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Physics - Topic 2 - Electricity (Series Circuits (Resistance adds up (This…
Physics - Topic 2 - Electricity
Current and Circuit Symbols
Current is the flow of electric charge
Electric current
is a flow of
electrical charge
. Electrical charge will
only flow
round a
complete circuit
if there is a potential difference. The unit of current is the
ampere
, A.
In a
single
, closed
loop
the current has the same value
everywhere
in the circuit.
Potential difference
(or voltage) is the
driving force
that
pushes
the charge around. It's measured in
volts
, V.
Resistance
is anything that
slows the flow
down. Unit:
ohm
The current flowing
through a component
depends on the
potential difference
across it and the
resistance
of the component.
Total charge through a circuit depends on current and time
The size of the
current
is the
rate of flow
of
charge
. When
current
flows past a point in a circuit for a length of
time
then the
charge
that has passed is given by the formula:
More charge
passes around the circuit when a
larger current
flows.
Learn these Circuit Diagram Symbols
Use this link for a photo of all the circuit symbols you need to know from the revision guide:
https://drive.google.com/file/d/1ITs_h9rBhoAWgVEVyWiZ_XurUN12RmW2/view?usp=sharing
Resistance and V = IR
There's a formula linking potential difference and current
REMEMBER THIS EQUATION
Potential difference = Current x Resistance
OR
V = IR
Potential difference, V, in volts, V
Current, I, in amps, A
Resistance, R, in ohms
Resistance and I-V Characteristics
Ohmic conductors have a constant resistance
The
resistance
of
ohmic conductors
(e.g. a
wire
or a
resistor
) doesn't change with the current. At a
constant temperature
, the current flowing through an ohmic conductor is
directly proportional
to the potential difference across it.
The resistance of
some
resistors and components
does
change, e.g. a
diode
or a
filament lamp.
When an
electric charge
flows through a filament lamp, it
transfers
some energy to the
thermal energy store
of the filament, which is designed to
heat up.
Resistance
increases with
temperature
, so as the
current
increases, the filament lamp heats up more and the resistance
increases
.
For
diodes
, the resistance depends on the
direction
of the current. They will happily let current flow in one direction, but have a
very high resistance
if it's
reversed
.
Circuit Devices
LDR is short for Light Dependent Resistor
An LDR is a resistor that is
dependent
on the
intensity
of
light
.
In
birght
light, resistance
falls
.
In
darkness
, the resistance is
highest
.
They have lots of
applications
including
automatic night lights
, outdoor lighting and
burglar detectors
.
The Resistance of a thermistor depends on temperature
A
thermistor
is a
temperature dependent
resistor.
In
hot
conditions, resistance
drops
.
In
cool
conditions, resistance
goes up.
Thermistors make useful
temperature detectors
, e.g.
car engine
temperature sensors and electronic
thermostats
.
You can use LDRs and Thermistors in sensing circuits
Sensing circuits
can be used to
turn on
or
increase the power
to components depending on the
conditions
that they're in.
Series Circuits
Current is the same everywhere
In series circuits the
same current
flows through
all
the components, i.e. I1 = I2 = ...
The
size of the current
is determined by the
total pd
of the cells and the
total resistance
of the circuit, i.e. I = V / R
Resistance adds up
This is because by
adding a resistor
in series, the two resistors have to
share the total pd.
The
pd across each resistor is lower
, so the
current
through each resistor
is also lower.
In series, the
current is the same
everywhere so the
total current is reduced when a resistor is added.
This means the
total circuit resistance increases.
In series circuits the
total resistance
of two components is the
sum
of their resistances:
R total = R1 + R2.
The
bigger
a component's
resistance
, the bigger its
share
of the
total potential difference.
Potential Difference is shared
In series circuits, the
total pd
of the supply is
shared
between the various
components
. So the potential differences round a series circuit always
add up
to
equal
the
source
pd.
V total = V1 + V2 + ...
Cell potential differences add up
There is a
bigger pd
when
more cells
are in series, if they're all
connected
in the
same way.
For example, when two cells with a potential difference of 1.5 V are connected in series, they supply 3 V between them.
Series Circuits -all or nothing
If you
remove
or
disconnect
one
component
, the circuit is
broken
and they
all stop.
This is generally not very handy.
In
series circuits
, the different components are connected
in a line
,
end to end
, between the +ve and -ve of the
power supply
(except for voltmeters, which are always connected
in parallel
, but they don't count as part of the circuit).
