Current, potential difference and resistance

• What is this circuit symbol

  Cell

  
• What’s this circuit symBol

  Battery

  
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  Lamp

  
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  Fuse

  
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  Voltmeter

  
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  Ammeter

  
• What’s this circuit symbol

  Diode

  
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  Resistor

  
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  Thermistor

  
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  Variable resistor

  
• What’s this circuit symbol

  LDR

  
• What’s this circuit symbol

  LED

  
• What is an electric current

  The flow of electrical charge
• What equation links charge,current and time

  Q= I x t
• What can be said about the value of current at any point in a single closed loop

  Current is the same at all points in a closed loop
• What two factors does the current in a circuit depend on

  1. Potential difference
  2. resistance
• What is an ohmic conductor ? State the condition required

  • A conductor for which current and potential difference are directly proportional
  • resistance remains constant as current changes
  • temperature must be constant
• List 4 components for which resistance is not constant as current changes

  1. Lamps
  2. diodes
  3. thermistors
  4. light dependent resistors(LDR)
• What happens to the resistance of a filament lamp as the temperature increases? Why?

  • 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
• What is different about current flow through a diode

  • The current only flows in one direction
  • resistance is very high in the other direction, preventing current flow
• State what happens to the resistance of a thermistor as temperature increases

  The thermistors resistance decreases
• Give two examples of when a thermistor may be used

  • 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
• State what happens to the resistance of a LDR as light intensity decreases

  The LDRs resistance increases
• Give an application for a LDR

  • Street lights often use LDRs
  • when light levels become too low, the light gains sufficient current to turn on