Physics paper 1 questions

Explain, with reference to the processes within an atom, the difference between an emission spectrum and an absorption spectrum. (3)

Explain, with reference to the processes within an atom, the difference between an emission spectrum and an absorption spectrum. (3)

• Photon is an energy carrier

• In absorption, atom becomes excited

• In emission, atom de-excites

Pulley M is pulled to the left as the gate is opened.

Explain why this increases the tension in the horizontal cable C. (2)

• Angle to horizontal decreases

• So horizontal components increase

Explain what happens to the brightness of a lamp as resistance in a variable resistor connected in series increases. (2)

• Resistance of resistor increases

• Reducing current throughout the circuit and dimming the lamp

Explain what happens to the brightness of a lamp as resistance in a variable resistor connected in parallel increases. (2)

• Resistance throughout circuit increases

• Reduces current in circuit

• Meaning less volts are lost in the battery so lamp gets brighter

The potassium isotope can also decay by a second decay process to form a calcium-40 nuclide.

Suggest how the emissions form a nucleus of decaying potassium can be used to confirm which decay process is occurring. (3)

• Beta emission

• Electron released

• Electrons detected via cloud chamber or absorption

• Antineutrino released

• No photon released

Explain how the actual frequencies produced in an experiment investigating vibrations in stretched nylon string of fixed length would be different to those that a student can predict. (2)

• Diameter will decrease as string is stretched

• Causing a lower mass per unit length so the frequency increases

Explain how the pattern produced on the screen supports the idea that the electron beam is behaving as a wave rather than as a stream of particles. (3)

• Particle behaviour would only produce a patch of light

• Wave property shown by diffraction

• Graphite causes the electrons to spread out

• Bright rings are formed due to constructive interference

• For a diffraction grating maxima when sin =n/d

Explain how the emission of light from the fluorescent screen shows that the electrons incident on it are behaving as particles. (3)

• Electrons transfer energy in one-to-one collisions

• Idea of light emission due to exitation and de-excitation of electrons

• Idea of collisions by incident electrons moving electrons in atoms between energy levels

• Light emitted when atoms de-excite

Discuss the consequences for the forces acting on the pole when one acrobat has a much greater mass than the other. (3)

• Vertical force on the pole increases

• Increase in mass increases weight and so tension in the rope increases

• Centripetal force on the acrobats will be different

• Unbalanced forces

• causing pole to sway towards the more massive acrobat

Explain why virtual photons cannot be detected in a laboratory but are nonetheless required by particle physics. (3)

• In order to detect it, it must be measured

• This would block the exchange of the force so the interaction will not be able to take place

• They must still exist as forces do not act instantaneously so they must still be exchanged

The weak force acts over a much shorter distance than the strong force.

Explain 2 differences between the relevant exchange particles that account for this. (3)

• Exchange particle for weak force is W boson, for strong force it is the gluon

• The gluon is lighter than the W boson

• Therefore it has a larger range

The electric field is going from A to B.

Suggest and explain an experimental change to the setup as shown that would reduce the photocurrent measured (3)

• Reverse terminals of power supply

• So photoelectrons would have to do work against the electric field

• This reduces photocurrent as only photoelectrons with sufficient energy can reach plate A

During electron capture, the nucleus ‘captures’ an orbiting inner-shell electron. The resulting nucleus has one less proton and one more neutron. Sometimes this process is accompanied by the emission of a characteristic X-ray photon.

Explain why an X-ray photon is produced in this process and suggest why it is called characteristic (3)

• An outer electron replaces the missing electron

• Transition to a lower energy state releases an X-ray photon

• Characteristic because energy = difference in energy levels

Explain the significance of an electron at an energy level of 0eV (3)

• The atom has become ionised

• Enough energy has been transferred to the electron so that it can escape

• It is no longer bounded by the nucleus

In a fluorescent tube, mercury atoms are excited, leading to a series of events which result in the emission of photons in the visible region of the electromagnetic spectrum.

