Cambridge ALevel Definitions
This post contains the definitions used in the Alevel curriculum.
Motion in a circle
Radian
One radian is defined as the angle subtended at the centre of a circle by an arc equal in length to the radius.
Angular speed
The angular speed is defined as the angle swept out by the radius per second.
Angular velocity
The angular velocity is defined as the angle swept out in a certain direction (e.g. clockwise or anticlockwise) by the radius per second.
Simple Harmonic Motion
Simple harmonic motion
Simple harmonic motion is defined as the motion of a particle about a fixed point such that its acceleration is proportional to its displacement from the fixed point and is directed towards the point.
Period
Period is the time taken for the oscillator to complete one oscillation.
Frequency
Frequency is the number of oscillations completed per unit time
Displacement
Displacement is the distance of the oscillator from equilibrium position
Amplitude
Amplitude is the maximum displacement of the oscillator from equilibrium position
Free oscillation
An object is said to undergo free oscillation if the only external force acting on it is the restoring force
Damped oscillation
An object is said to undergo damped oscillation if friction and other forces, other than the restoring force, also acts on the object such that the oscillator energy is eventually converted to heat.
Resonance
Resonance occurs when the natural frequency of vibration of an object is equal to the driving frequency, giving a maximum amplitude of vibration
Phase
Phase refers to the point that an oscillating mass has reached within the cycle of a complete oscillation
Phase difference
Phase difference is the fraction that one wave is out of step with another wave. It can be measured in fraction, radians or degrees
Gravitational Field
Newton’s law of gravitation
Newton’s law of gravitation states that two point masses attract each other with a force that is proportional to the product of their masses and inversely proportional to the square of their separation
Gravitational field
Gravitational field is a region which a mass experiences a gravitational force
Gravitational field strength
The gravitational field strength at a point is defined as the force per unit mass acting on a small mass placed at that point
Gravitational potential
Gravitational potential at a point is defined as the work done by an external agent in bringing unit mass from infinity to the point
Kepler’s third law of planetary motion
For planets describing circular orbits about the same central body, the square of the period is proportional to the cube of the radius of the orbit
Geostationary orbit
A geostationary orbit is an equatorial orbit with the same period of revolution as the Earth rotation and move in the same direction as the Earth rotation
Electric field
Law of electrostatic charges
Like charges repel, unlike charges attract
Coulomb’s law
The force between two charges is proportional to the product of the charges and inversely proportional to the square of the distance between them
Electric field
Electric field is a region which a charge experiences an electric force
Electric field strength
Electric field strength at a point is the force per unit charge acting on a small positive charge placed at that point
Electric potential
Electric potential at a point is defined as the work done by an external agent in bringing unit positive charge from infinity to that point
Relationship between field strength and potential
The field strength is equal to the negative of the potential gradient at that point
Capacitance
Capacitance is the ratio of charge to potential for a conductor
Farad
One farad is one coulomb per volt
Factors affecting capacitance of a pair of parallel plates
The capacitance is directly proportional to the area of the plates and inversely proportional to the distance between them
Relative permittivity
The relative permittivity is defined as the capacitance of a parallelplate capacitor with a dielectric between the plates divided by the capacitance of the same capacitor with a vacuum between them,
Time constant of a capacity
The time constant of a capacitor is the time for the charge to decrease to 1/e of its initial charge
Temperature
Kelvin
One kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water
Equation of state of a gas
The volume of a gas is inversely proportional to its pressure, provided that the temperature is held constant
Charles’ law
For a given mass of gas maintained at constant pressure, the volume of the gas is directly proportional to its thermodynamic temperature
Pressure law/GayLussac’s law
For a given mass of gas maintained at constant volume, the pressure of the gas is directly proportional to its thermodynamic temperature
Mole
The mole is the amount of the substance which contains as many elementary particles as there are atoms in 12 g of carbon12
Avogadro constant
Avogadro constant is the number of atoms in 12 g of carbon12. It has a value of 6.02 × 10^{23} per mole.
Ideal gas
An ideal gas is a gas which obeys the equation of state pV = nRT at all temperatures, pressures and volumes
RootMeanSquare speed
R.M.S. speed of a molecule is the average speed of an ensemble of molecules obtained by the square root of the mean of the squaredspeeds of all the molecules
<c^{2}>
Mean of the square speeds of all the molecules in the gas system
Assumptions of an ideal gas

All molecules behave as identical, hard, perfectly elastic spheres.

