Physics II - Energy & Electricity Key Notes
Energy is the ability to ‘do some work’ - everything that happens needs energy (e.g.
heating; cooking; lighting; movement of vehicles; and keeping us alive)!
Energy cannot be created or destroyed, only transferred from place to place in a
variety of ways
1. Electrical energy - a current in a circuit etc…
2. Light energy - from the sun; a light bulb etc…
3. Sound energy - from a loudspeaker; a drum etc…
4. Kinetic (movement) energy - anything which moves!
5. Nuclear energy - transferred during a nuclear reaction
6. Thermal (heat) energy - transferred from hot objects to colder ones
7. Radiant heat (infra red) energy - transferred as electromagnetic radiation
by hot objects
8. Gravitational potential energy - possessed by anything with the ability to fall
9. Elastic potential energy - stretched springs; elastic and rubber bands etc…
10. Chemical energy - possessed by food; fuels; batteries etc…
Different types of energy can be transferred from one type to another
Energy transfer diagrams show each type of energy, whether it is stored or not,
and the processes taking place as it is transferred
Sankey diagrams show the relative amounts of each type of energy
They summarise all the energy transfers taking place in a process – the thicker the
line or arrow, the greater the amount of energy involved
Energy can be 'wasted' during energy transfers
Energy cannot be created or destroyed, only transferred from one form to
another
Energy that is ‘wasted’ (e.g. heat energy from an electrical lamp) does not
disappear – instead it is transferred to the surroundings, spreading out so much
that it becomes difficult to do anything useful with
Gravitational potential energy

Any object that is raised against the force of gravity stores gravitational
potential energy

For example, if you lift a book up onto a shelf, you have to do work against
the force of gravity – the book has gained gravitational potential energy
Kinetic energy

Every moving object has kinetic energy (sometimes called movement energy)

The more mass an object has, and the faster it is moving, the more kinetic
energy it has

Kinetic energy is energy of movement – anything which is moving has kinetic
energy
Kinetic energy = ½ x mass x velocity2
The greater the mass of the object, and the faster its velocity then the greater
kinetic energy it will possess
Electricity is the flow of electrical power (charge) in the form of electrons
Electricity is a useful secondary energy source – most energy sources (like coal, oil,
nuclear, wind etc…) can be converted into electricity
A bulb in the circuit is like a radiator – an electrical device uses electrical energy,
supplied by the circuit
The wires are like pipes - they carry the flow of electricity (current) around the
circuit
The electrical current is pushed by the cell (battery) – this is the voltage
The electrons flow from –ve to +ve
An electric current needs two things: 
Something to make the electricity flow (battery or power pack)

A complete circuit
Without these two basic things, an electric current will not flow
Components that are connected one after another on the same loop of the circuit
are connected in series
If you remove or disconnect one component, the circuit is broken and they all stop
Series Circuits

The same current flows through all parts of the circuit

The total resistance is the sum of all the resistances

The size of the current is determined by the total p.d of the cells and the
total resistance of the circuit (I = V/R)

The total p.d of the supply is shared between the various components, so the
voltages around a series circuit always add up to equal the total voltage of
the supply

The bigger the resistance of a component, the bigger its share of the total
p.d
Parallel Circuits

All components get the full source p.d, so the voltage is the same across all
the components

The current through each component depends on its resistance – the lower
the resistance, the greater the current which flows through it

The total current flowing around the circuit is equal to the total of all the
currents in the separate branches (the total current going into a branch
always equals the total current leaving the branch (no current is lost))

The total resistance of the circuit is always less than the branch with the
smallest resistance
An electric current flows when electrons move through a conductor
The moving electrons can collide with the atoms of the conductor – this makes it
more difficult for the current to flow, and causes resistance
Resistance is measured in Ohms and is an indication of how easily electrons can
travel through a material
Electrons collide with atoms more often in a long wire than they do in a short one –
a thin wire has fewer electrons to carry the current than a thick wire
This means the resistance in a wire increases as: 
The length of the wire increases

