Physics Key laws and facts

Hooke’s law

Extension is directly proportional to force until the spring reaches its elastic limit.

F=kx

Ohm’s law

The current in a metal wire at constant temperature is directly proportional to the potential difference across it.

V=IR

Stefan’s law

The luminosity of a star is proportional to the fourth power of the star’s temperature and directly proportional to its surface area.

L∝T^4∝A

L=4π²x

Principle of conservation of momentum

Assuming no external forces act, momentum is always conserved. Total momentum before collision = total momentum after the collision.

Newton’s third law

If an object A exerts a force on object B then B exerts a force equal of magnitude in the opposite direction.

Newton’s second law

The rate of change of momentum of an object is directly proportional to the net force which acts on the object.

Force = Δp / Δt

Newton’s first law

Velocity of an object won’t change unless another force acts on the object.

Snells law

Use this to calculate refractive index

N1xsin1=N2xsin2

Conservation of Mass-Energy

Energy can’t be lost or gained only transferred between sources.

E=mc²

Newtons law of gravitation

The gravitational force between 2 particles is proportional to the product of their masses.

Mm∝1/r²

Coulomb’s law

The electrostatic force ( F) between 2 small bodies is proportional to the product of their charges and inversely proportional to their separation squared.

F∝Qq∝1/r²

Kepler’s first law

The orbit of each planet around the sun is an ellipse with the suit at one foci

Kepler’s second law

A line joining the sun to a planet sweeps out equal areas in equal times

Keplers third law

The square of the orbital period of a planet is directly proportional to the cube of the mean distance between the planet and the sun.

T² ∝r³

Hubble’s law

The recessional velocity of a galexy is directly proportional to its distance away from earth.

v= Ho d

Weins Displacement law

Maximum wavelength is inversely proportional to the stars temperature.

λmax= k/T

Lenzs law

Induce EMF opposes the direction of the change of flux linkage that caused it.

The minus sign in Faradays law

Boyles law

Pressure is inversely proportional to volume for a fixed mass of gas at a constant temperature.

P ∝ 1/v

Charles’s law

Volume is directly proportional to the absolute temperature for a fixed mass of gas at constant pressure.

V∝T

Pressure law

Pressure is directly proportional to absolute temperature for a fixed mass of a gas at constant volume

P ∝ T

Kirchhoffs First law

The sum of the current entering any junction is equal to the sum of the currents leaving the junction.

Conservation of charge

Kirchoffs Second law

The sum of the emf is equal to the sum of the potential difference in a closed loop.

Conservation of energy

Doppler effect

Change in frequency due to relative motion between source and observer.

Archimedes principle

When an object is partly submerged in fluid it experiences an upthrust equal to the weight of the fluid that has been displaced.

Ideal gas assumptions

• Perfectly elastic collisions

• Large number of particles

• The volume of the particles is negligible compared to the volume of gas

• Rapid and random motion

• No forces between particles except collisions

Luminosity

Total radiant power of a star

Radio waves

• 10^1 - ^-6

• Pass through matter

• Radio transmissions

Microwaves

• 10^-3 - ^-1

• Mostly pass through matter, some heating

• Radar

• Microwave cooking

• TV transmissions

Infrared (IR)

• 7×10^-7 - ^-3

• Mostly absorbed by matter, heating

• Heat detectors

• Night vision cameras

• Remote controls

• Optical fibres

Visible light

• 4×10^-7 - 7x 10^-7

• Absorbed by matter, causing some heating

• Human sight

• Optical fibres

Ultra violet (UV)

• 10^-8 - 4×10^-7

• Absorbed by matter, slight ionisation

• Sun beds

• Security marks

X-rays

• 10^-13 - 10^-8

• Mostly pass through matter, causes ionisation

• See damage to bones and teeth

• Airport security

• Kill cancer cells

Gamma rays

• 10^-16 - 10^-10

• Mostly pass through matter, cause ionisation

• Irradiation of food

• Sterilisation

• Kill cancer cells