Physics Key laws and facts

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33 Terms

1
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Hooke’s law

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

F=kx

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Ohm’s law

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

V=IR

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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

4
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Principle of conservation of momentum

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

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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.

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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

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Newton’s first law

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

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Snells law

Use this to calculate refractive index

N1xsin1=N2xsin2

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Conservation of Mass-Energy

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

E=mc²

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Newtons law of gravitation

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

Mm∝1/r²

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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

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Kepler’s first law

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

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Kepler’s second law

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

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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.

∝r³

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Hubble’s law

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

v= Ho d

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Weins Displacement law

Maximum wavelength is inversely proportional to the stars temperature.

λmax= k/T

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Lenzs law

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

The minus sign in Faradays law

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Boyles law

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

P ∝ 1/v

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Charles’s law

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

V∝T

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Pressure law

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

P T

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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

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Kirchoffs Second law

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

Conservation of energy

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Doppler effect

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

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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.

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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

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Luminosity

Total radiant power of a star

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Radio waves

  • 10^1 - ^-6

  • Pass through matter

  • Radio transmissions

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Microwaves

  • 10^-3 - ^-1

  • Mostly pass through matter, some heating

  • Radar

  • Microwave cooking

  • TV transmissions

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Infrared (IR)

  • 7×10^-7 - ^-3

  • Mostly absorbed by matter, heating

  • Heat detectors

  • Night vision cameras

  • Remote controls

  • Optical fibres

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Visible light

  • 4×10^-7 - 7x 10^-7

  • Absorbed by matter, causing some heating

  • Human sight

  • Optical fibres

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Ultra violet (UV)

  • 10^-8 - 4×10^-7

  • Absorbed by matter, slight ionisation

  • Sun beds

  • Security marks

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X-rays

  • 10^-13 - 10^-8

  • Mostly pass through matter, causes ionisation

  • See damage to bones and teeth

  • Airport security

  • Kill cancer cells

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Gamma rays

  • 10^-16 - 10^-10

  • Mostly pass through matter, cause ionisation

  • Irradiation of food

  • Sterilisation

  • Kill cancer cells