# ELECTRICITY NOTES UNIT 2

```By: Tehmina Bilal
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EDEXCEL IGCSE
PHYSICS
Notes
Topic:
ELECTRICITY
By: Tehmina Bilal
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2.1.1 Charge &amp; Current
Current


Electric current is defined as the rate of flow charge
o In other words, the size of an electric current is the amount of charge
passing through a component per second
Current flows from the positive terminal to the negative terminal of a cell
Charge flows from the positive terminal to the negative terminal
Charge


The wires in an electric circuit are made of metal, because metal is a
good conductor of electric current
In the wires, the current is a flow of negatively charged electrons
In metal wires, the current is a flow of negatively charged electrons. This
image shows the electrons flowing through a lattice of metal ions
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Calculating Electric Charge

The charge, current and time are related by the equation:

Where:
o Q = charge measured in Coulombs (C)
o I = current measure in amps (A)
o t = time measured in seconds (s)

This equation can be rearranged with the help of the following formula
triangle:
Worked example
When will 8 mA of current pass through an electrical circuit?
A
When 1 J of energy is used by 1 C of charge
B
C
When a charge of 4 C passes in 500 s
When a charge of 8 C passes in 100 s
D When a charge of 1 C passes in 8 s
Step 1: Write out the equation relating current, charge and time
Q = It

o
This can be rearranged to make current I the subject of the equation:
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Step 2: Rule out any obviously incorrect options

o
Option A does not mention time, so can be ruled out
Step 3: Try the rest of the options by applying the equation to determine the

o
Consider option B:
I = 4 / 500 = 8 &times; 10–3 = 8 mA

o
Consider option C:
I = 8 / 100 = 80 &times; 10–3 = 80 mA

o
Consider option D:
I = 1 / 8 = 125 &times; 10–3 = 125 mA

o
Therefore, the correct answer is B
Exam Tip
Electric currents in everyday circuits tend to be quite small, so it's really common for
examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA
(10 milliamperes).Make sure that you are on the lookout for these prefixes and that
you can convert them into standard units, so 10 mA = 10 &times; 10-3 A
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2.1.2 Voltage &amp; Energy
Voltage



The terminals of a cell make one end of the circuit positive and the
other negative
This sets up a potential difference across the circuit
o This is sometimes known as the voltage
Potential difference is defined as:
The amount of energy transferred per unit of charge passing through the
terminals

This means that one volt (the unit of potential difference) is equivalent to
one joule (the unit of energy) per coulomb (the unit of charge):
1V=1J/C
Calculating Voltage

The equation linking the energy transferred, voltage and charge is given
below:

Where:
o V = potential difference, measured in volts (V)
o E = energy transferred, measured in joules (J)
o Q = charge moved, measured in coulombs (C)

This can be rearranged using the formula triangle below:
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Worked example
The normal operating voltage for a lamp is 6 V.Calculate how much energy is
transferred in the lamp when 4200 C of charge flows through it.
Step 1: List the known quantities

o
o
Voltage, V = 6 V
Charge, Q = 4200 C
Step 2: State the equation linking potential difference, energy and charge

o
The equation linking potential difference, energy and charge is:
E=V&times;Q
Step 3: Substitute the known values and calculate the energy transferred
E = 6 &times; 4200
E = 25 200 J

o
Therefore, 25 200 J of energy is transferred in the lamp
Exam Tip
Don't be confused by the symbol for potential difference (the symbol V) being the
same as its unit (the volt, V). Learn the equation and remember especially that one
volt is equivalent to 'a joule per coulomb'.
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2.1.3 Resistance
Calculating Current, Resistance &amp; Potential
Difference

Resistance is defined as the opposition to current:
o The higher the resistance of a circuit, the lower the current
o This means that good conductors have a low resistance and insulators
have a high resistance


The symbol for resistance is R
It is measured in Ohms (Ω)
o Ω is the Greek capital letter ‘Omega’
o An Ohm is defined as one volt per ampere (1 V / A)

The resistance of a circuit can be increased by adding resistors (or variable
resistors) to it
Every electrical component has a resistance, even wires
o In exam questions, the resistance of the wires and batteries are
assumed to be negligible

High resistance means there is a lower current and vice versa

The current I through a component depends on both the resistance R of the
component and the potential difference V across the component
o The greater the resistance R of the component, the lower the
current I for a given potential difference V across the component
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The lower the resistance R of the component, the greater the
current I for a given potential difference V across the component

The current, resistance and potential difference of a component in a circuit are
calculated using the equation:

This equation can be rearranged with the help of the following formula
triangle:
Voltage, current, resistance formula triangle
Worked example
Calculate the voltage across a resistor of resistance 10 Ω if there is a current of 0.3 A
through it.
