physics paper 1

• specific heat capacity- the amount of energy needed to raise the temperature of 1kg of a substance by 1 degree celcius
• efficiency of a system can be increased by:

  • reducing waste output (lubrication, thermal insulation etc)
  • recycling waste output (eg absorbing thermal waste and recycling as input energy)
• The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component.
• resistance changing with current

  • as current increases, electrons (charge) has more energy
  • when electrons flow through the resistor, they collide with the atoms in the resistor
  • this transfers energy to the atoms, causing them to vibrate more
  • this makes it difficult for the electrons to flow through the resistor
  • so resistance increases and current decreases
• resistance changing with temperature

  • with normal wires, same process as current
  • with thermistors: higher temp means lower resistance
  • thermistors are often used in temperature detectors/thermostats
• resistance changing with length

  • greater length: more resistance, less current
  • electrons have to make their way through more resistor atoms, so it is harder than using a shorter wire
• resistance changing with light

  • for LDR: greater intensity with light, lower resistance so when it is dark, the resistance is greater
  • used for automatic night lights
• resistance changing with voltage

  • Diode allows current to flow freely in one direction
  • In the opposite direction, it has a very high resistance so no current can flow
• series circuit

  • same current
  • pd is shared
  • resistance is the sum of the resistance in each component
• parallel circuits

  • pd is the same
  • current is the sum of the current for all the components
  • total resistance is less than the branch with the smallest resistance
• live wire

  • brown
  • 230V
  • carries alternating pd from the supply
  • can be dangerous even if main circuit is off as current could still be flowing through it
• neutral wire

  • blue
  • 0V
  • completes the circuit
• earth wire

  • green and yellow stripes
  • 0V
  • only carries current if there is a fault
  • safety wire to stop appliance from becoming live
  • connected to earth and casing
  • If the live wire touches the metal casing of the appliance, it will become live (you’ll get a serious electric shock if you touch it, as current flows through you to the ground)
• step up transformers

  • increase pd from power station to the national grid
  • so as power is constant (P=IV) current decreases so less energy is lost
• step down transformers

  • decrease the pd
  • from national grid to the consumers
  • for consumer safety
• static electricity

  • When two insulators are rubbed together
  • Electrons are transferred from one object to the other
  • Forming a positive charge on one object and a negative charge on the other
• If conductors were rubbed, electrons will flow in/out of them cancelling out any effect, so they stay neutral

  Insulators become charged because the electrons cannot flow

  A positive static charge forms on object which loses electrons

  A negative static charge forms on object which gains electrons
• specific latent heat: the amount of energy needed to change the state of 1kg of a substance without changing its temperature
• graph shows temp of ice

  • At A it is Solid.
  • At B, reaches 0°C .
  • From B to C there is no temperature change because the energy is used through melting.
  • From C to D it is in liquid state.
  • From D to E the water is boiling. This takes longer, because evaporation takes more energy
  • From E to F the gas is heating.

  
• Pressure


  The total force exerted by all of the molecules inside the container on a unit area of the walls
• Gas molecules


  • In constant random motion
  • Temperature is related to the average kinetic energy of the molecules
• As temperature increases


  The average kinetic energy and average speed of the molecules increases
• Gas pressure


  1. Molecules collide with the wall of their container

  2. Exert a force on the wall
• Pressure law


  Changing the temperature of a gas, held at constant volume, changes the pressure exerted by the gas
• • A gas can be compressed or expanded by pressure changes. The pressure produces a net force at right angles to the wall of the gas container (or any surface).
  • Increasing the volume in which a gas is contained, at constant temperature, can lead to a decrease in pressure (known as Boyle’s law), this is due to the reduced number of collisions per unit area.
• adding more particles to a fixed volume

  • Doing work on a gas means compressing or expanding the gas, so changing the volume
  • Pumping more gas into the same volume means more particles are present, so more collisions occur per unit time with the walls, so pressure increases.
  • Energy is transferred to the particles when more gas is added into the fixed volume, so this heats the gas
• a fixed number of particles for a smaller volume

  • The particles collide with the wall which is moving inward
  • So the particles gain momentum, as the rebound velocity is greater than the approaching velocity
  • So as the particle has a greater velocity, the pressure increases as the particles collide with the walls more frequently (time between collisions decreases)
  • And the temperature also increases, as the kinetic energy of each particle increases.
• When electrons move to a higher orbit (further from the nucleus), the atom has absorbed EM radiation
• When the electrons falls to a lower orbit (closer to the nucleus), the atoms has emitted EM radiation
• gold foil experiment

  • Most 𝛼 particles went straight through (So most of atom is empty space)
  • Some 𝛼 particles were slightly deflected (So nucleus must be charged, deflecting positive 𝛼)
  • Few 𝛼 particles were deflected by >90° (So nucleus contained most of the mass)
• Activity is the rate at which a source of unstable nuclei decays

  So a sample with high activity has a fast rate of decay

  Measured in Becquerel, Bq
• Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay.
• Count-rate is the number of decays recorded by a detector per second. - E.g. a Geiger-Muller Tube
• alpha (α)

  • a helium nucleus
  • highly ionising
  • weakly penetrating (~5cm of air)
• beta minus (β)

  • medium ionising
  • medium penetration (~50cm of air, sheet of paper)
• gamma rays (γ)

  • low ionising
  • highly penetrating (very far in air, few cm of lead)
• The half-life of an isotope is the time taken for half the nuclei in a sample to decay or the time taken for the activity or count rate of a sample to decay by half.
• a short half life

  • The source presents less of a risk, as it does not remain strongly radioactive
  • This means initially it is very radioactive, but quickly dies down
  • So presents less of a long-term risk
• long half life

  • source remains weakly radioactive for a long period of time
• contamination

  • lasts for a long period of time
  • Radioactive contamination is the unwanted presence of radioactive atoms on other materials – the hazard is the decaying of the contaminated atoms releasing radiation