Chapter 19
Electric circuits
Standard circuit symbols
Electric circuits
The electric circuit provides a path through which the current can flow. Current
flows continuously through a complete circuit transferring energy from the power
supply to the device. There are two types of current
Direct current: Cells and batteries supply direct current which flows continuously
around the circuit in the same direction.
Alternating current: Alternating current oscillates or reverses to and fro in the
circuit. The frequency of the a.c supply states how often the current changes the
direction.
Resistors
A passive electrical component with two terminals that are used for either limiting
or regulating the flow of electric current in electrical circuits.
Fixed resistors: These resistors have a specific value and these values cannot be
changed.
Variable resistors: These resistors do not have a specific value and the values can
be changed with the help of dial, knob, and screw. These resistors find applications
in radio receivers for controlling volume and tone. Following are the different
types of variable resistors:
Potentiometers
Rheostats
Trimmers
Thermistor
A thermistor is a type of resistor (a thermal resistor) which changes its resistance
when it is heated up:
o As a thermistor gets hotter, its resistance decreases
o As it gets colder, its resistance increases
Light dependent resistor (LDR)
An LDR (Light Dependent Resistor) is a type of resistor which changes its
resistance when light is shone on it:
o When more light is shone on an LDR its resistance decreases
o If an LDR is covered up (reducing the light hitting it) its resistance
increases.
Relay
A relay consists of two parts:
o A coil (electromagnet)
o A magnetic switch
When there is a current in the coil, it creates a magnetic field which attracts
the switch, closing it
This can be used to control a switch in a separate circuit, as shown in the
above diagram
Sensing circuits
Thermistor or LDR may be used to create heat or light sensitive circuits which
perform certain actions when the heat or light reaches a certain value.
Eg: Alarm circuit that uses a relay
In the above circuit a thermistor is used along with a relay to set off an alarm
when temperature rises.
When the temperature rises, the resistance of the thermistor decreases. This
causes large current through the relay coil, pulling the relay switch closed. So
current will pass through the right-hand side of the circuit and this makes the bell
rings.
Light controlled circuits may be created by replacing the thermistor with a LDR.
Diodes
A diode is a component which only allows charge to flow in one direction
through it.
Diodes are used to convert an alternating current into a direct current. This
process is called rectification
Series circuits
A series circuit consists of a string of two or more components, connected end to
end.
In a series circuit the current is the same at all points
When several cells are connected together in series, their combined emf is
equal to the sum of the individual emfs
In a series circuit, the components share the emf of the power supply.
Parallel circuits
A parallel circuit consists of two or more components attached along
separate branches of the circuit
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
Note that the current does not always splits equally, often there will be more
current in some branches than in others.
The current in each branch will only be identical if the components along
each branch are identical.
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
Total voltage = V₁ +V₂ +V₃
Total resistance = R₁ + R₂ + R₃
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 current from the source is greater than the current through either
resistor.
The current from the supply is the sum of the currents flowing through the
resistors.
I = I₁ + I₂ + I₃
The effective resistance R for three resistors in parallel is,
Potential divider circuits
When two resistors are connected in series, the potential difference across
the power source is shared between them
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. 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. 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.
The p d across each resistor adds up to the e m f of the cell
Vin = V₁+ V₂
Resistance for two resistors used as a potential divider
V₁/ V₂ = R₁/R₂
Electrical safety
Common electrical 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
In order to protect the user or the device, there are several safety features
built into domestic appliances, including:
o Double insulation
o Earthing
o Fuses
o 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
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
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
The earth wire provides a low resistance path to the earth
It causes a surge of current in the earth wire and hence also in the live wire
The high current through the fuse causes it to melt and break
This cuts off the supply of electricity to the appliance, making it safe
Fuses
Fuses usually consist of a glass cylinder containing 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
Circuit breakers
A circuit breaker consists of an automatic electromagnet switch that breaks
the circuit if the current exceeds a certain value
This has a major advantage over a fuse because:
o It doesn’t melt and break, hence it can be reset and used again
o It works much faster
Residual current devices (RCDs) are used for protection against direct and
indirect contacts. An RCD can detect low leakage currents that could flow
through the body of a person. It thus provides additional protection if the
normal protection means fail, e.g. old insulation, human error etc
Self-assessment questions
1a.
