Definitions Flashcards
Radians
the angle subtended at the centre of a circle by an arc of length equal to the radius of the circle
Symbol: rad
Angular speed
angular displacement per unit
Unit: rad s⁻¹
Symbol: ω
Centripetal force
the resultant force acting on an object towards the centre of the circle when the object is rotating around that circle at constant speed
Unit: N
Symbol: Fc
Centripetal acceleration
the acceleration of an object towards the centre of a circle when the object is rotating at constant speed round that circle
Unit: ms⁻²
Symbol: a𝒸
Natural frequency
the frequency at which a body vibrates when there is no resistive force acting on it
Unit: Hz
Symbol: f
Phase
a point that an oscillating particle has reached within the complete cycle of an oscillation
Unit: rad
Symbol: φ
Phase difference
the difference in phases of two oscillating particles measured in degrees or radians
Unit: rad
Symbol: φ
Simple harmonic motion (2)
a body executes simple harmonic motion if its acceleration is directly proportional to its displacement and if it acts in the opposite direction to its displacement
oscillation where there is acceleration to a fixed point
Damped oscillation
a damped oscillation is an oscillation in which resistive forces cause the energy of the system to be transferred to the surroundings as internal energy
Resonance
resonance occurs when the frequency of the driving force is equal to the natural frequency of the oscillating system
Critical damping
the minimum damping that causes the oscillating system to return to its equilibrium position in the minimum time and without oscillating
Internal energy
the sum of the random distribution of kinetic and potential energies of the atoms or molecules in a system
Unit: J
Symbol: U
Isothermal change
change of a system in which temperature remains constant
Thermal energy
energy transferred from one object to another because of a temperature difference (heat energy)
Unit: J
Symbol: Q
Thermal equilibrium
a condition where two or more objects in contact have the same temperature so that there is no net flow of energy between them
Absolute zero
the temperature 0k, at which a system has minimum internal energy, equivalent to -273.15ºC
Thermocouple
a device consisting of wires of two different metals across which an emf is produced when the two junctions of the wires are at different temperatures, measures temperature
Specific heat capacity
the energy required by unit mass per unit temperature change
Unit: Jkg⁻¹ºC⁻¹
Symbol: c
Specific latent heat of fusion
the amount of heat energy per unit mass needed to convert unit mass of solid to liquid without change in temperature
Unit: Jkg⁻¹
Symbol: L𝒻
Specific latent heat of vaporisation
the amount of heat energy per unit mass needed to convert unit mass of liquid to gas without change in temperature
Unit: Jkg⁻¹
Symbol: Lᵥ
Avogadro constant
the number of particles in one mole of any substance
Symbol: Nᴀ
Ideal gas
a gas that behaves according to the equation pV ∝ T
Root-mean-square-speed
the square root of the average of the squares of the speeds of all the molecules in a gas
Unit: ms⁻¹
Symbol: cᵣ.ₘ.ₛ
Gravitational field
a region in space where a mass experiences a gravitational force per unit mass
Centre of mass
the centre of mass of an object is the point at which we consider the total mass of the object to be concentrated
Gravitational field strength
the gravitational field strength at a point is the gravitational force exerted per unit mass on a small object placed at that point
Unit: Nkg⁻¹
Symbol: g
Gravitational potential
the gravitational potential at a point is the work done per unit mass in bringing a point mass from infinity
Symbol: Φ
Electric field
a region where a charged object experiences a force per unit positive charge
Field of force
a region of space where an object feels a force; the force may be gravitational, magnetic, electric and so on
Electric field strength
the force per unit charge exerted on a stationary positive charge at that point
Unit: NC⁻¹ or Vm⁻¹
Symbol: E
Electric potential
the electric potential at a point is equal to the work done per unit charge in bringing unit positive charge from infinity to that point
Capacitance
the capacitance of a capacitor is the charge on the plates of the capacitor per unit potential difference across the plates
Unit: F or CV⁻¹
Symbol: C
Right-hand grip rule
rule for finding direction of magnetic field inside a solenoid
right hand grips solenoid with fingers following direction of conventional current and thumb points in direction of magnetic field
Right-hand rule
rule for finding the direction of the magnetic field around a straight, current-carrying wire
right hand grips wire with thumb pointing in the direction of the current and fingers curl around in direction of the magnetic field
Fleming’s left-hand (motor) rule
rule is used to predict force experienced by a current-carrying conductor placed in an external magnetic field
Thumb: motion (force)
Index finger: magnetic field
Middle finger: conventional current
Magnetic flux density
(magnetic field strength)
the force acting per unit current per unit length on a wire placed at right-angles to the magnetic field
Unit: T (tesla)
Symbol: B
Hall effect
the production of a potential difference across an electric conductor when an external magnetic field is applied in a direction perpendicular to the direction of the current
Hall voltage
the potential difference produced across the sides of a conductor when an external magnetic field is applied perpendicular to the direction of the current
Vʜ ∝ B
Unit: V
Symbol: Vʜ
Fleming’s right-hand (generator) rule
used to predict the direction of the current caused by an induced e.m.f in a conductor moved at right angles to a magnetic field
Thumb: motion (force)
Index finger: magnetic field
Middle finger: induced conventional current
Magnetic flux
the product of magnetic flux density normal to a circuit and the cross-sectional area of the circuit.
