9702/41 : Explain
21. State one advantage and one disadvantage of the use of alternating rather direct current in home
+ easy to change voltage because it changes direction every half cycle - requires greater rectification to overcome problems of reduced power
14. What are the advantages & disadvantages of thermocouple?
+fast response, wide range -metals can corrode; emf small
14. What are the advantages & disadvantages of liquid in gas thermometer?
+high precision, easy -fragile, restricted range
14. What are the advantages & disadvantages of thermistor?
+small size, low cost - slow response, so not suitable for rapid changing temp.
20. (1) Explain why a magnetic force is exerted on the parallel wires
- Because a MAGNETIC FIELD IS CREATED on each wire acting on the wire parallel to it [1] (B) - Current across each wire acts at RIGHT ANGLES TO THE MAGNETIC FIELD [1] (I)
18. Explain why the calculations of charge for plastic are not the same as for metal [3]
-Plastic is an insulator, not a conductor [1] -So charges DO NOT MOVE [1] -Therefore, charge cannot be assumed to be acting at the centre of the object! [1]
19. Explain why a capacitor stores energy but not charge [3/4]
-Since charges on both plates are equal in magnitude and opposite in polarity [1] , there is NO RESULTANT CHARGE [1] -So energy is stored because there is a CHARGE SEPARATION [1] between the plates due to work done [1] by the power supply
19. Explain how a capacitor stores charge [3]
-Since energy stored due to a CHARGE SEPARATION [1] between plates OPPOSITE IN POLARITY [1] -work MUST be done by power supply to SEPARATE CHARGES [1]
An alternating voltage is applied to a piezo electric crystal. What happens to the air surrounding it?
1. Alternating voltage causes crystal to vibrate 2. so air molecules will start vibrating too 3. at a frequency within ultrasound range
22. Describe three conclusions that can be drawn from the graph in Fig. 7.1. The conclusions may be qualitative or quantitative.
1. Energy of the emitted electrons is varies directly proportially to frequency 2. Gradient is the Planck constant (6.63x 10-34) 3. There is a frequency below where no electrons are emitted
22. State three further observations from photoelectric emission that provide evidence for a particulate nature of electromagnetic radiation.
1. Kinetic energy does NOT depend on intensity 2. Kinetic energy does depend on frequency 3. rate of emission of electrons does depend on intensity
14. What are some physical quantities (apart from density of liquid) determining temperature?
1. Resistance of metal 2. volume of gas at constant temperature
13. Total energy of satellite decreases. State and explain the effect of this decrease on 1. the radius r of orbit
As energy decreases, it becomes more negative, therefore radius R will decrease
13. Total energy of satellite decreases. State and explain the effect of this decrease on 2. The linear speed v of the satellite
As energy decreases, so does kinetic energy. Ek= -Gmm/2r decreases, therefore v decreases
20. Explain why the core is LAMINATED [2]
Because eddy currents need to be reduced [1] in order to reduce heating [1]
19. State the functions of a capacitor [2]
Block DC, Smoothing others: Stores energy, separate charges, prevent sparks
18. State why, although charge A is a point charge, the electric field strength between x=3 and x=7, does not obey an inverse square law.
Charge A is not an isolated charge as it is influenced by charge B
22. The experiment in (b) is repeated twice, each time making one change. State, with a reason, how the graph obtained would compare with Fig. 7.1 when: (i) a different metal is used, but keeping the intensity I of the radiation the same
Different threshhold frequency, therefore SAME GRADIENT but different y intercept.
20. Explain the use of uniform electric and magnetic fields for the selection of the velocity of a charged particle.
Electric field and Magnetic field are NORMAL to each other. The charged particle enters a region normal to both fields.For NO deflection , v= E/B
18. What is the relationship between electric field strength and the gradient of the graph?
Electric field strength is potential gradient, therefore field strength = - gradient of graph
20. The direct currents in wire A and B are replaced by sinusoidal alternating currents of equal peak values. Describe the variation of the force experienced by wire B.
