CIE Physics A-Level Definitions

The first law of thermodynamics can be represented by the expression: ΔU = q + w. State what is meant by the symbols in the expression. [2]

+∆U = Increase in internal energy +q = Heat energy transferred to the system +w = Work done on system

Briefly describe two phenomena associated with the photoelectric effect that cannot be explained using a wave theory of light. [2]

1. *Maximum* energy of electrons does not depend on intensity 2. *Maximum* energy of electron depends on frequency

State two conditions necessary for a mass to be undergoing simple harmonic motion. [2]

1. Acceleration proportional to displacement 2. Displacement and acceleration in opposite directions

State two effects of negative feedback. [2]

1. Greater bandwidth 2. Smaller gain

State three advantages of an optic fibre compared to a metal wire for the transmission of a signal. [3]

1. Greater bandwidth 2. Better security, harder to tap 3. Less attenuation

State one use of piezoelectric transducers, apart from ultrasound. [1]

1. Microphone 2. Pressure sensor 3. Spark generation 4. Watches/clocks

State two advantages of the transmission of data in digital form, compared with the transmission in analog form. [2]

1. Noise can be eliminated 2. Signal can be encrypted for better security

State what is meant by a photon. [2]

A discrete amount of energy of electromagnetic radiation.

What is meant by negative feedback? [2]

A fraction of output is returned to the input and subtracted from the input.

State what is meant by an ideal gas. [2]

A gas the follows PV/T=Constant P=Pressure V=Volume T=Thermodynamic Temperature

State what is meant by the de Broglie wavelength. [2]

A particle has a wavelength that is dependent on its momentum.

Explain what is meant by a magnetic field. [2]

A region where there is a force experienced by a current-carrying conductor, moving charge and permanent magnets.

State what is meant by a *Digital Signal*. [1]

A signal that consists of 1s and 0s.

What should the graph of displacement against acceleration look like for simple harmonic motion? [2]

A straight line through origin which indicates acceleration is directly proportional to displacement. A negative gradient shows acceleration and displacement are in opposite directions.

State the typical frequency for communication between the satellite and the Earth, for a geostationary orbit. [1]

A value in the range (1-300)×10⁹ Hz.

State the position of the satellite relative to the Earth's surface, for a geostationary orbit. [1]

Above the Equator.

Explain why, at the surface of a planet, gravitational field strength is numerically equal to the acceleration of free fall. [1]

Acceleration = Force/Mass and Field Strength = Force/Mass, so they are equal.

By reference to the action of a transformer, explain why the input to the transformer is an alternating voltage, rather than a constant voltage. [3]

Alternating voltage gives rise to changing magnetic flux in core. This flux links the secondary coil. Induced e.m.f. in secondary only when flux is changing.

State what is meant by amplitude modulation (AM) [2]

Amplitude of carrier wave varies in synchrony with displacement of information signal.

Explain why, for an ideal gas, the change in internal energy is directly proportional to the change in thermodynamic temperature of the gas. [3]

An ideal gas has no intermolecular forces so therefore no potential energy. So the internal energy is solely kinetic energy of particles. Therefore mean kinetic energy of particles proportional to thermodynamic temperature of gas.

Use band theory to explain why, as temperature of the semiconductor material rises, electrical resistance of a sample of material decreases. [5]

As temperature rises electrons gain energy, they then enter the conduction band. Positively charged holes are left in the valence band. This increase in charge carriers decreases resistance. As temperature rises, lattice vibrations increase. Effect of electrons and holes outweighs effect of lattice vibrations so resistance still decreases.

By reference to the question, "Describe the basic principle of CT scanning" suggest why the radiation dose for a CT scan is much larger than for an X-ray image of a leg bone. [1]

CT scan consists of many single X-ray images.

What is meant by the capacitance of a parallel plate capacitor? [3]

Capacitance = Charge/Potential (C=Q/V). Charge (Q) is the charge on one plate. Potential (V) is the potential difference between the plates.

State why the transformer has an iron core, rather than having no core. [1]

Core reduces loss of magnetic flux linkage.

State what is meant by the contrast of an X-ray image. [2]

Difference in the degrees of blackening between the structures.

