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Leaving Cert. Physics Definitions (In italics are what you can not
get from formulas)
Mechanics
Displacement:
The displacement of a particle from a fixed point is its distance from
that point in a particular direction. (m)
Velocity:
The rate of change of displacement with respect to time. (m/s)
Acceleration:
The rate of change of velocity with respect to time. (m/s2)
Scalar quantity: A quantity that has no direction associated with it. Eg mass
Vector quantity: A quantity that has direction and magnitude associated with it. Eg
Velocity.
Force:
A force is something that causes or tries to cause acceleration. (N)
Momentum:
The product of its mass (m) and velocity (v). (Kgm/s)
Conservation of momentum: In any interaction within a closed system the total momentum
before the interaction is equal to the total momentum afterwards.
Newton’s Laws of Motion:
1.
Every body remains in a state of rest, or of uniform motion in a straight line, unless
acted on by an external force.
2.
The rate of change of momentum of a body is proportional to the applied force and
takes place in the direction in which the force acts.
3.
If one body A, exerts a force on a second body B, then B exerts and equal and
opposite force (called the reaction) on A.
Weight: The weight of a body is the gravitational force exerted on it by the earth. (N)
Newton’s Law of Universal Gravitation states:
Every two bodies attract each other with a force that is directly proportional to
the product of their masses and inversely proportional to the square of the distance
between them.
Angular Velocity:The rate of change of angular displacement with respect to time (rad/s).
Centripetal Force: The force towards the centre that keeps a body moving in a circular
path (N).
Simple Harmonic motion: The motion of a particle is simple harmonic if its acceleration
towards a particular point is proportional to its displacement form that point.
Hooke’s Law:
The extension of an elastic material is directly proportional to the
force producing the extension.
Couple:
A couple is a system of forces which has a turning effect only – the
resultant of the forces is zero.
Archimedes Principle: When a body is partially or wholly immersed in a fluid the upthrust
is equal in magnitude to the weight of the fluid displaced.
Law of floatation: The weight of a floating body is equal to the weight of the fluid
displaced.
Energy:
Energy is the ability to do work. (J)
Work:
The amount of work done is equal to the product of the force and the
distance moved.
Power:
Power is the rate at which work is done. (W)
Heat
Specific heat Capacity: The specific heat capacity of a substance is the amount of
energy needed the change the temperature of 1kg of the substance by
1 kelvin. (j/Kg/K)
Specific Latent heat:
The heat needed to change the state of 1kg of a substance.
(j/Kg)
Thermometric property: A physical property that changes measurably with changing
temperature.
Solar Constant: The amount of heat falling on 1m2 of atmosphere every second
U- Value:
Amount of heat energy transferred through a material when the
temperature difference is 1k
Boyle’s Law states: When a fixed mass of gas is kept at constant temperature its volume
is inversely proportional to its pressure.
Light and Sound
Wave:
A wave is a means of transferring energy from one point to another.
Harmonics:
Harmonics are multiples of a fundamental frequency.
Resonance: The transfer of energy between two bodies of the same natural frequency.
Sound Intensity: The rate at which energy is crossing through each square metre of
area, at right angles to the direction in which the sound is travelling.
(W/m2)
Threshold of hearing: The minimum intensity audible to the average human ear at a
frequency of 1khz. (1 X 10-12W/m2)
Doppler effect: The apparent change in frequency of a sound due to the relative motion
of the source of a sound compared with the observer.
The Laws of Reflection state:
1.
The angle of incidence is equal to the angle of reflection
2.
The incident ray, the normal, and the reflected ray are all in the same plane.
The Laws of Refraction state:
1.
(Snell’s Law) sin i/sin r is constant. This constant is know as the refractive index of the
medium, and is denoted by n.
2.
The incident ray, the normal, and the refracted ray are all in the same plane.
Critical angle:
The critical angle is the angle of incidence in the denser medium for
which the angle of refraction in the less dense medium is 900
Total Internal reflection: This occurs when the angle of incidence in the denser medium
exceeds the critical angle.
Transverse Wave: A wave where the vibrations are at right angles to the direction in
which the energy is travelling.
Longitudinal Wave: A wave in which the vibrations are parallel to the direction in which
the wave is moving.
