By Chris McGinlay, ©Sandwick Junior High School 2000, 2001,2002.
This is a glossary of the terms used in Standard Grade Physics. Let the entire
document load or get 'Transfer Interrupted' errors! Most diagrams or figures
will be kept in separate linked documents. There are approximately 275
definitions at present.
- Acceleration
TR, SP
- Acceleration is the rate of increase of velocity. Acceleration tells you
how much faster or slower a moving object gets every second. A negative
acceleration is called a deceleration. The SI Unit of
acceleration is the metre per second
per second (ms-2)
- Acceleration due to
Gravity
TR, SP
- This is the acceleration caused by the
gravitational pull of a planet. On Earth, the strength of the acceleration due to gravity is roughly 10
ms-2. The acceleration due to gravity is
often given the symbol 'g'. See also weight and mass
- Accommodation
HP
- Accommodation relates to the ability of the eye to
change its focal length and so focus on objects at a
range of distances. To focus on a close object, the eye lens will be fat
(having a short focal length). To focus on a far object, the lens will be
pulled into a thin shape having a long focal length. Failure of accommodation
gives rise to long-sightedness and short-sightedness.
- Activity
HP, EM
- The activity of a sample of radioactive material
is the number of disintegrations per second taking place in that particular
sample. The SI unit of activity is the becquerel
(Bq).
- Aerodynamic
TR
- Aerodynamic objects have minimal air resistance. This means that they can
move at greater speed through the air. Remember that as objects go faster the
aerodynamic drag increases. Therefore, an aerodynamic object will be able to
travel faster than a non-aerodynamic one!
- Alpha (a) Radiation
HP
- a (alpha) radiation
consists of slow moving helium nuclei. These nuclei are released during the
disintegration of an unstable nucleus. a radiation
will be stopped by a few centimetres of air, or a sheet of paper. It is a
strongly ionising radiation, due in part to its relatively
large mass - it is about 8000 times as massive as the electrons in b radiation! The
helium nuclei are positively charged.
- Alternating Current (a.c.)
UE, EM
- Electrical current continually changes direction in
an a.c. power supply (as opposed to direct current (d.c.))
In the UK, mains electricity is a.c., with a frequency of 50 Hz and an r.m.s
voltage of 230 V.
- Alternator
EM
- This is a device for generating a.c. electricity. It
consists of rotor coils (with a d.c.
supply) and stator coils which produce the a.c. electricity.
- Ampere
UE, EL
- The ampere is the SI unit of electrical current. At a simple level it can be defined as the number
of coulombs of electrical charge
flowing per second. The full definition is beyond the scope of the S-grade
course
- Amplifier
TC, UE, EL
- An amplifier is a component of an electronic system, such as a radio or TV which makes all the input signals louder.
Ideally, an amplifier will increase the amplitude of
all input signals by the same factor. Amplifiers require a power supply which
can be battery or mains. The voltage gain or the power
gain can be used to describe how much amplification is taking place.
- Amplitude (Diagram)
TC, UE, HP, EL
- This is the height of a wave, measured vertically from
the centre line to a crest or a trough. The SI unit is the metre, although in some contexts amplitudes may be given in volts.
- Amplitude Modulation (Diagram)
TC
- Amplitude modulation refers to the changing of the amplitude of a radio-frequency or microwave frequency
carrier wave. The other method of modulation is called frequency modulation.
- Analogue
EL
- Analogue signals should be compared with digital
signals. Analogue signals are continuous. As an example, a microphone is an
analogue input device. This is because it can
produce a range of voltage levels.
- AND Gate
EL
- An AND gate is a logical device. It usually takes two inputs (although
more are possible) and produces only one output. The logic state of the output depends on the logic state of
the inputs. This is shown in the truth table below.
A and B are the inputs and Z is the output state.
From this it is easy to see that the output is on only if both inputs are
both on.
- Angle of Incidence (Diagram 1) (Diagram 2
)
TC
- The angle between the normal line and a ray or
wave incident to a surface.
- Angle of Reflection (Diagram)
TC
- The angle between the normal line and a ray or
wave reflected from a surface
- Armature
UE
- A multicoil rotating assembly, used in commercial motors instead of the
single rotating coil of the model motor built in class. A segmented commutator
is used to connect each of the armature windings to the power supply in turn,
thus providing smoother rotation of the motor.
