 # All Physics Definitions for Leaving Certificate

## Light

• Refraction is the bending of light as it goes from one medium to another.
• Laws of Refraction: (1) Incident ray, refracted ray & normal are on the same plane.  (2) Sine I/Sine R is constant.
• Refractive Index: Sine I/Sine R = 1/Sin C = Real depth/Apparent depth = c1/c2
• Critical Angle: The angle whose angle of incidence is 90 when light goes from a denser to a rarer medium.
• Total Internal Reflection: When light travelling from a denser to a rarer medium strikes the second medium with an angle of incidence greater than the critical angle, it is reflected back.

## Mechanics

##### Newton’s laws:
1. A body will remain in a state of rest or travelling at a constant velocity unless an external force acts on it.
2. When an unbalanced force acts on a body, the rate of change of the body’s momentum is directly propotional to the foce and takes place in the direction of the force. [ F ≈ (mv-mu)/t –> F = km(v-u)/t –> F = kma –> F = ma ]
3. Every action has an equal and opposite reaction.
• Pressure is force per unit area. P = F/A -> Due to liquid: P = pgh
• Archimedes’ Principle states that when an object is partially or completely immersed in a fluid it experiences an upthrust equal in magnitude to the weight of the fluid displaced.
• Law of Flotation states that the weight of a floating body is equal to the weight of the fluid it displaces.
• Boyle’s law states that at constant temperature the volume of a fixed mass of gas is inversely proportional to to its pressure.
• Newton’s Law of Universal Gravitation states that any two point masses in the universe 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.
• Moment = Force x Perpendicular distance
• Couple – Two parallel forces with the same magnitude acting in opposite directions.
• Torque (Moment of a couple) T = Fd

## Work, Energy, Power

• Work = Force x Displacement (Joule = Newton Meter)
• Energy is the ability to do work.
• Principle of Conservation of Energy states that energy cannot be created or destroyed but can only be converted from one form to another.
• Ek = 1/2mv2
• Ep = mgh
• Power = Work done ÷Time taken || P = w ÷ t (Watt = Joule/Second)
• Percentage Efficiency = (Output/Input) x 100

## Circular Motion

• Angular Velocity is the rate of change of angle per unit time.
• Centripetal Force: If a body is moving in a circle, the force towards the centre needed to keep it moving is centripetal force.
• Centripetal Acceleration is acceleration a body has towards the centre of the circular path it is moving on.
• Period of an Orbit is the time taken for a satellite to go once around the central body.

## Simple Harmonic Motion and Hooke’s Law

A body is said to be moving with simple harmonic motion if:

1. It’s acceleration is directly proportional to its distance from a fixed point on its path.
2. Its acceleration is always directed towards that point.
• Hooke’s law states that when an object is bent, stretched or compressed by a displacement ‘s’, the restoring force ‘F’ is directly proportional to the displacement – provided the elastic limit is not exceeded.

## Heat & Temperature

• Temperature is the measure of the hotness or coldness of a body.
• Thermometric properties are any physical properties that change measurably with temperature.
• Specific Heat Capacity of a substance is the heat energy needed to change the temperature of 1kg of the substance by 1K. (J/kg/K)
• Specific Latent Heat of a substance is the amount of heat energy needed to change the state of 1kg of that substance without changing its temperature. (J/kg)
• Conduction is the movement of heat energy through a substance by the passing on of molecular vibration from molecule to molecule. There is no overall motion of the substance.
• Convection is the transfer of heat through a fluid by means of circulating currents of fluid caused by the heat.
• Radiation is the transfer of heat energy from one place to another in the form of electromagnetic waves.
• U Value of a structure is the amount of heat energy lost per second through 1m2 of the structure when a temperature difference of 1K is maintained between its ends.
• Solar Constant is the average amount of the sun’s energy falling per second perpendicularly on 1 m2 of the earth’s atmosphere. 1.35 kW/m2

