Topic 5 - Electricity and Circuits

• energy

  • never created or destroyed - only transferred
  • energy transferred when system changes
  • closed system - system which experiences no external forces - total energy remains the same at all times
• $\Delta GPE\ =$


  $mg\Delta h$

  change in gravitational potential energy = mass X gravitational field strength X change in vertical height
• $KE\ =$


  $\frac{1}{2}mv^2$

  kinetic energy = 1/2 X mass X speed^2
• changes

  • work done - when external force transfers energy to a system
  • electric - chemical potential energy forms electric
  • heating - electric transfers to thermal energy, as work is done against heating coils causing it to heat up
• E =

  Fd

  work done = force X distance moved in the direction of force
• waste

  • energy is rarely 100% transferred
  • some is dissipated so its stored in less useful ways
  • mechanical processes are wasteful when they get hot - usually due to friction
  • energy is dissipated as heat - rise in temp - energy lost to its surroundings
• power

  the rate at which energy is transferred
• P =

  E/t

  power = energy transferred or work done / time taken
• efficiency

  measure of how much energy is not lost as waste
• efficiency =

  useful output energy / total input energy
• gravitational attraction

  attraction between two objects with mass

  larger mass has greater attraction
• electrostatic attraction / repulsion

  larger charge gives greater force

  like charges repel - opposite attract
• magnetic attraction / repulsion

  stronger magnet gives stronger field - having greater force

  like poles repel - opposite attract
• normal contact force

  force is perpendicular to the plane of conduct
• friction

  surfaces that are rough cause friction when moved
• free body diagrams

  • shows the direction of forces that are present in a situation
  • reaction force always acts at the normal to the line of contact from the point of contact
  • friction acts in opposite direction to movement along with line of contact
  • weight always acts downwards acting from centre of mass
• scale drawings

  • length of each arrow represents its size - in relation to the other forces acting on the object
  • direction of larger force shows resultant force
  • if arrows are in opposite directions with equal length:
  • (equal in magnitude but opposite in direction)
  • forces cancel out - objects in equilibrium - object travels at constant velocity
• isolated solid systems

  no forces are present that come from a source outside a system
• skydiver

  • forces that act are air resistance and weight
  • initially - skydiver has no air resistance and the only force acting on him is weight
  • as he falls - accelerates - increasing his speed
  • this makes air resistance increase
  • so resultant force decreases
  • so acceleration decreases - F = ma - not speeding up as fast
  • eventually - weight and air resistance are equal and balance - no resultant force
  • so no acceleration - terminal velocity reached
• vehicle

  • initially - low are resistance and thrust is only hindered by friction
  • air resistance increases - decreasing resultant force
  • eventually - car is travelling at terminal velocity - thrust is balanced by drag and friction - no resultant force acts
  • (this is immediately changed when more thrust is added - it now becomes resultant force until drag increases to balance it again)
• atom - proton

  relative mass - 1

  relative charge - +1
• atom - neutron

  relative mass - 1

  relative charge - 0
• atom - electron

  relative mass - 1/1840

  relative charge - -1
• series circuit

  • closed circuit
  • current is the same everywhere
• parallel circuit

  • branched circuit
  • current splits into multiple paths
  • total current into a junction = total current in each of the branches
  • voltage is the same across each branch
• potential difference

  • measures in volts
  • measured across two points - amount of energy per unit charge to move from one point to the next
  • measured with voltmeter - placed in parallel across a component
  • can be voltage across a component - in closed or open circuit
  • when theres a closed circuit and potential difference - current will always flow
• current

  • measured in amps
  • rate of flow of charge (the flow of electrons in wires) - measured at any single point on the circuit
  • measured with ammeter - placed in series
• resistance

  • greater resistance - harder for charge to flow through component - current is smaller
  • variable resistor - changes amount of resistance of component - changes amount of current that flows in the circuit
• series - features

  • components are connected end to end
  • all current flows through all the components
  • can only switch them all off at once
  • pd is shared across whole circuit
  • current is the same through all parts of the circuit
  • total resistance is the sum of the resistance in each component
• parallel - features

  • components are connected separately to power supply
  • can switch off each component individually
  • pd is the same across all branches
  • current is shared between each of the branches
  • total resistance is less than the branch with the smallest resistance
• how resistance changes with current

  • as current increases - electrons (charge) have more energy
  • when electrons flow through resistor - collide with ions in resistor
  • current here is doing work against resistance
  • this transfers energy to ions - means they vibrate more (heating resistor)
  • makes it more difficult for electrons to flow through resistor - resistance increases and current decreases
  • may be a benefit - toasters use heating filaments that have a high resistance to get hot easily
• how resistance changes with temperature

  • normal wires - same as current - atoms vibrate when hot
  • thermistor only:
  • hotter temps - resistance lower
  • used in thermostats
• how resistance changes with length

  • greater length - more resistance - lower current
  • electrons make their way through more resistor atoms - harder to get through thsn if you were using a shorter wire
• how resistance changes with voltage

  • diode only:
  • diode allows current to flow freely in one direction
  • in opposite direction - has very high resistance - no current can flow
• plug - live wire

  • brown colour
  • carries voltage from mains to appliance
  • may be dangerous even if mains circuit is off - current may be still flowing through it
• plug - neutral wire

  • blue colour
  • completes circuit
• plug - earth wire

  • green and yellow stripes
  • safety wire - used to stop appliance becoming live
  • connected to earth and casing
  • if live wire touches metal casing of appliance - becomes live (will get serious electric shock if you touch it - current flows through you to the ground)
  • connected to metal casing - its low resistance means current will go from casing through earth wire to ground
• plug - fuse

  • connected to live wire
  • if large current passes through live wire - fuse heats up and melts - breaks circuit - prevents fire or damage