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63 Cards in this Set
- Front
- Back
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Why is gravitational potential negative? |
Gravitational potential is taken to be zero at infinity. Since gravitational forces are attractive, the work done by an external force in moving a mass from infinity to that point is negative as displacement and external forces are in opposite directions. |
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Escape velocity |
Occurs when total energy is 0 meaning gravitational potential energy = kinetic energy |
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Latent heat of vapourisation at 20°C vs 100°C |
Higher at 20°C because molecules are more tightly bound to one another and hence require more energy for substance to change into vapour. |
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Phase difference in a stationary wave |
Same segment: In phase Adjacent segment: 180° out of phase |
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Why does pressure increase when temperature increases? |
When temperature increases, kinetic energy increases hence speed of gas molecules increase. Greater frequency of collision and greater change in momentum per collision, greater average force per unit area hence pressure increases. Why |
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Why does pressure increase when volume decreases? |
Frequency of collision increases due to lesser space leading to greater force per unit area and hence greater pressure. |
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Why is there a threshold frequency? |
Photon energy needs to be more than equal to the work function energy of the metal for photoelectric emission to occur. This only takes place if photon frequency achieves a minimum frequency known as threshold frequency. |
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Why is there a minimum stopping potential to reduce current to 0? |
To stop moving electrons from reaching the cathode, and opposing potential difference needs to be applied. Kinetic energy of electrons will then be converted into electric potential energy. |
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Why does current not increase for positive values of V? |
Current is dependent on number of electrons through a point per unit time. Since number of electrons is dependent on number of photons incident on metal surface, value of current will stay constant since number of photons per unit time is constant. |
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Why is there a minimum wavelength for emitted X-Rays? |
To produce an X-ray photon, accelerating electron must decelerate suddenly when it collides with the target atom. Minimum wavelength comes about when 100% of kinetic energy of electron is lost and converted to a single X-ray photon of maximum energy. |
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Explain the formation of a stationary wave in the pipe (2 open ends) |
The incident sound wave travels along the pipe and is reflected at the other open end of the pipe. The superposition of the incident and reflected wave of same amplitude, speed and wavelength but travelling in opposite directions creates a standing wave in the pipe. |
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How to use a cross field for velocity selection? |
1. Set up a region with uniform electric field and uniform magnetic field. 2. Electric field must be directed perpendicular to the magnetic field. 3. Charged particles must enter the region in a direction that is perpendicular to both fields. 4. Charged particles will travel straight through the cross field if Fb=Fe. |
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How is emf obtained across secondary coil? |
Alternating voltage, changing magnetic flux linkage in soft iron core, primary and secondary coil linked by soft iron core, secondary coil experiences a rate of change of magnetic flux linkage, by Faradays law emf is induced in the secondary coil. |
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Increase in temperature due to decrease in volume |
Gas molecules rebound off incoming piston at a higher speed than before so there is an increase in average kinetic energy that corresponds to an increase in temperature. |
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Why does intensity of minima increase with fringe seperation? |
As fringe separation (x) increases, incident wave is stronger and reflected wave is weaker. Hence there is a more complete cancellation of incident and reflected waves at small values of x and less complete cancellation at larger values of x. |
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Why is work done by gravitational force in moving any mass along the equipotential line 0? |
Force is perpendicular to displacement along equipotential line so work done is 0. |
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Why does a scattered photon have a wavelength longer than that of an incident photon? |
Scattered photon has energy less than incident photon energy because some energy is transferred to electron. Since energy is proportional to frequency, frequency of scattered photon is smaller and hence wavelength is greater. |
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Describe movement of air particles at the open top of the air column |
Particles vibrate along the axis of the tube with maximum amplitude. |
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Free oscillations |
Oscillating body oscillates without any external force applied. |
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Forced oscillations |
Body is made to vibrate by an external periodic force or continuous energy is transferred to the vibrating body. |
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Neutral point |
Position along a straight line between the Earth and the moon where an object experiences no net gravitational force from both the Earth and the moon. |
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Path difference for constructive and destructive interference |
nλ means in phase hence constructive interference. (n + 0.5)λ means anti phase hence destructive interference. |
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Why is specific latent heat of vapourisation greater than specific latent heat of fusion? |
Increase in volume (intermolecular seperation) is more significant in vapourisation compared to melting. Hence increase in internal energy and work done by substance in expanding against atmospheric pressure is greater in vapourisation compared to melting. |
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Why does a ball take longer to drop with air resistance |
