Physics paper 2 questions

An isolated solid conducting sphere is initially uncharged. Electrons are then transferred to the sphere.

State and explain the location of the excess electrons. (2)

An isolated solid conducting sphere is initially uncharged. Electrons are then transferred to the sphere.

State and explain the location of the excess electrons. (2)

• They move to the outer surface of the sphere

• They are equally spaced

• Because electrons repel

Explain the function of the core in a transformer. (1)

• Provides a greater magnetic flux linkage

Explain the function of the secondary coil in a transformer. (2)

• Has an alternating magnetic flux passing through it

• Produces an induced emf that is determined by number of turns in the primary and secondary coil

Explain how the efficiency of the transformer is increased by constructing the core in this way. (3)

• Laminations made from high resistivity material - reduces eddy currents

• Laminations of iron used - smaller emfs induced

• Laminations of iron used - resistance increases, causing a decrease in eddy currents

What is meant by binding energy of a nucleus? (2)

• Energy required to separate nucleus

• Into individual nucleons

The graph shows how the amplitude of the oscillations of the mass varies with frequency for spring A.

The investigation is repeated with the mass attached to spring B with a spring constant of 3x spring A immersed in a beaker of oil.

A graph of the variation of the amplitude with frequency for spring B is different from the graph in Figure 2.

Explain two differences in the graph for spring B.

• Peak will be at a higher frequency - due to a higher spring constant

Define the electric field strength at a point in an electric field. (2)

• Force per unit charge

• On a small positive charge

Explain, without calculation, whether net work must be done in moving a proton from infinity to position P (2)

• Work must be done on a positive proton as P is at a positive potential

• Potential at infinity is 0

The thread breaks. Explain the motion of the ball (2)

• Experiences both horizontal and vertical accelerations/forces

• The acceleration is constant so it moves in a straight line

State Lenz’s law (1)

• The direction of induced emf will oppose the change of flux creating it

During the demonstration an induced current is detected by the ammeter. The induced current is in the direction E to F.

How does this demonstrate Lenz’s law? (2)

• Current produces a north pole on the right-hand side of the coil

• This opposes motion of the bar magnet

Define the tesla (1)

• Strength of magnetic field that produces a force of 1N in a wire of 1m with 1A

A slow-moving neutron is in collision with a nucleus of an atom of the fule which causes fission.

Describe what happens in the process. (3)

• A neutron is absorbed into a uranium nucleus

• Causing the nucleus to split into 2 smaller daughter nuclei

• Releasing fast moving neutrons

A thermal nuclear reactor produces radioactive waste.

State the source of this waste and discuss some of the problems faced in dealing wit the waste at various stages of its treatment, including:

• the main source of the most dangerous waste

• a brief outline for how waste is treated

• problems faced in dealing with the waste, with suggestions for overcoming these problems (6)

• Main source = control rods or fission fragments

• Left to cool in cooling ponds for a number of years

• Plutonium/uranium separated and recycled

• High level waste vitrified

• Stored in lead-lined containers deep underground

• Waste is initially very hot - must be placed into cooling ponds

• Waste initially very radioactive - needs to be screened and managed remotely

• In liquid form it may leak - needs to be vitrified

• Will be radioactive for thousands of years - needs to be vitrified and stored deep underground

• People do not want waste to travel through places in which they live - trucks must be heavily lead-lined or could be processed nearby

What is represented by gravitational field lines? (1)

• Direction of force on a mass

What is the direction of the force that acts on the electron due to electric field and what is the effect it has on its motion? (2)

• To the right

• Accelerates it

Describe how a beam of fast moving electrons is produced in the cathode ray tube of an oscilloscope (3)

• Thermionic emission by heating

• Cathode heated

• Accelerated by electric field between anode and cathode

State and explain 2 reasons why the horizontal acceleration of a crushed particle containing 2 minerals falling through 2 parallel charged plates is different for each particle. (2)

• Mass is not constant since particles mass will vary

• Charge on particle is not constant

• Acceleration = Eq/m

• E constant but charge and mass are ‘random’

State 2 features of a geo-synchronous orbit. (2)

• Fixed position relative to the surface of the Eart

• Same angular speed as Earth

State 2 factors that affect the gravitational field strength at the surface of a planet. (2)

• Density of planet

• Radius

Explain why the values for gravitational energy are negative. (2)

• Zero potential at infinity

• Work done by the field in moving the object from infinity to that point

Impulse engines are used to place the satellite into an orbit with a longer period.

