Standard Answers Flashcards
Newtons Second Law.
Resultant force on an object is directly proportional to the rate of change of momentum and occurs in the same direction as the change in momentum.
Use Newtons Third law to explain the impulse received by A and B.
- Force exerted on A due to B is equal in magnitude and opposite in direction.
- Time of contact is the same and Impulse is FT
- Hence the Impulses are equal in magnitude and opposite in direction
Define Impulse.
Force x time in which the force acts
Define linear momentum.
mass x velocity
Why is Linear momentum a vector?
- Velocity is a vector
- The product of a scalar and a vector is a vector.
State the principle of conservation of momentum.
Total momentum is conserved provided no external forces are applied
Which conditions must be upheld for Newtons 3rd Law?
- Forces must act on different objects
- Forces must be equal in magnitude, type and opposite in direction
Why does speed of an object undergoing circular motion remain constant even with a resultant force acting on it?
- Resultant force is at right angles to direction of motion.
- No work is done by the force.
- KE and speed therefore constant
What is a Geostationary Orbit?
- Always vertically above the same point on earth
- Located above the equator with the same time period as earth
- Velocity of satellite parallel to velocity of a point on the surface at all times.
Describe the pattern of uniform field lines.
The field lines are parallel and equally spaced.
What are the benefits of Geostationary orbits?
- Receiving dish on earth can be kept in the same position as satellite stays above the same point on earth.
- Allowing it to be used for radio communications, TV signals etc
What is SHM?
- Acceleration proportional to displacement from equilibrium.
- This acceleration is always directed towards the equilibrium position
Difference between frequency and angular frequency?
Frequency is the number of oscillations per unit time. Angular frequency is 2pi x frequency.
What is resonance?
- Occurs when the driving frequency matches the natural frequency
- The amplitude of vibrations is then maximum due to maximum energy transfer from driver to the system.
What are the effects of damping on resonance?
- Smaller amplitude of vibrations
- Also slightly reduces the frequency which corresponds to max amplitude
What is Thermal equilibrium?
There is no net heat flow between the objects in contact and they are at the same temperature
Define internal energy
The sum of the randomly distributed KE and PE of the atoms/molecules in the system
Difference between ideal gas and real gas?
- Internal energy is the the sum of the randomly distributed KE and PE of the atoms/molecules in the system
- In an ideal gas all the internal energy is kinetic as it is assumed there is no attraction between molecules
- In a real gas some of this energy is potential energy
State some of the conclusions of molecules from observing smoke particles under a microscope.
- Movement of smoke particles caused by the molecules of the gas moving randomly
- Gas molecules not visible so they are much smaller.
- Smoke is continuously moving meaning the molecules are continuously moving the
State the kinetic model assumptions.
- Collisions of molecules with walls are perfectly elastic.
- Force between molecules is negligible except during collisions
- Large number of particles moving rapidly and randomly
- Volume of molecules is negligible compared to the volume of the container.
- Time during a collision is negligible compared to time between collision.
How does a gas exert pressure on the container walls?
- There are many collisions with the molecules and the wall, causing a change in momentum.
To change momentum the wall must have provided a force as the rate of change of momentum is proportional to the force exerted. - by Newtons 3rd law, an equal force must have been applied in the opposite direction
- Pressure= Sum of all forces/area of wall
Why does force on wall increase with temperature?
- Speed of gas molecules increase
- More frequent collisions
- The change in momentum is also larger
- So total force exerted in the container increases.
Why must the volume of gas increase if the pressure is to remain constant as it is heated.
- As temperature increases, the speed of the molecules increases resulting in a greater rate of collisions and greater changes in momentum.
- For a constant pressure, fewer collision per unit time are needed.
- Can be achieved by increasing volume as the molecules will need to travel a greater distance between collisions
Describe how the scattering experiment provided evidence for the existence, charge and size of the nucleus.
- Most went straight through and some deviated through small angles, showing the atom is mostly empty space.
- A very small number scattered through large angles, showing the existence of a tiny positive nucleus.
- Size of nucleus is about 10^-14m
Why during closest approach does the gold nucleus have a velocity and the alpha particle does not?
