ELECTRIC CIRCUITS AND COMPONENTS

Circuit Components
 The diagram below shows the circuit symbols that could be used in circuit
diagrams
o You will be expected to know what each component is and how it
behaves in a circuit

Power supplies

 Cells, batteries, power supplies and generators all supply current to the circuit

Resistors

 Potential dividers, fixed and variable resistors, thermistors and light-dependent

resistors (LDRs) are all used to control current

Meters

 Ammeters and voltmeters are used to measure the current and potential
difference
o Ammeters are always connected in series whilst voltmeters are always
connected in parallel
Electromagnetic Components

 Magnetising coils, relays and transformers use electromagnetic effects

o Relays use a small current in one circuit to switch on a much larger
current in another
o Transformers 'step up' and 'step down' current and potential difference

Fuses

 Protect expensive components from current surges and act as a safety measure
against fire

Diodes
 In addition to the above, you should be able to recognise and draw the circuit
symbol for a diode:

A diode is a component that only allows a current in one direction

(Note: diodes are occasionally drawn with a horizontal line running through the middle
of them)

If a diode is connected to an a.c. (alternating current) power supply, it will only allow a
current half of the time

(This is called rectification)

A diode can be used to rectify an alternating current

 Current in Series Circuits
 In a circuit that is a closed-loop, such as a series circuit, the current is the same value at
any point

o This is because the number of electrons per second that passes through one part of
the circuit is the same number that passes through any other part
 This means that all components in a closed-loop have the same current

The current is the same at each point in a closed-loop

 The amount of current flowing around a series circuit depends on two things:
o The voltage of the power source
o The resistance of the components in the circuit

 Increasing the voltage of the power source drives more current around the circuit
o So, decreasing the voltage of the power source reduces the current
 Increasing the number of components in the circuit increases the total resistance
o Hence less current flows through the circuit

Current will increase if the voltage of the power supply increases, and decreases if the number
of components increases (because there will be more resistance)
 Current in Parallel Circuits
 A parallel circuit consists of two or more components attached along separate branches of
the circuit

Diagram showing two bulbs connected in parallel

 The advantages of this kind of circuit are:

o The components can be individually controlled, using their own switches
o If one component stops working the others will continue to function
 In a parallel circuit, the current splits up - some of it going one way and the rest going
the other
 This means that the current in each branch will be smaller than the current from the
power supply

At a junction in a parallel circuit (where two or more wires meet) the current is conserved

o This means the amount of current flowing into the junction is equal to the amount
of current flowing out of it
 This is because charge is conserved

 Note that the current does not always split equally – often there will be more current in
some branches than in others
o The current in each branch will only be identical if the resistance of the
components along each branch are identical
 Current behaves in this way because it is the flow of electrons:
o Electrons are physical matter – they cannot be created or destroyed
o This means the total number of electrons (and hence current) going around a
circuit must remain the same
o When the electrons reach a junction, however, some of them will go one way and
the rest will go the other
 Current is split at a junction into individual branches

Worked example

In the circuit below, ammeter A shows a reading of 10 A, and ammeter A shows a reading of 6
0 1

A.
What is the reading on ammeter A ? 2

Step 1: Recall that at a junction, the current is conserved

This means that the total amount of current flowing into a junction is equal to the total amount
flowing out

Step 2: Consider the first junction in the circuit where current splits

The diagram below shows the first junction in the circuit

Step 3: Calculate the missing amount of current

 Since 10 A flows in to the junction (the total current from the battery), 10 A must flow
out of the junction

o The question says that 6 A flows through ammeter A so the remaining current
1

flowing through ammeter A must be:

2
 10 − 6 A = 4 A ,Therefore, 4 A flows through ammeter A 2

Exam Tip

The direction of current flow is super important when considering junctions in a circuit.You
should remember that current flows from the positive terminal to the negative terminal of a cell /
battery. This will help determine the direction current is flowing 'in' to a junction and which way
the current then flows 'out'.

