Physics AQA mock revision

• Energy stores and systems

  A system is an object or group of objects. There are changes in the way energy is stored when a system changes.
• changes of energy involved when a system is changed:

  • heating
  • work done by forces
  • work done when a current flows
• equation of power:

  • power = energy transferred / time
  • power = work done / time
• equation for kinetic energy:

  • kinetic energy = 0.5 x mass x (speed)^2
• number equation for kinetic energy

  Ek = 1/2 x m x v^2

  (lower case 'k' that is small)
• equation for gravitational potential energy

  • g.p.e = mass x gravitational field strength x height
• Energy transferred in a system

  Energy can be transferred usefully, stored, or dissipated. But can not be created or destroyed.
• The higher the thermal conductivity of material, the higher the rate of energy transfer of conduction across the material.
• Efficency equation

  useful/output energy transnfer (useful power input) / total input energy transfer (total power imput)
• National and global energy resources

  main energy resources:

  • fossil fuels (coal, gas, oil)
  • nuclear fuel
  • wind
  • geothermal
  • sun + water waves
  • the tides
  • bio-fuel
  • hydro-electricity
• renewable energy source: one that is being (or can be) replaced.

  The uses of energy resources include: transport, heating, electricity generation.
• Electrical charge and current

  For electrical charge to flow through a closed circuit, the circuit must include a source of potential difference.

  Electric current is a flow of electrical charge. The size of the electric current is the rate of slow of electrical charge.
• Charge flow equation

  Charge flow = current x time
• The current through a component depends on the resistance of the component. The greater the resistance of the component, the smaller the current for a given potential difference across the component.
• Equation for potential difference

  V = I x R

  potential difference = current x resistance
• Equation for power

  power = (current)^2 x resistance
• Resistors.

  1. The current through an *Ohmic Conductor* is Directly Proportional. (first image)
  2. The resistance of a *Filament lamp* increases as the temp of the filament increases. (second image)
  3. The current through a *Diode* flows in one direction only. The diode has a very high resistance in the reverse direction. (third image)

  
• Resistors and applications

  The resistor of a thermistor decreases as the temp increases.

  The applications of thermistors in circuits -> thermostat is required.

  The resistance of an LDR decreases as the light intensity increases.

  The application of LDRs in circuits -> switching lights on when it gets dark is required
• series VS parallel (components) -series-

  Series:

  • Same current through each component.
  • total p.d of power supply is shared.
  • total resistance of two components is the sum of resistance of each component. (Rtotal = R1 + R2, etc.)
• series VS parallel (components) -parallel-

  Parallel:

  • p.d is the same.
  • total current = sum of currents through the seperate components.
  • total resistance of two resistors is less than the resistance of the smallest individual resistor.
• Direct and alternating current.

  Direct: flows backwards and forwards.

  Alternating: flows in only one direction.
• mains electricity - wire colours

  Live wire = brown. (230V)

  Neutral wire = blue.

  Earth wire = green + yellow stripes
• mains electricity

  The live wire carries alternating p.d from the supply.

  The neutral wire completes the circuit.

  The earth wire is a saftey wire to prevent electric shock or to provide a path for current to flow in case of a fault or malfunction.
• Energy transferred equation

  E = P x T

  Energy = Power x Time

  E = Q x V

  Energy transferred = Charge flow x Potential Difference
• The national grid

  The national grid is a system of cables and transformers linking power stations to consumers.

  Electrical power is transferred from power stations to consumers using the national grid.

  Step-up transformers are used to increase the p.d from the power station to transmission cables.

  Step-down transformers are used to decrease the p.d to a much lower value for domestic use. (use in homes)
• Equation for density


  p = m/v

  Density = mass/volume
• Particle model

  Used to explain: the different states of matter, the differences in density.
• Changes of state

  Solid -> liquid = melting

  liquid -> solid = freezing

  solid -> gas = sublimation

  liquid -> gas = evaporation/boiling

  gas -> liquid = condensation
• Internal energy

  Energy is stored inside a system by the particles (atoms and molecules) that make up a system. This is called internal energy.