- There is a repulsive electrical force between the gold nucleus and alpha particles.
- Linear momentum must be conserved as there are no external forces.
- Initial KE of alpha particle transformed into electric potential energy
Explain the Strong Nuclear Force.
- Acts between nucleons
- Repulsive at less than 0.5fm
- Attractive from 0.5fm to 3fm
- Very short range
Why is the probability of a proton and helium fusing less than two protons?
- Greater charge
- Greater electrostatic repulsive force
- Smaller change of fusion
Describe Carbon Dating.
- Living organisms take in carbon when they are alive, some of which is radioactive carbon 14.
- When they die they stop taking in carbon
- Ratio for dead carbon determined
- Ratio for todays ratio of carbon determined
Then use the formulae
Why is the activity of a radioactive material a major consideration when considering safety precautions?
- High activity means it will be highly ionising so precautions need to be taken.
- Small activities have long half life so need long term disposal.
Disadvantages of power from fission.
- Problems with reaction getting out of control
- Maintaining reaction so it runs continuously.
- Risks or radiation
- Long half life of waste products make disposal very difficult and dangerous
Why is the mass of the individual nucleons different to the mass of the nucleus?
- Mass of the nucleus is less than the total mass of the individual nucleons.
- The nucleus has binding energy, so energy must be supplied to free the nucleons from the nucleus
- This binding energy is equal to the mass difference
Describe the process of induced nuclear fission.
- Thermal neutron absorbed by a massive nucleus
- Massive nucleus splits into two daughter nuclei and one or more fast moving neutrons
What are fuel rods?
Contain the fissile material eg U235
What is the purpose of a moderator in the core of the nuclear reactor?
- Fission reactions produce fast moving neutrons
- Neutrons make collision with the moderator nuclei and transfer some of their kinetic energy, so that they slow down.
- Slow moving neutrons have a greater possibility of causing fission.
Purpose of control rods?
- They absorb some of the neutrons
- So as to allow on average one neutron from a previous reaction to cause subsequent fission
- Materials include Boron or Cadmium
What is Fusion?
- Fusion is the joining together of lighter nuclei to make heavier nuclei
- This process releases energy as the total mass before the reaction is larger than the total mass after
Why can fusion occur at lower temperatures than calculated?
- Some nuclei have kinetic energy much greater than the mean kinetic energy.
- So they will have a larger energy to overcome the electrostatic repulsion and get close enough for the strong nuclear force to have an effect.
What is a non invasive technique? What are the advantages?
No cutting or incision of the patients body, no surgery required.
- Less rick of infections
- Less trauma, bleeding etc
Properties of X Ray photons?
- Can travel in a vacuum at the speed of light
- Have no charge
- Have no rest mass
- Are highly ionising
What causes the production of X Rays resulting in a continuous spectrum of energies?
- Fast moving electron decelerates rapidly
- Electron interacts with electric field around the nucleus
- An X ray photon equal to the energy lost by the incoming electron is emmitted
What causes the production of X Rays that give rise to the characteristic lines in the spectrum?
Fast moving electron ejects inner electron and another electron from a higher level falls to fill in the vacancy.
It releases a photon with a wavelength corresponding to this difference in energy levels
How are X rays produced in a hospital?
- Electron emitted from filament through thermionic emission.
- Electrons accelerated through a high potential difference from the cathode to the anode.
- Tube is evacuated so the X ray photons do not lose energy through collisions.
- Electrons collide with the target anode and release x ray photons.
Explain compton scattering
- X Ray photon interacts with a orbital electron.
- Electron emitted with a small fraction of the KE from the X Ray photon, and a photon of the remaining energy is emitted.
What is pair production?
When an incoming X Ray photon interacts with the nucleus, vanishes, and spontaneously produces an electron-positron pair.
What is the photoelectric effect? (X Rays)
X ray photon causes emission of free electrons from atoms.
- Work function can be ignored
What is simple scattering?
When low energy X Rays collide with orbital electrons, and they do not have enough energy to cause ionisation.
They are scattered with no loss in KE.
What is attenuation?
The gradual decrease in intensity as the X rays pass through matter.
