P2 markings

• Explain how the resistance of a LDR changes with light intensity and give an example of its use

  • as the light intensity increases, the resistance decreases
  • e.g. automatic street lights
• Explain how the resistance of a thermistor changes with temperature and give one example of its use

  • as the temperature increases, the resistance of a thermistor increases
  • e.g. thermostats
• Describe how current varies with potential difference for a fixed resistor and diode

  • fixed resistor - current is directly proportional to potential difference
  • diode - the current travels in one direction only after a threshold voltage
• Describe how a current-carrying conductor placed in a magnetic field experiences a force and explain how this is used in an electric motor

  • conductor experiences a force due to interaction between its magnetic field and external field
  • this force is perpendicular to both the current and the magnetic field
  • this principle is used in electric motors to create rotational motion
• Explain how electromagnetic induction is used in alternators

  • a potential difference is induced when there is relative motion between a conductor and a magnetic field
  • alternators - rotating coil cuts through a magnetic field, causing the magnetic flux through coil to change
  • changing magnetic flux induces an alternating current in coil, as the direction of current alternates with the rotation
• Explain how electromagnetic induction is used in dynamos to generate electricity

  • dynamos induce a potential difference using a rotating coil but they produce direct current using a commutator
  • a commutator in a dynamo reverses the connection of the coil to the external circuit, ensuring the current flows in one direction
  • the faster the rotation of the coil or the stronger the magnetic field, the greater the induced potential difference
• Explain how a transformer works, including the role of EM induction and function of the iron core

  • AC in primary coil produces changing magnetic field
  • this induces an alternating potential difference in secondary coil
  • iron core increases efficiency by concentrating the magnetic field
• Describe and explain the changes made to power before it travels through the pylons of the National Grid

  • voltage is increased using step-up transformers. Current is decreased as a result of voltage increase
  • increasing voltage reduces energy losses as heat in transmission cables
  • lower current reduces resistance heating in wires, improving efficiency
• Explain why doing work on a gas can increase its temperature in the example of a bicycle pump

  • compressing the air reduces the volume of gas
  • work is done on gas by applying a force to compress it, transfers kinetic energy into the gas particles
  • gas particles move faster as their kinetic energy increases, which raises the temperature (more collisions) causing pump and air inside to feel warm
• What is meant by term absolute zero and the effect this condition may have on particles

  • temperature where all particle motion ceases
  • it is -273 degrees Celsius or 0 kelvin
• Explain how levers can make tasks easier

  • levers increase the distance from the pivot
  • reduces the force needed to achieve the same moment
  • this makes tasks easier by providing a mechanical advantage
• Explain how an insulator can become charged by friction

  • electrons are transferred from one material to another when they are rubbed together
  • the material that gains electrons becomes negatively charged
  • the material that loses electrons becomes positively charged
• Explain why sparking is a danger during refuelling of vehicles and describe what could be done to prevent this

  • sparks occur when a static charge builds up and discharges suddenly
  • this can ignite fuel vapours, leading to explosions
  • earthing is used to safely discharge static charge and prevent sparks
• Insecticide sprayers use electrostatic charges to improve their efficiency. Explain how this works and why it is an advantage

  • droplets are given the same charge, causing them to repel and spread evenly
  • this ensures better coverage and reduces waste, making the spraying more efficient
• Explain why pressure in a liquid increases with both depth and density

  • pressure increases with depth because there is more weight of liquid above
  • pressure increases with density because denser liquids exert more force per unit area
• Explain why an object placed in a fluid experiences an upward force and describe how this force is related to the weight of the displaced fluid

  • upthrust = weight of fluid displaced by object
  • this force acts upwards, opposing the weight of the object
  • if upthrust is greater than the weight, the object floats ; otherwise, it sinks
• Upthrust equation

  density of water x volume of cube x gfs
• Specific latent heat

  the energy required to change the state of 1 kg of a substance without changing its temperature
• State what is meant by a moment

  the turning effect of a force
• Over time, the current through the resistor begins to decrease, why?

  • when current flows through the resistor, electrons collide with the ions in the lattice of the resistor
  • this transfers energy to the ions causing them to vibrate more and causing the resistor to heat up
  • the more the ions vibrate, the harder it is for electrons to pass through the lattice so the current decreases
• The hairdryer becomes less efficient the longer it is left on for, why?

  • the longer the hairdryer is left on for, the more the motor heats up
  • this increases its resistance
  • meaning less energy is transferred usefully
• A radio develops a fault such that the live wire is in electrical contact with the neutral wire. Explain whether you think the radio will work while this fault remains.

  No, radio won't work

  • closed loop has been formed where current from live wire is carried away by the neutral wire
  • so no current will flow through the radio
• State two purposes of the insulation that covers the live wire

  • to stop an electric current from flowing out of the live wire and potentially causing a fire from safety
  • also to make it easier to identify the live wire
• The socket is switched oof and the iron is unplugged. Explain whether there is still a danger of the man receiving an electric shock from the plug socket.

  Yes, although there is no current flowing when switched off, there is still a potential difference in the live wire inside the socket

  • touching it could cause a current to flow through you to the Earth
• Explain why houses have fuse boxes

  • to protect the wiring of the house
  • prevent fires if there is a fault
• Explain why replacing fuses with pennies is dangerous

  • pennies will not melt like a fuse
  • if there is a current surge
  • so circuit won't be broken
• Adv and Dis of using circuit breakers instead of fuses

  • breaks the circuit more quickly than fuses
  • more expensive than fuses
• Explain how the earth wire and fuse work when a fault develops with a metal appliance

  • if live wire comes loose and touches metal, large current will flow through the fuse, live wire and earth wire
  • current melts the thin wire in fuse
  • cuts of electricity supply to device
• Suggest why devices need fuses with different ratings

  • fuse needs to be rated slightly higher than the normal operating current
• Give one reason why earthing reduces the risk of fires

  • discharges the aircraft
  • to prevent sparking which could cause a fire
• Explain how an aircraft can become electrically charged as it flies through the air

  • friction between aircraft and air
  • causes electron transfer between aircraft and air
• Explain how charging the door helps the paint to form an even coating on both sides of the door. Use ideas of forces and fields

  • negatively charged door attracts positively charged paint droplets
  • therefore since the electric field is directed towards the door, the positive paint will move to the door
• Explain why the force experienced by the cable changes

  • the transmission wire carries an alternating current
  • the force is caused by this current which varies in size and direction
  • direction of the force depends on direction of current so direction of force also changes
  • magnitude of this force depends on the magnitude of the current so the magnitude of the force also changes
• Suggest one reason why the atmospheric pressure changes with height above the Earth's surface

  air becomes less dense
• Explain why there is less of the ball below the surface of the seawater than below the surface of the fresh water

  • weight of ball = weight of water displaced
  • seawater is more dense than fresh water
  • smaller volume of seawater needs to be displaced
  • to produce the same weight of water displaced