Step 1: List the known quantities

o
o
Resistance, R = 10 Ω
Current, I = 0.3 A
Step 2: Write the equation relating resistance, potential difference and current
V = IR
Step 3: Substitute in the values
V = 0.3 &times; 10 = 3 V
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2.2.1 Current in Series &amp;
Parallel
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 number (and type) of 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

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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




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
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Current is split at a junction into individual branches
Worked example
In the circuit below, ammeter A0 shows a reading of 10 A, and ammeter A1 shows a
reading of 6 A.
What is the reading on ammeter A2?
Step 1: Recall that at a junction, the current is conserved

o
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

o
The diagram below shows the first junction in the circuit
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Step 3: Calculate the missing amount of current

o
o
Since 10 A flows in to the junction (the total current from the battery),
10 A must flow out of the junction
The question says that 6 A flows through ammeter A1 so the remaining
current flowing through ammeter A2 must be:
10 A − 6 A = 4 A

o
Therefore, 4 A flows through ammeter A2
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'.
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2.2.2 Voltage in Series &amp;
Parallel
Voltage in Series &amp; Parallel

In a series circuit:
o The current is the same at all points ie. through each component
o The total potential difference of the power supply is shared between
the components
Lamps connected in a series circuit

In the above circuit:
o The current from the power supply is the same as the current in both
lamps I = I1 = I2
o If the battery is marked 12 V, then the potential difference would be 12
&divide; 2 = 6 V across each lamp

In a parallel circuit:
o The total current through the whole circuit is the sum of the currents
through the separate components
o The potential difference across each component is the same
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Lamps connected in a parallel circuit

In the above circuit:
o Because the current splits up, the sum of currents in each branch will
equal the current from the power supply I = I1 + I2
o If the battery is marked 12 V, then the potential difference would be 12
V across each lamp
Series Circuits



A series circuit consists of a string of two or more components connected in a
loop
The advantages of a series circuit are:
o All of the components can be controlled by a single switch
o Fewer wires are required
The disadvantages of a series circuit are:
o The components cannot be controlled separately
o If one component breaks, they will all stop working as well
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In the series circuit above, only one switch is needed to control all of the
lamps. This can be seen as an advantage or as a disadvantage
Parallel Circuits



A parallel circuit consists of two or more components attached
across different branches of the circuit
The advantages of a parallel circuit are:
o The components can be individually controlled, using their own
switches
o If one component breaks, then the others will continue to function
The disadvantages of a parallel circuit are:
o Many more wires involved so much more complicated to set up
o All components have the same voltage as the supply, so harder to
control if components need to have different voltages
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In the parallel circuit above, the lamps are connected in parallel and can be
switched on and off by their own switch
Exam Tip
You may have noticed that for a parallel circuit, all of the components can be
controlled by a single switch - like a series circuit. Nevertheless, the exam board still
considers this an advantage of series circuitsNote that the current does not always
split equally in a parallel circuit – often there will be more current in some branches
than in others. The current in each branch will only be identical if the resistance of
the components along each branch are identical. However, the voltage across two
components connected in parallel is always the same
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2.2.3 Resistors in Series
Resistors in Series





When two or more resistors are connected in series,
the total (or combined) resistance is equal to the sum of their individual
resistances
For example, for three resistors of resistance R1, R2 and R3, the total
resistance can be calculated using:
Where R is the total resistance, in Ohms (Ω)
Increasing the number of resistors increases the overall resistance, as the
charge now has more resistors to pass through
The total voltage is also the sum of the voltages across each of
the individual resistors
o In a series circuit, the voltage of the power supply is shared between
all components
Three resistors connected in series. The total voltage is the sum of the
individual voltages, and the total resistance is the sum of the three individual
resistances
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Worked example
The combined resistance R in the following series circuit is 60 Ω.What is the
resistance value of R2?