What component goes before an output device? [1]

Digital-to-Analogue Converter (DAC).

State what is meant by quantisation of charge. [1]

Discrete and equal amounts of charge.

Use kinetic theory to explain why, during the melting process, thermal energy is required although there is no change in temperature. [3]

During melting, bonds between molecules are broken. This means the potential energy of molecules is increased. Little work done so required input of energy is thermal.

A stone of mass m has gravitational potential energy E at a point X in a gravitational field. The magnitude of the gravitational potential at X is φ. State the relation between m, E and φ. [1]

E = -mφ

Explain why an electromotive force (e.m.f.) is not induced at the output when a constant direct voltage is at the input. [2]

E.m.f. induced only when flux is changing. A constant direct voltage gives constant a flux.

Explain how a uniform magnetic field and a uniform electric field may be used as a velocity selector for charged particles. [3]

Electric and magnetic fields at right-angles to one another. The particle enters fields with velocity normal to the two fields. Forces on particle due to fields are in opposite directions. Forces are equal for particles with a particular speed.

The spectrum of the light emerging from a cloud of cool gas is viewed using a diffraction grating. Explain why this spectrum contains a number of dark lines. [4]

Electrons in gas atoms interact with photons. These photon energy causes electron to move to higher energy level. The photon energy = difference in energy of energy levels. When electrons de-excite, photons emitted in all directions, so therefore dark line.

Use band theory to explain why, unlike a copper wire, the resistance of an intrinsic semiconductor decreases with an increase of temperature. [5]

Electrons need energy to enter conduction band from valence band. Positive holes are left in the valence band. Moving charge carriers are current. The increase in charge carriers gives rise to less resistance. The increase in temperature causes greater vibrations of atoms. The effect of more charge carriers is greater than the effect of greater vibrations and so resistance decreases.

Define specific latent heat of fusion. [2]

Energy per unit mass. Change of state from solid to liquid at constant temperature.

Suggest why, when a capacitor is connected across the terminals of a battery, the capacitor stores energy, not charge. [2]

Equal and opposite charges on the plates so no resultant charge. +ve and -ve charges separated so energy stored.

State one disadvantage of a greater bandwidth. [1]

Fewer possible stations in any frequency range.

State the relationship between electric potential at a point and the electric field strength at the point. [2]

Field strength = -potential gradient.

State Newton's law of gravitation. [2]

For point masses. Force is proportional to the product of masses and inversely proportional to the square of the separation.

State what is meant by electric field strength. [1]

Force per unit charge.

Define magnetic flux density. [3]

Force per unit current Force per unit length Current normal to magnetic field

State what is meant by gravitational field strength. [1]

Force per unit mass.

State Coulomb's law. [2]

Force proportional to product of point charges and inversely proportional to the square of their separation.

State what is meant by nuclear fusion and nuclear fission. [3]

Fusion: Two nuclei combine to form a single nucleus. Fission: A single large nucleus divides to form smaller nuclei.

State what is meant by infinite bandwidth. [2]

Gain is constant for all frequencies.

State how the hardness of an X-ray beam from an X-ray tube is increased. [1]

Greater accelerating potential difference.

Explain why in order for consistent measurements of magnetic flux density to be made, the current in the probe must be constant. [1]

Hall voltage depends on current in slice.

Explain why a Hall probe is made from a thin slice of material.

Hall voltage depends on thickness of slice. Therefore thinner slice, larger Hall voltage.

State what is meant by the hardness of an X-ray beam. [2]

Hardness is the penetration of beam. Greater hardness means greater penetration.

The fraction of the indecent intensity of an ultrasound beam that is reflected at a boundary between two media depends on the specific acoustic impedances Z₁ and Z₂ of the media. Discuss qualitatively how the relative magnitudes of the two specific acoustic impedances affect the reflected intensity. [2]

If Z₁ about equal to Z₂, it will result in negligible reflection. If Z₁ is very different from Z₂, it will result in mostly reflection.

