Download National 4 Summary Notes Unit 3 Electricity and Energy

North Berwick High School
Department of Physics
National 4
Summary Notes
Unit 3
Electricity and Energy
Physics N4 Unit 1: Pupil Notes
Page 1 of 22
Section 1: Generation of Electricity
What is electricity?
Electricity is a form of energy associated with stationary or moving charges in a
material. Electricity is the general name we give to the supply of electrical energy.
Electrical energy is carried (or transferred) around a circuit by tiny particles called electrons.
Think about a lamp and a switch connected by wires to a battery. When the switch
is closed, the lamp lights up – negative charges called electrons from the negative
side (terminal) of the battery move through the wires and the lamp to the positive
terminal of the battery. This movement, or flow, of negative charges is called an
electrical current (or current for short). A current is a movement of electrons. As
the electrons move through the lamp, some of their electrical energy is changed into
heat and light.
Electricity is used to power many devices at home, school, or industry.
Generating electricity
Fuel burning power stations lose a lot of energy in the form of waste heat. Electrical
energy is lost as electricity is transmitted through power lines; the greater the
distance we need to send the energy the greater the power loss.
Using a transformer we can change the output
Voltage from the power station up to 400kV for
connection to the super grid. A transformer consists
of a primary coil, a core and a secondary coil. By
using a higher voltage to transfer the electrical
energy from the power stations to the National Grid
we lose less energy to heat. Electricity pylons carry the electricity high above us as
the high voltages used are very dangerous.
Physics N4 Unit 1: Pupil Notes
Page 2 of 22
Types of Fuel
Non - renewable Uranium and the fossil fuels (coal, oil and gas.)
Renewable
Wind, waves, tidal, hydro, geothermal, bio fuels, etc
Energy
Advantage
Disadvantage
Wind
Cheap to run
clean
Not always windy
Wind farms use large area of land
Turbines can be unslightly
Solar
Cheap to run
Clean
Expensive to install
Sun is not always shining
Wave
Cheap to run
Clean
Wave size variable
Suitable sites are distant form user
Possible shipping hazard
Tidal
Cheap to run
Clean
Expensive to set up
Few suitable locations
Hydroelectric
Cheap to run
Clean
Expensive to build
Reservoirs use a large area of land
Environmental damage
Geothermal
Cheap to run
Clean
Expensive to set up
Few suitable locations
Physics N4 Unit 1: Pupil Notes
Page 3 of 22
Efficiency
Not all of the energy we change (chemical, kinetic, heat, potential, etc.) to make
electricity in Power Stations ends up as electrical energy. Some of this energy is
wasted or lost, usually in the form of heat.
Efficiency calculations are shown later in this booklet.
Physics N4 Unit 1: Pupil Notes
Page 4 of 22
Section 2. Practical electrical and electronic circuits
Conductors and Insulators
Negative charges (electrons) can only move from the negative terminal to the
positive terminal of a battery if there is an electrical path between them. Materials
that allow negative charges to move through them easily, to form an electrical
current, are known as conductors .
Materials, which do not allow electrons to move through them easily, are called
insulators
Conductors are mainly metals, such as copper, silver, gold, however, carbon is also a
good conductor.
Glass, plastic, wood and air are examples of insulators.
Current
Electrical current is a flow of negative charges called electrons. Electrons will flow
through a conductor if there is a potential difference, i.e. if there is a power supply.
Electrical current is given the symbol I and is measured in Amperes (A).
Voltage
Think about a lamp connected to a battery. The battery changes chemical energy
(from the materials inside it) into electrical energy. This electrical energy is carried
by the charges (electrons) that move around the circuit and is given up as heat and
light as they pass through the wire of the lamp.
The voltage supplied to a circuit is a measure of the electrical energy given to each
electron in a circuit. When the supply voltage is increased, a larger current flows
through the circuit.
Voltage is measured in Volts (V).
Physics N4 Unit 1: Pupil Notes
Page 5 of 22
Direct Current
When a circuit is connected to a battery, the electrons always flow round the circuit
in one direction from negative to positive This is called direct current (d.c.).
The battery supplies electrical energy to the electrons?
Using Ammeters and Voltmeters
We use an Ammeter to measure current in Amperes.
We use a Voltmeter to measure voltage in Volts.
When an ammeter is connected in a circuit to measure the current through a
component, it is connected in series with the component. Remember, we have to
‘break’ the circuit to insert the ammeter.
When a voltmeter is connected in a circuit to measure the voltage across a
component, it is connected in parallel with the component. Remember, we do not
break the circuit; we simply attach the voltmeter across the component we are
measuring.