Discuss any changes this makes to the orbital motion of the satellite. (4)

• Radius must increase

• So velocity decreases

• T² is proportional to R³

• v² is inversely proportional to R

As a spacecraft approaches a comet, the speed can be reduced to match that of the comet. This can be done in stages using four ‘thrusters’. These are fired simultaneously in the same direction.

Explain how the propellant produces the thrust. (3)

• Chemical combusion of propellant

• Gas is expelled

• There is a change in momentum of gases

• Causing an equal and opposite change in momentum of the spacecraft

• Thrust = rate of change of momentum

Explain the meaning of g and G. (3)

• G is gravitational constant and g is the gravitational field strength

• G is a constant value relating attractions of any 2 masses to their separation

• g is a force of 1kg of an object on Earth’s surface

State the time period for a geostationary satellite. (1)

• 1 day

The amount of energy required to move a manned spacecraft from the Earth to the Moon is much greater han that required to reurn it to the Earth. By reference to the forces involved, to gravitational field strength, gravitational potential and point X, the point between the Earth and the Moon, explain why this is so. (6)

• The spacecraft experiences gravitational attractions to both the Earth and the Moon during its journey

• These forces pull in opposite directions on the spacecraft

• Becasue E is much more massive than M, for most of the outwards journey the force towards E is greater than that towards M

• Only in the later stages of the outward journey is the resultant force directed towards M

• On the return journey the resultant force is predominantly towards E

• During the outward journey E’s gravitational fied becomes weaker and M’s becomes stronger

• The resultant field is the vector sum of those due to E and M separately

• X is reached when these 2 component fields are equal and opposite, giving 0 resultant

• X is much closer to M than to E

• Once X has been passed, the spacecraft will be attracted to M by M’s gravitational field

• On the return journey the spacecraft will ‘fall’ to E once it is beyond X

• The gravitational potential due to E increases as the spacecraft moves away from E

• The resultant gravitational potential is the sum of those due to E and M separately

• At X the gravitational potential reaches a maximum value before decreasing as M is approached

• In order to reach M on the outward journey, the spacecraft has to be given at least enough energy to reach X and vice-versa for the return

• Much more work is needed to move the spacecraft from E to X than from M to X, since a larger force has to be overcome over a larger distance

Compare the principal features of the geosynchronous and polar orbits and explain the consequences for possible uses of satellites in these orbits. Explain:

• why a low polar orbit is suitable for a satellite used to monitor conditions on the Earth

• why a geosynchronous circular orbit above the Equator is especially suitable for a satellite used in communications (6)

Low polar orbit:

• Orbital period is a few hours

• Earth rotates relative to the orbit

• Many orbits with different radii and periods are possible

• Orbit height is less than geosynchronous satellite

• Satellite scans the whole surface of the Earth

• Applications: surveillance of conditions/installations on Earth, mapping, weather observations, environmental monitoring

• Gives access to every point on the Earth’s surface every day

• Can collect data from regions inaccessible to man

• Contact with transmitting/receiving aerial is intermittent

• Aerial is likely to need a tracking facility

• Lower signal strength required than that for geosynchronous satellite

Geosynchronous orbit above the Equator:

