Physics HL Key concepts

velocity

rate of change of position with time

speed

rate of distance travelled along a path

acceleration

rate of change of velocity with time

translational equilibrium

a body in equilibrium has zero resultant force acting on it and therefore has zero acceleration

conservation of energy

The total energy of an isolated system can not be created or destroyed only converted from one form to the other.

Newton’s first law

A body at rest remains at rest, and a body in motion will remain in motion at a constant velocity unless acted upon by an unbalanced force.

Newton’s second law

The acceleration of an object depends on the mass of the object and the amount of force applied.

Newton’s third law

Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first.

Linear momentum

The product of a body’s mass and its velocity p =mv

Impulse

The change in momentum of a body. ∆p = mv - mu

law of conservation of momentum

If the total external force acting upon a system is zero, the momentum of the system is constant.

Work

Work is the ability to exert a force causing displacement of an object. W = fs

Power

The rate of work. P= work/time

Energy

Ability to do work

Kinetic energy

The energy an object has due to its translational motion.

Potential energy

Energy stored by a system due to the relative positions of its component parts. E = mgh (cannot be used for objects in space)

Elastic collision

A collision in which the total kinetic energy is conserved.

Inelastic collision

A collision in which some kinetic energy is transferred to other forms (eg internal energy, sound), therefore the total kinetic energy is less after the collision than before.

Gravitational field strength (g)

Force per unit mass experienced by a small point mass at the point. g = GM/r²

Weight

Force due to gravity

Universal law of gravitation

Every single point mass attracts every other point mass with a force that is directly proportional to the product of the masses and inversely proportional to the square of the radius. Gravitational force between two objects (F) = Gm₁m₂/r²

Test mass

A small mass which has a negligible effect on the gravitational field in which it is placed.

Uniform gravitational field

Where all field lines are always the same distance apart, almost exactly true close to the earths surface. Field is always in the same direction and magnitude.

Field lines

The strength of the field decreases as the lines get further apart as what happens when you get further from Earth.

Gravitational potential energy

The work done to move a body from infinity to a point a gravitational field. E_p = -Gm₁m₂/r

Escape speed

Speed of object at the surface of a planet so that it will escape from the gravitational field and travel to infinity.

Keplers laws

1. Planets orbit the sun in elliptical paths

2. A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time.

3. The square of the orbital period is directly proportional to the cube of the semi-major axis of its orbit. T² =kr³

Keplers second law in depth

A line segment joining a planet and the Sun sweeps out equal areas during equal intervals of time. As the orbiting object approaches what it is orbiting, its potential energy will convert into kinetic energy so its speed will increase. The orbiting body then covers an equal area in an equal amount of time.

Deriving keplers third law

Using Newton’s law of gravitation and circular motion: mω²r=GM₁M₂/r² and ω=2π/T

T²/r³ = 4π²/GM

Energy of an orbiting body

E_k=GMm/2r = 1/2mv²

E_p= -GMm/r

total energy = E_p + E_k = -GMm/r + GMm/2r = -GMm/2r

Brownian motion

Any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations. Average value.

Temperature

Measure of the average kinetic energy of molecules, the natural flow of thermal energy is from high to low temperature.

Thermal energy

Thermal energy is the kinetic energy of the component particles of an object measured in jeaules.

Heat

Energy transferred from one body to another due to a temperature difference.

Thermal equilibirum

2 bodies that are in thermal contact are in thermal equilibrium when the net heat flow between them is zero, therefore the 2 bodies must have the same temperature.

Microscopic

On the scale of atoms and molecules (eg. particle mass, velocity, kinetic energy, momentum)

Macroscopic

On the scale of people (eg, temperature, volume, pressure, density)

Ideal gas

A gas that obeys the equation pV=nRT with the assumptions:

• molecules are identical spheres of negligible volume (their total volume is much smaller than the volume of the container)

• collisions are perfectly elastic

• no forces between the molecules (except when they collide) so they move at constant velocity between collisions

Ideal gas assumptions

• the collisions between molecules are perfectly elastic

• the molecules are spheres

• the molecules are identical

• there are no forces between the molecules (except when they collide) so they have constant velocity between collisions

• the molecules are very small, their total volume is much smaller than the volume of the container

Kinetic theory of gases

A model of the microscopic behvaior of gas particles that explains the macroscopic behavior of the gas.

Internal energy (U)

The sum of all random kinetic energies and mutual potential energies of the particles of the body or system. Does not include the kinetic and potential energy of the body as a whole. An ideal gas has no intermolecular forces (no mutual potential energies) therefore the ideal gas depends only on the kinetic energy of the particles.

Mole

Amount of substance of a system which contains as many elementary units as there are carbon atoms in 12×10^-3 kg of Carbon-12. 6.02 × 10^-23.

Molar mass

The mass of one mole of a substance.

