Module 5 Flashcards
When are two objects in thermal equilibrium?
When there is no net flow of thermal energy between them. Any two objects in thermal equilibrium must be at the same temperature.
What is the zeroth law of thermodynamics?
If two objects are in thermal equilibrium with a third, then all three are in thermal equilibrium with each other.
What does the absolute temperature scale use as its fixed points?
The triple point of water and absolute zero
How do you convert between temperature in Kelvin and degrees centigrade.
T(K)=T(C)+273
What does the kinetic model describe?
How all substances are made up of atoms or molecules, which are arranged differently depending on the phase of the substance.
How are atoms or molecules arranged in a solid?
Regularly arranged and packed closely together, with strong electrostatic forces of attraction between them holding them in fixed positions, but they can vibrate and therefore have kinetic energy.
How are atoms or molecules arranged in a liquid?
Very close together, but they have more kinetic energy than solids, and can change positions and flow past each other.
How are atoms or molecules arranged in gasses?
Have higher kinetic energy than in liquids, and are much further apart. They are free to move past each other as there are negligible electrostatic forces between them, unless they collide with each other or the container walls. They move randomly with different speeds and in different directions.
What is the internal energy of a substance?
The sum of the randomly distributed kinetic and potential energies of atoms or molecules within the substance.
What are the electrostatic potential energies of the atoms or molecules within a gas?
The electrostatic potential energy is zero because there are negligible electrical forces between the atoms or molecules.
What are the electrostatic potential energies of the atoms or molecules within a liquid?
The electrostatic forces between atoms or molecules give the electrostatic potential energy a negative value. The negative means that the energy must be supplied to break the bonds.
What are the electrostatic potential energies of the atoms or molecules within a solid?
The electrostatic forces between atoms or molecules are very large, so the electrostatic potential energy has a large negative value.
What is specific heat capacity?
The energy required per unit mass to change the temperature by 1 K.
What is specific latent heat?
The energy required to change the phase per unit mass while at a constant temperature. When changing from solid to liquid it is the specific latent heat of fusion, and when it is liquid to gas it is the specific latent heat of vaporisation.
How do you calculate the total number of atoms or molecules in a substance?
N = n x NA
N is number of atoms or molecules, n is number of moles, NA is Avogadro’s constant
What are the assumptions made in the kinetic model for an ideal gas?
- the gas contains a very large number of atoms or molecules moving in random directions with random speeds
- the atoms or molecules of the gas occupy a negligible volume compared with the volume of the gas
- the collisions of atoms or molecules with each other and the container walls are perfectly elastic
- the time of collisions between the atoms or molecules is negligible compared to the time between the collisions
- electrostatic forces between atoms or molecules are negligible except during collisions
What is Boyle’s law?
If the temperature and mass of a gas remain constant then the pressure of an ideal gas is inversely proportional to its volume.
What is Charles’ law?
If the volume and mass of a gas remain constant, the pressure of an ideal gas is directly proportional to its absolute temperature.
What is the distribution of particle speeds at different temperatures?
Maxwell-Boltzmann distribution.
The hotter a gas becomes, the greater the range of speeds. The modal speed and r.m.s. speed increase.
What is the Boltzmann constant?
Molar gas constant/Avogadro constant
What is angular velocity?
The rate of change of angle, therefore w = theta/t
w=2pi/T
w=2pif
What is a centripetal force?
Any force that keeps a body moving with a uniform speed along a circular path.
What is centripetal acelleration?
The acceleration of an object travelling in a circular path at a constant speed.
What is simple harmonic motion?
A kind of oscillating motion where the acceleration is given by a=-w^2x where w^2 is a constant for the oscillator.
What are two key features of all objects moving with simple harmonic motion?
- the acceleration of the object is directly proportional to its displacement
- the acceleration of the objects acts in the direction opposite to the displacement.
What is a displacement-time graph for simple harmonic motion?
cos graph
What is a velocity-time graph for simple harmonic motion?
-sin graph
What is an acceleration-time graph for simple harmonic motion?
-cos graph
How do you know which equation to use to find the displacement in simple harmonic motion?
If the object is beginning from its amplitude at t=0, then use x=Acoswt
If the object begins oscillating from its equilibrium position, then use x=Asinwt
Calculators will need to be in radian mode before completing SHM calculations.
What is damping?
When an external force that acts on the oscillator has the effect of reducing the amplitude of its oscillations.
What is light damping?
When the damping forces are small, the amplitude of the oscillator gradually decreases with time, but the period of oscillations is unchanged.
What is heavy damping?
Larger damping forces decrease the amplitude significantly, and the period of oscillations increases slightly.
What is free oscillation?
When a mechanical system is displaced from its equilibrium position and then allowed to oscillate without any external forces.
What is the natural frequency?
The frequency of the free oscillations.
What is forced oscillation?
A periodic driver force is applied to an oscillator.
What is the driving frequency?