Wavelength:
The distance between two successive crests. (m)
Amplitude:
The maximum displacement from the baseline. (m)
Frequency:
The frequency of a wave is the number of complete cycles of a wave
passing a point per second. (Hz)
Period:
The time taken for one complete cycle of a wave. (s)
Refraction:
The bending of a wave as it passes from medium to the next
Diffraction:
The spreading out of a wave beyond an obstacle.
Interference:
Interference occurs when two or more waves meet- the amplitude of
the resultant wave is equal to the algebraic sum of the amplitudes of
the interfering waves.
Coherent sources: Sources are coherent if they are in phase and have the same
frequency.
Polarisation:
This is when vibrations of a wave are confined to one plane only.
Dispersion:
The separating out of white light into its constituent colours.
Primary colours: Red, green, blue- together they combine to give white light.
Secondary colours: Yellow, cyan and magenta- these are produced when primary colours
are mixed in equal intensity.
Complementary colours: When a primary colour and secondary colour combine to give
white light.
Electricity.
Electric field:
The region in which electric charges at rest will experience a force.
Potential difference: The potential difference between two points is the work done in
bringing unit charge from one point to the other. (v)
Volt:
The potential difference is one volt if the work done in bringing a
charge of 1C from on point to the other is 1J.
Capacitance:
Capacitance is the ratio of charge to potential. (F)
Electric field strength: The force per unit positive charge at that point. (N/C)
Coulomb’s Law states:The force between two point charges is directly proportional to the
product of the magnitude of the charges and is inversely proportional to the square of the
distance between them.
Electric Current: An electric current is a flow of charge. (A)
E. M. F. :
This is the work done in bringing unit charge around a complete circuit.
(V)
Resistance:
The ratio of the potential difference across a conductor to the current
flowing through it (ohms)
Resistivity:
The resistivity of a conductor of length l and of cross-sectional area A,
with a resistance R is given by p= RA/l
Ohm’s Law states:
The current flowing through a wire is proportional to the potential difference
across it, i.e., V & I, or V = IR where R is the resistance of the wire.
Ampere:
The ampere is that constant current which if maintained in two straight
parallel conductors of infinite length, of negligible cross sectional area placed 1m apart in a
vacuum, causes each other to exert a force of 2 x 10-7N per meter length of each other.
Faraday’s Law states:
The induced E.M.F. in a conductor is directly proportional to the rate at which the
conductor cuts the magnetic flux.
Lenz’s Law states:
The direction of the induced current is such that it opposes the motion or change
causing it.
Joule’s Law states:
The heat produced electrically is proportional to the square of the current, the
resistance of the heating wire and the time of heating.
Magnetic Flux Density
Magnetic flux density is the amount of magnetic flux per unit area of a section,
perpendicular to the direction of flux
Modern Physics:
Isotope:
Atoms that have the same atomic number, but have different mass
numbers.
Atomic number: The number of protons in the nucleus of an atom of that element.
Mass number:
The total number of protons and neutrons in the nucleus of an atom of
the element.
Thermionic emission:
The emission of electron from the surface of a hot metal.
Photoelectric emission: The emission of an electron from the surface of a metal when
light of the correct frequency falls on it.
Threshold frequency: The minimum frequency required for photoemission to occur.
Work function:
The minimum energy required by a photon to remove an electron from
the surface of a metal. (j or eV)
X-Rays:
High frequency photons of electromagnetic radiation that are produced
when high speed electrons strike a heavy metal target.
Radioactivity:
The decay of a nucleus of an atom with the emission of one or more of
the three types of radiation.
Activity:
The number of nuclei of a radioactive substance that are decaying per
second. (Bq)
Bequerel:
The unit of activity. One bequerel is one radioactive disintegration per
second.
Law of radioactive decay: The number of disintegrations per second is directly
proportional to the number of nuclei present.
Half life:
The half life of a radioactive isotope is the time taken for half of the
nuclei present to decay. (s)
Nuclear fission: The splitting of a large nucleus into two smaller nuclei with the
emission of large amounts of energy.
Nuclear fusion:
The joining together of two light nuclei to form a larger nucleus with
the emission of large amounts of energy.