  Internal energy is the total kinetic energy and potential energy of all particles that make up a system.

  Heating changes the energy stored within a system by increasing the energy of particles that make up the system.

  This either raises the temp of the system or produces a change of state.
• changes of state, temp changes in a system, and specific latent heat.

  The energy needed for a substance to change state is called latent heat. When a change of state occurs, the energy supplied changes the energy stored (internal energy) but not the temp.

  Specific latent heat of a substance is the amount of energy required to change the state of 1kg of the substance with no change in temp.

  The increase in temp depends on the mass of the substance heated, the type of material and energy input to the system.
• Particle model and pressure

  The molecules of a gas are in constant random motion. The temp of the gas is related to the average kinetic energy of the molecules.

  Changing the temp of a gas, held at a constant volume, changes the pressure exerted by the gas.
• Isotopes and atoms

  Isotopes are forms of an element that have the same number of protons but a different number of neutrons.

  Atoms turn into positive ions if they lose one or more outer electrons.

  They can also turn into negative ions if they gain one or more outer electrons.
• Plum pudding model, alpha scattering experiment, and Niels Bohr.

  The plum pudding model suggested that the atom is a ball of positive charge with negative electrons embedded in it.

  The results of the alpha scattering experiment led to the conclusion that the mass of an atom was concentrated at the nucleus and that it was charged. This nuclear model replaced the plum pudding model.

  Neils Bohr adapted the nuclear model by suggesting that electrons orbit the nucleus at specific distances.
• Radioactive decay and nuclear radiation

  Some atomic nuclei are unstable. This nucleus give out radiation as it changes to become more stable. This is a random process called radioactive decay.

  The nuclear radiation emitted may be a neutron.

  Alpha (a) - 2 neutrons, 2 protons

  Beta (β) - a high speed electron from the nucleus as a neutron turns into a proton.

  Gamma (Y) - electromagnetic radiation from the nucleus.
• Nuclear equations - Alpha

  For alpha, the helium symbol is needed to be remembered. (The last one in the photo where the equation is written)

  -YOU HAVE TO ADD THE NUMBERS TO GET THE ANSWER-

  
• Nuclear equations - Beta

  For beta, the symbol is needed to be remembered. (The last one in the photo from the equation)

  -YOU HAVE TO ADD THE NUMBERS TO GET AN ANSWER-

  
• Radioactive decay and half-life

  Radioactive decay is random.

  The half life of a radioactive isotope is the time is takes for the number of nuclei of the isotope in a sample to halve, or the time is takes for the count rate (or activity) from a sample containing the isotope to fall to half its inital level.

  
• Example of half-life graph with explanation on how to work it out.

  [LOOK AT GRAPH WHILE READING THIS]

  --

  From the start of timing it takes two days for the count to halve from 80 down to 40. It takes another two days (so in total 4 days) for the count rate to halve again, this time from 40 to 20.

  [YOU JUST LOOK AT THE GRAPH AND GO ACROSS TO SEE THE ACTIVITY AND HOW LONG IT TAKES]

  
• example of half-life. (QUESTION)

  The half-life of cobalt-60 is *5* years. If there are *100g* of cobalt-60 in a sample, how much will be left after *15* years?

  [AFTER WRITING THIS EQUATION] - or you can replace the 1/2 and 3 in indicies with 1/8 as 15 years is three half-lifes (because 5,10,15 is three times)

  So, after 15 years, there will be 12.5 g of cobalt-60 remaining.

  
• example of half-life (QUESTION 2 - EASIER)

  What is the half-life of a sample where the activity drops from 1,200 Bq down to 300 Bq in 10 days?

  You half 1,200 which is 600.

  half of 600 is 300.

  So it takes two half-lives to drop from 1,200 Bq to 300 Bq, which is 10 days.

  So one half-life is five days.

  (YOU JUST HALF IT)