Specific heat capacity

The amount of energy required to raise the temperature of unit mass through 1K.

Heat (thermal) capacity

The amount of energy required to raise the temperature of a substance through 1K.

Evaporation

The escape of molecules from the surface of the liquid.

Boiling

Occurs when molecules escape in the form of bubbles of vapour from the body of the liquid.

Specific latent heat

Energy per unit mass required to change the phase of a substance at its phase change temperature.

Pressure

The force per unit area exerted by something on the surface of a body. p = F/A

Indicator diagram

Graph of pressure against volume for agas

Isochoric (isovolumetric)

A process where the volume remains constant, therefore no work is done.

Isobaric

A process where the pressure remains constant

Isothermal

A process where the temperature remains constant, therefore the internal energy remains constant and change in U = 0

Adiabatic

A process in which there is no energy exchange between the system and surrounding during a compression or expansion of a gas. Work done is equal to the internal energy of the gas.

Entropy

Measure of disorder of a system

1st Law of Thermodynamics

Energy cannot be created or destroyed

2nd Law of Thermodynamics

In any process the overall entropy of the universe/a closed system increases.

Displacement (x) waves

Distance in a particular direction of a particle from its mean position

Amplitude (x₀)

Magnitude of the maximum displacement from the equilibrium position.

Frequency (f)

Number of oscillations per unit time.

Period (T)

Time taken for one complete oscillation

Monochromatic

Single frequency/color

Simple harmonic motion

The motion that takes place when the acceleration of an object is always directed to the equilibrium position and is proportional to the displacement from a central position.

Damping

The process whereby energy is taken from the oscillating system (usually due to friction). Decreases the amplitude and there is a small decrease in resonant frequency.

Over-damped

The system tends to equilibrium slowly

Critically damped

The mass returns to the equilibrium position as quickly as possible without crossing it

Natural frequency

The frequency at which a system oscillates when disturbed from its equilibrium state.

Resonance

Maximum amplitude of oscilation when a periodic force is applied to it and the frequency of the force is equal to the natural frequency of vibration of the system.

Wavefront

Line joining neighbouring points that have the same phase/displacement

Ray

direction in which the wave energy is travelling

Travelling (progressive) wave

A wave that transfers energy between points in a medium.

Transverse wave

Motion of the particles is perpendicular to the direction of wave travel.

Longitudinal wave

Motion of the particles is parallel to direction of wave travel.

Wave frequency

The number of vibrations performed in each second by the source.

Wave period

The time for one complete vibration performed by the source.

Wavelength

Distance travelled by a wave during one oscillation of the source. (distance between successive crests or troughs)

Wave speed

Distance travelled per unit time by the energy of the wave (by a wavefront)

Refractive index

Ratio of speed in electromagnetic waves in vacuum to their speed in the medium.

Dispersion

Splitting/separation of white light into its component colours because different frequencies have different refractive indices.

Doppler effect

Change in received frequency of sound as a result of relative motion of source and observer.

Diffraction

Spreading out of light which occurs when the disturbance is of the same order of magnitude as the wavelength, and does not alter any properties of the wave.

Coherent waves

Two waves have the same frequency and a constant phase difference.

Principle of superposition

If two or more waves overlap the total displacement is the sum of the displacement of each wave. This can result in constructive and destructive interference.

Constructive interference

Occurs when occurs when the phase difference between the waves is an even multiple of π (nλ = path difference). Amplitude doubles

Destructive interference

Occurs when waves have the same amplitude in opposite directions so when they meet they completely cancel each other out. (n + 1/2)λ = path difference

Electric potential difference

Energy per unit charge to move positive test charge between points.

Electronvolt

The work done to move one electron through a potential difference of 1V.

Electric current (I)

The rate of flow of charge past a given cross-section of a conductor.

Resistance (R)

The ratio of potential difference across a load to current in the load.

Electromotive force (emf)

The power supplied per unit current or work done per unit charge in moving charge completely around the circuit.

Ohm’s law

The resistance of a conductor is constant and the current is proportional to the potential difference if its temperature is constant.

Electric field strength

The force per unit charge felt by a positive test charge placed in the field.

Electric potential

The work done per unit charge in bringing a small positive charge from infinity to that point.

Magnetic flux

The number of magnetic field lines passing trhough the region.

Magnetic flux linkage

Product of number of turns in a coil and the flux through the coil.

Faraday’s law of electromagnetic induction

emf induced is proportional to the rate of change of flux linking the coil.

Lenz’s law

The induced emf is in such a direction that its effect is to oppose the change causing it.

rms voltage

The value of the direct voltage that gives the same power dissipation as the average ac power dissipation.

Direct current (DC)

Occurs when the current flows in one constant direction.

Alternating current (AC)

Occurs when the electric current periodically inverts its direction.