Why are contrast materials used?
- Contrast medium absorbs X Rays because it has a large attenuation coefficient
- Ideal for imaging the outline of soft tissues with very similar attenuation coefficients.
Why are Barium and iodine used as contrast materials?
- Large atomic numbers mean the electrons are bound by an energy equivalent to that of X ray photons and so absorb more x rays.
Describe the operation of a CAT scanner.
- An X ray beam rotates around the patient at different angles as the X ray tube rotates around the patient.
- A thing fan shaped beam is used
- Images of slices through the patient are produced.
- X ray tube moves along patient for next slice.
- Computer can form an image using these slices.
Advantages and Disadvantages of CAT over X Ray?
- 3D image
- Better contrast between soft tissues
- Image can be rotated
- More exposure to X rays means more ionisation
- Takes much longer to complete scan
What is a medical tracer?
- A radioactive substance injected into patient which is then absorbed by an organ or tissue.
Requirements of tracers.
- Gamma must be used
- Relatively short half life
- Non toxic
- High enough activity to allow for monitoring
Main components of gamma camera
Collimator - Only gamma ray photons travelling along the axis of the lead are not absorbed
Scintillator - Incident gamma ray photon produces many photons of light
Photomultiplier - Amplifies photoelectric effect to release many more electrons to give out an electrical pulse for every incident photon.
Computer - Uses signals from the photomultiplier tubes to generate an image.
Explain PET scanning
- Body surrounded by a ring of gamma cameras
- Positrons from the fluorodeoxyglucose annihilate electrons
- Produces two identical gamma photons travelling in opposite directions
- Delay time used to determine location
- Computer connected to camera camera and an image is formed.
Describe the sequence of events that occur to form a star such as our sun.
- Interstellar dust and gas clouds are drawn together by gravitational forces
- Loss in gravitational potential increases the temperature
- Fusion of protons occurs at the correct temperature
- Energy released, where protons then fuse into helium
- Stable star formed when gravitational pressure is equal to the radiation pressure
- Outer layers of star expand and core contracts when hydrogen runs out
- Red giant formed, eventually becoming a white dwarf
Explain the evolution of a star much more massive than our sun
When the hydrogen/helium runs out the outer layers of the star expand whilst the core collapses to form a supernova.
Electron degeneracy pressure is not enough to prevent further collapse
turns into neutron star r black hole depending on mass
What is the big bang?
The creation of the universe, from which time and space evolved
Why do galaxies not collapse on each other?
- Galaxies are moving away from each other
- Other galaxies may be pulling in the opposite direction
Describe the evolution of the big bang up until the present day
- Initially very hot and very dense
- Fundamental particles first formed.
- More matter than antimatter in the universe
- Expansion led to cooling
- Quarks combined to form hadrons
- Atoms then formed.
- Gravitational force between clouds of dust caused stars and then galaxies to form
- Continued to expand until present day where it is 2.7K
Which observations directly support the big bang?
- Spectra from galaxies shift to longer wavelengths
- More distant galaxies are moving away from earth faster than closer ones
- 2.7K CMBR, same intensity in all directions, initially gamma radiation and has redshifted to microwave as it expands
- More helium in the universe than expected. Universe must have been hot enough for fusion to occur
Why is g an estimate in the trap door experiment?
- The drag on the ball is ignored
- Reaction times when starting stopwatch (if being used manually)
- Trapdoor takes time to open and magnetism could delay release of ball
Why does the acceleration of a spaceship increase after lift off?
- F=MA
- Mass of ship decreases as it burns fuel
- Weight decreased so net force on ship increases
- Also because g decreases with increased altitude
Why can F=ma not be used for very high speeds?
Mass of particles increase as they approach the speed of light
How does wearing a seatbelt protect a driver?
- Increases the time for the driver to stop
- Smaller deceleration
- F=ma means F is then smaller
- Also prevents driver form hitting steering wheel
Advantages of using bulbs in parallel circuit?
- One bulb going out means the other bulbs remain lit
- Easier to detect broken bulb in parallel circuit
Advantages of a data logger?
- Can be fed directly to computer
- Can start and stop automatically
- Can take many readings in a short amount of time
What is terminal PD?
