Physics - Topic 2 - Electricity (Series Circuits (Resistance adds up…
Physics - Topic 2 - Electricity
Current and Circuit Symbols
Current is the flow of electric charge
is a flow of
. Electrical charge will
if there is a potential difference. The unit of current is the
the current has the same value
in the circuit.
(or voltage) is the
the charge around. It's measured in
is anything that
slows the flow
The current flowing
through a component
depends on the
across it and the
of the component.
Total charge through a circuit depends on current and time
The size of the
rate of flow
flows past a point in a circuit for a length of
that has passed is given by the formula:
passes around the circuit when a
Learn these Circuit Diagram Symbols
Use this link for a photo of all the circuit symbols you need to know from the revision guide:
Resistance and V = IR
There's a formula linking potential difference and current
REMEMBER THIS EQUATION
Potential difference = Current x Resistance
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
) doesn't change with the current. At a
, the current flowing through an ohmic conductor is
to the potential difference across it.
The resistance of
resistors and components
change, e.g. a
flows through a filament lamp, it
some energy to the
thermal energy store
of the filament, which is designed to
, so as the
increases, the filament lamp heats up more and the resistance
, the resistance depends on the
of the current. They will happily let current flow in one direction, but have a
very high resistance
LDR is short for Light Dependent Resistor
An LDR is a resistor that is
, the resistance is
They have lots of
automatic night lights
, outdoor lighting and
The Resistance of a thermistor depends on temperature
Thermistors make useful
temperature sensors and electronic
You can use LDRs and Thermistors in sensing circuits
can be used to
increase the power
to components depending on the
that they're in.
Current is the same everywhere
In series circuits the
the components, i.e. I1 = I2 = ...
size of the current
is determined by the
of the cells and the
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.
pd across each resistor is lower
, so the
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
of two components is the
of their resistances:
R total = R1 + R2.
, the bigger its
total potential difference.
Potential Difference is shared
In series circuits, the
of the supply is
between the various
. So the potential differences round a series circuit always
V total = V1 + V2 + ...
Cell potential differences add up
There is a
are in series, if they're all
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
, the circuit is
This is generally not very handy.
, the different components are connected
in a line
end to end
, between the +ve and -ve of the
(except for voltmeters, which are always connected
, but they don't count as part of the circuit).
Potential difference is the same across all components
In parallel circuits
full source pd
, so the potential difference is the
same across all components
: V1 = V2 = ...
This means that
connected in parallel will all be at the
Current is shared between branches
In a parallel circuit, there are
where the current either
. The total current going
a junction has to equal the total current
are connected in parallel then the
will flow through each component.
In parallel the
flowing around the circuit is equal to the
of all the currents through the
Parallel circuits - independence and isolation
one of them, it will
the others at all.
things must be connected in
for example in cars and in household electrics; you have to be able to switch everything on and off separately.
to the +ve and -ve of the
, which are always connected in
Adding a resistor in parallel reduces the total resistance
If you have
two resistors in parallel,
the resistance of the
Electricity in the home
Most cables have three separate wires
Neutral wire -
. The neutral wire
completes the circuit
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,
stops the appliance casing from becoming live.
It doesn't usually carry current - only when there's a
. It's also at
Live wire -
. 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
body is at 0 V
which means if you
the live wire, a
large potential difference
is produced across your body and
current flows through
This causes a
large electric shock
between live and earth can be dangerous. If the link creates a
low resistance path
to earth, a
will flow, which could
result in a fire.
if a plug socket or light switch is
A current may not be flowing but there's
still a pd in the live wire.
Mains supply is ac, battery supply is dc
UK mains supply is an ac
supply at around
of the ac mains supply is
50 cycles per second or 50 Hz (hertz).
the current is
are produced by
in which the
By contrast, cells and batteries supply
two types of electrical supplies
- alternating current (
) and direct current (
is a current that is always flowing in the
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
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.
higher the current
more energy is transferred
energy stores of the components.
Energy transferred depends on the power
power of an appliance
is the energy that it transfers
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
lower the power rating
less electricity an appliance uses in a given time
and so the
it is to run.
doesn't necessarily mean it
transfers more energy usefully.
appliance may be more powerful than another, but less efficient.
More on Power
Potential difference is energy transferred per charge passed
to the charge at the
to 'raise' it through a
this energy when it '
' through any
elsewhere in the circuit.
Energy Transferred (J) = Charge flow (C) x Potential Difference (V)
When an electrical
goes through a
in potential difference, then
That means that a
battery with a bigger pd
to the circuit for
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
when the most electricity will be used.
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
to cope with a
Throughout the day,
electricity usage (the
. Power stations have to produce
to have it when needed.
power stations that can start up quickly are kept in standby just in case.
The National Grid uses a high pd and low current
boost the pd
up really high and keep the
current as low as possible.
increasing pd decreases current
, which therefore
decreases the energy lost by heating
the wires and the surroundings. This makes the national grid an
way of transferring energy.
problem with a high current
energy is transferred
energy store of the
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
cover the UK
connects power stations to consumers.
Transfers electrical power from
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
as well as
- but it's still
The transformers have to
the pd up at one end, for
, and then bring it back down to
safe, usable levels
at the other end.
Potential difference is
/ stepped up using a
Potential difference is
/ stepped down using a
for domestic use.
Only electrons move - never positive charges
Both +ve and -ve electrostatic charges are only ever produced by the movement of
positive charges DO NOT move!
positive static charge
is always caused by electrons
away elsewhere. The material that
the electrons loses some negative charge, and is
left with an equal positive charge.
Too much static causes sparks
If the pd gets
then electrons can
between the charged object and the earth - this is the
They can also
that is nearby - which is why
getting out of a
builds on an object, the
between the object and the earth
happens when the gap is fairly
Build-up of static is caused by friction
This will leave the materials
, with a
charge on one and an
equal negative charge
on the other.
the electrons are transferred
together, negatively charged electrons will be
scraped off one
on the other.
The classic examples are
rods being rubbed together with a
Like charges repel, opposites attract
Two things with
electric charges are
to each other, while two things with the
electric charge will
These forces get
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
if they can. This is known as
electrostatic attraction / repulsion
and is a
Charged objects in an electric field feel a force
is placed in the
of another object, it feels a
This force causes the
The force on an object is linked to the
of the electric field it is in.
The force is caused by the
of each charged object
with each other.
the distance between charged objects, the strength of the field decreases and the force between them gets
Sparking can be explained by electric forces
are caused when there is a
high enough pd
A high pd causes a
strong electric field
The strong electric field causes
is normally an
, but when it is
it is much
, so a
can flow through it. This is the
Electric charges create an electric field
to the object you get, the
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
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
Q = It
Charge flow, Q, in coulombs, C
Current, I, in amps, A
Time, t, in seconds, s