2.1.3 Current, Resistance & Potential Difference

• Electric current

  The flow of electric charge around a circuit
• Electric current represents how much charge flows through a conductor per unit time
• Conventional Current

  Flows from the positive to the negative terminal of a power source
• Electron Flow


  Actual movement of electrons from the negative to the positive terminal
• Conventional current is used in circuit diagrams and analysis
• Resistance

  The opposition that a material offers to the flow of electric current
• Factors Affecting Resistance

  • Material
  • Length
  • Cross-Sectional Area
  • Temperature
• Material

  Conductors like copper have low resistance, while insulators like rubber have high resistance
• Length

  Longer conductors have higher resistance
• Cross-Sectional Area

  Wider conductors have lower resistance
• Temperature


  For most materials, resistance increases with temperature
• Potential Difference (Voltage, V)

  The work done to move a unit charge from one point to another in a circuit
• Ohm’s Law


  A fundamental principle that relates current (I), resistance (R), and potential difference (V) in a circuit
• Using Ohm’s Law

  1. To find Voltage

  2. To find Current

  3. To find Resistance
• To find Voltage

  If you know the current and resistance, you can find the voltage: V=I×R
• To find Current

  If you know the voltage and resistance, you can find the current: I=V/R
• To find Resistance

  If you know the voltage and current, you can find the resistance: R=V/I
• Series Circuits


  • The same current flows through all components
  • Total resistance is the sum of individual resistances
  • Total potential difference is the sum of the potential differences across each component
• Parallel Circuits

  • Total current is the sum of the currents through each parallel branch
  • Potential difference is the same across each parallel branch
  • Total resistance is found using: 1/Rtotal=1/R1+1/R2+⋯
• Calculating Voltage, Current, and Resistance

  • Example 1: V=I×R=2 A×10 Ω=20 V
  • Example 2: I=V/R=12 V/6 Ω=2 A
  • Example 3: R=V/I=9 V/3 A=3 Ω
• Electrical Power


  The rate at which electrical energy is transferred or converted in a circuit