Describe the series of events, following excitation of mercury atoms, which results in the emission of visible photons (6)

• Electrons de-excite and relax back to original energy levels

• They emit photons of energy equal to the energy difference between levels

• These are photons of UV light

• These photons then get absorbed by the fluorescent coating, exciting the atoms

• These atoms then de-excite indirectly

• The energy difference in these smaller levels is equal to energy of visible light so visible light is emitted

When X holds the ball, they remain still. When X throws a ball to Y, they move to the left. When Y catches the ball, they move to the right and then eventually come to rest.

Explain, using Newton’s 3 laws, the motion of both skateboarders.

Ignore all effects of air resistance and friction. (6)

• When X holds ball, both are stationary as there is no resultant force

• When X throws ball, he exerts force on ball to the right. Ball exherts an equal and opposite force on him to the left

• This is a resultant force so he accelerates to the left

• When Y catches ball, ball exherts force on him pushing right. He exherts force back on ball pushing left

• Resultant force to right so he accelerates right

• Ball also produces resultant force to left so Y decelerates till he stops moving

Weightlessness is a term used to describe the experience of astronauts in orbit. It is an effect of the astronauts being in a state of constant free fall.

By referring to Newton’s laws of motion, discuss the term ‘weightlessness’ as applied to astronauts in orbit. Include

• The misconception of an absence of gravity

• What it means to be in a state of constant free fall (4)

• Astronauts experience weightlessness but the Earth still exherts a gravitational force on them

• This is a resultant force causing them to accelerate towards the Earth

• The space station accelerates at the same rate

• So there is no floor to stop astronauts which makes it seem like there is 0 gravity

When an aircraft lands, its jet engines exert a decelerating force on the aircraft by making use of deflector plates, cuasing the air leaving the engines to be deflected at an angle.

The speed of the air leaving B is the same as the speed of the deflected air.

Explain why momentum of air changes and suggest why in practice the decelerating force provided by the deflector plates may not remain constant. (4)

• Momentum is vector

• Air changes direction

• Rate of intake of air decreases

• Due to a smaller rate of change of momentum

Explain, with reference to the conservation of momentum, the effect that the motion of a tennis ball released from a height and bouncing on a hard plate has on the mostion of the Eart from the instant it is released until it bounces at the plate. (3)

• Total momentum before an event = total momentum after provided no external force acts

• Momentum of ball towards Earth = momentum of Earth to ball

• On impact, the momentum of both is 0

• Momentum of ball away from Earth = momentum of Earth in opposite direction

Collisions can occur between neutrons and stationary uranium nuclei, for example, during nuclear fission. Discuss how this collision would be if this was inelastic compared with elastic. (4)

• Neutrons would be absorbed

• Both will now continue to travel in the same direction as the initial neutron

• Kinetic energy lost

• Neutron will rebound with velocity

• The momentum transferred to the uranium nucleus will be greater

• No kinetic energy lost

Explain, with energy and momentum considerations, why swinging one ball from the left will not release 2 balls on the right.

Assume the Newton’s cradle is in a vacuum. (5)

• Conservation of momentum states total momentum before = total momentum after

• If 1 ball is swung, momentum can be conserved if 2 are released as long as speed is halved

• Conservation of energy states total kinetic energy before = total KE after

• If 2 balls are released, speed must be 1/2v

• This results in 1/2KE

Define the volt (1)

• 1 joule per coulomb

What is the benefit of using a high value of resistance in potential divider circuits? (1)

• Reduced current

What are the advantages of using a potential divider arrangement over a variable resistor? (2)

• Provides more sensitive control of current

• Variable resistor can provide higher current but cannot get near 0A

Explain, in terms of charge carriers, why the resistance of the thermistor deccreases as the temperature rises. (3)

• Extra charge carriers released as temperature increases

• Increased thermal agitation of atoms resists flow of charge carriers

• 1st effect overwhelms 2nd

Explain one advantage and one disadvantage for using a potential divider and using a rheostat. (4)

• Better control with potential divider

• Potential divider wastes energy - the lower half of the resistor always carriers current

• Rheostat is easier to connect

• Cannot get values for 0A with rheostat

Define work function (2)

• Minimum energy

• Needed to release an electron from a metal surface

State and explain one reason why the core of the optical fibre is made as narrow as possible (2)

