QCE U3&4 Physics

Centripetal acceleration

The acceleration experienced by an object moving in a circular path, directed towards the centre of motion.

Centripetal force

The force acting on an object travelling in circular motion that constantly either pulls or pushes the object towards the centre of motion.

Gravitational field

A region of space around a body where another body will experience a gravitational force of attraction

Kepler’s first law

All planets move in an elliptical orbit with the sun at one foci and nothing at the other

Kepler’s second law

The line connecting the sun and the earth sweeps out equals areas in equal time intervals

Kepler’s third law

Ratio of period squared to radius cubed equals 4pi squared over GM

Electric field

A region of space around an object with charge within which other objects with charge experience an electrostatic force

Electrical potential energy

The capacity of electric charge carriers to do work based on their location within an electric field

Magnetic field

A region of space near a magnet, electric current or electrically charged particle in which a magnetic field acts on any other magnet, electric current or electrically charged particle.

Which direction do magnetic field lines go

From north to south

State three Right hand rules

First: thumb as conventional current, fingers magnetic field

Second: thumb as north of solenoid, fingers as induced current

Third: thumb as conventional current, fingers as magnetic field, palm as force

Magnetic flux

The amount of magnetic field that passes through a specific area.

Magnetic flux density

The number of magnetic field lines per unit area

EMF

A difference in potential that tends to give rise to an electrical current

Electromagnetic induction

The production of an EMF across an electric field due to its dynamic interaction with a magnetic field.

Faraday’s law

When the magnetic flux linking a circuit changes, an emf is induced which is proportional to the rate of change of magnetic flux

Lenz’s law

The direction of the induced electrical current due to the change in circuit or magnetic flux opposes the change that produced it

The process of inducing an emf across a moving conductor in a magnetic field.

A change in magnetic field induces an electric field. If a conductor exists in this electric field, the charge carries inside the conductor experience an electrostatic force due to their position in the E field. This experienced electrostatic force will have an associated potential difference, EMF.

How does Lenz’s law adhere to the conservation of energy.

As a magnet moves towards a conductor it has kinetic energy. As it nears the conductor an electric current is induced so now it has electrical energy as well. The kinetic energy is being converted into electrical energy. Kinetic energy is slowly decreasing and the magnet is slowing down as the induced magnetic field is pushing it away. If the induce current didn’t oppose the change in magnetic flux both kinetic energy and electrical energy would increase breaking the conservation of energy.

Transformers in terms of faradays law and electromagnetic induction

An alternating current passes through the first coil generating a change in magnetic flux. This changing magnetic flux is also applied to the secondary coil. The changing magnetic flux induces an emf proportional the the number of coils and rate of change of magnetic flux.

What is electromagnetic radiation

It is energy transmitted at the speed of light through oscillating electric and magnetic fields perpendicular to each other. They are self propagating due to Faradays law (changing B causes changing E) and Amperes law (changing E causes B).

Explain the presence of muons on the earths surface

Muons life span is short however when they are moving at relativistic speeds the distance between the atmosphere and the earth contracts and the muon can make the journey. From an observers perspective time is dilated so the muon has longer to travel down to the surface.

Reference frame

An abstract set of coordinates that defines the location of an observer

Inertial reference frame

Any reference with respect to which the acceleration of the object is 0.

2 postulates of special relativity

1. All laws of Newtonian physics apply in all inertial references frames.

2. The speed of light, c, is constant

Simultaneity

Two events are considered to be simultaneous if they occur at the same time

Time dilation

The difference in elapsed time between two events as measured by observers moving relative to each other

Proper time

Measured in the object moving

Relativistic time

Measured from the observer

Length contraction

At relativistic speeds an object will contract along the direction in which it is moving

Relativistic length

Contracted length seen by observer

Proper length

Length measured by moving object at rest

Rest mass

The mass of an object as measured by an observer at rest relative to the object

Relativistic momentum

Momentum of an object measured in a reference frame in which it is moving

Why can’t objects with mass travel at the speed of light

As their speed approaches c, their relativistic momentum approaches infinity, meaning it cannot be accelerated further.

