Module 5 Standard Answers

Question 1

Describe the motion of atoms in a solid, and the effect that a small increase in temperature would have on this

Answer 1

The atoms vibrate about their fixed positions
Increasing the temperature of the solid would cause the atoms to vibrate with greater amplitude and frequency, still about their fixed positions

Question 2

Describe the effect on the internal energy and temperature of a solid when it melts

Answer 2

By Εk=3/2Kt, the kinetic energy is directly proportional to the absolute temperature. As their is no change in temperature, the kinetic energy of the solid’s particles remains constant
The potential energy of the particles increases as work is done in breaking the bonds between the particles and increasing the separation between particles
As the internal energy is the sum of the randomly distributed kinetic energy and potential energy of the particles, the internal energy increases

Question 3

State and explain some conclusions that can be drawn about the motion of A(liquid/gas) by observing the motion of B(pollen grains/smoke particles) under a microscope

Answer 3

The B move about in a random haphazard motion known as a random walk. This means that B must be colliding with the A.
The B are visible but B is not. This means that A is much smaller than B and is moving with a greater speed
The B are continuously moving. this means that the A is continuously moving

Question 4

State the assumptions made in the kinetic model of an ideal gas

Answer 4

The collisions between the particles walls of the container are totally elastic
The force between the particles is negligible except during collisions
The volume of the particles is negligible compared to the volume of the container
The time during collisions is negligible compared to time between collisions
There is a large number of particles in a random rapid motion

Question 5

Explain why ideal gases have no potential energy

Answer 5

There are no intermolecular forces of attraction or repulsion in an ideal gas, except during collisions

Question 6

Using the K model of ideal gases and NLs to explain how a gas exerts pressure on the walls of a container

Answer 6

When a particle collides with a wall its momentum is changed
By N2L the wall must have exerted a force on the particle to change its momentum. the rate of change of momentum is directly proportional to the force exerted
By N3L, the force exerted on the wall must be equal in magnitude and opposite in direction to the force exerted on the particle by the wall
The total pressure experienced on the wall is Forces sum/wall Area

Question 7

A constant mass of gas occupies a container of constant volume. Use the K model to explain the increase in the force exerted on the wall of the container by the gas when the temperature in increased

Answer 7

As the temperature is increased the speed of the gas particles increases
As. A result collisions between particles and the walls of the container are more frequent
By ΔP=mΔv, th exchange in momentum of the particles will increase
By F is directly proportional to Δp/Δt, the total force exerted on the walls of the container by the gas particles increases

Question 8

Explain how the speed of an object undergoing circular motion remains constant even though there is a resultant force acting on it

Answer 8

The resultant force acting on the object is perpendicular to the direction of motion of the object (velocity)w=Fxd moved int he direction of the force and this means no work is done on the object
Hence the KE and speed of the object doesn’t change

Question 9

Distinguish between free and forced oscillations

Answer 9

Free: the system is displaced from the equilibrium position and then oscillates with no external periodic driving force applied. The system will oscillate at its natural frequency
Forced: The system oscillates due to continuous energy input by an external periodic driving force. The system oscillates at the driving frequency

Question 10

Explain what is meant by the resonance of a mechanical system

Answer 10

Resonance occurs when the driving frequency applied to the system matches the natural frequency of the system
When this happens, the amplitude of oscillations is then at a maximum and the system absorbs the greatest amount of energy from the driving input causing the resonance

Question 11

Explain what is meant by damping and state its effect on resonance

Answer 11

Damping is the loss of energy from the system due to a force which always opposes motion
Damping causes the amplitude of the oscillations to decrease causing a decrease in the natural frequency of the system
To achieve resonance, the driving frequency must also decrease to match the natural frequency of the system once more

Question 12

Describe the pattern of field lines in a uniform field

Answer 12

They’re parallel to each other
They’re equally spaced
They all point in the same direction

Question 13

Explain why is there is a point between the Earth and Moon at which the gravitational field strength is zero

Answer 13

The gravitational fields of the Earth and the Moon are in opposite directions
The resultant gravitational field strength is found by subtracting the field strength of each of the Earth’s and Moon’s gravitational field
There is a point where the two field exactly cancel, making the resultant field zero

Question 14

The total energy of the satellite E gradually decreases. State and explain the effect of this decrease on…

Answer 14

Radius…………. By E directly proportional to -r^-1, as the total energy becomes more negative, component r will become more negative.
Therefore r will decrease
Linear speed………By GMm/r²=mv²
An increase in r will decrease v and vice versa

Question 15

State some properties of a geostationary orbit

Answer 15

Orbit is equatorial so the satellite must always be at the same altitude above the surface of the planet
Velocity of the satellite is always parallel to the velocity of a point on the surface of the planet
The satellite orbits in the same direction as planets rotation

Question 16

Polar-orbiting satellites are used for communication on Earth. State and explain one advantage and one disadvantage of the use of polar-orbiting satellites, rather than geostationary satellites.