- Asteroid
SP
- An asteriod is a lump of rock which orbits around the sun. (The biggest
asteroids are about 100km across, although many are 1km or less across.
Asteroids orbit the Sun in the asteroid belt, between the orbits of Mars and
Jupiter.
- Atom
HP, EL, EM, SP
- An atom is the smallest possible particle of any chemical element. For
example, an atom of carbon is the smallest possible unit of carbon. All of the
atoms of an element are identical to each other, but different from the atoms of
any other element. For example, all carbon atoms are essentially identical to
each other, but carbon atoms and oxygen atoms are different from each other.
The size of an atom is approximately 10-10m.
The reason why atoms of oxygen are different from carbon (or any other
element) is down to the structure within the atom itself. Atoms are made up
from protons, neutrons and electrons. Atoms of different elements have different
numbers of protons. Atoms of the same element have the same number of protons.
For example, atoms of carbon always have six protons. Atoms of oxygen always
have eight protons.
The protons and neutrons are found in the nucleus of the atom. The tiny
electrons 'orbit' the nucleus. The nucleus is very small, approximate diameter
is 10-14m.
See also: Ion.
This link has more
information on atoms
- Angle of Refraction (Diagram)
TC
- The angle between the normal line and a ray or
wave refracted at a surface
- Average Speed
TC, HP, TR, SP
- The average speed of a vehicle is usually calculated over the whole
distance and time of the journey. For example a train may cover a distance of
240 km in a time of 3 hours. The average speed can be worked out as follows:
In kilometres per hour: speed = 240 km / 3 hr = 80 km/hr
In metres per second: speed = 240 000 m / 10 800 s = 22.2 m/s
Remember that average speed is different from instantaneous speed.
- Audio Wave
TC
- A sound wave, but usually in the context of modulation and radio
transmission
- Cancer
HP
- Cancer is a growth of out-of-control cells.
Localised cancer growths are called tumours. a, b, g or X-ray radiation can be used to diagnose and treat cancer.
- Capacitor (Diagram)
TC, UE, EL
- A capacitor is a device which can store electrical charge. Capacitors are often used to introduce a time delay
in electronic circuits. The SI unit of capacitance is
the farad.
- Carbon Dating
HP
- See radiometric dating.
- Carrier Wave
TC
- Carrier waves are used to carry information from
transmitter to receiver. The carrier wave will be radio or
microwave frequency. There
are two methods of using the carrier wave to carry information - amplitude modulation and frequency modulation. In radio transmissions
for example, the carrier is of a much higher frequency
than the audio wave which is to be transmitted. The
higher the frequency of the carrier, the higher the quality of the transmitted
signal.
- Cell (Diagram)
UE
- In electricity, a cell is a source of d.c. electricity.
A cell can be made using two different metals inserted into a salt solution, for
example. The metals react with the salt solution at different rates and in so
doing, generate a small electrical current. Many cells
can be combined to produce a battery of cells.
In biology, cells are the basic building blocks of all living things. Plant
and animal cells alike have a tiny nucleus which controls the behaviour of the
cell, a cell membrane which controls the entry and exit of substances. The
inside of the cell is filled with a liquid called cytoplasm. These cells can be
damaged or destroyed by ionising radiation such as a, b, or g. The amount of cell damage caused by radiation
depends on:
- the type of radiation
- the type of tissue or cells
- the total amount of energy absorbed
See also: Dose Equivalent.
- Charge
UE
- There are two types of electrical charge. One type is called 'positive',
the other is called 'negative'. Like charges repel each other, whilst unlike
charges attract each other. The amount of charge affects how strong this
attraction or repulsion actually is.
The SI unit of charge is the coulomb (C)
See also the references for the electron, the proton and the neutron.
In electrical circuits, charge is related to current
by the equation Q = I t
- Chemical Potential Energy
UE, EM
- Chemical energy is a form of potential
energy because it is stored energy. Energy stored in food, torch batteries
and explosives like dynamite is all chemical energy. Chemical energy is
released in a chemical reaction (like combustion of explosives or respiration in
plant and animal cells).