The frequency of the driving force.
When will an object resonate?
When the driving frequency is equal to the natural frequency. This will cause the amplitude to increase dramatically.
What are some examples of resonance?
- clocks keep time using the resonance of a pendulum
- musical instruments have bodies that resonate to produce louder notes
- Magnetic Resonance Imaging enables diagnostic scans of the insides of our bodies
What is gravitational field strength?
the gfs at a point within a gravitational field is the gravitational force exerted per unit mass on a small object placed at that point within the field.
What are the gravitational field lines around a spherical mass?
Radial field.
How do you represent a stronger gravitational field using field lines?
Put the lines closer together.
What are the gravitational field lines close to the surface of a planet?
There is said to be a uniform gravitational field, which means that the field lines are parallel and equidistant.
What is Newton’s law of gravitation?
The force between two point masses is:
- directly proportional to the product of the masses
- inversely proportional to the square of their separation.
F=-GMm/r^2
What is Kepler’s first law?
The orbit of a planet is an ellipse with the sun at one of the two foci.
What is Kepler’s second law?
A line segment joining a planet and the sun sweeps out equal areas during equal intervals of time.
What is Kepler’s third law?
The square of the orbital period of a planet is directly proportional to the cube of its average distance from the sun.
What is an astronomical unit?
The mean distance between the earth and the sun.
What are the uses of satellites?
- communications ( satellite phones, TV, some types of satellite radio)
- military reconnaissance
- scientific research
- weather and climate
- global positioning
What is a polar orbit?
An orbit that circles the poles. It offers a complete view of earth over a given period as the earth rotates beneath the satellite. It is useful for mapping and reconnaissance.
What is low earth orbit?
Satellites in orbit close to earth. Because of Kepler’s third law, they can orbit the earth in a very short time less than 2 hours.
What are the conditions required for geostationary orbit?
- be in orbit above the earths equator
- rotate in the same direction as earths rotation
- have an orbital period of 24 hours
What is gravitational potential?
The work done per unit mass to move an object from infinity to that point.
What does the gravitational potential in a radial field depend on?
- the distance from the point mass producing the field to that point
- the mass of the point mass
What is gravitational potential energy?
The work done to move the mass from infinity to a point in a gravitational field.
What are nebulae?
Gigantic clouds of dust and gas.
How do stars form?
- nebulae are formed over millions of years as the tiny gravitational attraction between dust and gas pulls the particles together
- as the dust and gas get closer together this gravitational collapse accelerates, and due to variations in the nebula, denser regions begin to form
- the denser regions pull in more dust and gas, gaining mass and getting denser, and also getting hotter as gravitational energy is transferred to thermal energy
- in one part of the could a protostar forms- this is a very hot, dense sphere of dust and gas
- nuclear fusion begins when the temperatures inside the core are high enough to overcome the electrostatic repulsion between nuclei.
Why do main sequence stars remain in stable equilibrium?
the radiation pressure and the gas pressure push outwards, and this is balanced by the force of gravitational attraction.
What are planets?
An object in orbit around a star with three important characteristics:
- it has a mass large enough for its own gravity to give it a round shape
- it has no fusion reactions
- it has cleared its orbit of most other objects
What are planetary satellites?
A body in orbit around a planet. This includes moons and man-made satellites.
What are comets?
Small irregular bodies made of ice, dust, and small pieces of rock. They all orbit the sun.
What are solar systems?
The sun and all its planets.
What are galaxies?
A collection of stars, and interstellar dust and gas.
What is the universe?
All electromagnetic radiation, energy and matter, all of space-time, and everything that exists within it.
What is the lifecycle of stars with low mass?
Stars between 0.5M and 10M will evolve into red giants. The reduction in energy released by fusion in the core means that the gravitational force is now greater that the outward force. The core then begins to collapse, increasing the pressure in the core enough to start fusion in a shell around the core. This causes the outside of the star to expand and cool, giving it a red colour.
Eventually the layers of the red giant drift off into space as a planetary nebula, leaving behind the hot core as a white dwarf.
When the core collapses, the electrons are squeezed together, and this creates electron degeneracy pressure.
The electron degeneracy pressure is only sufficient to prevent collapse up to the Chandrasekhar limit (1.44M). This is the maximum stable limit for a white dwarf. Above this, they will explode into a supernova or explode and collapse into a neutron star or blackhole.
What is the Chandrasekhar limit?
1.44M
What is the lifecycle of stars with masses greater than 10M?
Greater mass makes their cores hotter, they consume hydrogen much quicker. When the hydrogen in the core runs low, the core begins to collapse. However, as the cores of these stars are much hotter, the helium nuclei are moving fast enough to overcome electromagnetic repulsion, so fusion of helium can occur. Once this happens, the star starts to expand forming a red supergiant, where the temperatures become high enough to fuse massive nuclei together, forming a series of shells inside the star. This process continues until the star develop an iron core. As the star cannot fuse iron, there is a catastrophic implosion of the layers of the core, leading to a shockwave which ejects all of the core matter into space called a supernova.
