Edexcel Physics Alevel Definitions

Accuracy

How close a measurement is to its true value, influenced by the systematic and random errors of that measurement.

Base Units

The set of seven basic measures from which all other SI units can be derived.

Estimation

Making a reasonable approximation of a value in order to check a calculation or make a quick comparison to another value.

Precision

How close a set of repeated measurements are to one another but not the true value, influenced by the random errors of those measurements.

Random Error

The unpredictable variation in a measurement. These can be reduced by taking many repeated measurements and calculating their mean.

Repeatability

An experiment is said to be repeatable if the same person with the same equipment obtains the same result when doing the same experiment a number of times, over a short time period.

Reproducibility

An experiment is said to be reproducible if different people with different equipment, measuring the same quantity, get a similar result.

Resolution

​The smallest interval that a given measuring device can measure.

Systematic Error

A consistent shift in readings causing a deviation from the true value. This shift is due to the equipment or method being used and cannot be reduced by repeated measurements.

Uncertainty

The range of values that could reasonably contain the true value of a measurement, based on the confidence an experimenter has about their result.

Centre of Mass

The average point of all parts of an object weighted according to their mass. It is the point through which the total mass of the object is said to act.

Conservation of Energy

​In a closed system with no external forces the energy of the system before an event is equal to the energy of the system after the event. The energy does not need to be in the same form after the event as it was before the event.

Conservation of Momentum

​In a closed system with no external forces the momentum of the system before an event is equal to the momentum of the system after the event.

Drag

The resistance against the motion of an object through a fluid. It is usually proportional to the speed of the object.

Efficiency

The useful output (e.g. power, energy) of a system divided by the total output.

Elastic Collision

When the kinetic energy of a system before an event is equal to the kinetic energy of the system after the event.

Equilibrium

An object is at equilibrium when the moments on it about a point are balanced and the resultant force on the object is zero.

Force

The rate of change of momentum of an object. The product of the object's mass with its acceleration.

Friction

The resistance against the motion of an object. This could be caused by the air passing over the object or by the contact of the object with the surface it is moving on.

Gravitational Potential Energy

The energy gained by an object when it is raised by a height in a gravitational field.

Impulse

The change of momentum of an object when a force acts on it. Equal to the area underneath a force-time graph.

Inelastic Collision

When the kinetic energy of a system before an event is not​​equal to the kinetic energy of the system after the event. The kinetic energy has been transferred to other forms.

Kinetic Energy

The energy an object has due to its motion. It is the amount of energy that would be transferred from the object when it decelerates to rest.

Lift

A force acting perpendicular to the flow of air or liquid around an object, typically upwards and against the force of gravity.

Moment

The product of a force and the perpendicular distance from the line of action to the pivot about which the force is acting.

Momentum

The product of an object's mass and its velocity.

Newton's First Law

​An object at a constant velocity will remain at a constant velocity unless acted on by a resultant force. (If this constant velocity is zero the object is at rest).

Newton's Second Law

If an object is acted upon by a resultant force it will accelerate. The acceleration is inversely proportional to the mass of the object and directly proportional to the force acting upon it.

Newton's Third Law

Every action has an equal and opposite reaction. If an object A exerts a force on object B, object B will exert a force of equal magnitude but of opposite direction on object A.

Power

The work done by a system divided by the time taken for that work to be done.

Resultant Force

The sum of all forces on an object

Scalar

A quantity with only magnitude and no direction (e.g. mass, energy, length).

Terminal Velocity

The maximum velocity an object can achieve. It is the point at which frictional forces and driving forces are balanced and so no acceleration occurs and the resultant force on the object is 0 N.

Vector

A quantity with magnitude and direction (e.g. velocity, acceleration, force).

Weight

The force of gravity on an object, the product of the object's mass and the acceleration due to gravity.

Work Done

A force applied over a distance, it is the energy transferred in that distance.

Conventional Current Flow

Flow from positive to negative, used to describe the direction of current in a circuit.

Current

​The rate of flow of charge in a circuit, measured in Amperes and has symbol I.

Detecting Circuit

A circuit with a potential divider with one of the resistors being a semiconductor which when an external condition changes will change its resistance and change the voltage across the other resistor in the potential divider.

Diode

Components that allow current through in one direction. In the correct direction, diodes have a threshold voltage (typically 0.6 V) after which current flows normally.

Electromotive Force (EMF)

The energy supplied by a source per unit charge passing through the source, measured in volts.

Electron Flow

The flow of electrons in a circuit, from negative to positive.

Internal Resistance

The unavoidable resistance any power source will have that makes it harder for current to flow through the source. It causes energy to be dissipated in the source.

Kirchoff's First Law

The total current entering a junction is equal to the total current leaving it.

Kirchoff's Second Law

The sum of EMF in any loop of the circuit is equal to the sum of the potential differences of each component.

Light Dependent Resistors (LDR)

When these components are illuminated with light their resistance goes down. As light intensity increases resistance decreases.

Ohmic Conductor

A conductor following Ohm's law where current flowing through it is directly proportional to the potential difference between each end of the conductor. This only holds if the conductor is kept at a constant temperature.

Ohm's Law

Electric current is proportional to potential difference and inversely proportional to resistance.

Parallel Connection

When two electrical components are on separate loops to one another in a circuit. Potential difference over each loop is the same, current is split between branches.

Potential Difference

The difference in electrical potential between two points in a circuit and the work done that is required per coulomb to move a charge from the lower potential point to the higher potential point. It is measured in Volts.

Potential Divider

A combination of two or more resistors in series. These result in the potential difference of the circuit being split into a specific ratio depending on the resistance of the resistors. They can be used to get a specific output voltage from the circuit.