Answer 16

• Advantage: Polar satellites cover/map the whole Earth in several orbits, whereas a
geostationary satellite is only ever above the same point on the Earth’s surface. Also, polar
orbits are much lower and so there is a much shorter time delay in receive communication.
• Disadvantage: Polar satellites must be tracked, and more satellites are needed for continuous
operation. Whereas geostationary satellites maintain continuous communication with the same
point on the Earth’s surface and so are used for television communications and weather
pattern tracking.

Question 17

Suggest one advantage of launching geostationary satellites from the Equator in the
direction of rotation of the Earth.

Answer 17

The satellite will already have some speed in the correct direction (less work must be done on
the satellite to launch it into geostationary orbit).

Question 18

Explain why values of gravitational potential are always negative whereas values of
electric potential may be positive or negative.

Answer 18

• The potential is defined as being zero at infinity.
• Gravitational forces are always attractive.
• For gravitational forces, work is done on the masses in bringing them closer together
(decreasing their separation).
• However, electric forces can be attractive or repulsive.
• For electric forces, work is done by the charges if they have the same sign (potential is
positive), and work is done on the charges if they have opposite signs (potential is negative).

Question 19

The escape velocity is the minimum vertical velocity a particle must have in order to
escape from the Earth’s gravitational field. Explain why atoms, which have a mean
speed lower than the escape velocity, still escape from the Earth’s atmosphere.

Answer 19

• The atoms have a range of speeds and kinetic energies.
• Some of the atoms will have a velocity greater than the escape velocity and will therefore be
able to leave the atmosphere.

Question 20

Explain how emission spectra provide evidence for the existence of discrete energy
levels in atoms.

Answer 20

• Orbital electrons can only absorb and emit photons of particular energies, not just any energy.
• Discrete wavelengths evidence that these photons have particular energies, by E = hc / λ.
• The energy of these photons is determined by the energy change of orbital electrons in the
atom and so the electrons must exist in discrete energy levels within atoms. The differences in
the energy levels are equal to the energy of the photons that the orbital electron can
emit/absorb

Question 21

Describe briefly the sequence of events which occur in the formation of a star, such as
our Sun, from interstellar dust and gas clouds.

Answer 21

• Interstellar dust and gas clouds are drawn together by gravitational forces.
• The loss in gravitational potential energy increases the temperature of the gas.
• Fusion of protons occurs at high pressures and temperatures of the order 107 K.
• Energy is released from the fusion reactions, where protons fuse together into helium.
• A stable star is formed when the gravitational pressure is equal to the radiation pressure

Question 22

Explain what is meant by fusion

Answer 22

• Fusion is the joining together of lighter hydrogen nuclei to make heavier helium nuclei.
• This process releases energy (in the forms of kinetic energy and electromagnetic
energy/photons) as the total mass before the reaction is larger than the total mass after the
reaction

Question 23

Explain the conditions necessary for fusion to occur in the core of a star. Explain your
reasoning for these conditions.

Answer 23

• High temperatures of the order 107 K are needed for fusion to occur.
• The hydrogen protons within the core repel each other because they have like charges.
• When the hydrogen nuclei are close enough, the strong nuclear force causes the protons to
fuse together and release energy.
• High pressures are also required in the core.
• This ensures that there is a greater rate of fusion reactions

Question 24

Describe and explain the evolution of a star similar in mass to our Sun

Answer 24

• When the hydrogen fuel runs out in the star, the outer layers of the star expand, and the star
becomes a red giant.
• The red giant is cooler, but it has a larger surface area and so radiates a larger amount of
energy, making it very bright/luminous.
• The mass of the star is below the Chandrasekhar limit, so the star cannot collapse and
overcome the electron degeneracy pressure to form a neutron star (or black hole).
• So, once fusion has ceased, the outer layers of gas are ejected into space, which forms a
planetary nebula.
• The remnant core of the original star is a white dwarf.
• The white dwarf is an extremely dense remnant of a low-mass star. It is very hot and has low
luminosity. No fusion reactions occur

Question 25

Describe and explain the evolution of a star much more massive than our Sun.