- Clock Pulse Generator
EL
- A simple circuit using a capacitor, resistor and transistor. The circuit
generates 'clock pulses' - simply a regular sequence of logic LO and logic HI.
The capacitor charges and discharges through the resistor. As the capacitor
charges to a certain level, the inverter's input goes HI and its output goes LO.
This this causes the capacitor to start discharging. When its voltage gets low
enough, the inverter output goes back to HI and the cycle starts again.
By increasing the resistance or increasing the capacitance, the pulses can
be generated more slowly.
- Combined Heat and Power
EM
- Most power stations end up trying to turn heat energy into electrical
energy. This can never be done with 100% efficiency (due to the second law of
thermodynamics). Most of the energy produced (by combustion for example) will
be lost as waste heat. A combined heat and power plant uses the waste heat to
heat nearby homes and factories. This is why half of Lerwick was dug up around
2000-2001 - for the installation of pipes for the CHP plant.
- Comet
(Image 1)
SP
-
A comet is a small body orbiting our Sun, usually at great distance from
Earth and with a highly elliptical orbit. They are composed of 'ices' and dust.
As a comet approaches the inner solar system, the heat of the Sun vapourises
volatile material from the surface and pushes the material directly away
from the Sun. This gives rise to the 'ion tail', usually a ghostly
blue colour and quite straight, it always points directly away from the Sun and
has nothing to do with the direction of motion of the comet. A second tail
exists, usually white and curved. This is dust which falls off the comet and is
left in track of its orbit. Here is a photo of comet Hale-Bopp, taken from
Hoswick, Shetland in 1998. Scientists recently deliberately crashed a space
probe onto the surface of a comet in order to learn more.
Comets and their dust tails give rise to regular meteor showers.
- Conductor
UE, EL
- A material that allows electricity to flow easily through it. Copper, for
example is a good conductor. Good conductors have a very low resistance. See also insulators
and semi-conductors
- Conduction (Heat)
EM
- This refers to the ability of heat energy to travel through a material.
For example, copper is a good conductor of heat as it will heat up quickly if
left on a radiator. Wood, for example does not conduct heat so well.
Conduction is not possible in a vacuum. See also convection and radiation.
- Consumer Unit
UE
- Commonly known as the fusebox, the consumer unit is the part of the meter
board which separates the incoming electrical supply into the various lighting
and ring main circuits. Each of these circuits is
protected by either a fuse or miniature circuit breaker (MCB)
- Continuous Spectrum
SP
- The light from a tungsten filament lamp has a
range of colours present within it. Each colour corresponds to a wavelength of
visible (EM) radiation. In a continuous spectrum like this, there are no
missing wavelengths. However, the light from the Sun (or from any star) has
certain wavelengths missing. The missing wavelengths can be used to identify
the 'fingerprint' of chemical elements in stars' atmospheres, or in the
intervening dust and gas clouds between us and a particular star. In class, we
use a direct vision spectroscope to observe the missing colours of
sunlight.
- Control Rods
EM
- These are used in nuclear reactors. They control the rate of nuclear
reactions by absorbing the neutrons which cause the reactions to happen. By
lowering the rods into the reactor, the rate of reaction slows down.
- Convection
EM, SP
- This is a way of moving heat energy around. In a fluid (i.e. a liquid or
gas) heat can travel by convection. For example, hot air is less dense than
cold air, so the hot air rises up, carrying its heat energy with it. Convection
is not possible in a vacuum. See also conduction and radiation.
- Core (Soft Iron)
UE, EM
- The presence of an iron core in a solenoid will produce a stronger magnetic
field than the solenoid would manage on its own. The iron core thus magnifies
the effect of the solenoid's magnetic field.
- Coulomb
UE, EL
- The coulomb is the SI Unit of electrical charge. It can be defined as the amount of charge which
flows in 1 second due to a current
of 1 ampere.
- Crest (Diagram)
TC
- This refers to waves. The crest of a wave (also called
the peak) is simply the part of the wave with the greatest amplitude.
- Critical Angle
TC, HP
- The critical angle is associated with refraction of light. This angle is the smallest angle of incidence at which total
internal reflection (TIR) occurs.
- Current
UE, EL, EM
- Electrical current is the rate of flow of electrical charge round a circuit. It is measured in amperes and should be measured in series with an ammeter.
