Unit B (1-5) Flashcards
What is the difference between an Insulator and a Conductor?
conductors: have lots of charge, free electrons, to allow current flow
insulators: do not have charge carriers so current cannot flow
What is the difference between conventional current and electron flow?
conventional current: positive charge flows from the positive terminal of a voltage source, through the circuit, and into the negative terminal
electron flow: Electrons flow from the negative terminal of a voltage source, through the circuit, and into the positive terminal
What is the difference between potential difference and EMF?
PD: the work done per unit charge in converting electrical energy into (fill in blank) energy in coulumbs
EMF: the work done per unit charge in converting chemical energy into electrical energy in volts
Define the Electronvolt
If 1 electron travels through a potential difference of 1V then it will gain 1.6x10^-19 J of energy
Summerize what a LDR, thermistor, stress sensor, potential divider circuit - what they are and what they do and for the first three, under what conditions does R increase or R decrease?
LDR: when light intensity increases, resistance decreases, used on iphone brightness
Thermistor: when the temperature increases, the resistance decreases, used to sensor an oven
Stress sensor: as the length increases, the area decreases and resistance increases, piece of metal placed under a bridge
Potential Divider: changes the amount of voltage that passes through different parts of the circuit, control of temperature in a fridge or as audio volume controls
Ammeter and Voltmeter - what are they used for? What resistance properties do they have? Where do we put them in a circuit?
Ammeter: measures the current in a circuit, connected in series
Voltmeter: measures the voltage or potential difference between two places in a circuit, connected in parrallel
What is Ohm’s Law?
If the potential difference across a resistor is proportional to the current flowing through it then the device is Ohmic, provided the physical conditions stay the same
If a conductor is Ohmic, current is proportional to V (you can’t get R from the instantaneous gradient of a V-I graph)
What is the definition of internal resistance? How do we rearrange the formula E = I(R + r) to obtain the EMF and the internal resistance?
A cell consists of conductive components and a store of chemical energy. All these materials have resistance.
V = e-Ir
Define heat and thermal energy
Heat - energy that is transferred from one body and into another as a result of a difference of temperature
Heat always flows from hot to cold
Thermal energy - the transfer of energy from a hotter object to a colder object
Define thermal equilibrium
If two bodies are in thermal equilibrium – no heat flows from one body to another
Define absolute zero
the lowest temperature that is theoretically possible, at which the motion of particles that constitutes heat would be minimal. It is zero on the Kelvin scale, equivalent to –273.15°C or –459.67°F.
Define absolute temperature
measure of the average kinetic energy of the molecules of a substance. The average kinetic energy of the molecules is directly proportional to the absolute temperature in kelvin
How do we convert from Kelvin to Celsius?
K = C + 273
What is Internal energy?
the total random kinetic energy of the molecules of a substance plus any potential energy associated with forces between the molecules
What is specific heat capacity?
The specific heat capacity is the amount of energy needed to raise 1kg of a substance by 1K
The units are J kg-1 K-1
Define latent heat
Latent heat is the energy required to change the state of 1kg of as substance WITHOUT a change in temperature
The energy from the heat source goes into separating the molecules instead of heating the substance.
What is the difference between latent heat of fusion and vaporization?
latent heat of fusion is about the transition from solid to liquid, like ice melting into water, while latent heat of vaporization is about the transition from liquid to gas, such as water boiling into steam
What is a phase change?
A change of state of the matter (melting, freezing, boiling, condensation, sublimation)
What do flat parts of a time – temperature graph mean?
The flat parts are when the phase changes are occurring and the slope lines are when the matter is in one state
What are the three mechanisms that transfer heat? When is each one used?
Thermal Conduction - heat energy is transmitted through collisions between neighboring atoms or molecules
Convection - the movement of particles through a substance, transporting their heat energy from hotter areas to cooler areas. Mass movement of particles due to differences in density
Radiation - transfer of energy that travels in the form of waves or particles, EM radiation does not need a medium, can travel to us from the sun, Stefan – Boltzman
What does it mean to be a blackbody?
an idealized object that absorbs all electromagnetic radiation that falls on it, regardless of the wavelength or angle of incidence. It is a perfect absorber and emitter of radiation
What does thermal conduction depend on?
