CIE Physics A-Level Definitions

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

The direction of force on a small test mass.

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.

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.

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.

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

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.

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.

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

and state its unit

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

Weber (Wb) (Tm²)

Describe the photoelectric effect. [2]

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

State what is meant by a photon. [2]

A discrete amount of energy of electromagnetic radiation.

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

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.

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.

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.

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.

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

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

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.

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

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

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

West to East.

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

Above the Equator.

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

Force per unit positive charge.

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

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

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.

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

Difference in the degrees of blackening between the structures.

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.

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.

Define electric potential at a point. [2]

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

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 radioactive decay. [3]

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

State what is meant by gravitational field strength. [2]

gravitational force per unit mass in bringing the mass from infinity to a point

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.

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

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

Define specific latent heat of fusion. [2]

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

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

Field strength = -potential gradient.

Define magnetic flux density. [3]

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

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

Mostly dark background with coloured lines.

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

Fusion: Two smaller and lighter nuclei combine to form a single larger and heavier nucleus.
Fission: A single large and heavy nucleus divides to form two smaller nuclei of approximately the same size.

State the first law of thermodynamics. [2]

it can be represented by the expression: ΔU = q + w.
where:

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

(the increase in internal energy of a system is equal to the thermal energy transferred to the system plus the work done on the system)

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.

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

To increase flux linkage.

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.

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.

Define the binding energy of a nucleus. [2]

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

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

The gravitational force provides a centripetal force.

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

Random motion, with constant velocity between collisions.

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 incident on a very thin gold foil. Why must the foil be very thin? [1]

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

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.

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.

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

Core reduces loss of magnetic flux linkage.

Explain why the core is laminated. [2]

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

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.

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 photons energy causes electrons to move to higher energy levels.

The photon energy = difference in energy of energy levels.

When electrons de-excite, photons emitted in all directions, so therefore dark lines form

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.

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 Coulomb's law. [2]

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

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.

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.

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.

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

(or instantanous emission of electrons)

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.

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

A wavelength associated with a moving particle

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

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

Discrete and equal amounts of charge.

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.

Explain why emitted electrons may have kinetic energy less than the maximum at any particular frequency. [2]

Interaction with electron may be below surface energy required to bring electron to surface.

A microwaves warm the food in the cooker by causing water molecules in the food to oscillate with a large amplitude at the frequency of the microwaves. State the name given to this phenomenon. [1]

Resonance.

Define the radian. [2]

The angle subtended at the centre of a circle by an arc equal in length to the radius.

Define radioactive half-life. [2]

The time for number of atoms of the isotope to be reduced to one half.

(the time takes for the activity to halve)

Distinguish between an X-ray image of a body structure and a CT scan. [5]

X-ray: Flat, 2D image, does not show depth.
CT scan: Built up from images at different angles. The final image is three-dimensional. The image can be viewed at different angles.

Explain what is meant by a geostationary satellite. [3]

Satellite is in equatorial orbit.
Travelling from west to east.
Period of 24 hours.

State what may be deduced from the difference in the temperatures of two objects. [1]

The rate of transfer of thermal energy.

State the basic principle by which temperature is measured. [1]

Uses a property of the substance that changes with temperature.

Explain why two thermometers may not give the same
temperature reading for an object. [2]

Temperature scale assumes linear change of measured properties with temperature. So the thermometers may agrees only at fixed points.

State, by reference to simple harmonic motion, what is meant by angular frequency. [1]

Angular frequency = 2π × Frequency

Suggest why the Hall voltage is difficult to detect in a thin slice of copper. [2]

n is very large, therefore V is small.

The mean value of an alternating current is zero.
Explain why heating occurs when there is an alternating current in a resistor. [2]

Heating depends on I² and I² is always positive.

Transmission of electrical energy is frequently achieved using alternating high voltages. Suggest why high voltages are used. [2]

In this power transmission, higher voltage → lower current. And lower current → less power loss in cables.

Transmission of electrical energy is frequently achieved using alternating high voltages. Suggest why the voltage is alternating. [2]

Transformers only work with A.C.
Therefore voltage can be stepped up/down.

A radiation detector is placed close to a radioactive source. The detector does not surround
the source. Radiation is emitted in all directions and, as a result, the activity of the source and the measured count rate are different.
Suggest two other reasons why the activity and the measured count rate may be different. [2]

1. Emission from radioactive daughter products.
2. Absorption in air before reaching detector.

A sample of a isotope has readings obtained at room temperature. A second sample of this isotope is heated to a temperature of 500°C. The initial count rate at time t = 0 is the same. The variation with time t of the measured count rate from the heated source is determined.
State, with a reason, the difference, if any, in the half-life. [2]

No change because decay is spontaneous.

A sample of a isotope has readings obtained at room temperature. A second sample of this isotope is heated to a temperature of 500°C. The initial count rate at time t = 0 is the same. The variation with time t of the measured count rate from the heated source is determined.
State, with a reason, the difference, if any, in the measured count rate for any specific time. [1]

Count rate could be different but is random so cannot be predicted.

Suggest one advantage of the use of high-frequency ultrasound rather than lower-frequency ultrasound. [1]

Small structures can be observed.

For any point outside a spherical conductor, the charge on the sphere may be considered to act as a point charge at its centre. By reference to electric field lines, explain this. [2]

Electric field lines are normal to surface of sphere. Electric field lines appear to originate from centre of sphere.

Explain why a constant hall voltage is developed between two faces of a metal. [4]

Charge carriers moving normal to magnetic field.Charge carriers experience a force normal to current.
Charge build-up sets up electric field across the slice.
Charge stops building up when F_B = F_E.

Two slices have similar dimensions. One slice is made of a metal and the other slice is made of a semiconductor material. For the same values of magnetic flux density and current, state which slice, if either, will give rise to the larger Hall voltage. Explain your reasoning. [2]

Hall voltage inversely proportional to number density of charge carriers. Number density of charge carrier lower in semiconductors so
hall voltage larger for semiconductor slice.

State what is meant by a field of force. [2]

A region of space where a particle experiences a force.

State what is meant by radioactive. [2]

an unstable nucleus emits radiation randomly and spontaneously

State what is meant by decay constant. [2]

Probability of decay per unit time.

Explain why the emitted β^- particles have a range of energies. [2]

Energy is shared with antineutrino.

Suggest why the electric field strength in a charged sphere is zero. [2]

In an electric field, charges would move. No movement of charge so zero field strength.

State the type of field, or fields, that will give rise to a force acting on a moving uncharged particle. [1]

Gravitational.

State the type of field, or fields, that will give rise to a force acting on a stationary charged particle. [1]

Gravitational and electric.

State the type of field, or fields, that will give rise to a force acting on a charged particle moving at an angle to the field or fields. [1]

Gravitational, electric and magnetic.

define angular velocity [2]

the rate of change of angular displacement swept out by radius

define gravitational field [2]

a region of space where a unit mass experiences a gravitational force

define amplitude [1]

maximum displacement from the equilibrium position

what is damping [2]

the loss of energy due to the resistive forces which causes the amplitude to decrease gradually

define resonance

System forced to vibrate close to its natural frequency, the amplitude of vibration increases rapidly. Frequency of external force equals to the natural frequency, and amplitude is maximum.