Physics WACE

Artificial satellites

used for communication, navigation, remote sensing and research

Low orbit satellite

160 to 2,000 km above earth. For low latency communications and earth observation

medium orbit satellite

2,000 to 36,000 km obove earth. GPS, telecommunications and weather forecasting

high orbit satellite

36,000 to 47,000 km above earth. communication, navigation, research

high speed muons

prove einstein’s theory of special relativity. They should die before reaching earth if not for time dilation.

Einstein’s theory of general relativity

Explains black holes, gravity waves, and gravitational lensing.

Gravity not as force but the curvature of space time caused by mass and energy.

Black holes are just where the curvature becomes infinite.

Gravitational waves are just ripples caused by massive objects

Einstein’s first postulate

speed of light in a vacuum is constant

Einstein’s second postulate

laws of physics are the same in all inertial reference frames

Back emf

As the coil turns it cuts flux lines creating a voltage opposing the original force from the coil. This regulates the motor speed and prevents overheating

regenerative braking

As the engine stops the wheels keep turning creating back EMF. This current produces torque that opposes the rotation so slowing the vehicle. The electrical energy is then fed back into the battery of the vehicle.

induction stove

generates a rapidly changing magnetic field through a AC supply which creates a changing magnetic field so induces eddy currents in magnetic pots that create heat.

Lenz’s law

An induced current always flows in a direction that opposes the original change in flux

Faraday’s law

a changing magnetic flux induces an emf

why objects with mass can’t reach the speed of light

Because of relativistic momentum if the velocity is c then the lorentz factor becomes 1/0 which is infinite. as Ek is (lorentz - 1)mc² it suggest the kinetic energy has to be infinite and this is impossible

lorentz approaches infinity as speed approaches infinity so Ek and p approach infinity which is impossible as infinite energy is impossible

Fluorescence

when a material absorbs high energy photons and immediately re-emits lower energy photons that are visible light.

1: A high energy photon his an atom or molecule and if the photon’s energy exactly matches the gap between two energy levels an electron absorbs, jumping levels, and gets excited

2: It falls through intermediate energy levels and some energy is transferred as heat without light

3: the electron then drops to a lower level and emits a photon. this photon has less energy than the one absorbed as some was lost as heat. lower energy means lower frequency and longer wavelength that is visible light.

phosphorescence

absorbs photons then re emits lower energy light slowly.

1: a photon with matching energy gap is absorbed and the electron gets excited

2: some energy lost as heat as electron drops levels

3: electron lands in a metastable energy level. dropping to the ground state is very unlikely

4: electron escape from metastable state at random times each emitting a photon as it drops

x-rays

high energy electromagnetic radiation

1: a heated cathode emits electrons by thermionic emission

2: a large potential difference between the cathode and a metal anode accelerates the electrons

3: an electron is slowed down by the electric field of a target nucleus and loses some or all of its kinetic energy. as the electrons lose different amounts of energy it gives a continuous spectrum of wavelengths called bremsstrahlung radiation. an incoming electron knocks an inner shell electron out of a atom and an outer shell electron drops into the vacancy and emits a photon with energy equal to the gap. as the gap depends on the element it gives sharp lines at specific wavelengths called characteristic x-rays.

wave particle duality

light can’t just be a particle as it can diffract and interfere with each other and only a wave can do these things, but it also can’t just be a wave and light travels through vacuum.

Young’s double split experiment:

1: when a wave passes through a gap it diffracts and spreads out. The effect is strongest when the gap is similar to the wavelength. particles would travel in a straight line and make a sharp-edged patch

2: when two waves overlap constructive and destructive waves create bright and dark patches

polarisation

when a transverse wave oscillates in only one plane. In an unpolarised light the electric field oscillates in all directions perpendicular to the direction of travel but a polarising filter only lets through the component of the oscillation along its transmission axis.

electromagnetic waves

transverse waves made of mutually perpendicular oscillating electric and magnetic fields.

quantisation of matter and energy

a quantity can only take certain discrete values not any value in a range. Light is quantised as it can only be in energy steps. emission spectra prove this as a gas emits light only at specific wavelengths. absorption spectra also prove this as light passing through a gas gets absorbed and dark lines appear at exact wavelengths of the gas

black body radiation

an ideal object that absorbs all radiation falling on it and emits a continuous spectrum of thermal radiation that depends only on its temperature. This disproves the classical wave model as brightness drops to zero at short wavelengths and classical physics says energy can be given out in any size amount so the brightness should keep rising at shorter wavelengths heading to infinite energy which is impossible.

photoelectric effect

only light of a certain frequency can emit an electron from a metal surface. Intensity increases the number of electrons while frequency increases the energy of the electron.

Bohr model

The problem: earlier models suggested electrons orbiting the nucleus but an orbiting electron is accelerating and classical physics says accelerating charges radiate anergy, so the electron should loose energy and spiral into the nucleus so atoms shouldn’t be stable

Key ideas: electrons only exist in certain stationary orbits (energy levels), while in an orbit the electron does not radiate, so it is stable, the electron can jump between orbits, going up needs a photon to be absorbed and going down emits a photon, the photons energy must exactly equal the gap between two levels

cosmic background radiation

thermal radiation that fills the whole universe. the leftover light from the early universe which has been stretched by the expansion of space until it is now in the microwave region.

1: after big bang universe was really hot as shown by the black body photons found today. matter was a plasma of free electrons and nuclei and photons constantly scattered off the electrons

2: as space expanded the universe cooled. at 3000K electrons could be captured by nuclei to form neutral atoms (mostly hydrogen). the radiation cooled to 2.725K and peaks in the microwave and the spectrum has a black body curve

3: neutral atoms dont scatter photons as much so light could travel free. these photons have been travelling ever since and the cmb is that light.

expansion of space

as the universe is expanding now then in the past it has to have been smaller, denser, and hotter

red shift

the lengthening of the wavelength and drop in frequency of light from a source that is moving away from us or space stretching between us. because of doppler effect visible light shifts to red end as if moves away and blue as it comes closer

abundance of light elements

big bang predicts the universe was hot enough to fuse protons and neutrons into light nuclei. predicts 75% hydrogen and 25% helium. universe cooled too quikcly for heavier elements to form.

hubble’s law

the recession speed of a galaxy is proportional to its distance from us. every galaxy is moving away and the further ones move faster, which is what is expected if space is expanding and not planets moving away.

LED

1: has a p and n junction. the n has extra free electrons and the p has holes (missing electrons)

2: p is positive and n is negative. once the voltage exceeds the threshold voltage electrons and holes are pushed towards the junction and cross the barrier

3: at the junction electrons drop from the higher energy conduction band into holes in the lower energy valence band. the energy loss is equal to the band gap

4: lost energy emitted as a single photon. the band gap depends on the material so each material emits a different colour.

why objects with mass can’t reach the speed of light

Because of relativistic momentum if the velocity is c then the lorentz factor becomes 1/0 which is infinite. as Ek is (lorentz - 1)mc² it suggest the kinetic energy has to be infinite and this is impossible

lorentz approaches infinity as speed approaches infinity

threshold frequency

the minimum frequency of light that can eject electrons from a metal surface