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National 5 Physics Summary Notes
Electricity & Energy
Energy
Forms of energy
Energy can never be created or destroyed. It can only be transformed from one
type to another (or other types).
There are many different forms of energy:
•
•
•
•
•
•
•
•
•
Kinetic (movement) Energy
Gravitational Potential Energy
Chemical Potential Energy
Nuclear Potential Energy
Elastic Potential Energy
Heat Energy
Light Energy
Sound Energy
Electrical Energy
Work Done
Power
Work done is a measure of the energy
transferred during an energy
transformation.
Power is the rate at which energy is
transferred during an energy
transformation.
Work done has the symbol Ew and is
measured in joules (J).
Power has the symbol P and is measured in
watts (W).
Power, Energy and time
Work Done, Force & Distance
work done = force× distance
Power =
EW = Fd
F=
EW
d
d=
EW
F
P=
Energy
time
E
t
E = Pt
t=
E
P
1
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Mechanical Energy
Kinetic Energy
Gravitational Potential Energy
Kinetic energy, Ek, is the energy an object
has due to its motion.
The change in gravitational potential energy,
Ep, of an object is the work done against
gravity (lifting) or by gravity (falling).
Kinetic energy is measured in joules (J)
The kinetic energy of an object depends on
both its mass and speed. Increasing the
mass and/or the speed of the object
increases its kinetic energy.
as
Gravitational potential energy is measured
in joules (J).
Gravitational Potential Energy, mass
and height
Kinetic Energy, mass and speed
E P = mgh
m=
EP
gh
g=
EP
mh
h=
EP
mg
×
=
1
EK = mv 2
2
v2 =
2E K
m
m=
2E K
v2
Conservation of Energy
During any energy transformation the total amount of energy stays the same. This is known
as the principle of conservation of energy.
The principle of conservation allows numerical problems involving different forms of energy
to be solved. For example, when a vehicle rolls down a slope, gravitational potential energy
is converted into kinetic energy and heat (work done by friction). Calculations can be
performed on each of the spate forms of energy (EP=mgh, EK=½mv2, and EW=Fd) but it is also
known that the total amount of energy stays the same and so (in this case) EP = EK + EW.
Energy “Loss”
In some situations it may appear that energy has been lost. In these cases it will have been
turned into less useful (or sometimes less obvious) forms. The most common example is
when energy is “lost” as heat, due to friction.
2
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Electrical Charge Carriers and Electric Fields
Electrical Charges
There are 2 types of charges, positive and negative.
negative Electric fields exist around charges.
These fields cause other charges to experience a force, either attraction or repulsion.
Opposite charges attract.
attract Like charges repel.
repel
In electricity we follow the flow of the negative charges. These are known as electrons.
electrons
Current
Direct and Alternating Current
An electrical current is the quantity of
charge that flows past a fixed point every
second.
current =
A direct current is one which flows in one
direction. A good example of this is
current from a battery or cell. The flow is
always from the negative terminal to the
positive terminal.
charge
time
An alternating current is when the
negative and positive terminals reverse
polarity at regular intervals. This causes
the current to flow in one direction and
then the opposite direction. The mains
electrical supply is an alternating current.
It has a frequency of 50 Hz.
Q
I=
t
Current is measured in amperes (A), charge is
measured in coulombs (C) and time is
measured in seconds (s).
Potential Difference (Voltage)
Circuits
When a charge is moved by an electric
field, the charge is given energy. This
energy is then used elsewhere in the
circuit. The energy per unit charge is
known as the voltage.
voltage =
V=
A current will only flow in a circuit which is
complete. There must be at least one
continuous connection between the two
terminals of the power supply. Energy is
supplied by the power supply and is
converted into other forms of energy by
the components in the circuit.
energy
charge
E
Q
Voltage is measured in volts (V), energy is
measured in Joules (J) and charge is measured
in coulombs (C).
In this circuit the electrons flow in a
clockwise direction.
3
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Components
Circuits may consist of many components. In a component, an energy change normally
takes place. For example, in a lamp electrical energy is converted into light (and heat).
Component
Function and use
Most common energy source in an electric circuit. Cells have
chemically stored energy. Energy is given to the electrons which
pass through the cell. This energy is then passed on to components
A combination of two or more cells connected in series. The
voltage supplied by a battery is the sum of the voltages of each of
the cells.
A lamp converts electrical energy into light energy (and some heat
too). It is an example of an output device.
A switch is used to “break” the circuit. This creates an open circuit
and stops electrons from flowing.
A motor converts electrical energy into rotational kinetic energy. It
does this using magnets and coils of current carrying wire. A motor
is used as an output device.
A microphone converts sound energy into electrical energy. It is an
example of an input device.
A loudspeaker converts electrical energy into sound energy. It is an
example of an output device.
