Physics Scriptures Flashcards
Combining uncertainties
Adding or subtracting- add absolute uncertainties (must be of the same units).
Multiply or Dividing data- Add percentage uncertainties.
Raising to power- Multiply percentage uncertainty by power.
how to find uncertainty in a graph
by finding the difference between gradient of line of best fit and worst fit, then divide it by the line of best fit, and multiple by 100 if you want the percentage uncertainty.
The charge on most objects usually due to what
the loss or gain of electrons
A larger current may be due to what?
Greater number of electrons moving past a given point each second (i.e wire with large cross sectional area).
Same number of electrons moving faster through the metal
Electrolytes
liquids that can carry electrical current (which isn’t the flow of electrons by ions).
Number density (n)
the no. of free electron per unit vol.
The faster the electrons move in a wire
the quicker it heats up
Drift velocity is due to what?
the repeated collisions between positive ions and free electrons causing the random motion of charged carriers.
Centre of gravity
point where the entire weight of the object APPEARS to act.
what happens to mass of particle as it approaches to the speed of light?
Mass particle increases as it gets closer to the speed of light.
For stopping distance, the distance travelled is directly proportional to what two factors?
to the speed^2
and also the mass
cell function in a circuit
push the electron round the circuit.
Ohms law states that…
for any metallic conductor at constant temp, the p.d across it directly proportional to the current passing through it.
Energy is lost within the cell in the form of …
heat, lost volts
why is it that a change in current won’t affect internal resistance?
V and I are directly proportional.
Why do Rechargeable batteries and cars have low internal resistance?
allows higher currents used, without overheating and wasting energy, so recharging is faster.
what are Potential Dividers
simple circuit that uses resistors / thermistors / LDR to alter the output p.d.
Loading refers to what?
Connecting a component or circuit to Vout, placing a component in parallel with R2. This lowers the resistance of this part of the circuit, and so lowers Vout.
Electron gun
electrical device used, produce narrow beam electrons, used ionise particles by adding or removing electrons, from atoms.
How do electron guns work?
need source electrons, electron gain KE, some enough to escape from surface metal (thermionic emission). Heated filament placed in vacuum and high p.d applied between filament (cathode) and an anode, freed electrons accelerate towards anode (if hole present, beam electrons created).
Kirchhoffs first law states what about charge?
charge in circuit not used up, just flows around circuit.
Kirchhoffs second law
in any circuit, sum emf = sum p.d, in any closed loop.
Total energy transferred to charged carriers (emf) = Total energy transferred by charged carriers (terminal p.d) as they move around circuit.
LED’s act as what to the current
one way valves
equation for electrical energy
E=eV
Perfectly elastic collision
no deformation and momentum conserved, and KE
Inelastic collision
deformation occurs, momentum is conserved, KE is not, but energy always is.
Photoelectric emission
the release of electrons when photons of light hit the metal surface electrons are on.
Generally work function lower for what type of materials?
more reactive metals
Reflection is evidence for what
wave-particle duality
Lights also acts as particle, called a…
(what’s its charge?)
photon, has no charge
EM waves can only be released in what?
what did Einstein suggest?
discrete packets called quanta (single - quantum)
Einstein suggested EM waves and energy they carry, only exist in discrete packets, called wave-packets photons.
Photons act as particles, and will either transfer all or none of…
its energy when interacting with other particles.
Electronvolt definition
the KE gained by an electron when, accelerated through a p.d of 1 volt.
what can the threshold voltage be used to find?
Threshold Voltage used to find planks constant, current will only pass through LED the threshold voltage applied.
E = hc / lambda = e x Vo
what do you plot on a graph, to find planks constant?
Plot graph of voltage against 1/ wavelength.
E = hc / lambda = e x Vo
what happens to the free electrons on a metallic surface if they absorb enough energy?
bonds broken and released as photoelectrons.
How can we demonstrate the photoelectric effect?
gold-leaf electroscope, electroscope plate initially -ve, so gold leaf is repelled. Zinc plate exposed to UV light, free electrons ejected, zinc plate loses -ve charge, gold leaf drops.
what determines whether a photoelectron gets emitted or not?