## Waves

• Transverse wave is a wave where the direction of the wave is perpendicular to the direction of vibration.
• Longitudinal wave is a wave where the direction of the wave is parallel to the direction of vibration. (e.g. Sound)
• Reflection is the bouncing of waves off an obstacle in their path.
• Refraction is the changing of direction of a wave as it enters a region where its speed is different.
• Diffraction is the sideways spreading of waves into the region beyond a gap or around an obstacle.
• Interference: When two waves from two sources meet, a new wave is produced. The displacement produced at any point by this wave is the algebraic sum of the displacements that each wave would produce on its own. This is called interference of waves.
• Constructive interference: When waves from two sources meet and the amplitude of the resulting wave is greater than the amplitudes of the individual waves.
• Destructive Interference: When waves from two sources meet and the amplitude of the resulting wave is less than the amplitude of the individual waves.
• Coherent Sources: Two sources of waves are said to be coherent if they are in phase or if there is a constant phase difference between waves from each of the sources. If this is so, the sources must also have the same frequency.
• Interference Pattern is the resulting pattern when waves from two or more coherent sources meet.
• Stationary wave: When two periodic travelling waves of the same frequency and amplitude moving in opposite directions meet, they interfere with each other. The resulting wave is a stationary wave or a standing wave.
• Doppler effect is the apparent change in frequency of waves due to the motion of the source or the observer.

## Sound

• Frequency of a vibration is the number of cycles occurring per second.
• Amplitude is the maximum displacement of any molecule from its rest position.
• Overtones are multiples of the fundamental frequency.
• Characteristics: Loudness = Amplitude , Quality  = No. of Overtones, Pitch = Frequency.
• Frequency limits of Audibility are the highest and lowest frequencies that can be heard by the human ear. 20Hz – 20kHz.
• ResonanceIf the frequency of a periodic force applied to a body is the same as or very near to its natural frequency that body will vibrate with very large amplitude. This phenomenon is called resonance.
• Sound Intensity: The rate at which sound energy is passing through unit area at right angles to the direction in which the sound is travelling at that point. I = P/A (W/m2)
• Threshold of Hearing is the smallest sound intensity detectable by the average human ear at a frequency of 1kHz. 1 x 10-12W/m2.
• Fundamental Frequency of a String: A string vibrating with an antinode at its center and a node at each end is vibrating at its fundamental frequency.

## Electricity

• Conductor: Any substance through which charge can flow.
• Insulator: A substance through which electric charge cannot flow.
• Coulomb’s Law states that the force of attraction or repulsion between two point charges is directly proportional to the product of the charges and inversely proportional to the distance between them.
• Electric field is any region of space where a static electric charge experiences a force other than the force of gravity. Caused by other static charges in the vicinity.
• Electric Field Strength at a point in an electric field is the force per unit charge at that point. E = F/Q (also known as electric field intensity).
• Potential Difference between two points in an electric field is the work done in bringing a charge of +1C from one point to the other. (V) (J/C)
• Electric Current is the flow of electric charge.
• Capacitance of a conductor is the ratio of the charge on the conductor to its potential. C = Q/V. (F)
• Size of an Electric Current is the amount of charge passing any point of that conductor per second.
• Q = It : Charge gone past = Current x Time
• Ampere = Coulomb per second
• Potential Difference between two points in a circuit is the energy lost by one coulomb as it moves from one point to the other.
• EMF is voltage when applied to a circuit.
• Resistance of a conductor is the ratio of potential difference across it to the current flowing through it. (Ohm)
• Joule’s Law states that the rate at which heat is produced in a conductor is directly proportional to the square of the current provided its resistance is constant.
• Fuse is a piece of wire that will melt when a current of a certain size passes through it.
• MCB’s are a bimetallic strip and electromagnet which seperate and break the circuit when current is larger than a certain value.
• RCD’s detect the difference between current in the live and the neutral.
• Semiconductor is a substance whose resistance is between that of a good conductor and a good insulator. The resistivity of a semiconductor decreases as temperature increases.
• Intrinsic Conduction: Conduction in a pure semiconductor due to electrons moving from negative to positive and an equal number of holes moving in the opposite direction.
• Extrinsic Conduction: Increased conduction in a semiconductor due to the addition of impurities.
• Doping is the adding of small controlled amounts of certain impuritities to a pure semiconductor to increase its conductivity.
• N type semiconductors are those in which the impurity added produces more free electrons available for conduction.
• P type semiconductors are those in which the impurity added produces extra holes which are available for conduction.
• P-N Junction: A piece of semiconductor with a part of it doped p type and the other part doped n type.
• Depletion layer is the region at both sides of a p-n junction that contains no free majority charge carriers. It thus behaves as an insulator.
• Junction Voltage is the potential difference that exists across a p-n junction caused by holes and electrons moving across the junction when it was formed.
• Rectification is the conversion of a.c. to d.c.