Physics N4 Unit 1: Pupil Notes
Page 6 of 22
Series Circuits
In a series circuit there is only one path for the current to flow.
If one of the lamps is unscrewed, the rest go out because the circuit is no longer
complete. Adding more lamps reduces the overall current in the circuit and also
reduces the voltage available for each lamp.
We can write the Series circuit rules as:
Is = I1 = I2 = I3 = I4...
Where Is is equal to the total current of the supply – in a series circuit, the current is
the same at ALL points in the circuit.
Vs =
V1 + V2 + V3...
Where Vs is equal to the voltage of the supply – the Voltage of the supply is split
across the different components in the circuit.
Parallel Circuits
Physics N4 Unit 1: Pupil Notes
Page 7 of 22
In a parallel circuit there is more than one path (called a branch) for the current to
flow.
If one lamp is unscrewed, the others remain lit.
We can write the Parallel circuit rules as:
Is =
I1 + I2 + I3 + I4...
Where Is is equal to the total current of the supply – in a parallel circuit, the current
is split between the branches in the circuit.
Vs = V1 = V2 = V3...
Where Vs is equal to the voltage of the supply – the voltage across the different
branches in the circuit are equal to the supply voltage.
Adding more lamps in parallel does not affect the voltage of the individual branches.
It will, however, increase the current drawn from the supply.
Physics N4 Unit 1: Pupil Notes
Page 8 of 22
Circuit Symbols
Circuit symbols are used in electrical circuits to represent circuit components or
devices to make them easier to draw and understand.
Some of the circuit symbols you might need to know are shown below.
Name
Symbol
Name
Cell
Fuse
Battery
Light Bulb
DC Supply
Motor
AC Supply
Switch (open)
Switch (closed)
Buzzer
Resistor
Variable
Resistor
Light
Dependent
Resistor
Thermistor
Voltmeter
Ammeter
Light Emitting
Diode (LED)
Diode
Physics N4 Unit 1: Pupil Notes
Symbol
Page 9 of 22
Photodiode
Capacitor
Loudspeaker
Microphone
NOT Gate
AND Gate
OR Gate
Physics N4 Unit 1: Pupil Notes
Page 10 of 22
Section 3. Ohm's Law
Resistance
All materials oppose current passing through them. Note that wires / components
with low resistance are generally termed conductors; those with high resistance are
generally termed insulators.
In circuits, the components’ resistance, the current and the voltage are all linked. To
investigate this the following circuit is set up
The voltage is varied and readings are taken as
follows.
Voltage (V)
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
Current (A)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
Plotting voltage against current for these results gives the following graph:
Physics N4 Unit 1: Pupil Notes
Page 11 of 22
Potential Difference / V
y = 50x
R² = 1
Current / A
The gradient of the line in the graph is 50, which matches the resistance of the
resistor in the circuit. This means that V/I = R.
We can state, then, that the voltage is equal to the current times the resistance.
Symbol
V
I
R
Name
Potential
Difference
Current
Resistance
Unit
volts
Unit Symbol
V
amperes
ohms
A
Ω
This relationship is known as Ohm’s law and can be used for circuits with many
types of components.
Physics N4 Unit 1: Pupil Notes
Page 12 of 22
Resistors in series
R1
R2
R3
The resistors here are shown in series.
If you imagine that each resistor is a tunnel and lots of people want to get from the left to the
right, each tunnel is going to slow them down. In other words, the delay at each resistor will add
up.
In equation form, this is
Symbol
Rtotal
R1
R2
R3etc
Name
Total Resistance
Resistance 1
Resistance 2
Other resistors
Unit
ohms
ohms
ohms
ohms
Unit Symbol
Ω
Ω
Ω
Ω
In this equation we put … at the end as there can be any number of resistances.
Worked examples
1. Calculate the resistance of the following circuit
A
Rtotal
R1
R2
R3
=?
= 60 Ω
= 35 Ω
= 22 Ω
Rtotal
Rtotal
Rtotal
= R1 + R2 + R3
= 60 + 35 + 22
= 117 Ω
2. The total resistance of this circuit is 25 kΩ. Calculate the value of Resistor 2
V
Rtotal
R1
R2
R3
= 25 kΩ
= 12 kΩ
=?
= 500 Ω
Rtotal
25000
R2
R2
R2
= R1 + R2 + R3
= 12000 + R2 + 500
= 25000 – 12500
= 12500
= 12.5 kΩ
Physics N4 Unit 1: Pupil Notes
Page 13 of 22
A variable resistor is a resistor whose resistance can be changed. The resistance is
normally changed by altering the length of the wire in the resistor (the longer the
wire, the higher the resistance). Variable resistors are often used a volume or
brightness controls on televisions or dimmer switches for lights.
