Current, PD and resistance

• symbol for open switch 


  .

  
• symbol for closed switch 


  .

  
• symbol for a single cell 


  .

  
• symbol for lamp 


  /

  
• symbol for fuse 


  .

  
• symbol for voltmeter


  .

  
• symbol for ammeter 


  .

  
• diode symbol 


  .

  
• thermistor symbol 


  .

  
• resistor symbol


  .

  
• variable resistor symbol 


  .

  
• LDR symbol


  .

  
• LED symbol


  .

  
• what does a resistor do?

  limits the flow of current
• what does a variable resistor do?

  • can vary the current
• what does a thermistor do

  as temperature increases, its resistance decreases
• what does an LDR do?

  • as the light intensity increases, its resistance decreases
• what does a diode do?

  allows current to flow in only one direction
• what does a light emmittign diode (LED) do?

  emits light when current passes through
• what are sources of pd

  • cell
  • battery
  • generator
• what is the definition of potential difference across a component

  • the energy transferred per unit charge flowing from one point to another
• what is the equation linking pd, wd and charge

  V = W/Q
• how should a voltmeter be set up?

  in parralel to the component you are measureing
• what is current

  flow of electrical charge

  amps

  I
• what is a coulomb

  • the quantity of charge that passes a fixed point per second when a current of 1A is flowing
• what equation links current, charge and time

  Q = It

  Q - charge flow in coulombs
• which way do electrons flow in a circuit

  • away from the negative terminal
• where should ammeters be set up

  • in series with the part of the circuit you are measuring
• what is the current like in a closed loop?

  constant throughout all components
• what is resistance

  the oppsition to current - the higher the resistance, the lower the current
• what equation links pd, current and resistance

  V=IR
• What is Ohms law?

  the current through a conductor is directly proportional to the potential difference across it

  • conductors which abide to this are called ohmic conductors - ie wires and fixed resistors
  • this law is only relevant at constant temperatures
• IV graph for ohmic conductor 


  straight line

  
• filiment lamp IV graph 


  • lamp is not an ohmic conductor so current and pd are not directly proportional
  • increased resistance of lamp = increased temperature
  • As current increases, temp increases - due to more atoms in the metal to vibrate, which increases the resistance as less electrons can get through

  
• IV diode graph

  • When the direction of the current is in the same direction as the arrows on the graph, this is forward bias causing an increase in pd and current
  • when the direction of the diode is switched, this is reverse bias which is shown by there being a zero reading of pd and current

  
• what are applications of a thermistor

  • non ohmic conductor
  • can be used as a thermostat as it regulates temperature and dosent exceed a certain point
  • ovens, fridges, fire alarms, boilers
• what are applications of LDRs

  • automatically regulate the light intensity
  • automatic lights, alarm clocks, security lights