See also: Ohm's Law
- Curved Reflector (Diagram)
TC, SP
- Curved reflectors, with a parabolic shape, are used to produce parallel beams, or to collect light, radio waves or similar, over the area of the reflector and
focus this energy onto a detector. This makes the
received signal at the detector much greater than if no reflector was used.
- Cycle
TC
- A complete cycle of a wave is equivalent to one
complete wavelength of that wave. The time taken to
produce one complete cycle is called the period of the
wave. The number of cycles per second is called the frequency of the wave
- Lamp
UE
- A device intended to produce light energy. Modern
lamps convert electrical energy to light energy via one of two main mechanisms -
the filament lamp or the gas discharge lamp. Note that filament lamps are
less efficient than discharge lamps (i.e. fluorescent lamps) as they produce more waste heat
energy.
- Laser
TC, HP
- The word laser was originally an abbreviation for Light Amplification by
the Stimulated Emission of Radiation. Lasers have countless applications, many
in medicine and communications. Lasers produce light which is monochromatic,
coherent and tightly focussed. Lasers are used in the technique of
photocoagulation (used to treat retinae).
- Latent Heat
EM
- Latent heat is heat energy used or released during change of phase.
When matter changes phase, energy must be given out or taken in. Change of
phase takes place at constant temperature. For example, if a bunsen is used to
bring 100ml of water to the boil, it is easy to understand that the heat energy
from the bunsen increases the heat energy of the water - because the water gets
hotter. During boiling however, the temperature of the water does not increase.
The energy from the bunsen no longer causes an increase in temperature, but is
now carried away in the steam. The energy is used to break the bonds between
the water molecules instead of increasing the temperature. (Remember: heat and
temperature are different concepts!)
- Liquid Crystal Display (LCD)
EL
- This is an output device. They run on almost no power since they don't
produce their own light, but work by reflecting room or daylight.
- Light Dependent Resistor (LDR) (Diagram)
EL
- LDRs are designed so that their resistance
depends on the intensity(brightness) of light falling on them.
When in low intensity illumination (dark!), the resistance of an LDR is
high.
When in bright illumination, the resistance of an LDR is low.
- Light Emitting Diode (Diagram)
EL
- LEDs are simply diodes which emit light when connected
correctly to a power supply.
- Light-year
SP
- The light-year (ly) is a unit of distance which is useful in astronomy.
It is the distance which light (or any other form of electromagnetic radiation)
travels in one year. Note that real astronomers do not use light-years,
but use a unit called the parsec instead.
It is easy to work out how far a light-year is as follows:
distance = speed x time
1 ly = 3 x 10 8 x 365x24x60x60 metres
1 ly = 9.46 x 10 15 metres
To appreciate the scale of this unit, recall that the distance from the Earth to
the Sun is 1.5 x 10 8 metres. This is about 8 light-minutes. The
distance to the next-nearest star (Proxima Centauri) is about 4 ly!
- Line Spectrum
SP
- Plural: Line Spectra. A spectrum of light with only certain wavelenghts or
colours present. (See also continuous spectrum). If you use a direct vision
spectroscope to view light from a (yellow) sodium vapour street-lamp, you will
see that only a few wavelengths are present. This is an emission line spectrum
and is unique to sodium. Sodium can also absorb these wavelengths too, so if a
continuous spectrum of white light were to be shone through sodium vapour and
then observed through a spectroscope, certain colours or wavelengths would be
missing. This is called an absorption spectrum.
These facts can be used to identify which elements are present in stars'
atmospheres, by matching lines in their spectra with the spectra of elements
tested on Earth.
- Logic Diagram
EL
- A simplified diagram of an electronic system, using rectangles to represent
input and output devices along with the usual logic gate symbols for the AND, OR and NOT gate etc. Inputs and outputs are given
letters (A,B,C,.. for inputs; Z for output) to help layout a truth table.
- Logic Gate
EL
- For standard grade, we study AND, OR and NOT gates.
Other common gates are the NAND, NOR and XOR (or EOR). Logic gates allow an
electronic system to make decisions based on a number of inputs. For example an
OR gate can be used to illuminate a warning light if engine temperature is too
high or if it is over-revved.
