so the
voltmeter
should be connected in
parallel
with the componet you're investigating
Work Done
on a Charge Equals the
Kinetic Energy
it Gains
When a
charged particle
is accelerated by a
potential difference
, the energy transferred to it is equal to the work done on the aprtic;e,
W = VQ
. For an electron (with charge of size
e
), thus cab be wrutteb as
W = Ve
The energy transferred is equal to the
kinetic energy
gained by the electron
Kinetic energy = 1/2mv^2
eV = 1/2mv^2
This gives you the velocity of a single electron accelerated through a potential difference - don't get it confused with mean drift velocity
The
Mean Drift Velocity
is the
Average
Velcoty of the
Charge Carriers
When
current
flowsthrough a wire, ypu might imaginr thr
electrons
all moving uniformly in the
same dircetion
. In fact, they move
randomly
in
all directions
, but tend to
drift
one way. The
mean drift velocity
is just the
average velocity
and it's
much, much less
than the elctrons'
actual speed*
The Current Depends on the MEan Drift Velocity
If you're using difference charge carriers, just replace
e
with the cjarge on each carrier, and let
n
be the number density of charge carriers
The
current
is given by the continuity equation
I = Anev
If you double the
number of electrons
, the current doubles**
If you double the
are
the
current doubles
IF the electrons move
twice as fast
, the
curent doubles*
as twice as many ekectrons move past a point in the same amount of time
Different Materials
have Different Numbers of
Charge Carriers
In a
metal
, the
charge carriers
are
free electrons
- they're the oness from the
outer shell
of each atom. Thinkning abnout the fomula
I = Anev
, there are
loads
of charge carriers per unit volume, making
n big
. The
drift velocity
is
small
, even for a
high current
Semiconductors
have
fewer charge carriers
, si the
drift velocity
needs to be
heighter
to give the same current**
A
perfect insulator
wouldn't have any
charge carriers
, so n= 0 ub the firnyka and ger
no current
.
Real
insulators have a
very small n
Charge Carriers
in Liquids and Gases are
Ions
Ionic crystals
like sodium chloride are
insulators
. Once
molten
, though, the liquid
conducts
, Positive and negative
ions
are the
charge carriers
. The
same thing
happanes in an
ionic solution
like copper sulfate solution.
A substance containing ions that conducts electricity like this is called an
electrolyte
Everything
has
Resistance
If you put a
potential difference
(p.d.) across an
electrical component
, a
current
will flow
How much
current you get get for a particular
p.d.
depends on the
resistance
of the component
You can think of a component's
resistance
as a
measure
of how
difficult
it is to get a
current
to
*flow
through it
Mathematically,
resistance
is
R = V/I
This equation really
dines
what is mean by resistance
The is the
circuit symbol
for a resistor
Resistance
is measured in
ohms
(Ω)
A component has a resjsta ce of
1 Ω
if a
potential difference
of
1 V
makes a
current
of
1 A
flow through it
Three
Things
Determine Resistance
IF you think about a nice,
simple electrical com[pnent
like a
length of wire
, its
resistance
depends on
Length (L)
. The
longer
the wire the
more difficult
it is to make a
current flow
Waves
Quantum Physics