A photovoltaic cell converts light energy into electrical energy. It
may be used as an input device. It is the basis for the solar cells
used in solar panels.
A fuse is a safety device. It contains a piece of wire which melts
when the current reaches a certain level. This isolates an appliance
from the mains and protects the appliance and the user. Appliances
usually have a 3A (under 700W) or 13A (over 700W) fuse.
A diode is an electrical one-way-street. It only allows the current to
flow in one direction. In the case of the one shown, electrons
would only be able to flow from right to left.
A capacitor is able to “store” charge. It is initially neutral, however
when connected to a DC power supply, like a cell, one plate
becomes positively charged while the other becomes negatively
charged. If it is then connected across a lamp, the capacitor will
discharge and the lamp will come on for a short period. It is often
used in electronics for timing circuits, since its charge/discharge
time can be controlled by its capacitance and the resistance of a
resistor.
A relay is an electrical switch. It allows a low voltage (6V) primary
control circuit (see electronics section) to control a high voltage
(230V) secondary circuit.
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F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Series Circuits
Parallel Circuits
A series circuit is one in which the
component are connected in a single loop.
In a parallel circuit, current can take more
than one path.
In a series circuit:
In a parallel circuit:
• the currents in the branches add up to
the current from the supply
• the voltage is the same across each
branch and is equal to the supply
voltage
• the current is the same at all points
• the voltages across the separate
components add up to the supply
voltage.
Ammeters and Voltmeters
Each meter is responsible for measuring a different quantity
and each meter is connected in a different way. Ammeters are
connected in series (in line with) with components, while voltmeters
are connected in parallel (across) components.
Resistance
Ohm's Law
Resistors reduce the current (flow of
charge) in a circuit.
Ohm's Law describes the relationship
between voltage, current and resistance.
For a fixed value resistor there is a direct
proportion relationship between the
current through the resistor and the
voltage across it.
Resistance is measured in ohms (Ω).
An ohm meter is used to measure the
resistance of a component directly. The
ohm meter does not require a separate
power supply and so is connected as
shown in the diagram.
0
This relationship is known as Ohm's Law.
V = IR
5
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Resistors is Series
Resistors is parallel
In a series circuit, the total resistance is the
sum of all the individual resistances.
The total resistance in parallel circuits is
more complicated. The following equation
can be used to calculate the total
resistance.
R T = R 1 + R 2 + R 3 + ...
1
1
1
1
=
+
+
+ ...
R T R1 R 2 R 3
Electronics
Electronic Systems
All electronic systems consist of 3 main parts.
Input Devices
Input devices convert one form of energy into electrical energy. This allows an external signal
to be converted into an electrical one, for use in the circuit.
Examples include:
solar cell
thermocouple
light energy → electrical energy
heat energy → electrical energy
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F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Dependant resistors as Input Devices
Light Dependant Resistors
Light Dependant Resistors, often abbreviated to LDR. These resistors have a varying
resistance which depends on light level. For an LDR as light level increases, resistance
decreases. This is often abbreviated to L.U.R.D. to help you remember.
Thermistors
These resistors have a varying resistance which depends on temperature. For a thermistor as
temperature level increases, resistance decreases. This is often abbreviated to T.U
T.U.R.D. to
help you remember.
Variable resistors
Variable resistors which can be controlled by the user.
Analogue Systems
Digital Systems
An analogue system is often described as
continuously variable. The signal can exist
at any state between its maximum and
minimum value. It is often described as a
wave:
A digital system is one in which the signal
can only exist at one of two states, off and
on, also often described as 1 and 0:
1
0
The Light Emitting Diode
The LED is an example of an output device. It converts electrical energy into light energy.
Unlike a lamp it produces very little heat energy. The LED only allows a current to flow
through it in one direction and so it must be connected the right way round.
The LED should always be connected in series with a resistor. The purpose of the resistor is
to reduce the current passing through the LED since too large a current will damage the LED.
7
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
LED Protective Resistor Calculation
Consider an LED rated at 2V and 10mA connected to a 6V supply.
This circuit is like a voltage divider. If there needs to be 2V across
the LED then there must be 4V across the resistor.
In a series circuit the current is the same through each of the
component. We can now do a calculation using Ohm's Law.
V = IR
4 = 0.01 × R
4
R=
0.01
R = 400Ω
Voltage Dividers
In a circuit with more than one resistor the voltage of the
supply is split across the resistors in that circuit. There are 2
equations that can be helpful in finding missing values in these
types of circuits.
The ratio of the resistances is the same as the ratio of the
voltages across them:
V1 R1
=
V2 R2
The voltage across the second resistor can be calculated using:
 R2 
 × Vs
V2 = 
R
+
R
 1 2
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F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Alternative Diagrams
In these circuits the reading on the voltmeter is determined by the ratio of the resistors. The
ratio can be controlled by the use of variable resistors including thermistors and LDRs. This
makes them excellent as input devices.