The volume of the molecules is negligible compared with the volume of the container.

There are no forces if attraction or repulsion between molecules.

There are many molecules, all moving randomly.
First law of thermodynamics
The increase in internal energy of a system is equal to the sum of heat added to the system and the work done on it
Alternating Current
RootMeanSquare value
The r.m.s. value of the current or voltage is that value of the direct current or voltage that would produce heat at the same rate in a resistor
Rectification
Rectification is the process of converting an alternating current to a direct current
Smoothing
Smoothing is the process of reducing the fluctuations in the unidirectional output of an a.c. voltage
Magnetic field
Law of magnets
Like poles repel. Unlike poles attract
Motor effect
Motor effect is the phenomenon when a currentcarrying conductor is at an angle to a magnetic field and experiences an electromagnetic force
Magnetic field
Magnetic field is a region which a magnetic pole experiences a magnetic force
Magnetic flux
Magnetic flux is the product of the magnetic flux density and the area normal to the lines of flux
Weber
One weber is the magnetic flux such that when linking to a circuit of one turn, an e.m.f. of 1 V will be induced in the circuit when the magnetic flux is reduced at a uniform rate to zero in one second
Magnetic flux density
Magnetic flux density is numerically equal to the force per unit length acting on a conductor carrying unit current at right angle to a magnetic field
Tesla
One tesla is the magnetic flux density which, acting normally to a long straight wire carrying 1 A current, causes a force per unit length of 1 N m^{–1} on the conductor
Magnetic flux linkage
Magnetic flux linkage is the product of the number of turns of a conductor and the magnetic flux passing through the conductor
Faraday’s law of electromagnetic induction
The e.m.f. induced is proportional to the rate of change of magnetic flux linkage
Lenz’s law
The direction of the induced e.m.f. is such as to cause effects to oppose the change producing it
Photoelectric effect
Photoelectric effect
Photoelectric effect is the emission of electrons from the surface of a metal when electromagnetic radiation is incident on its surface
Observations from photoelectric effect

If photoemission takes place, it does so instantaneously. There is no delay between illumination and emission.

Photoelectric emission takes place only if the frequency of the incident radiation is above a certain minimum value called the threshold frequency.

Different metals have different threshold frequencies.

Whether or not emission takes place depends only on whether the frequency of the radiation is above the threshold for that surface. It does not depends on the intensity of radiation.

For a given frequency, the rate of emission of photoelectrons is proportional to the intensity of the radiation.
Conclusion from photoelectric effect

The photoelectrons have a range of kinetic energies, from zero up to some maximum value. If the frequency of the incident radiation is increased the maximum kinetic energy of the photoelectrons also increases.

For constant frequency of the incident radiation, the maximum kinetic energy is unaffected by the intensity of the radiation.

When the maximum kinetic energy of the photoelectrons is zero, the minimum frequency required to cause emission from the surface may be found.
Photon
A photon refers to a quantum of energy when the energy is in the form of electromagnetic radiation
Work function
Work function energy is the minimum amount of energy necessary for an electron to escape from the surface of a metal
Nuclear Physics
Halflife
The halflife of a radioactive nuclide is the time taken for the number of undecayed nuclei to be reduced to half its original number
Activity
The activity of a radioactive source is the number of nuclear decays produced per unit time in the source. Activity is measured in becquerels (Bq), where 1 Bq is 1 decay per second
Decay constant
Decay constant is defined as the probability per unit time that a nucleus will undergo decay
Atomic mass unit
One atomic mass unit is defined as being equal to 1/12 of the mass of a carbon12 atom
Mass defect
Mass defect of a nucleus is the difference between the total mass of the separate nucleons and the combined mass of the nucleus
Binding energy
Binding energy is the energy equivalent of the mass defect of a nucleus. It is the energy required to separate to infinity all the nucleons of a nucleus
Binding energy per nucleon
Binding energy per nucleon is defined as the total energy needed to completely separately all the nucleons in a nucleus divided by the number of nucleons in the nucleus
Nuclear fission
Nuclear fission is the splitting of a heavy nucleus into two lighter nuclei of approximately the same mass
Observation from probing matter with alpha particles(Rutherford scattering)

the vast majority of the alphaparticles passed through the foil with very little or no deviation from their original path

a small number of particles were deviated through an angle of more than about 10º.

an extremely small number of particles (one in ten thousand) were deflected through an angle greater than 90º.
Conclusions from probing matter with alpha particles

the majority of the mass of an atom is concentrated in a very small volume at the centre of the atom. Most alphaparticles would therefore pass through the foil undeviated.

the centre of an atom is positively charged. Alphaparticles which are also positively charged, passing close to the nucleus will experience a repulsive force causing them to deviate.