Unit: Wb (weber)
Symbol: Φ
Magnetic flux linkage
the product of magnetic flux for a circuit and the number of turns.
Unit: Wb (weber)
Root-mean-square (r.m.s) value
the r.m.s value of an alternating current is the steady current that delivers the same average power as the a.c to a resistive load
Rectification
the process of converting from ac to dc
Photoelectric effect
an interaction between a photon and an electron in a metal, in which the electron is removed from the surface of a metal
Planck constant
a fundamental constant that links the energy of a photon E and its frequency f,
E = hf
Unit: Js
Symbol: h
electronvolt (eV)
the energy gained by an electron travelling through a potential difference of 1 volt
1 eV = 1.60 x 10⁻¹⁹ J
Threshold voltage
the minimum forward bias voltage across a light-emitting diode (LED) when it starts to conduct and emit light
Threshold frequency
the minimum frequency of the incident electromagnetic radiation that would eject electrons from the surface of a metal
Threshold wavelength
the longest wavelength of the incident electromagnetic radiation that would eject electrons from the surface of a metal
Work function of energy
the minimum energy needed by an electron to free itself from the surface of a metal. Work function (together with threshold frequency and threshold wavelength) is a property of the metal.
Emission line spectrum
a spectrum with bright-coloured lines of unique wavelengths
Absorption line spectrum
a spectrum with dark lines of unique wavelengths seen against the background of a continuous spectrum
Quantised
a quantity is said to be quantised when it has a definite minimum magnitude and always comes in multiples of the magnitude
restricting a variable, observable quantity to discrete values
Mass defect
the difference between the total mass of the individual nucleons separated to infinity and the mass of the nucleus
Binding energy
the minimum external energy required to completely separate all the neutrons and protons of a nucleus to infinity
Fission
the process in which a massive nucleus splits into two smaller nuclei
Fusion
the process in which two light nuclei join together to form a heavier nucleus
Decay constant
the probability that an individual nucleus will decay per unit time
Unit: s⁻¹
Symbol: λ
Activity
the rate of decay of a nuclei of a radioactive source
Unit: Bq (becquerel)
Symbol: A
Count rate
the number of particles (beta or alpha) or gamma-ray photons detected per unit time by a Geiger–Müller tube
count rate is always a fraction of the activity of a sample
Exponential decay
the decrease of a quantity where the rate of decrease is proportional to the value of the quantity
Half life
the half life t₀.₅ of an isotope is the mean time taken for half of the active nuclei in a sample to decay
Attenuation
the gradual decrease of intensity of radiation as it passes through a medium
Attenuation (or absorption) coefficient
the attenuation (or absorption) coefficient of a material is a constant that depends on the material and the frequency of the x-rays
Unit: m⁻¹
Symbol: µ
Image intensifiers
a device used to change low intensity X-ray image into a bright visual image
Contrast media
materials, such as barium, which are good absorbers of X-rays; they are used to show the edges of soft tissues in X-ray images
Computerised axial tomography (CAT or CT)
a technique in which X-rays are used to image the body in order to produce a three-dimensional image
1. x-ray tube rotates around patient
2. thin fan shaped beam of x-rays is used
3. x-rays absorbed differently by different tissues
4. intensity of x-ray recorded by detector
5. images of “slices” of the patient are taken
Piezoelectric crystal
a material that produces an e.m.f when it is stressed, causing its shape to change
also, when a voltage is applied across it in one direction, it changes dimensions slightly
Piezoelectric effect
the production of an e.m.f across a crystal by putting the crystal under stress
the opposite effect is applying a p.d. across the crystal causing it to change
Acoustic impendence
the product of the density of the substance and the speed of the ultrasound in the substance
Unit: kgm⁻²s⁻¹
Symbol: Z
Impedance matching
the use of a gel to reduce the intensity of unwanted, reflected ultrasound between 2 materials. The gel has a similar acoustic impedance to the reflecting material
Radiotracer
a substance that consists of radioactive material that is attached to a natural chemical, such as glucose, introduced to the body to study tissue
Line of response
a line drawn from the PET detector triggered by one gamma-ray to the opposite detector that was triggered by the second gamma-ray
Luminosity
the total radiant power emitted by a star
Unit: Watts
Symbol: L
Radiant flux intensity
the radiant power passing normally through a surface per unit area
Unit: Wm⁻²
Symbol: F
Black body
an idealised object that absorbs all electromagnetic radiation falling on it
it has a characteristic emission spectrum and intensity that depends only on its thermodynamic temperature
Big bang theory
a model of the creation of the universe from an extremely hot and dense state and its subsequent evolution
Redshift
a term used to describe the increase in the observed wavelength of the electromagnetic waves due to the recession of the source
Annihilation
when a particle interacts with its antiparticle so that mass is converted to energy
Photon
a quantum of energy of electromagnetic radiation
Standard candle
a reference for measuring cosmic differences as it has a known and unchanging luminosity
Magnetic field
a region where a force acts on a magnetic material