EMF & Potential Difference in Circuits

Potential Difference in Series Circuits

 When several cells are connected together in series, their combined EMF is
equal to the sum of their individual EMFs

The total EMF of these cells is equal to the sum of their individual EMFs

Potential Difference in Series Circuits

 In a series circuit, the sum of potential differences across the components is
equal to the total EMF of the power supply
 In a series circuit the components share the EMF of the power supply

Potential Difference in Parallel Circuits

 A parallel circuit consists of two or more components attached along separate
branches of the circuit

Diagram showing two bulbs connected in parallel

 The advantages of this kind of circuit are:

o The components can be individually controlled, using their own
switches
o If one component stops working the others will continue to function
 In a parallel circuit, the current splits up - some of it going one way and the
rest going the other
 This means that the current in each branch will be smaller than the current from
the power supply
 Combined Resistance
Resistors in Series & Parallel
Resistors in Series

 When two or more components are connected in series:

o The combined resistance of the components is equal to the sum of
individual resistances

When several components are connected in series, their combined resistance is

equal to the sum of their individual resistances

Resistors in Parallel

 When resistors are connected in parallel, the combined resistance decreases

and is less than the resistance of any of the individual components
 If two resistors of equal resistance are connected in parallel, then the combined
resistance will halve

The above resistors will have a combined resistance of 2 Ω − half the value of
each resistor
 Determining Resistance in Parallel
 More generally, to determine the combined resistance of any combination of two
resistors, you must use the equation:

The above equation is not the same as R = R + R – a common, incorrect

1 2

simplification that is made

 To calculate the resistance:

o First find the value of 1/R (by adding 1/R + 1/R )
1 2

o Next find the value of R by using the reciprocal button on your calculator
(labelled either x or 1/x, depending on your calculator)
-1

Potential Dividers
Variable Potential Dividers

 When two resistors are connected in series, the potential difference across the
power source is shared between them

A potential divider splits the potential difference of a power source between two components
  The potential difference across each resistor depends upon its resistance:
o The resistor with the largest resistance will have a greater potential
difference than the other one
o If the resistance of one of the resistors is increased, it will get a greater
share of the potential difference, whilst the other resistor will get a smaller
share

A potentiometer is a single component that (in its simplest form) consists of a coil of
wire with a sliding contact, midway along it

A potentiometer is a kind of variable resistor

 The sliding contact has the effect of separating the potentiometer into two parts –
an upper part and a lower part – both of which have different resistances

Moving the slider (the arrow in the diagram) changes the resistances (and hence
potential differences) of the upper and lower parts of the potentiometer

 If the slider in the above diagram is moved upwards, the resistance of the lower
part will increase and so the potential difference across it will also increase
 Resistors as Potential Dividers
 When two resistors are connected in series, through Kirchhoff’s Second Law, the
potential difference across the power source is divided between them
 Potential dividers are circuits which produce an output voltage as a fraction of its
input voltage
 Potential dividers have two main purposes:
o To provide a variable potential difference
o To enable a specific potential difference to be chosen
o To split the potential difference of a power source between two or more
components
 Potential dividers are used widely in volume controls and sensory circuits using
LDRs and thermistors
 Potential divider circuits are based on the ratio of voltage between components.
This is equal to the ratio of the resistances of the resistors in the diagram below,
giving the following equation:

Potential divider diagram and equation

 The input voltage V is applied to the top and bottom of the series resistors
in

 The output voltage V is measured from the centre to the bottom of resistor R
out 2

 The potential difference V across each resistor depends upon its resistance R:
o The resistor with the largest resistance will have a greater potential
difference than the other one from V = IR
o If the resistance of one of the resistors is increased, it will get a greater
share of the potential difference, whilst the other resistor will get a smaller
share
 In potential divider circuits, the p.d across a component is proportional to its
resistance from V = IR
Worked example

The circuit is designed to light up a lamp when the input voltage exceed a preset value.

It does this by comparing V with a fixed reference voltage of 5.3 V.

out

V is equal to 5.3
out

Calculate the input voltage V . in

Exam Tip

When thinking about potential dividers, remember that the higher the resistance the
more energy it will take to 'push the current through' and therefore the higher the
potential difference.