Power

Rate of energy transfer in a circuit. It can be calculated as the product of the current and the potential difference between two points. Measured in Watts.

Resistance

A measure of how difficult it is for current to flow in a circuit or component, measured in Ohms and has symbol R.

Resistivity

​A measure of how difficult it is for charge to travel through a material, depending on the material's cross sectional area, length and resistance. It is measured in Ohm metres and has symbol ⍴.

Semiconductors

Materials that change their resistance depending on external conditions.

Series Connection

When two electrical components are on the same loop to one another in a circuit. Potential difference is split between components depending on their resistance, current is the same across all components.​

Superconductor

​A material that has zero resistivity below a critical temperature. They are used for very efficient electricity transmission or to create very strong magnetic fields but require extreme cooling to reach their critical temperatures.

Terminal Potential Difference

The actual potential difference across the terminals of a power source. It is the source's EMF minus the voltage drop due to the source's internal resistance.

Thermistor

When these components are heated up their resistance goes down. As temperature increases resistance decreases.

Variable Resistors

A resistor that can have its resistance changed. Typically, these are used with power supplies to change the voltage of a circuit without using a transformer.

Voltmeter

A device used to measure the potential difference between two points on a circuit, ideally it has infinite resistance so no current passes through it.

Archimedes' Principle

When a body is fully or partially submerged in a fluid, it experiences an upthrust equal to the weight of the fluid it has displaced.

Breaking Stress

The maximum stress a material can withstand without fracturing.

Brittle

A material that fractures without plastic deformation first.

Density

Mass per unit volume, with units kgm​⁻³

Ductile

​A material that can withstand large plastic deformation without breaking, this allows these materials to be stretched into long wires.

Elastic Deformation

When a material can return to its original shape, after an applied stress, without a permanent change to its shape. Any work done in elastically deforming an object is stored as elastic strain energy.

Elastic Limit

The maximum stress that can be applied to an object without plastic deformation.

Hooke's Law

The extension of an object is directly proportional to the force being applied to the object.

Laminar Flow

A state of flow where layers of fluid move together in parallel with little or no mixing between layers.

Limit of Proportionality

The point at which the stress on an object is so great that Hooke's law no longer applies to an object.

Plastic Deformation

When a material is permanently deformed after an applied stress due to the atoms moving relative to one another in the material. Work is done in plastically deforming the material and is dissipated as heat.

Stoke's Law

The magnitude of the force of viscosity acting on a spherical body as it moves through a fluid is proportional to its radius, its velocity and the fluid's viscosity. This only applies to spherical bodies travelling in laminar, non-turbulent, flow.

Tensile Strain

The extension of an object divided by its original length.

Tensile Stress

The internal resistance of an object against a force that acts to deform it. It is the force applied per unit cross-sectional area.

Turbulent Flow

A state of flow where layers of fluid mix together unpredictably causing a chaotic state.

Viscosity

A quantity measuring the internal friction of a fluid, it acts to reduce the flow of a fluid. It is temperature dependent.

Upthrust

The force felt against the weight of an object when fully or partially submerged in a fluid. It is equal to the weight of the fluid displaced by the object.

Yield Point

The point on a force-extension graph at which a material begins to rapidly extend without any additional stress.

Young's Modulus

​The ratio of stress to strain of an object. It is a measure of how stiff a material is.

LightAbsorption Spectrum

The frequencies at which a certain element absorbs photons at a higher rate as these frequencies correspond to the spacings between energy levels in the element's atoms.

Amplitude

The maximum displacement of a vibrating particle or wave from its equilibrium position.

Angle of Incidence

The angle at which a light ray hits a medium. Measured from a line perpendicular to the surface of the medium.

Angle of Reflection

The angle at which a light ray reflects off a surface. Measured from a line perpendicular to the surface of the medium.

Angle of Refraction

The angle at which light rays travel after transferring into a different refractive index material. Measured from a line perpendicular to the surface of the medium.

Antinode

The point on a stationary wave where the incoming and reflected wave are in phase forming a maximum point.

Coherence

Waves with the same frequency and constant phase difference.

Constructive Interference

Interference when the two waves are in phase. If the two waves are both at their peak the addition of the two peaks becomes large.

Converging Lens

A lens that takes a parallel set of light rays and causes them to meet at a point.

Critical Angle

​The angle of incidence when the angle of refraction is exactly 90​ degrees​. It is when the refracted ray travels along the boundary line.

De Broglie Hypothesis

All particles have a wave-like nature and a particle-like nature. The wavelength of a particle is inversely proportional to the particle's momentum.

De-excitation

​The movement of an electron from a high energy level to a lower energy level. This occurs in excited atoms over time and causes photons to be released.

Destructive Interference

Interference when the two waves are in antiphase. When one wave is at a peak and one is at a trough their addition results in a minimum point.

Diffraction

​The spread of a wave as it passes through a gap or over an edge.

Displacement (Waves)

The distance and direction that a vibrating particle or wave has travelled from its equilibrium position.

Diverging Lens

​A lens that takes a parallel set of light rays and causes them to travel away from each other and not meet.

Electronvolt

The kinetic energy gained by one electron that is accelerated through a potential difference of 1 volt. Equal to 1.6x10​⁻¹⁶​ J

Emission Spectrum

When a certain element's atoms de-excite they move from one energy level to another with a specific energy gap between these levels. This creates a photon of that specific energy and it is the frequencies of these photons that make up the emission spectrum.

Excitation

The movement of an electron from a low energy level to a higher energy level. This occurs when another electron or photon transfers energy to an orbital electron causing it to move to a higher energy level.

Focal Length

The distance from the centre of the lens to the focal point.