Parallel Circuits
Potential difference is the same across all components
In parallel circuits
all
components get
full source pd
, so the potential difference is the
same across all components
: V1 = V2 = ...
This means that
identical bulbs
connected in parallel will all be at the
same brightness.
Current is shared between branches
In a parallel circuit, there are
junctions
where the current either
splits
or
rejoins
. The total current going
into
a junction has to equal the total current
leaving
.
If two
identical components
are connected in parallel then the
same current
will flow through each component.
In parallel the
total current
flowing around the circuit is equal to the
total
of all the currents through the
separate components.
Parallel circuits - independence and isolation
If you
remove
or
disconnect
one of them, it will
hardly affect
the others at all.
Most
things must be connected in
parallel
,
for example in cars and in household electrics; you have to be able to switch everything on and off separately.
In
parallel circuits
, each
component
is
separately connected
to the +ve and -ve of the
supply
(except
ammeters
, which are always connected in
series
).
Adding a resistor in parallel reduces the total resistance
If you have
two resistors in parallel,
their
total resistance
is
less than
the resistance of the
smallest
two resistors.
Electricity in the home
Most cables have three separate wires
Neutral wire -
blue
. The neutral wire
completes the circuit
and
carries away current
- electricity normally flows in through the live wire and out through the neutral wire. It is
around 0 V.
Earth wire -
green and yellow.
It is for
protecting the wiring,
and for
safety
- it
stops the appliance casing from becoming live.
It doesn't usually carry current - only when there's a
fault
. It's also at
0 V.
Live wire -
brown
. The live wire
provides the alternating potential difference
(at about 230 V) from the mains supply.
The live wire can give you an electric shock
Your
body is at 0 V
which means if you
touch
the live wire, a
large potential difference
is produced across your body and
current flows through
you.
This causes a
large electric shock
which could
injure
or even
kill
you.
Any connection
between live and earth can be dangerous. If the link creates a
low resistance path
to earth, a
huge current
will flow, which could
result in a fire.
There is
still danger
if a plug socket or light switch is
turned off.
A current may not be flowing but there's
still a pd in the live wire.
Mains supply is ac, battery supply is dc
The
UK mains supply is an ac
supply at around
230 V.
The
frequency
of the ac mains supply is
50 cycles per second or 50 Hz (hertz).
In
ac supplies
the current is
constantly
changing direction.
Alternating currents
are produced by
alternating voltages
in which the
positive
and
negative
ends keep
alternating
.
By contrast, cells and batteries supply
direct current
(dc).
There are
two types of electrical supplies
- alternating current (
ac
) and direct current (
dc
).
Direct current
is a current that is always flowing in the
same direction
and is
created
by a
direct voltage.
Power of electrical appliances
Energy is transferred from cells and other sources
Electrical appliances are
designed to transfer energy
to components in the circuit when a
current flows.
Kettles transfer energy electronically from the mains to the thermal energy store of the heating element inside the kettle.
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy of the fan's motor.
No appliances transfer all energy completely usefully.
The
higher the current
, the
more energy is transferred
to the
thermal
energy stores of the components.
Energy transferred depends on the power
The
power of an appliance
is the energy that it transfers
per second.
So the
more energy it transfers
in a given time, the
higher its power.
Energy transferred (J) = Power (W) x Time (s)
The total energy by an appliance depends on how long it is on for and its power.
Power rating - maximum amount of energy transferred between stores per second when the appliance is in use.
Helps customers choose between models
- the
lower the power rating
, the
less electricity an appliance uses in a given time
and so the
cheaper
it is to run.
A
higher power
doesn't necessarily mean it
transfers more energy usefully.
An
appliance may be more powerful than another, but less efficient.
More on Power
Potential difference is energy transferred per charge passed
Energy is
supplied
to the charge at the
power source
to 'raise' it through a
potential
.
The charge
gives up
this energy when it '
falls
' through any
potential drop
in
components
elsewhere in the circuit.
Energy Transferred (J) = Charge flow (C) x Potential Difference (V)
When an electrical
charge
goes through a
change
in potential difference, then
energy
is
transferred
.
That means that a
battery with a bigger pd
will supply
more energy
to the circuit for
every coulomb
of charge which flows around it, because the
charge is raised up 'higher' at the start.
Power also depends on current and potential difference
Power (W) = Potential Difference (V) x Current (A)
The National Grid
Electricity production has to meet demand
They can
predict
when the most electricity will be used.
