IB Physics SL Definitions

Fundamental Units

seven basic units of the SI measurement system: kilogram, second, mole, meter, ampere, Kelvin, candela.

Derived Units

units that are combinations of fundamental units. These combinations may or may not have a separate name. (eg. 1 kg m/s2 = 1 N)

Accuracy

An indication of how close a measurement is to the accepted value (a measure of correctness).

Precision

An indication of the agreement among a number of measurements made in the same way (a measure of exactness).

Random Uncertainty

An uncertainty produced by unknown and unpredictable variations in the experimental situation, such as temperature fluctuations and estimations when reading instruments. (Affects the precision of results - Can be reduced by taking repeated trials but not eliminated - shows up as error bars on a graph)

Systematic Error

An error associated with a particular instrument or experimental technique that causes the measured value to be off by the same amount each time. (Affects the accuracy of results - Can be eliminated by fixing source of error - shows up as non-zero y-intercept on a graph)

Vector

a quantity with both a magnitude and a direction

Scalar

a quantity with magnitude only

*Displacement (s)

distance traveled in a particular direction (change in position)

*Velocity (u,v)

rate of change of displacement

*Speed (u,v)

rate of change of distance

*Acceleration (a)

rate of change of velocity

*Newton's First Law of Motion

An object at rest remains at rest and an object in motion remains in motion at a constant speed in a straight line unless acted on by an unbalanced force.

*Newton's Second Law of Motion

An unbalanced force will cause an object to accelerate in the direction of the net force. The acceleration of the object is proportional to the net force and inversely proportional to its mass. (F¬net = ma or Fnet = Δ p/Δ t (net force = rate of change of momentum))

*Newton's Third Law of Motion

When two bodies A and B interact (push or pull), the force that A exerts on B is equal and opposite to the force that B exerts on A.

Translational Equilibrium

net force acting on a body is zero

*Linear Momentum (p)

product of mass and velocity

*Impulse (J)

change in momentum

*Law of Conservation of Linear Momentum

The total momentum of an isolated system (no external forces) remains constant.

*Work (W)

The product of a force on an object and the displacement of the object in the direction of the force.

Kinetic Energy (EK)

product of ½ times the mass of an object times the square of an object's speed

Change in Gravitational Potential Energy

product of an object's mass times the gravitational field strength times the change in height

*Principle of Conservation of Energy

The total energy of an isolated system (no external forces) remains constant. (OR - Energy can be neither created nor destroyed but only transformed from one form to another or transferred from one object to another.)

*Elastic Collision

a collision in which kinetic energy is conserved

Inelastic Collision

a collision in which kinetic energy is not conserved

*Power (P)

The rate at which work is done or the rate at which energy is transferred.

*Efficiency (eff)

The ratio of the useful energy (or power or work) output to the total energy (or power or work) input.

*Newton's Universal Law of Gravitation

The force of gravity between two objects is directly proportional to the product of the two masses and inversely proportional to the square of the distance between them and acts along a line joining their centers. (NOTE: The objects are point masses. If they are not point masses but are very far apart, that is, the distance between them is very much greater than their radii, they can be treated like point masses.)

*Gravitational Field Strength (g)

gravitational force per unit mass on a point mass (g = Fg / m)

*Gravitational Potential Energy (EP)

the work done in moving a mass from infinity to a point in space (NOTE: the work done is path independent)

*Gravitational Potential (V)

the work done per unit mass in moving a mass from infinity to a point in space

Equipotential Surface

every point on it has the same potential

*Escape Speed

minimum speed of a rocket needed to escape the gravitational attraction of a planet

*Kepler's Third Law

the ratio of the orbital period squared to the average orbital radius cubed is constant for all planets

Weightlessness in free-fall

a sensation of weightlessness because a person is falling freely toward the Earth, hence there is no normal force (reaction force) acting on the person due to gravity

Weightlessness in orbital motion

a sensation of weightlessness due to the spacecraft and all objects in it being in constant free-fall together as they circle Earth

Weightlessness in deep space

a sensation of weightlessness due to the minimal pull of gravity very far from any massive object

*Temperature (T)

The property that determines the direction of thermal energy transfer between two objects. A measure of the average random kinetic energy of the particles of a substance.

Thermal Equilibrium

two objects are in thermal equilibrium when they are at the same temperature so that there is no transfer of thermal energy between them

*Internal Energy of a substance (U)

The total potential energy and random kinetic energy of the molecules of the substance.

*Thermal Energy (Heat) (Q)

Energy transferred between two substances in thermal contact due a temperature difference.

*Mole

An amount of a substance that contains the same number of atoms as 0.012 kg of 12C.

*Molar Mass

The mass of one mole of a substance.

*Avogadro constant (NA)

The number of atoms in 0.012 kg of 12C ( = 6.02 x 1023).