The thickness of the wire decreases
A graph of current against potential difference shows you how the current flowing
through a component varies with the potential difference across it
The current is plotted on the vertical (y) axis; and the potential difference on the
horizontal (x) axis
The current flowing through a resistor at a constant temperature is directly
proportional to the potential difference across it – it is said to follow Ohm’s Law
The filament lamp is a common type of light bulb (it contains a thin coil of wire
called the filament)
The filament heats up when an electric current passes through it, and produces
light as a result
The filament lamp does not follow Ohm’s Law – its resistance increases as the
temperature of its filament increases
As voltage increases the bulb gets hotter, and its resistance increases
Diodes are electronic components that can be used to regulate the potential
difference in circuits and to make logic gates
Light-emitting diodes (LEDs) give off light and are often used for indicator lights
in electrical equipment
The diode has a very high resistance in one direction – this means that current can
only flow in one direction
For the straight-line graphs the resistance of the component is steady and is equal
to the inverse of the gradient of the line (1/gradient)
The steeper the graph, the lower the resistance
If the graph curves the resistance is changing – calculated by taking the voltage
divided by the current
The resistance value of a resistor is shown by a series of coloured bands – each
colour represents a number
Most resistors have 4 bands: 
The first band gives the first digit

The second band gives the second digit

The third band indicates the number of zeros
The fourth band is used to shows the tolerance (precision) of the resistor: 
Red band = 2%

Gold band = 5%

Silver band = 10%
Light-dependent resistors (LDRs) are used to detect light levels, e.g. in automatic
security lights, burglar detectors etc…

As light levels increase the resistance decreases

As light levels decrease the resistance increases (resistance is highest in
darkness)
Thermistors are used as temperature sensors, e.g. car engine sensors, fire alarm
sensors, fridges etc…

As temperature increases the resistance decreases

As temperature decreases the resistance increases
The UK mains electricity supply is about 230V and can kill if not used safely
Electrical circuits, cables, plugs and appliances are designed to reduce the chances
of receiving an electric shock
The more electrical energy used, the greater the cost, and electrical supplies can
be direct current (d.c.) or alternating current (a.c.)
Alternating current and direct current are different electrical supplies
The battery in a torch makes the current flow around the circuit in one direction
only (it is a direct current (d.c.))
Mains electricity is alternating current (a.c.) which repeatedly reverses its
direction (flowing one way, then in the opposite direction in successive cycles) – its
frequency is the number of cycles per second (in the UK the mains frequency is 50
cycles per second (50Hz))
If the current flows in only one direction it is called direct current (d.c.)
Batteries and cells supply d.c. electricity, with a typical battery supplying maybe
1.5V
The diagram shows an oscilloscope screen displaying the signal from a d.c. supply: -
If the current constantly changes direction, it is called alternating current (a.c.)
Mains electricity is an a.c. supply, with the UK mains supply being about 230V - it
has a frequency of 50Hz (50 hertz), which means it changes direction, and back
again, 50 times a second
The diagram shows an oscilloscope screen displaying the signal from an a.c. supply.
The potential difference of the live terminal varies between a large positive value
and a large negative value – however, the neutral terminal is at a potential
difference close to earth, which is zero
The diagram shows an oscilloscope screen displaying the signals from the mains
supply – the red trace is the live terminal and the blue trace the neutral terminal
Note that, although the mean voltage of the mains supply is about 230V, the peak
voltage is higher
The oscilloscope can measure the peak p.d. and the frequency of a low voltage a.c.
supply
For example, an oscilloscope may be set to the following: 
Y-gain control at 0.5V/cm

Time base control of 10 milliseconds per centimetre (ms/cm)

If the peaks are 8.4cm above the troughs, then they are 4.2cm above the
middle (0 p.d.)
If the Y-gain control is set to 0.5V/cm then we know each centimetre of height is
due to 0.5V – so the peak p.d. is 2.1V (0.5V/cm x 4.2cm)
Then each cycle across the screen is 8cm across, the time base control set at
10ms/cm tells us each centimetre across the screen is a time interval of 10ms, so
one cycle takes 80ms (frequency of 12.5Hz (1/0.08s)
The live wire alternates between +325V and -325V
In terms of electrical power, this is equivalent to a direct voltage of 230V
Each cycle takes 0.02 seconds, so the mains supply alternates at 50 cycles every
second (50Hz)
There are various electrical hazards within the home – most are common sense, and
can be eliminated easily, with a basic list compromising of: 
Long or frayed cables

Cables in contact with something hot or wet

Children and pets (hamsters, rabbits etc…)

Water near sockets

Shoving things into sockets

Damaged plugs / too many plugs within a socket

Lighting sockets without bulbs in

Appliances without covers
A mains electricity cable contains two or three inner wires – each has a core of
copper, because copper is a good conductor of electricity
The outer layers are flexible plastic, because plastic is a good electrical insulator
The inner wires are colour coded: 
Blue – neutral

