electricity

• the current in an electric circuit flows from negative terminal to the positive
• What is current? current is the rate of flow of charge around a circuit
• how can we measure current

  Using an ammeter
• what is current measured in

  Amps
• formula for charge flow: Charge flow = current x time

  
  

  Q=It
• what is resistance: the measure of how difficult it is for the charge to travel around a circuit or through a component in the circuit
• relationship between current and resistance

  inversely proportional
• What is resistance measured in?

  Ohms (Ω)
• what is a resistor: component designed to provide a certain amount of resistance
• what is a variable resistor: allows you to change the amount of resistance in the circuit by moving a slider
• What is potential difference measured in? the amount of energy per unit of charge flowing in the circuit
• what is the PD of a component measured by: voltmeter
• How is a voltmeter connected in a circuit? it is connected in parallel to the component
• Formula for potential difference

  V = IR
• series circuit rules: current flowing through each component is the same

  
  

  total PD of cell is shared between each component

  
  

  in a series circuit, total resistance is the sum of all resistance in all components
• parallel circuit rules: current flowing through each circuit is shared

  
  

  total pd is the same between components

  
  

  adding more resistors in parallel decreases the total resistance-> this is because the total PD is the same across each resistor

  
  

  if one pathway is broken the other still works
• Formula for energy transferred: Energy transferred = power x time

  
  

  E = Pt

  
  

  Energy transferred = charge flow x pd

  
  

  E = Qv
• what are parallel circuits: circuits with more than 1 loop
• what is DC: direct current - charge flows in one direction
• What is AC? alternating current - charge can flow back and then forwards
• what is the voltage & frequency of mains: 230V & 50Hz
• formula for frequency

  frequency = 1 / time period
• how do you get a large amount of power? by either having a large PD or large current
• what is the problem with having a large current to transport large amounts of power: the wires get hot and lots of energy is transferred as heat to the surrounding
• why do we use a large PD when transporting a large amount of power: this means that there is a very low current which delivers high power but loses much less energy as heat - this means it is more efficient
• role of power station: generates electricity

  
  

  need to meet the demand across the country
• step up transformer: increases the PD to around 400000 V

  
  

  reduces the current

  
  

  power is transmitted efficiently
• high-voltage transmission lines: above or below ground

  
  

  high voltage

  
  

  low current

  
  

  well insulated
• step down transformer: decreases the PD to 230V

  
  

  increases current

  
  

  power supply is safe for consumers and appliances
• formula for power: power = current x pd

  
  

  power = current ^2 x resistance
• formula for efficiency: efficiency = useful output work/total input work x 100%
• • Connect circuit as per diagram
  • Clip crocodile clips on resistance wire, 100cm apart
  • Record readings of ammeter and voltmeter
  • Move one clip to make the distance 90cm, record readings again
  • Repeat previous step, reducing length by 10cm each time until minimum length of 10cm is reached
  • Calculate resistance for each length of wire using R=V/I
  • Plot a graph of resistance vs. length of wire
  • Use results to analyze the relationship between resistance and wire length.

  
• static electricity is due to the build up of electric charge
• it is called 'static' electricity because electricity is unable to flow
• the build up of electric charge can cause dangerous sparks
• lightning is an electrical discharge that occurs when charge builds up in the clouds
• static electricity is due to an electric charge that builds up on an insulator
• an insulator is a material that does not conduct electricity
• friction can be used to create a static charge
• If an insulator is rubbed with a cloth, it can become charged in two ways:

  
  

  electrons move from the cloth to the insulator

  
  

  or

  
  

  electrons move from the insulator to the cloth