After a supernova, which stars become neutron stars?
Stars which have cores with masses greater than the Chandrasekhar limit but less than 3M.
After a supernova, which stars become black holes?
Stars which have cores with masses greater than 3M.
What do supernovae create?
All elements above iron.
What is the Hertzsprung-Russell diagram?
A graph of stars in our galaxy showing the relationship between their luminosity on the y-axis and their average surface temperature on the x-axis.
What is luminosity?
Total radiant power output.
What are energy levels?
When electrons are bound to their atoms in a gas they can only exist in one of a discreet set of energies.
What are the implications of energy levels in gas atoms?
- an electron cannot have a quantity of energy between two energy levels
- the energy levels are negative because external energy is required to remove an electron from the atom. The negative values also indicate that the electrons are trapped within the atom or bound to the positive nuclei
- an electron with zero energy is free from the atom
- the energy level with the most negative value is known as the ground level or the ground state
When is an electron excited?
When it moves from a lower to a higher energy level within an atom in a gas.
What are emission line spectra?
Each element produces a unique emission line spectrum because of its unique set of energy levels.
What are continuous spectra?
All visible frequencies or wavelengths are present.
What are absorption line spectra?
A series of dark spectral lines against the background of a continuous spectrum. The dark lines have exactly the same wavelengths as the bright emission spectral lines for the same gas atoms.
Why would you use a diffraction grating instead of a double slit when analysing starlight?
The fringes formed with a double slit are not very sharp, so it can be difficult to determine the centre of each maxima. Using a diffraction grating will result in a clearer and brighter interference pattern.
How do you determine the maximum number of fringes you can see?
The largest possible angle is 90, so the larges possible n you can see is n lambda = d. You have another n fringes on the other side, and one central maximum, so the total is 2n+1.
When finding n, make sure to round down.
What is a black body?
An idealised object that absorbs all the electromagnetic radiation incident to it and, when in thermal equilibrium, emits a characteristic distribution of wavelengths at a specific temperature.
What is Wien’s displacement law?
The peak wavelength of a black body is inversely proportional to the absolute temperature of the black body.
lambda T= constant
What is Stefan’s law?
The total radiated power per unit surface area of a black body is directly proportional to the fourth power of the absolute temperature of the black body .
What does Stefan’s law show that the luminosity of a star is directly proportional to?
- its radius squared
- its surface area
- its surface absolute temperature^4
What is a light year?
The distance travelled by light in a vacuum in one year.
What is a parsec?
The distance at which a radius of one AU subtends an angle of one arcsecond
What is parallax?
The apparent shift in the position of a relatively close star against the backdrop of much more distant stars as the Earth orbits the sun.
How can you use the parallax angle to find the distance to a nearby star?
d=1/p, where d is in parsecs and p is in arcseconds.
This is only valid up to 100pc from Earth, because as d increases the parallax angle decreases, becoming too small to measure accurately.
What is doppler shift?
Whenever a wave source moves relative to an observer, the frequency and wavelength of the waves received by the observer change compared with what would be observed without relative motion.
What is Hubble’s law?
The recessional speed of a galaxy is almost directly proportional to its distance from Earth.
What is the cosmological principle?
The assumption that, when viewed on a large enough scale, the universe is homogeneous and isotropic, and the laws of physics are universal.
What does homogeneous mean?
Matter is distributed uniformly across the universe. For a very large volume, the density of the universe is uniform. This means that the same type of structures are seen everywhere.
What does isotropic mean?
The universe looks the same in all directions to every observer. It follows that there is no centre or edge to the universe.
What evidence is there in support of the big bang?
- Hubble’s law: shows that the universe is expanding, the galaxies are receding from each other because space itself is expanding in all dimensions. It follows that, going back in time, the universe would be much smaller, denser, and hotter, and would eventually reach a single point.
- cosmic microwave background radiation: when the universe was young and hot, space was saturated with high-energy gamma photons. The expansion of the universe means that space was stretched over time, which stretched the wavelength of these photons, so we now observe this primordial electromagnetic radiation as microwaves.
How can you estimate the age of the universe?
1/hubble constant if we assume that the universe has expanded uniformly since the big bang.
What is dark energy?
A hypothetical form of energy that permeates all space. It is the best accepted hypothesis to explain the accelerating rate of expansion.
What evidence is there for dark matter?
The velocity of stars in galaxies does not behave as predicted. It was expected that their velocity would decrease as the distance from the galaxies centre increases, but this didn’t happen. This can be explained if the mass of the galaxy is not concentrated at the centre.
What evidence is there for dark matter?
The velocity of stars in galaxies does not behave as predicted. It was expected that their velocity would decrease as the distance from the galaxies centre increases, but this didn’t happen. This can be explained if the mass of the galaxy is not concentrated at the centre.