Answer 25

• When the hydrogen fuel runs out in the star, the outer layers of the star expand, and the star
becomes a red supergiant.
• The core of the star collapses rapidly under gravity, forcing the electrons and protons in the
star together, creating neutrons. A shockwave passes though the neutrons, exploding off the
outer shells. This is a supernova explosion.
• As the mass of the star is greater than the Chandrasekhar limit (approximately 1.4 solar
masses), the remnant is either a neutron star or a black hole, depending on the initial mass of
the star.
• A neutron star forms because the mass of the star is large enough so that gravitational effects
cause the remaining material to continue to collapse (overcoming the electron degeneracy
pressure). This collapse continues to cause the electrons and protons in the star together,
creating neutrons. The density of a neutron star is approximately 1x1018 kg m-3
.
• To form a black hole, a huge gravitational force is required to overcome the neutron
degeneracy pressure of a neutron star. To generate such a large force, the mass of the star
must be very large

Question 26

State the properties of a black hole.

Answer 26

• Very dense.
• They do not emit light/electromagnetic radiation since the gravitational field of a black hole is
so strong that light cannot escape from it.
• Curves space.
• Slows down time.
• Emits Hawking radiation

Question 27

Describe an absorption line spectrum and how such a spectrum is produced.

Answer 27

• Dark bands against a background of continuous spectrum.
• The type of spectrum is produced when light from a source producing a continuous spectrum
passes through a cooler gas (the bands correspond to the particular wavelengths of photon
absorbed by the orbital electrons in the gas)

Question 28

Describe a continuous spectrum and how such a spectrum could be produced.

Answer 28

• A spectrum in which all visible wavelengths are present.
• This type of spectrum is produced by a heated solid metal such as a filament lamp

Question 29

Describe an emission line spectrum.

Answer 29

Bright bands against a black background.

Question 30

State what is meant by the Big Bang.

Answer 30

The creation of the universe, from which space and time evolved.

Question 31

Explain why galaxies do not collapse on each other.

Answer 31

• The galaxies are moving away from each other due to the Big Bang.
• The gravitational force (and acceleration) between galaxies is too small to cause them to
draw together and collapse

Question 32

Describe qualitatively the evolution of the universe immediately after the Big Bang to the
present day.

Answer 32

• At the start the universe was very hot and extremely dense.
• All four fundamental forces were initially unified into one force, but then separated.
• The first particles began to form: they were quarks and leptons.
• There was (and still is) more matter than antimatter in the universe.
• Expansion of the universe led to cooling.
• The quarks combined to form hadrons, and hadrons then combined to form nuclei.
• Atoms formed once the universe had cooled further.
• The gravitational force of attraction between clouds of gas and dust caused stars and then
galaxies to form.
• The universe has continued to expand and cool to its present state of 2.7 K

Question 33

Describe observations that directly support the idea of the Big Bang.

Answer 33

• Spectra from galaxies show Doppler shift to longer wavelengths. This suggests galaxies are
moving away from the Earth.
• The more distant galaxies are moving away from the Earth faster than ones closer to Earth.
• If the galaxies have always been moving away from each other, then they will have been
much closer in the past.
• The existence of the microwave background radiation, which is the same in all directions,
supports the Big Bang model of the universe, as the initial gamma radiation in the universe has
become microwaves radiation as the universe expanded

Question 34

State some properties of the cosmic background microwave radiation observed from
the Earth

Answer 34

• The intensity of the microwaves is the same in all directions (isotropic).
• These microwaves correspond to a temperature of 2.7K

Question 35

Discuss how the cosmic microwave background radiation supports the cosmological
principle and the Big Bang model.

Answer 35

• Cosmic microwave background radiation is the same in all directions. This supports the
cosmological principle, claiming that there is no centre to the universe.
• The Big Bang model explains that the universe was very hot before expansion of the universe
lead to it cooling to the current temperature of 2.7K.
• The initial gamma radiation in the universe has become microwave radiation as the universe
expanded. Observing cosmic microwave background radiation today supports the Big Bang
model

Question 36

Suggest reasons why it is difficult to predict the future of the universe.

Answer 36

• The existence of dark matter, dark energy, black holes and neutrinos.
• Hubble’s constant is not known to a high enough precision

Question 37

Suggest how the microwave background radiation may evolve in the future

Answer 37

• Further expansion will lead to the temperature decreasing below 2.7K.
• The wavelength of the EM radiation will then increase, and the microwaves will become radio
waves