A
100 Ω
B
30 Ω
C
20 Ω
D
40 Ω
Step 1: Write down the equation for the combined resistance in series
R = R1 + R2 + R3
Step 2: Substitute the values for total resistance R and the other resistors
60 Ω = 30 Ω + R2 + 10 Ω
Step 3: Rearrange for R2
R2 = 60 Ω – 30 Ω – 10 Ω = 20 Ω
Worked example
Dennis sets up a series circuit as shown below.
The cell supplies a current of 2 A to the circuit, and the fixed resistor has a
resistance of 4 Ω.
(a) How much current flows through the fixed resistor?
(b) What is the reading on the voltmeter?
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Part (a)
Step 1: Recall that current is conserved in a series circuit

o
o
o
Since current is conserved in a series circuit, it is the same size if
measured anywhere in the series loop
This means that since the cell supplies 2 A to the circuit, the current is
2 A everywhere
Therefore, 2 A flows through the fixed resistor
Part (b)
Step 1: List the known quantities

o
o
Current I = 2 A
Resistance R = 4 Ω
Step 2: State the equation linking potential difference, resistance and current

o
The equation linking potential difference, resistance and current is:
V = IR
Step 3: Substitute the known values into the equation and calculate the
potential difference
V=2&times;4=8V

o
Therefore, the voltmeter reads 8 V across the fixed resistor
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2.2.4 IV Graphs
IV Graphs


As the potential difference across a component is increased, the current also
increases
o This is because potential difference and current are proportional
The precise relationship between voltage and current is different for different
components and can be shown on an IV graph, including in:
o Fixed resistors &amp; wires
o Filament lamps
o Diodes
Fixed Resistors &amp; Wires



The current through a fixed resistor or a wire increases as the potential
difference (or voltage) across it increases
In other words, current is directly proportional to the potential difference for
a fixed resistor (or a wire)
o This relationship is true because the resistance of the fixed resistor (or
wire) stays constant
An IV graph shows that the line is straight and goes through the origin, as
shown in the image below:
IV graph for a fixed resistor. The current is directly proportional to the
potential difference (voltage) as the graph is a straight line through the origin
Filament Lamps
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

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For a filament lamp, current and voltage are not directly proportional
o This is because the resistance of the filament lamp increases as
the temperature of the filament increases
The IV graph for a filament lamp shows the current increasing at a
proportionally slower rate than the potential difference
IV graph for a filament lamp

This is because:
o As the current increases, the temperature of the filament in the lamp
increases
o The higher temperature causes the atoms in the metal lattice of the
filament to vibrate more
o This causes an increase in resistance as it becomes more difficult
for free electrons (the current) to pass through
o Resistance opposes the current, causing the current to increase at
a slower rate

Where the graph is a straight line, the resistance is constant
o The resistance increases as the graph curves
Reversing the potential difference reverses the current and makes no
difference to the shape of the curve

Diodes


A diode allows current to flow in one direction only
o This is called forward bias
In the reverse direction, the diode has very high resistance, and
therefore no current flows
o This is called reverse bias

The IV graph for a diode is slightly different:
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o
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When the current is in the direction of the arrowhead symbol, this
is forward bias
 This is shown by the sharp increase in potential difference and
current on the right side of the graph
When the diode is switched around, this is reverse bias
 This is shown by a zero reading of current or potential difference
on the left side of the graph
IV graph for a semiconductor diode
Investigating IV Graphs Experimentally


In order to investigate the relationship between current and voltage different
components, the following equipment is required:
o An ammeter - to measure the current through the component
o A voltmeter - to measure the voltage across the component
o A variable resistor - to vary the current through the circuit
o Power source - to provide a source of potential difference (voltage)
o Wires - to connect the components together in a circuit
The image below shows the circuits set up to obtain IV graphs for a filament
lamp and a diode
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These circuits enable the investigation of current and voltage for a filament
lamp or diode to be investigated



The current is the independent variable
o The variable resistor is used to change the current flowing through
the filament lamp / diode
The voltage is the dependent variable
o The voltmeter is used to measure the voltage across the filament lamp
/ diode
Recording measurements of current and voltage as the current increases
enables an IV graph to be plotted for each component
Resistance




Resistance is the opposition to the flow of current
o The higher the resistance of a circuit the lower the current
Resistors come in two types:
o Fixed resistors
o Variable resistors
Fixed resistors have a resistance that remains constant
