Current, Potential Difference and Resistance

• Cell


  • +
  • 
    
• Battery


  • +
  • 
    

  +

  • 
    
• Lamp

  • 
    
• Fuse

  • 
    
• Voltmeter


  • V
• Ammeter


  • A
• Diode

  • 
    
• Resistor

  • 
    
• Thermistor

  • 
    
• Variable resistor


  • 
    
• LDR

  • 
    
• LED

  • 
    
• Electric current


  The flow of electrical charge
• Equation linking charge, current and time

  Q = I t

  Charge (Coulombs), Current (Amperes), Time (Seconds)
• Current is the same at all points in a closed loop
• Factors current in a circuit depends on

  1. Potential Difference (V)

  2. Resistance (R)
• Equation to calculate potential difference

  V = I R

  Potential Difference (V), Current (A), Resistance (Ω)
• Ohmic Conductor


  A conductor for which current and potential difference are directly proportional

  Resistance remains constant as current changes

  Temperature must be constant
• Components with non-constant resistance

  • Lamps

  Diodes

  Thermistors

  Light Dependant Resistors (LDRs)
• Resistance of a filament lamp as temperature increases

  Resistance increases

  Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow
• Current flow through a diode

  The current only flows in one direction

  Resistance is very high in the other direction, preventing current flow
• Resistance of a thermistor as temperature increases

  The thermistor's resistance decreases
• Uses of a thermistor

  • In a thermostat to turn a heater on below a certain temperature

  In a freezer to turn on a cooler when the temperature becomes too high
• Resistance of a LDR as light intensity decreases


  The LDR's resistance increases
• Application of a LDR


  • Street lights often use LDRs

  When light levels become too low, the light gains sufficient current to turn on