Demand increases
when people get up in the morning, come home from school or work and when it starts to get dark or cold outside.
Power stations often run at
well below their maximum power output
, so there's
spare capacity
to cope with a
high demand.
Throughout the day,
electricity usage (the
demand
)
changes
. Power stations have to produce
enough
for
everyone
to have it when needed.
Lots of
smaller
power stations that can start up quickly are kept in standby just in case.
The National Grid uses a high pd and low current
It's
much cheaper
to
boost the pd
up really high and keep the
current as low as possible.
For a
given power
,
increasing pd decreases current
, which therefore
decreases the energy lost by heating
the wires and the surroundings. This makes the national grid an
efficient
way of transferring energy.
The
problem with a high current
-
lose energy
as wires
heat
up and
energy is transferred
to the
thermal
energy store of the
surroundings
.
To
transmit
the huge amount of power needed, you need either a
high pd or a high current.
Electricity is distributed via the National Grid
The national grid is a giant system of
cables
and
transformers
that
cover the UK
and
connects power stations to consumers.
Transfers electrical power from
power stations
anywhere on the grid to anywhere else on the grid
where it's needed
,
e.g. homes and industry.
Potential difference is changed by a transformer
To get the potential difference to 400 000 V to transmit power requires
transformers
as well as
big pylons
with
huge insulators
- but it's still
cheaper
.
The transformers have to
step
the pd up at one end, for
efficient transmission
, and then bring it back down to
safe, usable levels
at the other end.
Potential difference is
increased
/ stepped up using a
step-up transformer.
Potential difference is
reduced
/ stepped down using a
step-down transformer
for domestic use.
Static Electricity
Only electrons move - never positive charges
Both +ve and -ve electrostatic charges are only ever produced by the movement of
electrons
. The
positive charges DO NOT move!
A
positive static charge
is always caused by electrons
moving
away elsewhere. The material that
loses
the electrons loses some negative charge, and is
left with an equal positive charge.
Too much static causes sparks
If the pd gets
large enough
then electrons can
jump
across the
gap
between the charged object and the earth - this is the
spark
.
They can also
jump
to any
earthed conductor
that is nearby - which is why
you
can get
static shocks
getting out of a
car
.
As
electrical charge
builds on an object, the
pd
between the object and the earth
increases
.
This
usually
happens when the gap is fairly
small
.
Build-up of static is caused by friction
This will leave the materials
electrically charged
, with a
positive
charge on one and an
equal negative charge
on the other.
Which way
the electrons are transferred
depends
on the
two materials
involved.
When
insulating
materials are
rubbed
together, negatively charged electrons will be
scraped off one
and
dumped
on the other.
The classic examples are
polythene
and
acetate
rods being rubbed together with a
cloth duster.
Like charges repel, opposites attract
Two things with
opposite
electric charges are
attracted
to each other, while two things with the
same
electric charge will
repel
each other.
These forces get
weaker
the
further apart
the two things are.
When two electrically charge objects are brought close together they
exert a force
on one another.
These forces will cause the objects to
move
if they can. This is known as
electrostatic attraction / repulsion
and is a
non-contact force.
Electric Fields
Charged objects in an electric field feel a force
When a
charged object
is placed in the
electric field
of another object, it feels a
force
.
This force causes the
attraction
or
repulsion
.
The force on an object is linked to the
strength
of the electric field it is in.
The force is caused by the
electric fields
of each charged object
interacting
with each other.
As you
increase
the distance between charged objects, the strength of the field decreases and the force between them gets
smaller
.
Sparking can be explained by electric forces
Sparks
are caused when there is a
high enough pd
between a
charged object
and the
earth
.
A high pd causes a
strong electric field
between the
charged object
and the
earthed object.
The strong electric field causes
electrons
in the
air particles
to be
removed
(
ionisation
).
Air
is normally an
insulator
, but when it is
ionised
it is much
more conductive
, so a
current
can flow through it. This is the
spark
.
Electric charges create an electric field
The
closer
to the object you get, the
stronger
the
field
is.
An electric field is created around any electrically charged object.
Field lines for an isolated, charged sphere.
They're always at a right angle to the surface.
The closer together the lines are, the stronger the field is.
Electric field lines go from positive to negative
The
greater the resistance
across a component, the
smaller the current
that flows (for a given potential difference across the component).
REMEMBER THIS EQUATION
Charge flow = Current x Time
OR
Q = It
Charge flow, Q, in coulombs, C
Current, I, in amps, A
Time, t, in seconds, s