- Logic State
EL
- In a digital system, there are two logical states. One state is 'ON', 'HI'
or 'logic 1' (all mean the same). The other is 'OFF', 'LO', 'logic 0' (again
all mean the same).
- Long-sighted
HP
- This refers to a problem with the accommodation ability of an eye.
Basically, the eye can focus on far away objects with no problem, but can't
properly focus on nearby objects. There can be two causes of this. The first
occurs in older people, where the aging eye lens becomes less flexible and so
cannot adopt the highly curved shape needed to focus at short range. This
condition is called presbyopia. The second cause is that retina is too near to
the lens - so that the focal point falls behind the retina. This is called
hypermetropia or hyperopia. Long-sightedness can be treated by wearing
spectacles which are convex in shape and therefore help to focus the image
properly by reducing the overall focal length of the
glasses and eye.
- Lubricant
TR
- A lubricant is a substance which reduces friction
between two surfaces. Examples are oil, water, air, bananas, seaweed.
- Parallel Beam (Diagram)
TC, SP
- A parallel beam is one which does not diverge or converge - its width
stays the same as the wave (which could be light or microwaves for example) travels out from its source.
Parallel beams are produced using curved (parabolic)
reflector dishes (e.g. in car headlamps, satellite transmitter dishes,
mobile phone mast relay dishes). Parallel light beams can also be produced
using convex lenses (e.g. in binoculars or telescopes). Parallel beams can be
used to send waves over much greater distances than would otherwise be possible.
This is because the wave energy stays much closer together when reaching its
destination. However, this would be no use for sending a signal out which was
intended to cover a wide area.
In astronomy, the light from stars has come from such a great distance that
the starlight does effectively form a parallel beam travelling down the tube of
a telescope.
- Parallel Circuit
UE, EL
- In a parallel circuit, each component is connected directly across the
terminals of the power supply. Therefore, the voltage across any component is
the same as the voltage across the power supply terminals. The rules for current, voltage and resistance in a parallel circuit are as follows:
Vsupply = V1 = V2 = V3 = ...
Isupply = I1 + I2 + I3 + ...
1/Rtotal = 1/R1 + 1/R2 + 1/R3 +
...
Note that the calculation for total resistance in parallel circuits requires
an extra step to get the final answer. Here is an example calculation for a 30
ohm and a 40 ohm resistor connected in parallel:
1/Rtotal = 1/R1 + 1/R2
1/Rtotal = 1/40 + 1/30
1/Rtotal = 3/120 + 4/120 (change to common denominator)
1/Rtotal = 7/120
Rtotal/1 = 120/7 (extra step)
Rtotal = 17.14 W (remember the
unit)
Calculators with a reciprocal button can make these calculations easier.
See also: Series circuits
- Period
TC, SP
- Satellite period is the time taken for a satellite to make one complete
orbit around its parent body (e.g the time taken for the Moon to orbit the Earth
is called the Moon's period). Satellite period is usually measured in hours or
days.
Wave period is the time taken for one complete wave to pass
a point. The SI unit of wave period is the second (s).
A closely related concept is that of frequency. See
also: cycle.
- Peak (Diagram)
TC,
UE
- This refers to waves. The peak of a wave (also called
the crest) is simply the part of the wave with the greatest amplitude.
- Peak Voltage
UE
- This the maximum voltage reached by an a.c. source in each cycle. It is very similar to the peak or crest of a wave. Compare with the r.m.s voltage of an a.c. source.
- Persistence of Vision
TC
- This is the effect which allows the human brain to be fooled into thinking
that the 25 frames per second of TV is actually continuous motion. The retina
retains each image for a fraction of a second and so
provided that the changes between the frames are small enough, smooth motion
will be seen. (The phosphor dots which make up the screen also continue to emit
light for a short time after the electron beam has
passed - this effect also helps to give the impression of a smooth motion)
- Photographic_film
HP
- Photographich film can be used to detect visible light, x-rays, or alpha, beta and gamma radiation. These radiations will blacken film due to a
chemical reaction. Photographic film badges are used by medical and nuclear
workers to monitor their level of exposure to
radiation.
- Planet
SP
- Our solar system has nine planets, including the Earth. Pluto's status as
a planet is a subject of current debate, with many scientists pointing out that
it is really just a big rock. The table below shows some data for the planets.