FORCE IS ALWAYS TOWARDS WIRE A (since currents are in the same direction) - it will go from 0 to maximum value -variation is sinusoidal
20. State and explain the direction of the Force on the magnets
Force direction for wire =/ by Newton's IIIrd for magnets
17. State the cause of damping
Force in opposite direction to motion
20. Explain why the path of the electron in the magnetic field is the arc of a circle
Force is always normal to the velocity and line of motion ,therefore force and speed of particle are constant; Magnetic force provides centripetal force and acceleration
13. What is the significance of the negative sign in (-) Gm/r ?
Gravitational force is always attractive, therefore work is done by the masses to come together / as r decreases, mass does work
13. Explain how a satellite may be in a circular orbit around the sun
Gravitational force provides centripetal force
13. What links Gravitational and Centripetal force?
Gravitational force provides centripetal force
22. Why difference in wavelength must always be positive?
In the collision, the incident photon loses some of its energy and gives it to the deflected electron. Because the photon has less energy and E=hc/lambda this means lambda has increased to make the energy smaller.
20. When a balance is present underneath a horseshoe magnet, what is the direction of the force on the wire due to the current?
It is first downwards, due to the balance. They form a Newton's third pair. Wen a current is switched on, force turns upwards
14. What can be changed in the design of a thermometer for more accurate measurments?
Liquid with specific heat capacity lower than mercury
12. How are centripetal force and radius of a string linked?
Net force is directly proportional to the radius, therefore centripetal force INCREASES as radius INCREASES
14. Describe what is observed when viewing Brownian motion that provides evidence for your answer in a) [2]
Particles move randomly because small specks of light can be seen
Why thermometer is not accurate measure for thermodynamic temperature?
Scale does not include absolute 0 it depends on the properties of a real substance
18. By reference to the direction of the electric field, state and explain whether the charges A and B have the same, or opposite, signs
Signs are the same because at point P the field is 0
17. While the pendulum is oscillating, the length of the string is increased in such a way that the total energy of the oscillations remains constant. Suggest and explain the qualitative effect of this change on the amplitude of the oscillations.
Since Et= 1/2mw2A2, (w= square root (k/L)) and L increases, amplitude increases to keep energy constant
18. Suggest the effect of the electric field on a single atom near the sphere's surface as electrical breakdown of the air occurs. [2 (B1)]
Since electrons are stripped off the atoms [B1], electrons and positive ions move in opposite directions [B1]
13. Explain why there is a point X on the line between the centres of the Earth and the Moon where the resultant gravitational field strength due to the Earth and the Moon is zero.
Since gravitational forces are always attractive, when their resultant gravitational field strength is 0, their potentials are equal and in opposite directions
20. Why magnet takes longer to pass through metal tube than plastic tube? [5]
Since magnet cuts through magnetic flux, an EMF is induced and eddy currents are formed. As eddy currents form a magnetic field that opposes the motion of the magnet, the Acceleration of magnet becomes smaller than g.
20. Explain why the motion of the sphere in the horizontal plane is circular.
Since magnetic field is always normal to the line of motion, magnetic force = centripetal force
20. Why does temperature rise in core? [4]
Since magnetic flux is changing, an EMF is induced in the core. As eddy currents are Formed,they give rise to temperature
22. How are black lines formed in the photoelectric effect? [4]
Since photons and electrons collide in gas molecules, photons excite electrons. As photon energy equals the difference in energies of the emitted electrons,when Electrons de-excite photons are emitted in all directions therefore producing blacklines
21. Explain, in terms of HEATING EFFECT, what is meant by the root mean square value of the alternating current [2]
The value of DC that DISSIPATES heat energy(P= I^2R) at the SAME RATE in a resistor
20. (2) State the direction of the force
Towards each other (if both currents are in same direction) Away from each other (if current are in opposite directions from one another)
16. What happens at constant TEMPERATURE of a gas?
U=0
20. (3)Explain whether the magnetic forces on the two wires are equal in magnitude, although their currents are not equal.
YES, they are, because they are similar and act towards each other, thus forming a Newton's Third pair. So using Newton's III Law, the magnetic forces are assumed to be equal.