• Orbital period matches Earth’s rotational period exactly

• Satellite maintains same position relative to the Earth

• Only one particular orbit radius is possible

• Travels west to east above the Equator

• Orbit height is greater than polar orbit satellite

• Speed is less than that of polar orbiting satellite

• Scans a restricted area of the Earth’s surface only

• Applications: tellecommunications generally, cable and satellite TV, radio, digital information

• Satellite is in continuous contact with transmitting/receiving aerial

• Aerial can be in a fixed position

What is meant by an ideal gas? (2)

• Molecules have negligible volume

• Collisions are elastic

• Gas cannot be liquified

• No interactions between molecules

• Obeys ideal gas law

State 2 assumptions about the movement of ideal gas molecules. (2)

• Motion of the molecules is random

• Collisions are elastic

• Newtonian mechanics apply

• Time for collisions to happen is negligible compared to time between collisions

• Effect of gravity is ignored

Define the Avogadro constant. (1)

• Number of particles in 1 mole of substance

Explain the significance of the temperature of absolute 0 (2)

• The temperature of the air at which volume occupied would be 0

• The gas has 0 kinetic energy

It is suggested that the relationship between the volume of water evaporated per unit time Y and the celsius temperature T of the water is:

Y = kT^s where k and s are constant.

Design a laboratory experiement to test the relationship between Y and T and to find k and s.

Include:

• A diagram

• A procedure

• Measurements to be taken

• Control variables

• Analysis of data

• Any safety precautions

• Temperature independent variable, VOlume per unit time is dependent

• Keep temperature of surroundings constant

• Labelled diagram including: beaker, water, method to heat water

• Method to determine volume - measuring cylinder/balance

• Meaure time with stop-watch

• Use thermometer to measure T

• Plot graph of lgY against lgT

• s=gradient

• k=10^y-intercept

• Protective gloves to handle hot beaker

• Keep SA constant

• Keep water temperature constant

• Method to keep temperature constant while water is evaporating - waterbath

• Large SA to increase rate of evaporation

• Y=change in volume/time

• Relationship valid if straight line

• Insulation around beaker

• Avoid draughts

When air inside a bell jar is gradually removed, the sound of a ringing bell inside the jar is heard to decrease in intensity. Design a laboratory experiment to test the relationship:

f=kp²

where f=frequency, p=pressure and k is a constant. Include:

• A diagram

• The procedure

• THe measrements

• COntrol variables

• Analysis of data

• Safety precautions

• Vary pressure

• Measure frequency

• Keep frequency of sound waves generated constant

• Keep temperature of air constant

• Diagram with container with source of sound, pump to remove air

• Pressure gauge to measure

• Use microphone connected to oscilloscope to measure f

• Method to determind period of the wave including use of timebase

• f=1/period

• Plot f against p²

• Relationship correct if graph is straight line through origin

• k=gradient

• Use safety screen to prevent glass entering eyes if container shatters

• Seal points where wires pass through bell jar

• Use loud sound to obtain measureable readings at low pressures

• Check temperature with thermometer

Explain why there is an increase in the energy stored by the capacitor when a dielectric is removed from between the plates. (2)

• Energy is increased

• In polar dielectric, molecules align in the field with the positive charged end towards the negative plate

• Work is done on the capacitor separating the positively charged surface from the negatively charged plate

Give 2 reasons why a capacitor is not a suitable soure for powering a cordless telephone. (2)

• Capacitors are too large

• They would need to be recharged very frequently

• Capacitor voltage would fall continuously while in use

Explain why the current that charges the capacitor falls as the capacitor charges. (3)

• As capacitor charges, voltage increases

• Pd = voltage of resistor + voltage of capacitor

• So current decreases as V=IR

The switch is moved to position 1 until capacitor is fully charged and is then moved to position 2.