Brown – live

Green / yellow stripes – earth
The features of a plug are: 
The case is made from tough plastic or rubber, because these materials are
good electrical insulators

The three pins are made from brass, which is a good conductor of electricity

There is a fuse between the live terminal and the live pin

There is a fuse between the live terminal and the live pin

The fuse breaks the circuit if too much current flows

The cable is secured in the plug by a cable grip – this should grip the cable
itself, and not the individual wires inside it
Many electrical appliances have metal cases, including cookers, washing machines
and refrigerators – the earth wire creates a safe route for the current to flow
through if the live wire touches the casing
You will get an electric shock if the live wire inside an appliance, such as a cooker,
comes loose and touches the metal casing
The earth terminal is connected to the metal casing so that the current goes
through the earth wire instead of causing an electric shock
A strong current surges through the earth wire because it has a very low
resistance – this breaks the fuse and disconnects the appliance
The fuse breaks the circuit if a fault in an appliance causes too much current flow,
protecting the wiring and the appliance
The fuse contains a piece of wire that melts easily – if the current going through
the fuse is too great, the wire heats up until it melts and breaks the circuit
Fuses in plugs are made in standard ratings (3A, 5A, 13A etc…)
The fuse should be rated at a slightly higher current than the device needs: 
If the device works at 3A, use a 5A fuse

If the device works at 10A, use a 13A fuse etc…
The circuit breaker does the same job as the fuse, but works slightly differently –
a spring-loaded push switch is held in the closed position by a spring-loaded soft
iron bolt
An electromagnet is arranged so that it can pull the bolt away from the switch
If the current increases beyond a set limit, the electromagnet pulls the bolt
towards itself, which releases the push switch into the open position
When an electrical appliance is used it transforms electrical energy into other
forms of energy
The power of the appliance (in watts (W)) is the energy it transforms in joules per
second (J sec-1)
power (W) = energy transformed (J)
time (s)
The power rating of an appliance is simply how much energy it uses every second (1
Watt = 1 Joule per second)
For any electrical appliance: 
Current through it is a measure of the number of electrons passing through
it each second (charge flow per second)

Potential difference across it is a measure of how much energy each
electron transfers
The power supplied (in watts (W)) is the current (I) multiplied by the potential
difference (V)
power (W) = current (I) x potential difference (V)
When an electrical appliance is on, electrons are forced through the appliance by
the potential difference of the voltage supply unit
The potential difference causes a flow of charge through the appliance, carried by
the electrons (-ve charge)
The rate of flow if charge is the electric current through the appliance
Charge (measured in coulombs) flowing through a component in a certain time
depends on the current and the time
charge (Q) = current (I) x time (s)
The amount of energy that flows in a circuit will depend on the amount of charge
carried by the electrons and the voltage pushing the charge around
When a resistor is connected to a battery electrons are made to pass through the
resistor by the battery
Each electron repeatedly collides with the vibrating atoms of the resistor,
transferring energy to them (heating the resistor)
The energy transformed to the resistor depends on the amount of charge passing
through it and the potential difference across it
In an atom the number of electrons is the same as the number of protons
This means the atom has no net charge (the –ve electrons and +ve protons cancel
each other out)
Atoms can gain or lose electrons, forming ions (groups 1, 2, 6 and 7 are the
elements which most readily form ions): 
Group 1 and 2 elements lose electrons to form +ve ions (cations)

Group 6 and 7 elements gain electrons to form –ve ions (anions)
Static electricity is caused by charges which are not free to move – this causes
them to build up in one area, and often ends with a spark / shock when they are
finally able to move…
When two insulating materials are rubbed together, electrons are scrapped off
one, and dumped onto the other
This leads to a positive static charge on one, and a negative static charge on the
other (only the electrons move)
Like charges repel, opposite charges attract

As charge builds up, so does the voltage – causing sparks!