A semiconductor has very high resistance in the darkness. Light incident on the semiconductor material causes its resistance to decreases. Explain the resistance of the semiconductor material in different light conditions. [5]

In the darkness the conduction band is empty so therefore a high resistance. In the light electrons in valence band absorb photons, causing the electrons 'jump' to conduction band this leaves holes in valence band. As there are now more charge carriers, resistance in the light decreases.

Explain the main principles of the generation of ultrasound waves for medical use. [4]

In the transducer there is a piezo-electric. An alternating p.d. is applied across crystal this causes crystal to resonate. When the crystal resonates it does so at ultrasound frequencies.

State Faraday's law of electromagnetic induction. [2]

Induced e.m.f. proportional to rate of change of magnetic flux linkage.

In a nuclear reaction, suggest the forms of energy into which the energy from the reaction is transformed. [2]

Kinetic energy of nuclei and γ-ray photon energy.

A relay is required as part of an output from an op-amp to a lamp. Explain why a relay is required. [2]

Lamp needs 'large' voltage, an op-amp can deliver only a small voltage.

State one advantage of a greater bandwidth. [1]

Less distortion.

The beam passes through some hydrogen gas. It then passes through a diffraction grating and an absorption spectrum is observed. All of the light absorbed by the hydrogen is re-emitted. Explain why dark lines are still observed in the absorption spectrum. [1]

Light is re-emitted in all directions, only part of the re-emitted light is in the direction of the beam.

State what is meant by the magnetic flux linkage of a coil. [3]

Magnetic flux density × cross-sectional area × number of turns on coil. For a magnetic flux density normal to cross-sectional area.

Describe the appearance of a visible line emission spectrum, as seen using a diffraction grating. [2]

Mostly dark background with coloured lines.

Suggest why, for small changes near the Earth's surface, gravitational potential is approximately constant. [2]

Near Earth's surface changes in height is much less than changes in the radius. The potential is inversely proportional to radius and radius approximately constant, so potential is approximately constant.

State what is meant by infinite slew rate. [2]

No time delay between change in input voltage and change output voltage.

State three further observations from photoelectric emission that provide evidence for a particulate nature of electromagnetic radiation. [3]

No time delay between illumination and emission. Max. energy of electron dependent on frequency. Max. energy of electron independent of intensity.

An electron is accelerated through a uniform magnetic field by potential difference *V*. The P.D is increases and the magnetic flux density *B* remains unchanged. By reference to the momentum of the electron, explain the effect of this increase on the radius *r* of the path of the electron in the magnetic field. [2]

P.D proportional to momentum, therefore greater momentum. Momentum proportional to radius, therefore greater radius.

What component goes before an optic fiber? [1]

Parallel-to-serial converter.

Explain the main principles behind the use of ultrasound to obtain diagnostic information about internal body structures. [6]

Pulses of ultrasound Reflect at the boundaries between media The reflection is then received by the ultrasound generator. Reflected wave processed and displayed The time between transmission and receiving gives information about the depth of structures The intensity of the reflection gives information about the type of structures A gel is used between the generator and the skin to minimise reflection at the surface.

Describe the motion of molecules in a gas, according to the kinetic theory of gases. [2]

Random motion, with constant velocity between collisions.

Explain why the core is laminated. [2]

Reduces size of eddy currents in core, so heating of core is reduced.

Suggest one similarity and one difference between the gravitational fields at the surface of a spherical comet and at the surface of a non-spherical comet. [2]

Similarity: Both attractive towards the comet Difference: Radial for spherical comet and non-radial for non-spherical comet.

State one similarity and one difference between the electric field lines and the gravitational field lines around an isolated positively charged metal sphere. [2]

Similarity: Lines are radial. Difference: Gravitational lines are directed towards the sphere and electric lines are directed away from the sphere.

Describe what is observed what viewing brownian motion that provides evidence for the motion of molecules in a gas. [2]

Small pollen grains moving randomly.

In an 𝛼-particle scattering experiment, the beam of 𝛼-particles is incent on a very think gold foil. What must the foil be very thin? [1]

So that single interactions between the nucleus and the 𝛼-particle can be studied.