18. State and explain how the final speed of a helium nucleus compares with the final speed of a proton after leaving the region of the electric field [3]
[1] (State) The helium nucleus has double the charge (2 protons) but 4 times the mass of a proton (molecular mass) [2] (Explain) So final speed is lower due to the decreasing acceleration perpendicular to the plates, although the velocity parallel to plates is the same
16. Explain why, for an ideal gas, the change in internal energy is directly proportional to the change in thermodynamic temperature of the gas
[1] No intermolecular ENERGIES, no potential energy. [1]Internal energy is solely kinetic energy [1] KE directly proportional to T
17. At any given frequency, the amplitude of oscillation of trolley is constant. Explain how this indicates that there are resistive forces opposing the motion of the trolley [2]
[1] Oscillator supplies energy [1]without energy loss, the amplitude would constantly increase
22. An emission spectrum is seen as a series of differently coloured lines on black background. Suggest how this information provides evidence for discrete electron energy levels in atoms [2]
[1] each coloured line corresponds to ONE frequency/wavelength, and since E=hf [1] implies specific energy changes between energy levels
23. Suggest why there is an additional force at short range between protons in a Helium nucleus
[1] outside nucleons there is repulsion between protons [1]so short range is needed to keep nucleons from sticking together
14. Explain why the change in kinetic energy of the molecules of this ideal gas is equal to the change in internal energy [3]
[1] there are no intermolecular forces between molecules [1] so potential energy = 0 [1] since internal energy = ek + ep , internal energy = ek
20. The procedure in (b) is repeated, but this time the terminals of the coil are connected together. State and explain the effect on the coil of connecting the terminals together during the change of magnetic flux density shown in Fig. 6.2.
[1]Since induced emf causes CURRENT to flow in coil [1]current causes DISSIPATION of energy in the resistance of the coil [1] As eddy currents are formed, these give rise to he temperature as heating (dis. of energy) , therefore TEMPERATURE RISES
15. Use kinetic theory to explain why, when the piston is moved so that the gas expands , this causes a decrease in the temperature of the gas.
[1]Speed of molecules DECREASES on impact with moving piston. [1] <c>2 directly p. to T [1] temperature decreases
20. Explain how a steady hall voltage VH is developed between exterior faces of the material [3]
[1]The force on charge carriers is perpendicular to both field and current. [1] As charge carriers are DEFLECTED to one side , an electric field is set up. [1] So a steady VH is developed only when electric forces and magnetic forces are EQUAL and OPPOSITE
20. The ring is now cut. Explain why, when procedure in b) is repeated, the cut ring completes more oscillations than the complete ring when oscillating between magnet's poles. [3]
[1]current CANNOT pass all the way around the ring [1]induced emf gets SMALLER [1] smaller rate of ABSORPTION of energy.
20. Use Lenz's law to state and explain the direction of the magnetic field due to the induced current in the small coil.
[1]magnetic field in solenoid is increasing [1]the field in coil is in opposite direction to OPPOSE INCREASE
What is indicated by the direction of field lines?
direction of force on a positive test charge
Explain how the emission spectrum provides evidence for the existence of discrete energy levels for the electron in a hydrogen atom
energy of photon has a corresponding frequency • change in electron energy level emits a single photon • photon energy = difference in energy levels discrete energy changes imply discrete energy levels
16. What happens when energy supplied by heating increases?
heat is ADDED to system
16. What happens when energy supplied by heating decreases?
heat is TAKEN FROM system
14. State 2 conditions for the equation pV= constant x T to be valid [2]
ideal gas [1] and constant amount of gas [1]
16. What happens when internal energy increases?
thermal energy (q) is transferred FROM gas
16. What happens when internal energy decreases?
thermal energy (q) is transferred TO gas
16. What happens at constant VOLUME of a gas?
w= 0
22. What happens to the maximum photoelectric current I if wavelength is decreased and intensity is constant?
when wavelength decreases, fewer photons are emitted per unit time, so current is smaller
16. What happens when work done decreases?
work is done BY system, meaning gas EXPANDS
16. What happens when work done increases?
work is done ON system, meaning gas COMPRESSES