The greater the charge on an isolated object, the greater the voltage
between it and the Earth

If the voltage gets big enough it can cause a spark which jumps across the
gap

High voltage cables can be dangerous for this reason

A charged conductor can be discharged safely by connecting it to Earth with
a metal strap
When something charged comes near something which isn’t charged it is induced
(electrons in the uncharged object move towards or away from the charged object)
The new arrangement of charge always makes the two objects pull together,
because repelling charges are now further apart than the attracting charges
The Van de Graaff generator has a dome which charges up when the generator is
switched on (massive sparks can occur if the charge on the dome builds up too
much)

The belt rubs against a felt pad, becoming charged

The belt carries the charge onto an insulated metal dome

Sparks are produced when the dome can no longer hold any more charge
Rain droplets fall to Earth with a positive charge – as they do a huge voltage builds
up, eventually leading to a huge spark!
Static electricity – clothing crackles

When synthetic clothes are dragged over each other (e.g. when they are in
the tumble dryer / being pulled over your head) electrons get scrapped off,
leaving a static charge on both parts

This leads to an attraction, as well as little sparks / shocks as the charges
rearrange themselves
Static electricity – car shocks

Air rushing over the car can give it a positive charge – getting out and
touching the door causes the electrons to flow from earth, through you and
to the car (which causes a shock for you)

Some cars have conducting rubber strips which hang down behind the car,
grounding it
Static electricity – chutes, rollers and fuel filling

As fuel flows out of a filler pipe, paper drags over rollers or grain shoots
from grain pipes then static electricity can build up leading to a spark (which
can be very dangerous around flammables)

This is why nozzles or rollers are made out of metal so the charge is
conducted away instead of building up (as well as having earthing strips
between the fuel tanks and fuel pipes)
Static electricity – surgery

In hospitals anaesthetic gases are used during surgery (if this were to
escape into the air then a tiny spark could make it explode)

To eliminate static charge an antistatic material is used for the floor
surface (the material is a poor electrical insulator, so it conducts charges to
Earth)

The surgical clothes are also antistatic, again preventing any charges
building up causing a spark
Photocopies and laser printers work in a similar way: -
Electrostatic precipitators: -
1. Smoke particles pick up a negative charge
2. Smoke particles are attracted to the collecting plates
3. Collecting plates are knocked to remove the smoke particles
The mass number (top number) shows the number of protons + neutrons
The atomic number (bottom number) shows the number of protons (and therefore,
the number of electrons)
An atom is made from a nucleus surrounded by electrons – the nucleus contains
protons and neutrons
Isotopes are atoms that have the same number of protons, but different numbers
of neutrons – the nuclei of some isotopes are unstable, emitting radiation and
breaking down to form smaller nuclei…
Isotopes are the atoms of an element with different numbers of neutrons – they
have the same proton number, but different mass numbers…
The nuclei of some isotopes are unstable – they can split up or ‘decay’ and release
radiation
Such isotopes are called radioactive isotopes or radioisotopes
When a radioactive isotope decays, it forms a different atom with a different
number of protons
An α particle consists of 2 protons and 2 neutrons
When an unstable nucleus emits an α particle its atomic number goes down by 2,
and its mass number down by 4
An β particle is an electron created and emitted by a nucleus which has too many
neutrons compared with protons
A neutron in its nucleus changes into a proton and a β particle – this is instantly
emitted at high speed by the nucleus
The relative mass of a β particle is effectively zero, and its relative charge is -1
When an unstable nucleus emits a β particle its atomic number goes up by 1, but its
mass number stays the same (the neutron has changed into a proton)
Uranium-230 nuclei emit alpha radiation and become nuclei of thorium-226
The mass number is reduced by 4 (2 protons + 2 neutrons gone)
The atomic number is reduced by 2 (2 protons gone)
* The alpha particle is identical to a helium nucleus
Hydrogen-3 nuclei emit beta radiation and become nuclei of helium-3
The mass number stays the same (2 protons + 1 neutron)
The atomic number increases by 1 (1 protons added)
Background radiation is all around us – most background radiation comes from
natural sources, while most artificial radiation comes from medical examinations,
such as X-ray photographs
Natural sources – radiation is all around us, coming from radioactive substances
including the ground, the air, building materials and food
Radiation is also found in the cosmic rays from space
Nuclear Fission: Energy is released in a nuclear reactor as a result of nuclear fission
The nucleus of an atom of a fissionable substance splits into two smaller ‘fragment’
nuclei
This event can cause other fissionable nuclei to split, leading to a chain reaction of
fission events
Two isotopes in common use as nuclear fuels are uranium-235 and plutonium-239
Fission is another word for splitting (splitting a nucleus is called nuclear fission)
Uranium or plutonium isotopes are normally used as the fuel in nuclear reactors,
because their atoms have relatively large nuclei that are easy to split, especially
when hit by neutrons
When a uranium-235 or plutonium-239 nucleus is hit by a neutron, the following
happens: 
The nucleus splits into two smaller nuclei, which are radioactive