Variable resistors can change the resistance by changing the length of wire
that makes up the circuit
o A longer length of wire has more resistance than a shorter length of
wire
Fixed and variable resistor circuit symbols
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2.2.5 Electrical Components
Thermistors &amp; LDRs
Thermistors


A thermistor is a temperature-dependent resistor
It is represented by the following circuit symbol:
Thermistor circuit symbol

The resistance of a thermistor changes depending on its temperature
o As the temperature increases the resistance of a
thermistor decreases and vice versa
The resistance through a thermistor is dependent on temperature
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LDRs

A light-dependent resistor (LDR) represented by the following circuit symbol:
LDR circuit symbol

The resistance of an LDR changes depending on the light intensity on it
o As the light intensity increases the resistance of an
LDR decreases and vice versa
The resistance of an LDR is dependent on the amount of light intensity on it
Lamps &amp; LEDs




Lamps illuminate (light up) when a current flows in a circuit
LEDs are types of diodes
o This means they only allow current to pass in one direction through
them and will only light if the current passes in that direction
LEDs also illuminate when a current flows in a circuit (provided the LED is
placed in the correct direction)
Since both electrical components have a visual response to current, they can
be used to indicate the presence of a current in a circuit
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LEDs can be used to indicate the presence of a current, because they
illuminate when current flows through them. The same is true for lamps
Exam Tip
Make sure you learn the various symbols mentioned on this page. Many of them are
very similar with small differences denoting what they do:


Two arrows pointing towards a symbol mean that it is light-dependent
Two arrows pointing away mean that it is light-emitting
Symbols are sometimes drawn with circles around them (e.g. the LDR). These
circles are often optional (although not in the case of meters and bulbs).
2.3.1 Electrical Power &amp; Fuses
Electrical Power

Power is defined as
The rate of energy transfer or the amount of energy transferred per second


The power of a device depends on:
o The voltage (potential difference) of the device
o The current of the device
The power of an electrical component (or appliance) is given by the equation:
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
The unit of power is the Watt (W), which is the same as a joule per second
(J/s)

This equation can be rearranged with the help of a formula triangle:
Power, current, voltage formula triangle
Worked example
Calculate the potential difference through a 48 W electric motor with a current of 4 A.
Step 1: List the known quantities

o
Power, P = 48 W
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Current, I = 4 A
Step 2: Write down the relevant equation
P = IV
Step 3: Rearrange for potential difference, V
Step 4: Substitute in the values
Exam Tip
Remember: Power is just energy per second. Think of it this way will help you to
remember the relationship between power and energyYou can remember the unit by
the phrase: “Watt is the unit of power?”
Selecting Fuses

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
This makes sure that more current doesn't keep flowing through the circuit
and causing more damage to the equipment, or, causing a fire
Fuses come in a variety of sizes, typically 3 A, 5 A and 13 A
o In order to select the right fuse for the job, the current through an
appliance needs to be known
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
If the power of the appliance is known (along with mains voltage), the
current can be calculated using the equation:

Where:
o I = current in amps (A)
o P = power in watts (W)
o V = voltage in volts (V)
The fuse should always have a current rating that is higher than the current
needed by the appliance, without being too high
o Because of this, the rule of thumb is to always choose the next size
up
If the fuse current rating is low, it will break the circuit even when an
acceptable current is flowing through
If the fuse current rating is too high, it will not be breaking the circuit in enough
time before damage occurs



Worked example
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2.3.2 Calculating Energy
Transfers
Calculating Energy Transfers




Work is done when charge flows through a circuit
o Work done is equal to the energy transferred
The amount of energy transferred by electrical work in a component (or
appliance) depends upon:
o The current, I
o The potential difference, V
o The amount of time the component is used for, t
When charge flows through a resistor, for example, the energy transferred is
what makes the resistor hot
The energy transferred can be calculated using the equation:
E=P&times;t

Where:
o E = energy transferred in joules (J)
o P = power in watts (W)
o t = time in seconds (s)

Since P = IV, this equation can also be written as:
E=I&times;V&times;t

Where:
o I = current in amperes (A)
o V = potential difference in volts (V)

When charge flows around a circuit for a given time, the energy supplied by
the battery is equal to the energy transferred to all the components in the
circuit
Worked example
Calculate the energy transferred in 1 minute when a current of 0.7 A passes through
a potential difference of 4 V.
