Current, Potential Difference and Resistance

• What is electric current?

  The flow of electrical charge
• What is the equation linking charge, current, and time?

  Q = I t
• What are the units for charge, current, and time?

  Coulombs, Amperes, Seconds
• What can be said about the value of current at any point in a single closed loop?

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

  Potential Difference (V) and Resistance (R)
• What equation should be used to calculate potential difference if current and resistance are known?

  V = I R
• What are the units for potential difference, current, and resistance?

  Volts, Amperes, Ohms
• What is an ‘Ohmic Conductor’?

  A conductor with constant resistance
• What condition is required for an ‘Ohmic Conductor’?

  Temperature must be constant
• List four components for which resistance is not constant as current changes.

  Lamps, Diodes, Thermistors, LDRs
• What happens to the resistance of a filament lamp as the temperature increases?

  Resistance increases
• Why does the resistance of a filament lamp increase with temperature?

  More collisions with electrons occur
• What is different about current flow through a diode?

  Current only flows in one direction
• What happens to the resistance of a thermistor as temperature increases?

  The thermistor’s resistance decreases
• What happens to the resistance of a LDR as light intensity decreases?

  The LDR’s resistance increases
• Give an application for a LDR.

  Street lights often use LDRs
• What are the key equations in electric circuits?

  1. Charge: Q = I t
  2. Potential Difference: V = I R
• What are the characteristics of Ohmic conductors and non-Ohmic components?

  • Ohmic Conductors:
  • Current and potential difference are directly proportional
  • Resistance remains constant
  • Temperature must be constant
  • Non-Ohmic Components:
  • Resistance changes with current
  • Examples: Lamps, Diodes, Thermistors, LDRs