Two or three more neutrons are released

Some energy is released
The additional neutrons released may also hit other uranium or plutonium nuclei and
cause them to split – even more neutrons are then released, which in turn can split
more nuclei
This is called a chain reaction – in nuclear reactors the chain reaction is controlled,
stopping it going too fast
In a nuclear bomb the idea is the opposite to this!
A nuclear reactor consists of uranium fuel rods, spaced evenly in the reactor core
The reactor core is a thick steel vessel containing the fuel rods, control rods and
water at high pressure
The fission neutrons are slowed down by the collisions with the atoms in the water
(the water acts as a moderator, slowing the fission neutrons down)
Without a moderator the fast neutrons would not cause further fission of the
nuclear fuel
Nuclear reactors use the heat from nuclear reactions in the nuclear fuel to boil
water – just as in conventional power stations, the steam from the boiling water in
the pressurised water reactor (PWR) makes a turbine spin, which in turn makes the
generator turn
Control rods (cadmium / boron) absorb surplus neutrons, controlling the chain
reaction
The fuel in a nuclear reactor must contain fissionable isotopes
Most reactors use enriched uranium which is ~97% non-fissionable U-238 and ~3%
fissionable U-235
In comparison natural uranium is >99% non-fissionable U-238
A nuclear bomb has two lumps of pure U-235 or Pu-239
Each lump cannot produce a chain reaction because it loses too many fission
neutrons, but bringing them together enables the reaction to occur...
Nuclear Fusion: Stars release energy as a result of fusing small nuclei such as hydrogen to form
larger nuclei
The energy released by this process is vast – water contains lots of hydrogen
atoms
If we could make a fusion reactor on Earth then a glass of water could provide the
same amount of energy as a tanker full of petrol!
2 small nuclei release energy when they are fused together to form a single, larger
nucleus
The process releases energy if the relative mass of the product nucleus is no more
than about 55 (the same as an iron nucleus)
Energy must be supplied to create bigger nuclei
The Sun consists of about 75% hydrogen (H) and 25% helium (He)
The core is so hot that it consists of a ‘plasma’ of bare nuclei with no electrons –
these nuclei move about and fuse together when they collide
When they fuse they release energy…
Nuclear fusion involves two atomic nuclei joining to make a large nucleus – energy is
released when this happens
The Sun and other stars use nuclear fusion to release energy – the sequence of
nuclear fusion reactions in a star is complex, but overall hydrogen nuclei join to
form helium nuclei, so the Sun is changing composition from hydrogen to helium: Hydrogen-1 nuclei fuse with hydrogen-2 nuclei to make helium-3 nuclei
An early model about the structure of the atom was called the plum pudding model
In this model, the atom was imagined to be a sphere of positive charge with
negatively charged electrons dotted around inside it like plums in a pudding
The positively charged matter in the atom was evenly spread about (pudding)
The electrons were buried inside (plums)
Rutherford, the father of nuclear physics, conducted an experiment which proved
the plum pudding idea was incorrect
Rutherford, along with Geiger and Marsden proved the plum pudding model
incorrect with their scattering experiment
A beam of alpha particles was aimed at very thin gold foil and their passage
through the foil detected
A beam of alpha particles was aimed at very thin gold foil and their passage
through the foil detected
The alpha particles were expected to pass straight through the foil, but instead
some of the alpha particles emerged from the foil at different angles, and some
even came straight back
The positively charged alpha particles were being repelled and deflected by a tiny
concentration of positive charge in the atom
As a result of this experiment, the plum pudding model was replaced by the nuclear
model of the atom

Most of the alpha particles passed straight through the metal foil

The number of alpha particles deflected per minute decreased as the angle
of deflection increased

About 1 in 10’000 alpha particles were deflected by more than 90o
Rutherford said this was like “firing naval shells at cardboard, and discovering the
occasional shell rebounds”
From this experiment Rutherford concluded that there is a nucleus at the centre
of every atom which: Is positively charged because it repels alpha particles
Is much smaller than the atom as most alpha particles passed through it
Is where most of the mass of the atom is located
The nucleus diameter was found to be about 100’000 times smaller than the atom