Describe what happens in this circuit after the switch is moved to position 1 and after it has been moved to 2. Refer to:

• the direction in which electrons flow in the circuit, and how the flow of electrons changes with time

• how the potential differences across the resistor and the capacitor change with time

• the energy changes which take place in the circuit (6)

Charging

• Electrons flow from plate P to terminal A and from terminal B to plate Q

• Electrons flow in the opposite direction to current

• Plate P becomes +and Q becomes -

• The rate of flow of electrons is greatest at the start and decreases to 0 when capacitor is fully charged

• Vr decreases from E to 0 whilst Vc increases from 0 to E

• at any time Vr + Vc = E

• Time variations are exponential decrease for Vr and exponential increase for Vc

• Chemical energy of battery is changed into electric potential energy stored in the capacitor and into thermal energy by resistor

• Half of energy supplied by batter is converted into thermal, half is stored in capacitor

Discharging

• Electrons flow back from plate Q via the shortening wire to plate P

• At the end of the process the plates are uncharged

• Rate of flow of electrons greatest at the start, decreases to 0 when capcitor is fully charged

• Vc decreases from -E to zero and Vr decreases from E to 0

• At any time Vc = -Vr

• Both Vc and Vr decrease exponentially with time

• Electrical energy stored by the capacitor is all converted to thermal energy by the resistor as the electrons flow through it and this energy passes to the surroundings

• Time constant of the circuit is the same for discharging as for charging

Explain why the rate of change of pd between the capacitor plates decreases as the capacitor discharges. (2)

• Current decreases

• Charge is lost more slowly so pd falls more slowly as voltage is proportional to charge

State 2 situations in which a charged particle will experience no magnetic force when plaed in a magnetic field. (2)

• It is not moving

• It is moving parallel to the magnetic field

Initially the variable resistor is set to its minimum resistance and S is open. Describe and explain what is observed on the ammeter when S is closed. (3)

With S still closed, resistance of the variable resistor is suddenly increased. Compare what is now observed on the ammeter with what was observed before. Explain why it differs. (2)

• Meter deflects than returns to 0

• Current produces magnetic flux

• Change in flux through Q induces emf

• Induced emf causes current in Q

• Meter deflects in opposite direction

• Flux through P decreases

• Induces emf in opposite direction

In the presence of a suitably directed uniform field in a particle accelerator, charged particles move at constant speed in a circular path of constant radius. By reference to the force acting on the particles, explain how this is achieved and why it happens. (4)

• Field applied is perpendicular to the path of charged particles

• Force acts perpendicular to path of charged particles

• Force depends on speed of particles

• Force supplies centripetal acceleration towards centre

• BQv = mv²/r

The speed of protons in a particle accelerator increases as their energy increases. State and explain how the magnetic field must change as the speed is increased. (2)

• Magnetic field must increase

• To increase centripetal force

Describe 2 causes of the energy losses in a transformer and dicuss how these energy losses may be reduced by suitable design and choise of materials (6)

1

• Ac currents induced in the coils

• Coils have resistance

• Currents cause heating which results in lost energy

• Can be reduced by using low resistance wires

• Thick wires of low resistivity have a reduced resistance

2

• Ac current in the primary coil constantly magnetises and demagnetises the coil in opposite directions

• Energy is required for this

• Energy is wasted in heating the core

• Choose a magnetically soft material like iron

3

• Magnetic flux in the core is changing continuously

• Core cut by flux and so emf is induced

• These emfs cause eddy currents which heat the core

• Laminate core - laminations separated by thin layers of insulator

• Currents cannot flow in a conductor that is discontinuous

4

• To be efficient as much magnetic flux as possible must pass through the secondary coil

• Will not happen if they are widely separated

• Magnetic losses can be decreased by having the coils closer together

• Better core design

An alternating current supply provides an output voltage of 12V rms at a frequency of 50Hz. Describe how you would use an oscilloscope to check the accuracy of the rms output voltage and frequency of the supply. (6)

• Connect oscilloscope to supply

• Keep timebase initially turned off

• Adjust y-gain to get as long a line as possible

• Length of line used to calculate peak to peak voltage

• Find rms voltage from this

• Timebase turned on and adjusted to get several cycles

• Find time period

• Frequency = 1/period

• Compare measured values with accepted values

What is meant by root mean square voltage? (1)

• Square root of the mean of the squares of all the values of the voltage in one cycle

Explain how the cyclotron produces a high-energy proton beam. (6)

• Alternating pd

• Pd accelerates protons between dees

• Magnetic field is perpendicular to the plane of dees

• Proton path is curved by the magnetic field

• As velocity of protons increases, radius increases

• Time for which proton is in a dee remains constant

When the switch is close there is a current in the coil in circuit X. The current is in a clockwise direction as viewed from position P.