Voltage Dividers as Input Devices
Light Sensing
Temperature Sensing
As light/temperature increases the resistance of the LDR/thermistor decreases. As a result
the voltage across the LDR/thermistor decreases. The reading on the voltmeter decreases.
Connected in reverse they carry out the opposite function:
As light/temperature increases the resistance of the LDR/thermistor decreases. As a result
the voltage across the LDR/thermistor decreases. The share of the voltage across the fixed
resistor increases and the reading on the voltmeter increases.
9
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
The npn transistors and the MOSFET
Transistors are electronic switches. When connected in a circuit with a voltage divider they
are controlled by the voltage across the lower resistor. There are two main types:
npn transistor
MOSFET
switches "on" at 0.7V
switches "on" at 2V
Control
Control Circuits
Control circuits use all of the pieces we have looked at so far. They include a sensing section,
dependant on light or temperature, an electronic switch and an LED (or other output device).
This is a temperature dependant circuit.
As temperature increases the resistance of the
thermistor decreases.
As the resistance of the thermistor decreases the
voltage across it also decreases.
As the voltage across the thermistor decreases the
voltage across resistor 2 increases.
When the voltage across resistor 2 rises above 0.7V
the transistor starts conducting and the LED goes on.
The control circuit can be reversed by switching the locations of the resistors. The circuit can
be calibrated by varying the resistance of the variable resistor.
Capacitors as time delays
In this type of circuit, the capacitor will take a set time to
“charge”. As it charges, the circuit current decreases, the
voltage across the resistor decreases and the voltage
across the capacitor increases (eventually reaching the 6V
of the power supply). When the voltage across the
capacitor reaches 0.7V the transistor will conduct,
allowing the LED to light. Swapping the locations of the
resistor and capacitor will cause the LED to go off after a
delay.
10
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Electrical Power
Power
Power is the quantity of energy transformed from one form into another form
(or other forms) every second.
Power =
Energy
time
E
P=
t
Power is measured in watts (W), energy measured in joules (J) and time
measured in seconds (s).
1 watt = 1 joule per second
Electrical Power
In electric circuits, the power transferred by a component can be calculated
from the voltage across the component and the current passing through it.
Power = Current× Voltage
P = IV
If resistance is known, then the following two equations can also be used to
determine power. They are produced by combining Ohm's Law with P=IV.
V2
P=
R
P = I2R
11
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Heat
Heat Energy
Heat is a form of energy.
energy Heat energy can be produced from another form of energy or
changed into another form, for example:
- kinetic energy → heat energy
- electrical energy → heat energy
(through friction)
(in a resistor)
- heat energy → electrical energy
(thermocouple)
As it is a form of energy heat is measured in Joules, J.
Temperature
Temperature is a measure of how hot or cold an object or substance is. Temperature is
measured in degrees Celsius, °C. Temperature can also be measured in Kelvin, K, (see Gas
Law and Pressure).
Temperature is a measure of the average kinetic energy of the particles in a substance. This
means that as the temperature of a substance increases, the kinetic energy (and therefore
the speed) of the particles in that substance increases too.
Changing Temperature
The temperature of a substance can be changed by supplying it with energy or removing
energy from the substance (heating or cooling it). The temperature change depends on the
energy supplied, the mass of the substance being heated and the material itself.
Different materials have different specific heat capacities.
capacities This is the energy required for 1kg
of a substance to achieve a temperature change of 1°C.
E H = cm∆T
EH - energy supplied/removed
c - specific heat capacity
m - mass of the substance
ΔT - temperature change
J
J kg-1 °C-1
kg
°C
12
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
States of Matter
There are 3 states of matter, solid, liquid and gas. A substance can change between states if
it receives or loses energy.
Changes of State
Increasing energy
solid → liquid
liquid → gas
melting
evaporating
Decreasing energy
gas → liquid
liquid → solid
condensation
freezing
Changing state also requires an increase or decrease in energy. When a substance is changing
state it does not experience a change in temperature.
temperature
Cooling graph of a substance
13
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Specific Latent Heat of Fusion (solid ↔ liquid)
Energy is required to change the state of a substance from a solid to a liquid. The energy
required depends on the mass of the substance and the substance itself. The property of the
substance is known as specific latent heat of fusion.
E H = ml f
EH - energy supplied/removed
m - mass of the substance
lf - specific latent heat of fusion
J
kg
J kg-1
Specific Latent Heat of Vaporisation (liquid ↔ gas)
Energy is required to change the state of a substance from a liquid to a gas. The energy
required depends on the mass of the substance and the substance itself. The property of the
substance is known as specific latent heat of vaporisation.