2a. Light Dependent resistor
b.
c. Decreases
3a.
b. thermometer
c. resistance changes with temperature. For an NTC (negative coefficient
thermistor), the resistance falls with temperature.
a Circuit diagram similar to Figure 19.8, with the thermistor replaced by an LDR
and the lamp replaced by a heater.
b When it is dark, the LDR has a high resistance. The current in the left-hand
circuit is small, so the relay remains open. There is no current in the
right-hand circuit.
c When light shines on the LDR, its resistance decreases. The current through
the relay coil increases, pulling the relay switch closed. Now a current flows in the
right-hand circuit and this turns on the heater.
5
6
7.Each lamp can be controlled by its own switch. If one lamp breaks and does not
allow current to flow, the other lamps will still work.
8 It is the same (1.4 A)
10. a Combined resistance = 4 +6 = 10 Ω
b. Current I = V/R = 6/ 10 = 0.6A
c. P.d across 4Ω resistor = IR = 0.6 x 4 = 2.4 V
p d across 6Ω resistor = IR = 0.6 x 6 = 3.6 V
a series
b To obtain a smaller fixed or variable p.d. than the source e.m.f.
a) V₁/V₂ = R₁/R₂
V₁/7.2 = 215/860
V₁= 1.8V
Vin = V₁+ V₂= 9V
b) V₁=Vin -V₂ = 36 – 8 =28V
V₁/V₂ = R₁/R₂
28/8 = R₁/80
R₁= 280Ω
13 Wire has resistance, like a resistor. A longer wire is like having resistors in
series (i.e. a chain of resistors) whose total resistance is the sum of the resistors, so
a longer wire has a higher resistance.
14 A thicker wire is like having a resistors in parallel whose combined resistance is
less than the smallest resistance value
15.a) Combined resistor = 15 + 30 = 45Ω
Current = V/R = 15/45 = 0.33A
b)30Ω because the bigger resistor
16.a) Effective resistance = 2.4Ω
b) Current through 6Ω = V/R = 6/6 = 1A
Current through 4Ω = 6/4 = 1.5 A
c)Current = 1+ 1.5 = 2.5 A
18 multi-way bar extension; block adapters
19 fuse
20 It can lead to a fire.
21 a 5 A; a fuse of 3 A would melt every time that the hairdryer was switched on.
Fuses rated higher than 5 would allow too big a current to flow before melting and
breaking the circuit.
22 a This is to prevent too high a current entering the house where it could melt the
insulation on wiring, which could give off poisonous fumes or cause a fire.
b circuit breaker
23 When the current gets too high the insulation can giving off poisonous fumes,
melt or catch fire.
24 The earth wire provides a low resistance electrical path to ground and reduces
the chances of a fatal electric shock.
25 If it was connected to the neutral or earth wire, a current could still pass into the
appliance even with the switch open. If the appliance was faulty and the live wire
touched the metal casing, then a person touching the casing would get an electric
shock.
26 If it was connected to the earth or neutral wire, a person could still get
electrocuted when they touch the appliance.
27 A fuse melts and breaks the circuit when the current passing through is too
high.
28 Double insulation is where the electric circuit for an electrical appliance is
inside a plastic case, which is inside the outer case.
Exam style questions
Ans: C
a) 7V
b) 12 – 7 = 5V
c) 23mA – 17mA = 6mA
d) R = V/I = 12/(23x 10⁻³) = 521 Ω
a A4 = 1.9A; A5 = 2.6 A
b A3 = 0.7A; A4 = 0.7 A;
A5 = 1.4 A;
a thermistor
b 14 mA
c7V
d 500 Ω
e The current increases
a To prevent excessive current that could lead to the insulation giving off
poisonous gases, melting or catching fire.
bC
c A fuse contains a thin section of wire. As current increases, temperature
increases.When current exceeds a certain value, the wire melts, which
stops current flowing in the circuit
d circuit breaker
e Someone with wet hands could be electrocuted (injured or killed) if they
touch a faulty electrical appliance. This is because water is a conductor
f earth wire;