A planet does not produce its own light but shines by reflected star-light
(sunlight!). The word 'planet' literally means 'wanderer' due to the way the
planets move across the night sky. From Earth, we can easily observe Mars,
Venus, Jupiter and Saturn using just binoculars or the naked eye. With 20x60
binoculars, firmly mounted it is also easy to observe Jupiter's four Galilean
moons - a treat well worth the effort- they will be visible as tiny pin-points
of light all in a row (but you may not see all four at once). These moons move
around pretty rapidly and if you sketch their positions, you should easily see
them shift position after several hours.
| Name | Distance from Sun
(approx)/AU | Period | Day/hr | Mass/ mass of
Earth | Radius / radius of Earth |
| Mercury | 0.4 | 88 days | 58.7
days | 0.055 | 0.382 |
| Venus | 0.7 | 226 days | 243
days | 0.815 | 0.949 |
| Earth | 1.0 | 1 year | 23.9
hours | 1.000 | 1.000 |
| Mars | 1.5 | 1.9 years | 24.6
hours | 0.107 | 0.533 |
| Jupiter | 5.2 | 11.9 years | 9.9
hours | 318 | 11.2 |
| Saturn | 9.5 | 29.5 years | 10.7
hours | 95.2 | 9.45 |
| Uranus | 19.2 | 84.0 | 23.9
hours | 14.6 | 4.1 |
| Neptune | 30.1 | 165.0 | 17.8
hours | 17.2 | 3.9 |
| Pluto | 39.4 | 248.0 | 6.4
days | 0.002 | 0.24 |
Mercury, Venus, Earth and Mars are called 'inner planets'. Jupiter, Saturn,
Uranus, Neptune and Pluto are 'outer planets'.
- Plug (Diagram)
UE
- A plug is correctly wired as follows:
Green/Yellow insulated core to the earth terminal
(top)
Brown insulated core to the live terminal. The live terminal can be
identified by the fuse. The fuse and the switch should
always be connected to live as it is the live which supplies electrical energy to the device. If the fuse and switch were connected
to neutral, the appliance could appear to be off, yet still be connected to
live.
Blue insulated core to the neutral terminal.
- Pluto
SP
- Planet Pluto usually has the furthest orbit from our star the Sun. It is the smallest planet, really just a large lump of
rock. Its status as a planet has been challenged by some, due to its similarity
to other similar sized objects. Currently the IAF still list Pluto has a
planet. Due to its highly elliptical orbit, Pluto is nearer to the Sun than
Neptune for part of its orbit.
- Potential Divider (potentiometer)
UE, EL
- A variable resistor, when used with all
three of its terminals, it is called a potentiometer or potential divider. This
arrangement is for controlling voltage. See also: Rheostat.
- Potential Energy
TR, EM, SP
- Potential energy means stored energy. Some examples
are chemical, elastic, nuclear and gravitational - all forms of potential
energy. For Standard grade, you should know the formula for gravitational potential
energy.
- Power
UE, EL, TR, EM, SP
- Power is the rate of transfer of energy. The SI unit of power is the watt (W). One
watt is the same as one joule per second
The formula which relates power (P), energy (E) and time (t) is:
E = P x t
In electrical applications, there are other equations for power, which
involve current, voltage and resistance. They are:
P = I V
P = I2R
P = V2 / R
- Power Gain
UE, EL
- Power gain has no units of its own. It expresses how many times more power
the output signal from a process has compared to its input power. A closely
related concept is that of voltage gain. Power Gain = Output Power / Input
Power.
- Power (of a lens)
HP
- Lens power describes the curvature of a lens. High power lenses have a
short focal length and are highly curved. A positive lens power indicates a
convex lens and negative power indicates a concave lens. Lens power, P, is
measured in Dioptres (D) and is related to the focal length, f, in metres
through the formula:
P = 1 / f
- Presbyopia (presbyopic eye)
HP
- See under long-sighted.
- Primary Colour (Diagram)
TC
- There are three primary colours (of light): red, green and blue. Any two
of these colours can be mixed to give a secondary colour as follows:
Red + Green = Yellow
Red + Blue = Magenta (pink)
Green + Blue = Cyan (Turquoise)
If all three colours are mixed together, white light can be obtained.
