Physics paper 1

• Current


  Flow of electrical charge
• Potential Difference


  Driving force that pushes charge round
• Resistance


  Anything that slows down charge flow
• Unit of current


  Ampere, A
• Unit of potential difference

  Volt, V
• Unit of resistance


  Ohm, Ω
• Current in Circuits


  1. No source of potential difference, no current flows

  2. Source of potential difference (battery), current flows
• Factors affecting current through a component

  • The component's resistance (greater resistance, smaller current)
  • The potential difference across the component (greater potential difference, larger current)
• Charge in Circuits


  • Size of the current = rate of flow of charge
  • Charge flow (coulombs, C) = It
• The size of the current is enormous
• Ohmic conductor (e.g. a resistor)


  • Current is directly proportional to potential difference, so resistance doesn't change
  • Graph is linear
• Components with changing resistance (when current through them varies)

  • Filament lamp (current increases, temperature of filament increases, resistance increases, graph is non-linear)
  • LDRs and Thermistors (resistance depends on light intensity or temperature, graph is non-linear)
  • Diode (high resistance in one direction, current only flows in the other direction, graph is non-linear)
• Series Circuits


  1. Current is the same everywhere

  2. Total source potential difference is shared between components

  3. Total resistance of components = sum of their resistances
• Parallel Circuits


  1. Total current flowing around a circuit = sum of the currents through each branch

  2. Potential difference across each branch is the same as the source potential difference

  3. The total resistance of resistors in parallel is less than the resistance of the smallest resistor
• Alternating Current (AC)


  Current that constantly changes direction and is produced by an alternating voltage, used in mains supply
• Direct Current (DC)


  Current that always flows in the same direction and is produced by a direct voltage, supplied by batteries
• Most electrical items have a three-core cable
• Three facts about UK mains AC supply

  • Frequency of 50 Hz
  • Voltage around 230 V
  • Provides alternating potential difference from the mains supply
• Colours of insulation on three-core cables

  • Brown - live wire
  • Blue - neutral wire
  • Green and yellow - earth wire
• Electric shocks


  • Current only flows through earth wire when there's a fault
  • Large potential difference (230 V) produced across body
  • Can cause injury or even death
• Even if a plug socket is turned off, there is still danger of an electric shock
• Any connection between the live wire and the earth can be dangerous - e.g. it could cause a fire
• Energy Transfers


  When charge flows, work is done (and so energy is transferred)
• Power


  Energy transferred per second
• Power Rating


  Maximum safe power an appliance can operate at
• Calculating energy transferred


  1. E = QV

  2. E = Pt

  3. P = VI

  4. P = I^2R
• National Grid


  • A system of cables and transformers that connect power stations to consumers
  • Electrical power transferred at a high potential difference and a low current (more efficient)
• Solid


  • Regular, fixed particle arrangement
  • Strong particle forces
  • Very small distance between particles
  • Vibration only particle motion
• Liquid


  • Irregular particle arrangement
  • Weak particle forces
  • Small distance between particles
  • Slow particle motion
• Gas


  • Irregular particle arrangement
  • Very weak particle forces
  • Large distance between particles
  • Fast particle motion
• Gas pressure


  Caused by gas particles colliding with a surface and exerting a net force on it
• Increasing temperature of a gas

  Increases the average energy in the kinetic energy stores of the gas particles
• Increasing temperature of a gas at constant volume

  Increases the gas pressure
• Density


  Mass per unit volume
• Internal Energy


  The total energy stored by the particles that make up a system
• Heating a system


  Increases the internal energy of the system, which can increase the temperature or change the state
• Changes of state


  • Freeze/melt (solid/liquid)
  • Sublimate (solid/gas)
  • Boil/evaporate (liquid/gas)
  • Condense (gas/liquid)
• Specific Heat Capacity


  The amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C
• Latent Heat


  The energy needed to change the state of a substance
• Specific Latent Heat


  The amount of energy needed to change 1 kg of a substance from one state to another, without changing its temperature