Explain the main principles behind the use of nuclear magnetic resonance imaging (NMRI) to obtain information about internal body structures. [8]

Strong uniform magnetic field, causes nuclei to precess. Radio frequency pulse causes resonance of nuclei. On relaxation nuclei emit RF pulse, this RF pulse detected and processed. Non-uniform field superposed on uniform field. This allows positions of resonating nuclei to be determined.

By reference to the question, "Describe the basic principle of CT scanning" suggest why CT scanning was not possible before fast computers with large memories were available. [1]

The combining of images involves a very large number of calculations.

What is meant by a line of gravitational force? [1]

The direction of force on a small test mass.

Describe the photoelectric effect. [2]

The emission of electron when electromagnetic radiation is incident on surface of a material.

Explain why the path of the electron in a magnetic field is the arc of a circle. [3]

The force acts normal to velocity and the force constant, so provides a centripetal force.

Explain how a satellite may be in a circular orbit around a planet. [2]

The gravitational force provides a centripetal force.

By reference to lines of gravitational force near the Earth's surface, explain why gravitational field strength is approximately constant near the Earth's surface. [3]

The lines of force are radial, as the Earth has a large radius at the surface the lines are approximately parallel. Parallel lines show a constant field strength.

By reference to intensity of light, state one piece of evidence provided by the photoelectric effect for a particulate nature of light. [1]

The maximum energy of the emitted electrons is independent of the intensity.

In the equation, ½m<c²>=(3⁄2)kT, What is meant by: <c²>. [1]

The mean square speed

Define the binding energy of a nucleus. [2]

The minimum work done to separate the nucleons in a nucleus to infinity.

Define specific acoustic impedance. [2]

The product of density and speed of ultrasound in medium.

What is meant by bandwidth? [1]

The range of frequencies of a signal.

State what is meant by radioactive decay. [3]

The spontaneous release of photons or particles from an unstable nucleus.

State the effect on the transmitted analogue signal when the number of bits in each sample is increased. [1]

The step height is reduced.

State the effect on the transmitted analogue signal when the sample rate of the analogue-to-digital converter (ADC) and of the DAC is increased. [1]

The step width is reduced.

State what is meant by the internal energy of a system. [2]

The sum of potential and kinetic energies of molecules undergoing random motion.

In the equation, ½m<c²>=(3⁄2)kT What is meant by: T. [1]

The thermodynamic temperature

The maximum energy *E* of electrons emitted from a metal surface when illuminated by light of wavelength λ is given by the expression. E = hc (1/λ- 1/λₒ) Identify the symbol λₒ. [1]

The threshold wavelength.

For a planet in orbit with a constant speed. Explain whether it is in equilibrium. [1]

The velocity and direction of motion changes so there is a resultant force therefore not in equilibrium.

Use one of the assumptions of the kinetic theory of gases to explain why the potential energy of the molecules of an ideal gas is zero. [1]

There are no intermolecular forces so therefore no potential energy.

Explain the function of the non-uniform magnetic field superposed on the large constant magnetic field in diagnosis using NMRI. [3]

This gives a unique magnetic field strength at each point. Resonant frequency depends on magnetic field strength, therefore this enables nuclei to be located.

Explain why the core of a transformer is made of iron. [1]

To increase flux linkage.

Suggest why frequencies of the order of gigahertz are used for satellite communication. [1]

To minimise attenuation by atmosphere.

Describe the function of a ADC. [2]

To sample the analogue signal at regular intervals and converts the analogue number to a digital number.

Suggest why the uplink frequency is different from the downlink frequency. [2]

Uplink signal is greatly attenuated, so different fequencies prevents downlink signal swamping the uplink signal.

State the direction of the orbit about the Earth, for a geostationary orbit. [1]

West to East.

Describe, with a labelled diagram, the structure of a metal-wire strain gauge. [3]

Wire in a 'grid' shape. The wire in very fine. There's insulating envelope containing the wire.

State what is meant by gravitational potential at a point. [2]

Work done per unit mass in moving the mass from infinity to the point.

Define electric potential at a point. [2]

Work done per unit positive in charge moving the positive charge from infinity to the point.

Describe the basic principle of CT scanning. [5]

X-ray image taken of one slice Many images taken from different angles This produces 2D image of slice This is repeated for many slices This builds up a 3D image of structure