If energy gained by electron from photon, greater than threshold freq, then emitted.
Interference and diffraction shows light as …., photoelectric effect (one-to-one interaction, all energy of photon given to one electron) shows light behaving as ….
wave
particle
Wave-particle Duality Theory
if ‘wave-like’ light showed particle properties (photons),
‘particle’ should be expected show wave-like properties.
what shows the wave nature of electrons?
Electron Diffraction
Note Particles don’t always show wave-like properties, only get diffraction if a particle with an object of about same size as…
its de Broglie wavelength.
De Broglie
all particles behave as waves, deduced particles have wavelength, and is only dependant on the momentum.
Tensile forces cause what?
Extension
F- kx can be used for … forces
compressive forces
what is the equation for work done, in terms of springs?
force x extension
equation for phase difference
distance / wavelength x 360 degrees
Two waves are said to be coherent if they have
the same frequency and the same constant phase difference
Path difference definition
difference in distances from 2 wave sources (measured in wavelengths).
Constructive interference, path difference equal to…
n x wavelength
where n is a whole number
Destructive interference, path difference equal to…
n/2 x wavelength.
Test mass
small enough, not able affect shape of field with own gravity.
Kepler’s Laws
the orbit every planet is an ellipse with the sun at one of the foci.
-Line segment joining planet and sun sweeps out equal areas during equal intervals time.
-Square of Orbital Period of a planet is directly proportional to the Cube of its average Distance from Sun.
How do satellites remain at constant height, if the only force acting upon is the gravitational force of the Earth?
Satellites always falling, however travel great distances, that as they fall, Earth curves away beneath it, keeping same height.
equation of the velocity for stable orbit.
V= square root Gm / r, for stable orbit.
Polar orbits offer…
complete view Earth, over given period, as Earth rotates beneath, they cover all parts globe after no. of orbits, useful mapping and reconnaissance.
Geostationary orbits
must be above equator, orbital period 24hrs, rotate same direction as Earth.
Gravitational potential definition
the potential energy at any point in the field. 0 potential is defined as infinity, so potential of a point close to a mass always -ve.
Gravitational potential energy definition
work done per unit mass in moving object from infinity to a point in the field.
what equation can you make from
Fc = mv^2 / r
Fg = -GMm / r^2
m = rv^2 / G
Prove T^2 directly proportional to r^3;
equate T= d / v
Square T, then sub in v^2 = MG / r.
Assumptions made for ideal gases (6)
1) Molecules are points- the vol of molecules is insignificant compared to vol of the ideal gas.
2) Molecules don’t attract each other- if they did then pressure exerted by the gas on its container would reduce.
3) All collisions between gas molecules and their container are elastic- no loss in KE.
4) Time taken for a collision is much shorter than the time between collisions.
5) Any sample of an ideal gas contains a very large no. of molecules.
6) The molecules in a gas have constant random motion.
N = n x Na
M= m / n
what third equation can you make
N / Na = m / M
Centripetal force, always acts towards …, and is … to liner velocity.
centre
perpendicular
Why does the speed in circular motion remain unchanged?
the net force directed perpendicular to direction of motion, no work being done by this force and so no change.
To derive escape velocity, what 2 equations do you equate?
Equate KE equation and GMm / r
Cosmological principles
isotropic, homogenous and laws of physics universal.
Luminosity of star
total power output of the star.
Electric Field Strength definition
force per positive unit charge
Electrical potential energy is equal to what?
EPE = Vq
Electrical potential energy definition
the work done per unit charge to move a POSITIVE charge from infinity to a specific point.
Protostars need to overcome what to become a star?
nuclear fusion in its core,
Fusion reaction produces energy form KE
Extremely high pressures and temps inside core needed to overcome electrostatic force repulsion between H nuclei, in order to fuse together and form He nuclei.
Flemings left hand rule normally used in terms of current, but can also be used for
charged particles, if +ve use left hand, if -ve use right.
In a series circuit, V at component / V total is equal to what?
V at component / V total = R at component / R total
For waves, intensity is directly proportional to what?