Note that these are the primary and secondary colours of light. The primary
and secondary colours of pigment (paint) are not the same.
- Primary Mirror
SP
- In a reflecting telescope, the primary mirror collects the light from the
objects under study in the night sky. Reflectors can be made much bigger than
the objective lenses of a refracting telescope. They are also easier to make,
since their is only one optical surface to prepare. The primary mirror itself
is usually made from glass, with an aluminium coating on its forward face
(unlike the mirror in your bedroom). The mirror is made as large as possible
for the same reasons as for the objective lens of a refractor. See also
eyepiece.
- Principle of
Reversibility
TC, SP
- This applies to light rays passing through any system of mirrors or lenses.
Basically, it means if a ray of light follows a path when going, say, from left
to right, then another ray of light going from right to left will follow the
same path in reverse.
- Proton
HP, EM
- Protons are sub-atomic particles. They carry a
positive electrical charge (qp = +1.602 x
10-19 C). They have a mass of mp = 1.673 x 10-27 kg.
- Pumped Storage
EM
- At times of low electricity demand, typically at night, the National Grid
has overcapacity. Since it is difficult and expensive to shut down power
stations, this extra energy would have to be wasted. However, by using it to
pump water back uphill into reservoirs, a small percentage of this energy can be
stored as gravitational potential energy of the water. The process is
inefficient, but dumping the excess power would be 0% efficient.
- Satellite
- In astronomy and space science, a satellite is simply any object which is
in orbit about a larger parent orbit. The force required to keep the satellite
in orbit is the graviational attraction between the satellite and parent object.
The moon and the ISS are
satellites of Earth. Earth is a satellite of the Sun.
- Saturn
- Planet Saturn has the sixth most distant orbit from our star the Sun. It is slightly smaller than Jupiter and is also a gas
giant planet. Saturn's ring system is easily seen in a small telescope from
Earth.
- Second
- The second is the SI unit of time. It is
defined as the time taken to produce 9 192 631 770 complete wavelengths of electromagnetic radiation from a
particular event in cesium atoms.
- Sievert (Sv)
- The sievert is the SI unit of dose equivalent. Since 1 Sv is a massive dose of
radiation, units of microsieverts are often used instead.
- Scintillation Counter
- These devices detect gamma rays and form part of the
gamma cameras used in radioactive tracer examinations. Scintillations are flashes of light
given out by fluorescent materials (e.g zinc sulphide. When radiation is absorbed by zinc sulphide, it causes the zinc
sulphide to produce a small burst of light. These can be electronically
counted.
- Series Circuit
- In a series circuit, each component is connected to the other to form a
'chain' between the terminals of the power supply. The rules for current, voltage and resistance in a series circuit are as follows:
Vsupply = V1 + V2 + V3 + ...
Isupply = I1 = I2 = I3 = ...
Rtotal = R1 + R2 + R3 + ...
The mnemonic VISA may help to remember the rules for voltage and current in
series and parallel circuits:
VISA: Voltages In Series Add
- Short Circuit
- This is one of two types of circuit fault (the other being an open circuit). A short circuit fault is often caused
by wires touching, allowing current to bypass a component
in the circuit. Short circuit faults register on an ohmmeter with almost zero
resistance.
- Short-sighted
- This refers to a problem with the accommodation ability of an eye.
Basically, the eye can focus on near-by objects with no problem, but can't
properly focus on far-away objects. This is called myopia. This is because the
retina is too far from the lens - so that the focal point falls short of the
retina. Short-sightedness can be treated by wearing spectacles which are
concave in shape and therefore help to focus the image properly by extending the
overall focal length of the glasses and eye.
- SI Units
- SI stands for Systeme International. This is the standard system of units
used worldwide and includes the metre, the second, the kilogram, the ampere, the kelvin, and the coulomb.
More information is available from the University of
Alberta, CA or the University of Exeter,
UK.
- Solenoid
- A solenoid is a coil of wire with an electrical current flowing through it. An iron core, inserted into the
coil, has the effect of increasing the strength of the magnetic field produced.