- It is the voltage recorded across the terminals of the power supply
- Equal to EMF when there is no current
- Less than EMF when current is flowing, as there is then a voltage drop across the internal resistance of the supply
How do Polaroid glasses prevent glare from reflected water surface?
- Light reflected from water is partially plane polarised
- Alignment of polaroid lens is at right angles to plane of polarisation for reflected light
- Polarised reflected light is then not transmitted
Advantages of using multiple slits?
- Sharper maxima
- Distance form central maximum to 1st order is greater
- Allows the more accurate measurements
How is a stationary wave formed?
- Wave travels to end and is reflected
- Reflected wave superposes with incident wave
- To form resultant wave with nodes (destructive interference) and antinodes (constructive interference)
Describe an experiment to demonstrate the photoelectric effect.
- Zinc plate on cap of gold leaf electroscope
- Plate initially negatively charges
- Shine UV radiation
Leaf collapses showing that electrons are being emitted
Describe how the photoelectric effect can be explained in terms of quantum behaviour.
- One to one interactions
- Only photons with energies above work function
- Energy conserved ( equation)
- Instantaneous emission
- Rate of emission depends on intensity
Why is photoelectric current proportional to intensity?
- One to one interactions
- Intensity = P/A
- Increasing intensity increases number of photons incident on the photocell per second and therefore the number of electrons emitted per second increases.
- Number of emitted electrons is proportional to current
How do photons interact with colder gasses?
- Some photons may have energy that matches an exact energy transition
- Some photons will be absorbed
- Atoms become exited and then reemit photons in all directions
What are the main properties of ultrasound?
Longitudinal wave with a frequency greater than 20kh
How is piezoelectric effect used in ultrasound to emit and receive ultrasound?
- Piezoelectric crystal connected to alternating potential difference making it resonate and emit ultrasound
- On receiving, ultrasound makes the crystal vibrate and produce an alternating pd across its ends
What are the principles of ultrasound?
- Piezoelectric transducer used to send pulses of ultrasound into patient
- Ultrasound reflected at boundaries between tissues
- Intensity of reflected signals depend on acoustic impedances at boundaries
- Time between transmitting and receiving the wave is used to determine the depth
Difference between A-Scan and B-Scan?
- A-Scan is one directional whereas B Scan involves different directions and angles to produce 3d or 2d images
Why is short wavelength ultrasound desirable?
Using smaller wavelengths allows for finer details to be seen and provided greater resolution
Explain method using ultrasound to determine the speed of blood in an artery.
- Transducer placed at angle to artery
- Ultrasound pulses reflected by moving blood cells
- Change in frequency of reflected ultrasound is related to the speed
Describe function of main components of a gamma camera
- Collimator only allows gamma photons to travel along axis of lead tubes
- Gamma photons hit scintillator which produces many photons of visible light.
- Photomultiplier initially emits one electron for each photon but amplifies effect so many more electrons are produced, and an electrical pulse is therefore produced for each incident photon
- Each photomultiplier tube connected to computer which is used to generate an image
How is the quality of a gamma camera image improved?
Longer, thinner collimators and a longer scanning time
Explain basic principles of PET scanning
- Body surrounded by ring of detectors
- Positrons from the fleurodeoxyglucose and annihilate with electrons to produce two identical gamma photons travelling in opposite directions
- The delay time between the detection of these two photons used to determine location
- Gamma cameras connected to a computer which forms an image
In terms of electrons, how does the capacitor acquire equal but opposite charge?
- Electrons move from one plate to the other plate, making the plate they leave positive and the plate they arrive at negative.
- Size of the charge is the same because the number of electrons that are removed from one plate is the same as the number of electrons acquired by the second plate.
Explain how an emf is induced in the secondary coil?
- Changing magnetic flux is produced in the primary coil.
- The iron core links this magnetic flux to the secondary coil.
- This changing magnetic flux through the secondary coil induces an EMF in the secondary coil
Describe in terms of energy conservation the polarity of a coil as a north pole is moved towards it and a current induced.
- There is a repulsive force and the work done against this force is transferred to electrical energy