Only alphaparticles that pass very close to the nucleus, almost striking it headon, will experience large enough repulsive forces to cause them to deviate through angles greater than 90º. The fact that so few particles did so confirms that the nucleus is very small, and that most of the atom is empty space.

Because atoms are neutral, the atoms must contain negative particles. These travel around the nucleus
Direct Sensing
Properties of an ideal opamp

infinite input impedance

zero output impedance

infinite openloop gain

infinite bandwidth

infinite slew rate
Virtual earth
In a inverting negative feedback circuit, the negative input voltage would tend towards the value of the noninverting input, which is earthed. Hence, the inverting input is very close to 0 V. This input is said to be virtual earth.
Remote Sensing
Intensity
Intensity is the wave power per unit area normal to wave direction
Hardness(Xray)
Hardness is the penetration of the Xray beam, which determines the fraction of the intensity of the incident beam that can penetrate the part of the body being Xrayed
Sharpness(Xray)
Sharpness is related to the ease with which the edges of structure can be determined
Contrast(Xray)
Contrast refers to the range of blackening in an Xray image. Good contrast has a wide range of blackening
Halfvalue thickness
Halfvalue thickness is the thickness of medium that will reduce the intensity of Xray beam by half
Tomography
Tomography is a technique whereby a threedimensional image through a body may be obtained
Specific acoustic impedance
Specific acoustic impedance is the product of the density of the medium and the speed of the wave in the medium
Communication
Modulation
Modulation is the variation of either the amplitude or frequency of the carrier wave
Amplitude modulation
In amplitude modulation, the carrier wave has constant frequency. The amplitude of the carrier wave is made to vary. These variations are in synchrony with the displacement of the information signal
Frequency
In frequency modulation, the amplitude of the carrier wave remains constant. The frequency of the carrier wave is made to vary in synchrony with the displacement of the information signal
Attenuation
Attenuation is the decrease in signal strength as it travels over a distance
Bandwidth
The bandwidth is the range of frequencies that the signal occupies
Analogue signal
An analogue signal can have any value, within limits, and is an exact representation of the raw information
Digital signal
A digital signal consists of a series of ones and zeros, with no values between them
Bit
Each digit in the binary number is known as a bit
Noise
Noise is the random, unwanted signal that adds to and distorts a transmitted signal
Sampling
Sampling is the measurement of the analogue signal at regular time intervals
Analoguetodigital converter
In an ADC, the analogue voltage is sampled at regular intervals of time, at what is known as the sampling frequency or sampling rate. The value of the sample voltage measured at each sampling time is converted into a digital number that represents the voltage value
Digitaltoanalogue converter
In a DAC, a digital signal is converted into an analogue signal
Characteristics of wirepair cable

used mainly for shortdistance communication

cause high attenuation of signal

easily pick up noise

suffer from crosstalk and are of low security

have limited bandwidth
Comparing a wire pair and coaxial cable
The coaxial cable

is more costly

causes less attenuation

is less noisy and is more secure

has a larger bandwidth
Advantages of fibre optics

large bandwidth, giving rise to large transmission capacity

much lower cost than metal wire

diameter and weight of cable is much less than metal cable, hence easier handling and storage

much less signal attenuation, so far fewer regenerator amplifiers are required, reducing the cost of installation

do not pick up electromagnetic interference, so very high security and negligible crosstalk

can be laid alongside existing routes such as electric railways lines and power
Geostationary satellites
Geostationary satellites are satellites orbiting the Earth with a period of 24 hours at a height of 3.6 × 10^{4} km above the Earth’s surface. The satellites orbit in the same direction as the rotation of the Earth (from west to east) and the orbit is above the equator
Polar satellites
Polar satellites are satellites that have low orbits and pass over the poles