An increased current or a greater number of turns on the coil will also increase
the strength of the magnetic field. See also: Electromagnet
- Solar Cell
- A solar cell is a photovoltaic semiconductor material which converts light
energy into electrical energy. Solar cells are transducers.
- Sound
- A longitudinal wave which travels through a medium,
usually air. Sound also travels through liquids (e.g. water) and solids (e.g.
steel). The volume (loudness) of sound is measured using the decibel (dB)
scale.
- Specific Heat Capacity
- The energy required to increase the temperature of 1kg of a material by 1
degree celsius. This is also the energy released during cooling. Water has a
high specific heat capacity at 4200 Jkg-1 oC-1.
Metals have much lower heat capacities. Physically, this is why metals heat up
and cool down more rapidly than water - less energy is required to bring about a
given change in temperature.
- Specific Latent Heat of
Fusion
- This the latent heat per kilogram of material required to change its phase
from solid to liquid (i.e. heat energy required to melt 1 kg of material). It
is also the heat energy released (in joules) if 1 kg of material 'freezes' from
liquid to solid.
- Specific Latent Heat of
Vapourisation
- This the latent heat per kilogram of material required to change the
material's phase from liquid to gas (i.e. heat energy required to boil or
vapourise 1 kg of material). It is also the heat energy released (in joules) if
1 kg of material condenses from solid to liquid.
- Spectroscope
- A device using the diffraction of light to split the visible spectrum up
into the rainbow of colours. The spectroscope is used in astronomy to study the
spectra of light from the stars.
- Spectrum
- (Plural: spectra) Usually refers to the 'rainbow' of colours seen by the
human eye, Violet, Indigo, Blue, Green, Yellow, Orange, Red
(in order of increasing wavelength). It is important
to remember that this visible spectrum is only a tiny part of the full electromagnetic spectrum.
- Speed
- Speed is the rate at which distance is covered. The SI unit of speed is
the metre per second. Other units include feet per second, kilometres per hour
and miles per hour.
The simple formula for movement at constant speed is: distance = speed x
time.
For waves, the formula v = f l can be used.
- Speed Time Graph
- This is a graph with time plotted along the horizontal axis, and with speed
up the vertical axis. You need to be able to interpret these graphs for
Standard grade. The area under the graph is the total distance travelled.
- Spectrum
- The electromagnetic spectrum includes all types of electromagnetic
radiation(EMR). All types of EMR travel at the same speed: the speed of light,
which is 3 x 108 ms-1. Starting with the lowest frequency
and longest wavelength radiation, the spectrum consists of
The visible spectrum includes red, orange, yellow, green , blue, indigo,
violet. Red light has a wavelength of approximately 700 nm and violet light's
wavelength is roughly 400 nm. All other colours have a wavelength between these
two limits.
- Star
- A star is a hot ball of gas which shines by producing its own light. The
closest star to Earth is called the Sun. The next closest is Proxima Centauri
at around four light-years distant.Other types of stars include red giants,
white dwarfs and neutron stars.
- Stator Coil
- In a dynamo, the stator coil is the the fixed coil which has an electric
current (a.c.) induced in it by the motion of the nearby rotor
(permanent magnet).
- Stethoscope
- The stethoscope is a device which allows a doctor or nurse to listen to
sounds coming from within a patient's body. The sounds can be used to diagnose
medical conditions. The two bells of a stethoscope are connected to the
earpieces via a flexible hollow tube. The bells can be selected by turning a
valve which allows only sound from one of the bells to enter the tubing at a
time. The open bell is used to listen to lower frequency (e.g. heart) sounds and the closed bell is used
to listen to higher frequency (e.g. breathing sounds).
- Streamlined
- Streamlined objects are designed to reduce fluid drag forces. All rough or
angular edges are removed, leaving only smooth curved surfaces. Fish, dolphins,
falcons, air interceptors and racing cars are good examples of streamlined
objects.
- Sun
- The Sun is our nearest star. It is a dim yellow dwarf star, not
particularly big or bright by astronomical standards. It has a mass of
2x1030kg and diameter 110 times that of the Earth. As an example for
comparison, Betelgeuse in Orion has around 20 times the mass of the Sun, and a
diameter 290 times that of the Sun. Also in Orion, the bright star Rigel is
intrinsically 60 000 times brighter than the Sun.