Ladder in the barn paradox

From runner: barn contracts and doors do not close at the same time

From observer: ladder contracts and it fits in the barn and both doors close at the same time

Solution: simultaneity is relative and both are correct from their perspective

Twins paradox

From earth: travelling twin is younger

From space: earth twin is younger

Solution: space twin accelerates so only the earth twin stays in a constant inertial reference frame and the space twin is younger when they return. There is no paradox when understanding the role of acceleration

Flashlights on the train paradox

From train: light reaches walls at the same time

From observer: light reaches the back of the train first

Solution: simultaneity is relative and both are correct from their perspectives

Young double slit showing wave nature of light

Light shone through two slits. Multiple Dark and bright bands appear not in line with slits. Only possible through the superposition of waves and constructive and destructive interference, these are properties of mechanical waves. Not explainable by particle theory.

Light as electromagnetic radiation

Light is a self propagating electromagnetic wave consisting of oscillating magnetic and electric fields perpendicular to each other.

Black body radiation

The emission of EMR as a result of an object’s thermal energy is black body radiation.

How does black body radiation show particle nature of light

Classical wave theory could not explain the observed distribution of wavelengths in black body radiation, leading to the ultraviolet catastrophe where it predicted infinite energy at high frequencies. Planck resolved this by proposing that the atoms within the black body could only emit or absorb energy in discrete packets proportional to the light’s frequency. This means light energy is quantised which is evidence for its particle nature.

Photon

Light with a quantised amount of energy, a quantum of all forms of electromagnetic energy

Photoelectric effect showing particle model of light

When photons are shone onto a metallic surface, photo electrons can be ejected from the metal if the photon meets the threshold frequency. Regardless of the intensity of the light if the frequency of the light does not meet the threshold frequency no electrons will be ejected. Increasing intensity increases the number of electrons emitted, increasing frequency increases the maximum kinetic energy of the electron emitted. This shows the quantised energy within a photon showing particle model of light.

Threshold frequency

Minimum frequency of light required to eject an electron

Work function

Minimum energy required to eject an electron from the metal

Rutherford model and its limitations

A dense positive nucleus with orbiting negatively charged electrons.

Lims: accelerating charged particles emit constant energy so the electrons should lose all their energy and implode. The emission spectrum of light from the atom is discrete however the model shows the electrons as having any energy level so it should be a continuous spectrum

Bohr model and its strengths

Electrons orbit the nucleus in stationary states in fixed orbitals with quantised energy levels

Strengths: electrons in stationary states do not emit constant energy and do not implode. Discrete spectrum explained by the energy required to jump between energy levels.

Wave - particle duality of light

Wave: young’s double slit

Particle: photoelectric effect and black body radiation

Elementary particles and antiparticles

Elementary particles are found in the standard model. Anti particles are its antimatter counterpart same mass opposite charge

Name the 6 quarks

Up down top bottom strange charm

Hadron

Particles made up of quarks

Meson

One quark one antiquark

Baryon

3 or more odd number of quarks

Name the 6 leptons

Electron, tau, muon, electron neutrino, tau neutrino, muon neutrino

4 gauge bosons and which forces they mediate

Gluon - strong nuclear force

Photon - electromagnetic force

W and Z boson - weak nuclear force

What forces do quarks and leptons experience

Quarks experience all 4 and leptons experience gravitational , weak nuclear force and electromagnetic if charged

Baryon number

Number of baryons

Lepton number

Difference between number of leptons and anti leptons

3 types of symmetry and what they conserve

Gauge symmetry - conserve lepton and baryons number

Time symmetry- conserve energy

Translational symmetry- conserve momentum

Electron - electron interaction

2 electrons, electrostatic repulsion mediated by photon

Electron - positron scattering

Shows scattering as both particles exist the whole time - mediated by photon

Electron - positron annihilation

Annihilation showed by particles not existing for a bit of time - mediated by photon

Neutron decay into proton

Neutron into proton - mediated by w boson, anti neutrino and electron

The anti neutrino conserves baryon number, lepton number and charge

What makes up protons and neutrons

Proton - 2 up quarks, 1 down quark

Neutron - 1 up quark, 2 down quarks