Explain how Lenz’s law predicts the direction of the induced current when the switch is opened and again when it is closed. (4)

• Induced current opposes change producing it

Switch on:

• Current increases flux through Y

• Current goes in the opposite direction to crease opposing flux

Switch off:

• Flux through Y due to X decreases so current reverses to create flux to oppose this decrease

Explain, using the laws of electromagnetic induction, how the device in the diagram acts as an electromagnetic brake. (3)

• Current produces magnetic field

• Rotating disc cuts flux inducing emf

• Induced eddy currents interact with magnetic field

• Force on eddy currents slows rotation

The table shows how the turning effect exerted on a load varies with angular peed for a particular distance between the copper disc and the plastic disc. Explain the trend shown by the data. (4)

• Relationship between angular speed and turning effect is proportional

• As speed increases, so does rate of change of flux

• This increases induced current in copper disc

• Which will lead to an increase in force

(6)

• Initial current is equal to battery emf divided by R

• Coil rotates

• Coil cuts magnetic flux linkage/there is a rate of change of flux linkage

• Which induces an emf

• This emf opposes original emf according to Lenz’s law

• The faster the coil rotates the larger this emf becomes

Describe how this hybrid electric vehicle device can be used as both a generator and an electric motor. (6)

Generator

• Coil has to be rotated

• Change in magnetic flux linkage

• Emf proportional to the rate of change of flux linkage induced

Motor

• Current provided to coil

• Force on coil perpendicular to magnetic field

• Coil rotates as forces provide moment

What is meant by the decay constant? (1)

• Probability of decay per unit time

Make comparisons between the 2 methods of estimating the radius of a nucleus - Rutherford scattering and electron diffraction. Consider:

• the principles on which each experiment is based, including a reference to an appropriate equation

• an explanation of what may limit the accuracy of each method

• a discussion of the advantages and disadvantages of each method

• Alpha scattering involves electrostatic repulsion

• Electron diffraction treats the electron as a wave having a de Broglie wavelength

• Some reference to relevant equation

• Reference to first minimum for electron diffraction

• Alphas only measure the least distance of approach, not the radius

• Alphas have a finite size which must be taken into account

• Electrons need to have a high speed

• To have a small wavelength - comparable to nuclear diameter

• Requirement to have a small collision region in order to measure the scaering angle accurately

• Importance in obtaining monoenergetic beams

• Cannot detect alpha particles with exactly 180 degree scattering

• Need a thin sample to prevent multiple scattering

• Alpha particle measurements are disturbed by the nuclear recoil

• Mark for alpha particle measurements are disturbed by the SNF when coming close to the nucleus

• SNF not relevant for electrons

• Alphas are scattered only by the protons and not all the nucleons that make up the nucleus

• Visibility - the first minimum of the electron diffraction is often difficult to determine as it superposes on other scattering events

Define atomic mass unit. (1)

• 1/12 the mass of an atom of 12-6C

State and explain how the mass of a 4-2He nucleus is different from the total mass of its protons and neutrons when separated. (2)

• Separated nucleons have greater mass

• Because of binding energy added to separate nucleons

Explain why nuclei in a star have to be at a high temperature for fusion to take place. (3)

• Nuclei need to be close together for strong force to be involved

• Electromagnetic force is repulsive

• If temperature is high then nuclei have high kinetic energies to overcome it