A variable resistor changes the amount of resistance in a circuit.
A resistor converts electrical energy into heat energy.
Worked example:
A 12 V battery supplies a motor which has a resistance of 18 Ω. What is the current
in the circuit?
V = 12 V
R = 18 Ω
I =?
V=
12 =
I=
I=
IR
I × 18
12/18
0.67 A
Physics N4 Unit 1: Pupil Notes
Page 14 of 22
Section 5 Electrical Components and Electronics
Practical series and parallel circuits.
Series circuits are often used in lighting circuits, continuity testers, old style
Christmas lights: benefits (simple to construct) and drawbacks (one component fails,
the whole circuit breaks).
Here are some examples of useful series circuits:
Stair lighting uses two or more 2-way switches in series:
Continuity testers:
Simple conductors can be tested for continuity by trying to pass a current through
them in series with some indicator device (bulb, buzzer, ammeter)
If there is a current, the series circuit is complete
If there is no current, there is a break in the circuit.
Never use a continuity tester with any equipment connected to the mains supply!!!
Physics N4 Unit 1: Pupil Notes
Page 15 of 22
Parallel circuits
These are often used in new style Christmas lights, car lighting circuits, and
household ring circuit: benefits (one component fails only that branch is broken) and
drawbacks (more complex).
Car lighting circuit:
If one bulb blows, the rest of the lights must stay on
Sidelights should come on first and together; headlamps come on after side lamps
Car often uses the car chassis as a ‘return’ instead of wire
Positive is the live and the negative is attached to chassis for return
Physics N4 Unit 1: Pupil Notes
Page 16 of 22
Electronic systems and Digital and Analogue signals.
What do electronic systems do? In their simplest form they respond to an input,
process this information in some way and then provide a response (an output). We
can draw a block diagram like this to represent any electronic system:
Input devices generally change some type of physical quantity (light, heat, sound,
touch) into an electrical signal.
Process devices take these electrical signals and change them in some way.
Output devices turn this electrical signal into something that we can use or react to;
for instance sound, light, heat, movement, etc.
There are two types of signals used by electronic systems: Analogue and Digital.
Analogue signals can have a continuous (changing) value:
Physics N4 Unit 1: Pupil Notes
Page 17 of 22
Digital signals can only have two possible values:
These are:
Maximum (ON): called logic 1 or logic high.
Minimum (OFF): called logic 0 or logic low.
Digital versus Analogue signals:
Digital signals carry more information per second than analogue signals.
Digital signals maintain their shape over distances far better than analogue signals
Logic Gates
Logic gates are digital devices. They are used to combine or change digital electronic
signals. There are 3 basic types of logic gate called the NOT gate (sometimes called
an inverter), the AND gate and the OR gate
They accept inputs of High (1) or Low (0)
They produce outputs of High (1) or Low (0)
Remember our simple electronic system block diagram:
Inputs and outputs are represented on a truth table.
Physics N4 Unit 1: Pupil Notes
Page 18 of 22
The NOT gate INVERTS its input signal – that is it flips a 1 to a zero and a zero to a
one! The output is always the opposite of the input.
E.g. Input HIGH (1) gives us an Output LOW (0)
NOT gate truth table:
INPUT
0
1
Physics N4 Unit 1: Pupil Notes
OUTPUT
1
0
Page 19 of 22
Physics N4 Unit 1: Pupil Notes
Page 20 of 22
The NOT gate only has ONE input!
The AND gate has TWO inputs: Input A and B.
It requires both inputs (A and B) to be high to give an output of high.
If both inputs are low the output will be low.
If the inputs are different the output is low.
AND gate truth table:
Input A
Input B
Output
0
0
0
0
1
0
1
0
1
1
1
1
Input A
Input B
Output
0
0
0
0
1
1
1
0
1
1
1
1
A simple way to remember this – the AND gate is
shaped like a D
The OR gate has 2 inputs:
The output will be logic 1 when either A or B
are logic 1
Real-life electronic systems
Electronic systems are part of our daily lives, so much so that we probably do not
even notice them. The following inputs and outputs can be combined with logic
gates, for example, how do streetlights turn on at night and off in the morning?
Physics N4 Unit 1: Pupil Notes
Page 21 of 22
A
We want the streetlight to turn on at night, but the light sensor switches on when it
detects light. A NOT gate can turn it into a darkness sensor.
Physics N4 Unit 1: Pupil Notes
Page 22 of 22