To amplitude^2
Do stationary and progressive waves transfer energy?
Stationary waves do not transfer energy at overall.
Progressive waves do, in the same direction of propagation.
For electric fields, direction of arrows go from…
positive to negative
Newton’s 3rd law only applies when acting on…
two separate objects, and two forces must be of same type.
Upthrust is directly proportional to…
the density of the fluid and volume of the object, therefore increases the deeper you go.
what evidence is there that waves carry energy (3)?
Waves carry energy: evidence, electromagnetic waves heat things, x-ray and gamma rays knock electrons out orbit, wave power can used generate electricity.
Oscilloscopes:
Oscilloscopes display waves, displayed wave called a trace, screen splits into squares called divisions, vertical axis in voltage, controlled by gain dial, horizontal axis in sec, controlled timebase dial, you alter both so wave becomes easier to read off measurements
why can’t longitudinal waves travel through a vacuum?
Longitudinal waves consists of alternate compression and rarefactions of medium its travelling through (hence why can’t travel through a vacuum).
Combining k (spring constant) in parallel and series
Series- 1/k = 1/k1 + 1/k2
Parallel k= k1 + k2
Elastic Deformation
When material put under tension, atoms of material, pulled apart from one another.
Atoms move slightly relative to their equilibrium position, without changing position in the material. Once removed, atoms return to their equilibrium distance apart
Plastic Deformation
some atoms in the material move position relative to one another, when load removed, don’t return original positions.
What is ultimate tensile strength
is the maximum stress a material can take
what is Elastic potential energy
energy stored in a stretched material
how do we get the equation:
Therefore work done (E) = 1/2 Fx
Can derive E = 1/2 kx^2
Work done on a spring equals the force x displacement, however force on material isn’t constant. It rises from 0 up to the force F. To calc work done use average force 1/2F
Stress and strain are proportional each other, up until…
limit of proportionality
what is Young’s Modulus, and how do engineers use it?
measure of stiffness of material. Used by engineers, make sure material can withstand sufficient forces.
Yield point
the stress at which a large amount of plastic deformation takes place with a constant or reduced load, here the material suddenly starts to stretch without any extra load.
Examples of polymeric materials
Rubber and polythene are polymeric materials
what does the loading and unloading curve for polythene look like?
Polythene behaves plastically, its a ductile material.
How does the loading and unloading curves for rubber vary?
The loading and unloading curves of rubber, different. Energy released when rubber is unloaded, less than work done to stretch rubber, because some of the EPE stored in stretch rubber, converted heat.
The Kinetic model of matter
idea that solids, liquids and gases are made up of tiny moving / vibrating particles.
Brownian’s motion supports what
The Kinetic Theory
what is potential energy caused by?
Potential energy, caused by interactions between particles, and is based on their positions, relative each other.
The absolute temp scale / thermodynamic scale is superior to the Celsius scale, how?
The absolute temp scale / thermodynamic scale - doesn’t depend on properties of any particular substance, unlike Celsius scale.
Calculate energy supplied by electrical heater with:
E = W = VIt
Learn, P = E x A
Power = Energy, of any form, times Activity
Learn equation for, Tension in string of a pendulum
mg + ( m x v max^2 / r ).
In SHM you can calc the total amount of energy in a closed system just by using…
Vmax, as the total energy at this point is just equal to the kinetic energy.
Are filament lamps ohmic conductors?
Filament Lamps aren’t ohmic conductors
If the external energy supplied to an electron from a photon isn’t the correct amount, for it to move up to a discrete energy level, then what happens?
It just passes through, isn’t absorbed.
what happens once electrons get excited to a higher energy level?
Once electrons are excited the want to return back to their ground state (can either be straight down, or down individual levels at a time, if so then releasing photons with different levels of energy).
How do fluorescent tubes work?
In fluorescent tubes, mercury atoms don’t get excited from incident photons, but an electron colliding with another electron in the mercury atom, then rebounds off with high energy still, however gives some energy to mercury atoms electron, exciting it.
what condition must be met if you are to measure the phase difference between two waves?
They must have the same constant freq.