• Reduces multipath dispersion

• Which would cause pulse broadening

State one appliaion of optical fibres and explain how this has benefitted society (2)

• Endoscope

• Improves medical diagnosis

Explain why total internal reflection will not occur when the ray travels from air to glass (1)

• Only happens when the ray travels from a higher n to a lower n

Give 2 reasons why optical fibres used for communications have a cladding (2)

• To protect the core from scratches

• To increase critical angle

• To prevent crossover of signals

• Increase rate of data transfer

Explain why the pulse coming out from an optical fibre has a lower amplitude and a longer wavelength (2)

• Reduced amplitude due to absorption

• Pulse broadening due to modal dispersion

Deep space probes often carry modules which may be ejected from them by an explosion.

Discuss how the principles of conservation of momentum and conservation of energy apply in this instance. (6)

• Momentum conserved as there are no external forces acting

• They are moving in free space so are far from large masses and so gravitational forces are neglible

• Equal and opposite forces acting between probe and capsule during explosion

• These are internal forces

• As momentum has to be conserved and is a vector, the capsule must move along the original line of movement after explosion

• Total energy is always conserved in any physical process as energy can neither be created nor destroyed

• Energy may be converted from one form to another

• Probe is already moving and has kinetic energy

• Some chemical energy is converted into kinetic

• So the system has more kinetic energy than it began with

A ball of mass is swung round on the end of a string so that the ball moves in a horizontal circle. Discuss the motion of the ball in terms of the forces that act on it.

• Explain how Newton’s 3 laws of motion apply to its motion in a circle

• Explain why, in practice, the string will not be horizontal (6)

• First law - ball does not travel in a straight line so a force acts on it

• Constant speed but changing velocity

• So it is accelerating

• Second law - Force on the ball causes it to accelerate

• Acceleration in the same direction as the force

• Force is centripetal

• Third law - ball must pull on the central point of support with a force that is equal and opposite to the force pulling on the ball from the centre

• Force acting on the point of support acts outwards

• Support of ball - supported because the rope is not horizontal

• Equilibrium in vertical plane

• Weight supported by tension

State the origin of this centripetal force and suggest why the operator has to incline his body towards the centre of rotation to avoid falling over (2)

• Force produced by friction between feet and roundabout

• Centripetal force has to act through the centre of mass of operator

Dust particle D is on a rotating vinyl disc. A combination of electrostatic and frictional forces act on the dust particle to keep it in the same position.

There is more dust concentrated on the inner part of the disc than the outer part. Suggest why. (3)

• Closer to the centre, particles need a smaller centripetal force

• v is proportional to r

• Friction is sufficient to meet requirements of particles close to the centre but not further away

What is simple harmonic motion? (2)

• Acceleration is proportional to displacement

• Acceleration is in the opposite direction to displacement

What are forced vibrations? (2)

• Repeated upwards and downwards movement

• Vibrations are at the frequency of the support rod

• The amplitude is smaller for a larger frequency

• When the frequency of the driver is out of phase by 180

What is resonance? (2)

• When the forced vibrations oscillate with the same frequency as the natural vibrations

• Oscillate with a large amplitude

• Max energy transfer

Are the oscillations of the pendulum more heavily damped when the cone oscillates with the metal ring on it or without? Explain. (4)

• More heavily damped when the cone oscillates without the ring

• Damping is caused by air resistance

• Cone has large surface area for small mass

• Loaded cone has more kinetic energy

• Smaller proportion of kinetic energy removed per oscillation

• Intertia of loaded cone is greater

Explain why, when the student rises above P, her motion is no longer simple harmonic. (2)

• Bungee cord becomes slack

• Student’s motion is under gravity

• Constant downwards acceleration

The bungee cord has a significant mass. Where along the bungee cord is the stress a maximum? (2)

• Uppermost point

• Stress=F/A and force at this point includes the weight of the whole cord

What is sound intensity? (1)

• Power per unit area

State an explain what happens to pd over R2 as potential difference over R1 is gradually increased

• Pd across R2 increases

• As R1 increases, pd across R1 increases

• Pd across R3 = 10V - pd across R1 so pd across R3 decreases

• Pd across R2 = 12 - pd across R3 so pd across R2 increases