Kinetic energy at any point is lower thus average velocity is lower hence given same vertical distance, time taken is lower. |
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Maximum velocity |
When acceleration is 0. |
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Why is amplitude highest at natural frequency? |
There is maximum energy transfer from driver to oscillating mass during resonance. |
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Explain why momentum is conserved using Newton's law of motions. |
By Newton's third law, when 2 particles collide, force that A exerts on B is equal in magnitude but opposite in direction to force that B exerts on A. According to Newton's second law, net force on a particle is proportional to the rate of change of its momentum. Since time of contact for A and B is equal, change in momentum of A is exactly equal in magnitude and opposite in direction to change in momentum of B. |
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Why does a cube bob in water when pushed down and released? |
Upthrust is greater than weight at instanr where weight on cube is removed, hence there is a net force upwards and a velocity in the upward direction. As cube moves upwards, upthrust will decrease, till its the same magnitude as the weight of the cube at equilibrium position hence velocity is a maximum. As upthrust decreases afterwards, there is a net force downwards hence velocity decreases as it reaches its highest point. |
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Why is it more difficult to demonstrate wave nature of neutrons than it is for electrons. |
For diffraction to be observable, wavelength must be comparable to the dimension of the aperture the wave is passing through. The wave nature of electron can be observed in electron diffraction experiments because typical inter atomic seperation in crystals is about 10^-10m which is comparable to the de Brogile Wavelength of the free electrons. It is difficult to observe diffraction effects involving neutrons since their wavelengths are much shorter compared to the inter atomic separation of crystals. |
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What happens to the diffraction pattern when visible light of a longer wavelength is used? |
Longer wavelength means it becomes more comparable to the slit width which results in a wider spread of the wave. |
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When white light is incident of a single slit, the central fringe is coloured at the edges and has a white central region. Explain this observation. |
White light is made of light of different wavelengths. When light passes through the slit, it spreads from the centre outwards. Thus light of all wavelengths will still fall onto the central region overlapping each other, forming white. Since light of different wavelengths spreads with a different amount with longer wavelength spreading out more, this causes edge of the central fringe to be coloured. |
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Why does intensity of light not affect stopping potential? |
If frequency/wavelength/energy of photon remains constant, when intensity changes, only number of photons per unit time arriving at the emitter changes. Since electrode is the dame, work function is constant. Each photon imparts the same energy to each electron hence maximum kinetic energy of electrons is the same. q |
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Why can Newton's Law of Gravitation be used to calculate force between two stars? |
If distance between stars is much greater than diameters of the stars, then the stars may be treated as point masses. |
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Why may force have negligible effect on the motion of the Sun? |
Since mass of sun is very huge, acceleration caused by the force is very small hence has negligible effect. |
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Can emf be induced in a non electrical conductor? |
Yes, but there will be no current in it. |
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Why is stationary wave only observed at particular frequencies? |
Resonance occurs when a stationary wave is formed as the vibrator matches with the natural frequency of the string. Nodes must located at the two ends. λ must be 2L/n where n= 1,2,3... and L is length of the string. So , λ can only take specific values. Since v is constant and f=v/λ, stationary waves can only form at specific frequencies. |
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Why is there no time lag in light bulb switching on although electron speed is very slow? |
There are electrons throughout the entire circuit including inside the lightbulb. So when switch is closed, lightbulb will immediately experience a flow of electrons. |
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Why does a coil experience a magnetic force? |
Because it is placed in a magnetic field and the direction of current flow is not parallel to the magnetic field. |
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By reference to flux pattern of the magnetic field in a solenoid, suggest the difference between the magnitude of the magnetic flux density at the ends of the solenoid compared with that at the centre. |
Spreading of magnetic field lines at the ends of the solenoid indicate that the flux density is lower at the ends than that at the centre of the solenoid. |
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By reference to heating effect, state what is meant by the root mean square value of an alternating current. |
The root mean square value of an alternating current is the equivalent value of a steady direct current that will dissipate heat at the same rate as the direct current in a given resistor. |
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Why does internal energy of air increase when heated? |
Internal energy of an ideal gas is equals to the sum of random distribution of kinetic energies associated with air molecules in a system. Since kinetic energy is directly proportional to thermodynamic temperature, when temperature increases after heating, kinetic energy and hence internal energy increases. |
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How does an induction cooker work? |
AC flowing in the coil produces a changing magnetic flux linkage in the pot. According to Faradays law, the pot experiences a rate of change in magnetic flux linkage causing an induced emf and hence current flows in the pot. As the induction current flows through the resistance of the pot, electrical energy is converted to thermal energy i.e. heating effect, causing pot to heat up. |