*Thermal Capacity (C)

energy required to raise the temperature of a substance by 1K

*Specific Heat Capacity (c)

energy required per unit mass to raise the temperature of a substance by 1K

Boiling

a phase change of a liquid into a gas that occurs at a fixed temperature

Evaporation

when faster moving molecules have enough energy to escape from the surface of a liquid that is at a temperature less than its boiling point, leaving slower moving molecules behind which results in a cooling of the liquid

*Specific Latent Heat (L)

energy per unit mass absorbed or released during a phase change

*Pressure (P)

force per unit area acting on a surface

*Ideal Gas

a gas that follows the ideal gas equation of state (PV = nRT) for all values of P, V, and T (an ideal gas cannot be liquefied)

Real Gas

a gas that does not follow the ideal gas equation of state for all values of P, V, and T (a real gas can approximate an ideal gas in some circumstances)

Absolute Zero of Temperature

temperature at which a gas would exert no pressure

Kelvin scale of Temperature

an absolute scale of temperature in which 0 K is the absolute zero of temperature

*First Law of Thermodynamics (U = ΔU + W)

The thermal energy transferred to a system from its surroundings is equal to the work done by the system plus the change in internal energy of the system. (an application of the principle of conservation of energy)

Isochoric (Isovolumetric)

a process that occurs at constant volume (ΔV = 0)

Isobaric

a process that occurs at constant pressure (ΔP = 0)

Isothermal

a process that occurs at constant temperature (ΔT = 0)

Adiabatic

a process that occurs without the exchange of thermal energy (Q = 0)

*Entropy

a system property that expresses the degree of disorder in the system

*Second Law of Thermodynamics

The overall entropy of the universe is increasing. (OR - All natural processes increase the entropy of the universe.) (NOTE: The second law implies that thermal energy cannot spontaneously transfer from a region of low temperature to a region of high temperature.)

*Displacement (for waves)

distance a particle moves in a particular direction from its mean (equilibrium) position

*Amplitude

maximum displacement from the mean position

*Frequency (f)

number of oscillations per unit time

*Period (T)

time taken for one complete oscillation (cycle) (OR: time taken for one cycle to pass a given point)

*Phase Difference

difference in phase between two points

*Simple Harmonic Motion

motion that takes place when the acceleration of an object is proportional to its displacement from its equilibrium position and is always directed toward its equilibrium position (NOTE: this motion is defined by the equation a = -ω2x)

Damping

involves a force that is always in the opposite direction to the direction of motion of the oscillating particle (NOTE: this force is a dissipative force)

Critical Damping

when a resistive force is applied to an oscillating system that causes the particle to return to zero displacement in a minimum amount of time

Natural Frequency of Vibration

when a system is displaced from equilibrium and allowed to oscillate freely, it will do so at its natural frequency of vibration

Forced Oscillations

a system may be forced to oscillate at any given frequency by an outside driving force that is applied to it

*Resonance

a transfer of energy in which a system is subject to an oscillating force that matches the natural frequency of the system resulting in a large amplitude of vibration

*Wave Pulse

single oscillation or disturbance in a medium

*Continuous Progressive (Traveling) Wave

series of periodic pulses (NOTE: involves a transfer of energy) (NOTE: each point on the wave has the same amplitude)

*Transverse Wave

wave in which the direction of motion of the energy transfer (the wave) is perpendicular to the direction of motion of the particles of the medium (NOTE: light waves are transverse) (NOTE: transverse waves cannot be propagated in gases)

*Longitudinal Wave

wave in which the direction of motion of the energy transfer (the wave) is parallel to the direction of motion of the particles of the medium (NOTE: sound waves are longitudinal)

Wavefront

collection of neighboring points on a wave that are in phase

Ray

line drawn perpendicular to a wavefront indicating the direction of motion of the energy transfer

Crest

top of a transverse wave

Trough

bottom of a transverse wave

Compression

area of high pressure in a longitudinal wave

Rarefaction

area of low pressure (expansion) in a longitudinal wave

*Wavelength (λ)

shortest distance along the wave between two points in phase with one another (OR: distance traveled by the wave in one period)

*Wave Speed (v)

speed of transfer of the energy of the wave

*Intensity (I)

power received per unit area (NOTE: for a wave, its intensity is proportional to the square of its amplitude)

Law of Reflection

The angle of incidence is equal to the angle of reflection when both angles are measured with respect to the normal line

*Snell's Law

The ratio of the sine of the angle of incidence to the sine of the angle of refraction is a constant, for a given frequency.

*Refractive Index (Index of Refraction) (n)

a. the ratio of the speed of the wave in the refracted medium to the speed of the wave in the incident medium b. the ratio of the sine of the angle of incidence to the sine of the angle of refraction

Diffraction

the bending of a wave around an obstacle or the spreading of a wave through an opening (NOTE: diffraction is only noticeable when the size of the opening is smaller than or on the same order of the size of the wavelength)

*Principle of Superposition

When two waves meet, the resultant displacement is the vector sum of the displacements of the component waves.

Constructive Interference

superposition of two waves which are in phase with each other

Destructive Interference

superposition of two waves which are out of phase with each other

Path Difference

difference in the distances two waves must travel from their sources to a given point

Standing (stationary) wave

resultant wave formed when two waves of equal amplitude and frequency traveling in opposite directions in the same medium interfere (NOTE: does not involve a transfer of energy) (NOTE: points on the wave have varying amplitudes)

*Node

locations of constant complete destructive interference on a standing wave