This means that if a component (often shown as a voltmeter in questions) needs to be

switched on by a change such as increased light or temperature, then the resistor it is in
parallel with needs to become larger compared to the other resistor.
Electrical Safety
Electrical Hazards
 Mains electricity is potentially lethal – potential differences as small as 50 volts
can pose a serious hazard to individuals

The risk of electrocution is indicated by hazard signs but other risks which would
not be signposted are listed below

 Common hazards include:

o Damaged Insulation – If someone touches an exposed piece of wire,
they could be subjected to a lethal shock
o Overheating of cables – Passing too much current through too small a
wire (or leaving a long length of wire tightly coiled) can lead to the wire
overheating. This could cause a fire or melt the insulations, exposing live
wires
o Damp conditions – If moisture comes into contact with live wires, the
moisture could conduct electricity either causing a short circuit within a
device (which could cause a fire) or posing an electrocution risk
o Excess current from overloading of plugs, extension leads, single
and multiple sockets when using a mains supply - If plugs or sockets
become overloaded due to plugging in too many components the heat
created can cause fires
 Mains Circuits
Mains Electricity

 Mains electricity is the electricity generated by power stations and transported

around the country through the National Grid
o Everyone connects to the mains when plugging in an appliance such as a
phone charger or kettle
 Mains electricity is an alternating current (a.c.) supply
 In the UK, the domestic electricity supply has a frequency of 50 Hz and
a potential difference of about 230 V
o A frequency of 50 Hz means the direction of the current changes back and
forth 50 times every second
 Mains electricity, being an alternating current, does not have positive and
negative sides to the power source
o The equivalent to positive and negative are called live and neutral and
these form either end of the electrical circuit

Three-pin Plug & Earth Connection

The plug socket and inside of a three-pin plug showing the three wires and their
connections. The live and neutral wires deliver the electricity to the device. The
Earth wire is for safety
In order to protect the user or the device, there are several safety features built into
domestic appliances, including:

 Double insulation
 Earthing
 Fuses
 Circuit breakers
Insulation & Double Insulation

 The conducting part of a wire is usually made of copper or some other metal
o If this comes into contact with a person, this poses a risk of electrocution
 For this reason, wires are covered with an insulating material, such as rubber

The conducting part of a wire is covered in an insulating material for safety

 Some appliances do not have metal cases and so there is no risk of them
becoming electrified
 Such appliances are said to be double insulated, as they have two layers of
insulation:
o Insulation around the wires themselves
o A non-metallic case that acts as a second layer of insulation

 Double insulated appliances do not require an earth wire or have been designed
so that the earth wire cannot touch the metal casing

Earthing

 Many electrical appliances have metal cases

 This poses a potential safety hazard:
o If a live wire (inside the appliance) came into contact with the case, the
case would become electrified and anyone who touched it would risk
being electrocuted
 The earth wire is an additional safety wire that can reduce this risk

 If this happens:
o The earth wire provides a low resistance path to the earth
o It causes a surge of current in the earth wire and hence also in the live
wire
o The high current through the fuse causes it to melt and break
o This cuts off the supply of electricity to the appliance, making it safe
 Fuses & Trip Switches
 A fuse is a safety device designed to cut off the flow of electricity to an
appliance if the current becomes too large (due to a fault or a surge)

The circuit symbol for a fuse - take care not to confuse this with a resistor

 Fuses usually consist of a glass cylinder which contains a thin metal wire.
 If the current in the wire becomes too large:
o The wire heats up and melts
o This causes the wire to break, breaking the circuit and stopping the
current
 A trip switch, found in the Consumer Box (where the electricity enters the
building) does the same job as a fuse
o When the current is too high the switch 'trips' (automatically flicks to the off
position)
o This stops current flowing in that circuit

Choosing Which Fuse to Use

 Fuses come in a variety of sizes (typically 3A, 5A and 13A) - in order to select
the right fuse for the job, you need to know how much current an appliance
needs
 If you know the power of the appliance (along with mains voltage), the current
can be calculated using the equation:

 The fuse should always have a current rating that is higher than the current
needed by the appliance, without being too high - always choose the next size
up

Example:

 Suppose an appliance uses 3.1 amps

o A 3 amp use would be too small - the fuse would blow as soon as the
appliance was switched on
o A 13 amp fuse would be too large - it would allow an extra 10 amps to
pass through the appliance before it finally blew
o A 5 amp fuse would be an appropriate choice, as it is the next size u