E H = mlv
EH - energy supplied/removed
m - mass of the substance
lv - specific latent heat of vaporisation
J
kg
J kg-1
Heat Transfer
Heat energy can be transferred by three methods:
Conduction - The vibrations of the hot particles are passed to their neighbours. This takes
place in solids.
Convection - Hot liquids or gases are less dense than colds ones and so they will rise and the
cold gases and liquids will fall. This sets up a convection current. This takes place in liquids or
gases.
Radiation - A form of electromagnetic radiation (infrared) Energy is transferred as rays. This
is the only method by which heat can be transferred through a vacuum.
The greater the temperature difference the greater the rate of heat transfer.
14
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Heat Loss
Heat always moves from hot objects to cold objects. The rate of heat loss depends on the
temperature difference between the two objects.
Reducing Heat Loss
Heat loss from the home can be reduced in the following ways:
Loft insulation - Reduces heat loss through by conduction through the ceiling.
Double Glazing - A vacuum between the panes of glass reduces heat loss by conduction.
Draught Excluders - Reduce heat loss by convection.
Foil-backed Plasterboard - Reduces heat loss by radiation from the walls.
Foil-backed Radiators - Reflects heat back into room and so reduces heat loss by radiation.
Cavity Wall Insulation - A foam filling between the walls prevents heat loss due to
convection or conduction.
Carpets - Reduces heat loss by conduction through floor.
Pressure
Defining Pressure
Pressure is defined as the force exerted per unit area by one medium on another. This
includes solids, liquids and gases. That's right, gases exert pressure too.
Calculating pressure
Pressure =
Force
Area
Pressure is measured in pascals (Pa) or newtons per square metre (N m-1)
Force is measured in newtons (N)
Area is measured in square metres (m2)
1 pascal = 1 newton per square metre
15
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Comparing Pressures
Pressure allows us to compare the effect that 2 very different objects may have on their
surroundings.
Take the example of an elephant and the stiletto heel of a shoe. Which would you rather
have step on your foot?
Elephant
Force exerted by elephant
Force is weight so:
W = mg
= 7000 × 9.8
= 68 600 N
Stiletto
Force exerted by stiletto
Force is weight so:
W = mg
= 50 × 9.8
= 490 N
Area of elephants foot ~0.2m2
F
P=
A
68 600
=
0.2
= 343 000 Pa
Area of stiletto heel ~0.0001m2
F
P=
A
490
=
0.0001
= 4 900 000 Pa
16
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Gas Laws
Kinetic Model of a Gas
Physicists often use models to describe situations
that are difficult to visualise. This includes the very
large and the very small. The kinetic model of a gas
describes the motion of the particles in the gas and
uses their behaviour to explain our observations of
the gas itself.
There are 3 important quantities about the gas that
we can observe, which can be explained by
visualising the particles within the gas.
Volume - The volume of a gas is the 3 dimensional space the gas occupies. This space is
usually determined by the container that the gas is in. We do not concern ourselves with
the volume of the particles, only the volume of the gas as a whole.
Temperature - The temperature of a gas is a measure of the average kinetic energy of the
particles in that gas. The greater the temperature, the greater the average kinetic energy of
the particles and therefore the greater their speed. (Note that it is the temperature of the
gas and the speed of the particles!)
Pressure - The pressure of the gas is caused by the particles colliding with the walls of the
container. The more frequent or violent the collisions, the more force is exerted on the
walls of the container and therefore the greater the pressure.
Relationship between Volume and Pressure of a Gas
For a fixed mass of gas at a constant temperature, the pressure of a gas is inversely
proportional to its volume:
0
0
P∝
1
V
P × V = constant
P1V1 = P2V2
17
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Relationship between Pressure and Temperature of a Gas
For a fixed mass of gas at a constant volume, the pressure of a gas increases as the
temperature increases:
0
This is not a proportional relationship, as the line does not go through the origin. If we
extend the graph into the negative temperature scale we see the following:
0
This is where we introduce the kelvin scale. -273°C is now equivalent to zero kelvin
0
We find that pressure is directly proportional to temperature, when temperature is measured
in kelvin.
kelvin
P∝T
P
= constant
T
P1 P2
=
T1 T2
18
F. Kastelein
Boroughmuir High School
National 5 Physics Summary Notes
Electricity & Energy
Converting between Celsius and Kelvin
To convert between the two temperature scales we either add or subtract 273:
Convert:
°C to K
K to °C
add 273
subtract 273
Relationship between Volume and Temperature of a Gas
For a fixed mass of gas at a constant pressure, the volume of a gas is directly proportional to
its temperature, when measured in kelvin:
elvin
0
V∝T
V
= constant
T
V1 V2
=
T1 T2
General Gas Equation
All three gas laws combine to form the general gas equation:
P1V1 P2V2
=
T1
T2
PV
= constant
T
Remember: temperatures must be converted to kelvin to work in this equation!
19
F. Kastelein
Boroughmuir High School