Atomic Vibration has larger amplitude at larger T so atoms collide until uniform T reached
Movement of free electrons diffuses the energy
What are the two blackbody laws?
Wien’s Law - For blackbodies, the peak wavelength of radiation determines the T and vice versa, the wavelength at which a blackbody emits maximum radiation is inversely proportional to its absolute temperature (λ_max = b / T)
Stefan-Boltzmann Law - the total energy radiated per unit surface area of a blackbody per unit time is directly proportional to the fourth power of the blackbody’s absolute temperature
(E = σ T^4)
Luminosity and Apparent brightness of stars?
Luminosity is defined as the amount of energy radiated by the star per second
apparent brightness – the amount of energy per second received per m2
Depends on the distance to the star
What is the greenhouse effect?
when gases in the Earth’s atmosphere absorb and trap heat from the sun that is radiated back from the Earth’s surface
What are the greenhouse gasses and why are they called this?
gases in the Earth’s atmosphere that absorb and trap heat radiated from the Earth’s surface, causing the greenhouse effect (carbon dioxide, methane, water vapor, nitrous oxide)
When the gases react with infrared radiation, Some gets reradiated back to earth and does not escape, heating the earth
What is resonance?
when an object vibrates at the same natural frequency as another object, leading to an amplification of vibrations (the greenhouse gases resonate with the particles in the atmosphere)
Define albedo and emissivity?
Albedo - amount of light reflected/amount of incident light
Emissivity - amount of light emitted/amount of light a black body at that T would emit (perfect black body = 1)
What is the solar constant?
Energy from the sun that changes due to:
Orbital variations Jan – June
Solar 11 year cycle
Other longer period cycles
Why is 1360W/m^2 incident but the total that converts to a T for earth only around 250W/m^2?
The energy has to be spread across the surface of the earth (4πd^2), from the perspective of the sun the earth is only a disk not a sphere and then some of this radiation is reflected back into the earths atmosphere
What are the three gas laws?
Boyle’s Law: for a fixed mass of gas at constant temperature, the pressure is inversely proportional to the volume
PV = constant
P proportional to 1/V
Charles’ Law: for a fixed mass of gas at constant pressure, the volume is directly proportional to the absolute temperature
Third Law (Guy-Lassac, Avagadro, Amontons): for a fixed mass of gas at constant volume, the pressure is directly proportional to the absolute temperature
What is a mole?
One mole of any substance contains the same number of atoms as in 12g of carbon-12.
What is Avogadro’s number?
One mole: The numerical value is NA – Avogadro’s number = 6.02x10^23
How do we measure the gas laws?
PV = nRT
PV = NkT
R is the gas constant = 8.31 J K-1 mol-1
k is the Boltzman constant = 1.38x10-23 J K-1
where P is pressure in Pa or N/m^2
V is volume in m^3
T is temperature in K
n - number of moles
N number of atoms
What is diffusion?
when molecules move from an area of high concentration to an area of low concentration
What are the assumptions of the ideal gas law? (kinetic theory)
A gas consists of a large number of molecules N which is large enough that averages can be made
The volume of the molecules themselves is negligible compared to the volume of the gas
Collisions between molecules and the walls of the container are elastic
There are no intermolecular forces between the molecules, except during collisions
What is the total energy of a gas?
Remember nR = NK so k = nR/N
And n = N/NA
So KE = 3/2 (R/NA) T
This gives the average KE of a particle of a gas of temperature T
When is an idea gas ideal? I.e. what are the conditions on temperature and pressure?
If T too low or P too high then a gas is not ideal
Under those conditions, gases liquefy and then cannot be compressed
Ideal gases cannot, therefore, be liquified
So a gas is ideal when T is high, P is low and density is low