## Magnetism

• Magnetic field is any region of space where magnetic forces can be felt. The direction of the magnetic field at a point is the direction of the force on a north pole if it were placed at that point.
• Right hand grip rule: Grasp a conductor with your right hand, with the thumb pointing in the direction of the current. The fingers give the direction of the magnetic field.
• Fleming’s Left Hand Rule states that if the thumb, first finger and second finger of the left hand are held at right angles, with the first finger pointed in the direction of the magnetic field and the second finger pointing in the direction of the current, the thumb points in the direction of the force.
• Magnetic Flux Density at a point in a magnetic field is a vector whose magnitude is equal to the force that would be experienced by a conductor of length 1m carrying a current of 1A at right angles to the field at that point and whose direction is the direction of the force on a north pole placed at that point. Unit = Tesla (T).
• Ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligble cross section and placed 1m apart in a vacuum would produce a force on each conductor of 2×10-7 Newtons per Meter of length.
• Electromagnetic Induction: Whenever the magnetic field passing through a coil changes an EMF appears in the coil.
• Faraday’s Law of Electromagnetic Induction states that the size of the induced EMF is directly proportional to the rate of change of flux. Induced EMF = Change in flux ÷ Time taken.
• Lenz’s Law states that the direction of an induced current is always such to oppose the change producing it.
• Electric Generator is a device that converts mechanical energy to electrical energy.
• Mutual Induction: If a changing magnetic field in one coil causes an induced EMF to appear in a nearby coil there is said to be mutual induction between the two coils.
• Self Induction: Whenever the current passing through a coil changes, the magnetic field surrounding the coil changes. This changing magnetic field induces an emf in the coil that opposes the changing current. This phenomenon is called self induction.
• Transformer: A device used to change the value of an alternating voltage.

## The Electron

• Thermionic Emission is the giving off of electrons from the surface of a hot metal.
• Electronvolt: The amount of energy gained or lost by an electron when it moves through a potential difference of one volt.
• The photoelectric effect is the emission of electrons from the surface of a metal by electromagnetic radiation of a suitable frequency.
• Threshold frequency: For a given metal the frequency below which photoemission will not occur is called the threshold frequency. Light above the threshold frequency will cause photoemission.
• The Work Function of a metal is the minimum energy needed to remove the loosest electron from the surface of that metal.
• A Photon is a packet of electromagnetic energy. E = hf.
• X-Rays are high frequency electromagnetic radiation produced when high speed electrons in a cathode ray tube strike a metal target that has a high melting point.
• Emission spectrum: When light from a luminous source undergoes dispersion the resulting pattern is called an emission spectrum.
• Energy level is the fixed amount of energy an electron in an atom may have.
• Atomic Number is the number of protons in the nucleus of an atom.
• Mass Number is the number of protons and neutrons in the nucleus of an atom.
• Isotopes are atoms of an element that have the same number of protons but different numbers of neutrons.
• Radioactivity is the spontaneous breaking up of the nuclei of certain atoms accompanied by the emission of one or more types of radiation.
• Alpha = Helium Nuclei.
• Beta = Electrons.
• Gamma = Electromagnetic radiation.
• Activity of a radioactive substance is the number of nuclei of that substance decaying per second. (Bq).
• Law of Radioactive Decay: The number of nuclei decaying per second is directly proportional to the number of nuclei undecayed.
• Half life of a radioactive isotope is the time taken for half of the undecayed atoms to undergo decay.
• A Mole of a substance is the amount of the substance that contains 6.02 x 1023 particles.

## Nuclear Physics

• Nuclear Fission is the splitting up of a large nucleus into two smaller nuclei of roughly the same size.
• Nuclear Fusion is the joining of two small nuclei to form a larger nucleus.

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