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What happens when you move the light source away from the double slit? |
Fringe separation will remain the same but intensity of bright fringe decreases while dark fringe remains dark. |
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What happens when you move the light source closer to one slit? |
The zeroth order will shift due to the path difference. |
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Why can we assume that the proton and the nucleus behave as point charges? |
Radii of protons and nucleus are very much smaller than the distance between them. |
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Difference between boiling and evaporation |
Boiling takes place at a specific temperature while evaporation occurs at all temperatures. |
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Explain why path of particle in the magnetic field is the arc of a circle. |
Since path of particle is normal to the direction of the magnetic field, the particle experiences a magnetic force when it enters the field. From Fleming's left hand rule, magnetic force in particle is acting upwards which is normal to its velocity. As such, magnetic force is always perpendicular to velocity, causing velocity to change direction but not its magnitude. |
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State how line spectra, together with the concept of a photon, provide evidence for discrete energy levels in isolated atoms. |
When an electron in the atom de-excites from a higher energy level to a lower energy level, the loss in energy of the electron is emitted out as a photon. The energy of the photon is equals to the energy difference between the two energy levels of a specific frequency given by E=hf. A line spectrum shows that only photons with certain frequencies and hence energy are emitted by the atoms and this is possible only if the electrons in the atom occupy discrete energy levels. |
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Why does temperature of gas decrease when gas expands? |
Kinetic energy of gaseous particles are transferred to the receding piston during expansion, causing average kinetic energy of gaseous molecules to decrease. Since average kinetic energy of gaseous molecules is proportional to thermodynamic temperature of the gas, temperature of gas decreases. |
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Explain why for the photoelectric effect, the existence of a threshold frequency provides evidence for the particulate nature of electromagnetic radiation, as opposed to a wave theory. |
According to wave theory, emission of electrons from metal surface can occur at any frequency if exposure time is sufficiently long as energy can be accumulated. However according to particulate model, electromagnetic radiation comes in discrete quantum where energy of each quantum is directly proportional to frequency of radiation. Hence emission of electrons can only occur if photon energy is greater than work function. |
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Suggest why electrons may assist with an understanding of the crystal structure. |
If De Brogile wavelength of electrons is of the same order of magnitude as the spacing between atoms in a crystal, then electrons can undergo diffraction. Electron diffraction pattern reveals the arrangement of atoms within the crystal. |
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Why is distance travelled further is space probe is launched from equator in eastward direction as compared to if it was launched from the geographical north pole? |
Earth rotates from west to east so when the space probe launches, it already has initial kinetic energy so it will travel further. |
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Why is acceleration at the north pole greater than at the equator? |
At north pole the object is not in circular motion so acceleration of object is the g-field strength. But at equator, acceleration is the g-field strength + acceleration from the circular orbit. |
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Effects of damping on amplitude-freuqency curve |
Peak will be lower, and will shift to the left. It will not however affect the natural frequency. |
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Use PCOM to explain how a forward thrust is produced on the rocket. |
Hot gas exits the rear of rocket with momentum hence rocket must acquire an equal and opposite momentum, resulting in the forward thrust. |
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Relation between electric field strength and potential |
Electric field strength is numerically equals to the potential gradient at that point. |
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Relation between electric field strength and potential |
Electric field strength is numerically equals to the potential gradient at that point. |
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Explain why an electron experiences a force due to the magnetic field. |
Electrons are moving perpendicularly to the magnetic field and hence experience a magnetic force. |
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Explain why a potential difference is established across two opposite faces after some time of electrons deflecting towards one face? |
Deflected electrons build up on one face so one side has a lower potential than the other. |
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explain why a body moving with uniform speed in a circle must experience a force towards the centre of the circle |
Body moving in circle experiences constant change in direction, so resultant force acts on body according to Newton's First Law. Since speed is constant, acceleration is normal to velocity. Resultant force is in the same direction as acceleration according to Newton's second law so force should be perpendicular to path and towards the centre of the circle. |
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If a boy stands on a weighing scale at the North pole, explain why the reading on the weighing scale is higher than that at latitude 36° north of the equator |
At the North Pole, the rotational effect of the Earth would be negligible thus no centripetal force. The normal contact force that the boy exerts on the scale at the North Pole is thus numerically the same as his true weight. |
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Assumptions in thermal energy calculations |
Specific heat capacity of the cup is not taken into consideration. Heat lost due to evaporation is negligible. |
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Which is most precise? |
See which max min difference gives the smallest difference |