Step 1: Write down the known quantities

o
o
o
Time, t = 1 minute = 60 s
Current, I = 0.7 A
Potential difference, V = 4 V
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Step 2: Write down the relevant equation
E=I&times;V&times;t
Step 3: Substitute in the values
E = 0.7 &times; 4 &times; 60 = 168 J
Exam Tip
'Energy transferred' and 'work done' are often used interchangeably in equations,
don't panic, they mean the same thing!Always remember that the time t in the above
equations must always be converted into seconds
2.3.3 Electrical Safety
Electrical Safety

Mains electricity is potentially lethal
o Potential differences as small as 50 V can pose a serious hazard to
individuals
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Signs, like the above, warn of the risk of electrocution

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

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 &amp; 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
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
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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
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A diagram showing the three wires going to a mains powered appliance: live,
neutral and earth

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 &amp; Circuit Breakers

Fuses and circuit breakers are safety devices 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 (not to be confused with a resistor)


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
By: Tehmina Bilal
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A circuit breaker consists of an automatic electromagnet switch that breaks
the circuit if the current exceeds a certain value
The main circuit breaker can quickly shut off electricity to the whole house.
The branch circuit breakers can shut off electricity to specific areas of the
house
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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
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For these reasons, circuit breakers are used in mains electricity in homes
o Sometimes they are misleadingly named &quot;Fuse boxes&quot;
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2.3.4 Electricity &amp; Heat
Electricity &amp; Heat
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When electricity passes through a component, such as a resistor, some of the
electrical energy is turned into heat therefore increasing its temperature
This is because energy is transferred as a result of collisions between:
o Electrons flowing in the conductor, and
o The lattice of atoms within the metal conductor
Electricity, in metals, is caused by a flow of electrons
o This is called the current
Metals are made up of a lattice of ions
As the electrons pass through the metal lattice they collide with ions
o The ions resist the flow of the electrons
As electrons flow through the metal, they collide with ions, making them
vibrate more
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When the electrons collide, they lose some energy by giving it to the ions,
which start to vibrate more
o As a result of this, the metal heats up
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This heating effect is utilised in many appliances, including:
o Electric heaters
o Electric ovens
o Electric hob
o Toasters
o Kettles
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The heating effect of current can be used for many applications such as
electric hobs
2.3.5 AC &amp; DC
AC &amp; DC
Direct Current
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A direct current (d.c.) is defined as
A current that is steady, constantly flowing in the same direction in a circuit,
from positive to negative
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The potential difference across a cell in a d.c. circuit travels in one direction
only
o This means the current is only positive or only negative
A d.c. power supply has a fixed positive terminal and a fixed negative terminal
Electric cells, or batteries, produce direct current (d.c.)
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Circuits powered by cells or batteries use a d.c. supply
Alternating Current

An alternating current (a.c.) is defined as
A current that continuously changes its direction, going back and forth around
a circuit
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An a.c. power supply has two identical terminals that switches between
positive and negative
o The current is therefore defined as positive or negative, depending on
which direction it is flowing at that time
The frequency of an alternating current is the number of times the current
changes direction back and forth each second
In the UK, mains electricity is an alternating current with a frequency of 50
Hz and a potential difference of around 230 V
On an oscilloscope, direct current and alternating current are represented in
the following way:
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Two graphs showing the variation of current with time for alternating current
and direct current
Comparing AC &amp; DC
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The following table summarises the differences between d.c. and a.c.
Direct Current vs. Alternating Current Table
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Exam Tip
If you are asked to explain the difference between alternating and direct current,
sketching and labelling the graphs shown above can earn you full marks.All the
circuits you have studied so far are d.c. circuits. Don't be put off by an exam question
if you are asked to calculate the current, potential difference or resistance in a d.c.
series circuits, you don't have to do anything different from what you have already
learned!
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