1. Intro

• Electric current


  Movement of drift of electrons along a conductor
• Electric current


  Current exists when electrons move around a closed conducting path by a source of electromotive force (emf)
• Electromotive Force (EMF)


  Unit: Volt (V), EMF of 1V causes a current of 1A to flow and dissipates energy at a rate of 1J/s (1 watt)
• Electromotive Force (EMF)


  Sources: chemical reactions in a battery, mechanical power in a generator, light energy in a solar cell

  
• Resistance


  Unit: Ohm (Ω), resistance of 1 ohm when a current of 1 ampere flows when there is a potential difference of 1 volt
• Ohm's Law


  V = IR
• Resistors


  Devices that obey Ohm's Law
• Conductance


  Unit: Siemens (S), Conductance (G) = 1/Resistance
• Conductors


  • Copper, aluminium, silver, gold, platinum pure metals
• Insulators


  • Most plastics, rubber, glass, ceramics, air
• Semiconductors


  • Carbon, metallic alloys, silicon
• Resistors in Series


  1. Vs = IR1 + IR2 + IR3

  2. Req = R1 + R2 + R3
• Resistors in Parallel


  1. 1/R = 1/R1 + 1/R2 + 1/R3

  2. For two resistors, R = R1*R2/(R1+R2)
• Voltage Divider


  V1 = IR1, V2 = IR2

  
• Current Divider


  I = Vs/R
• Potentiometer


  Variable potential divider with 3 terminals: "top", "bottom" and "wiper"
• Application of Potentiometer


  • Fuel level indicator
• Electrical Power


  Unit: Watt (W), P = VI = I^2R = V^2/R
• Resistance vs Temperature


  • Metallic conductors: resistance increases with increasing temperature
  • Semiconductors: resistance usually decreases with increasing temperature
• Temperature Coefficient of Resistance

  R_Q = R_0 (1 + α*Q), where α is the increase in resistance per K rise in Q, R_0 is resistance at 0°C, Q is temperature above 0°C
• Internal Resistance


  All EMF sources possess internal resistance which reduces the terminal voltage when a current flows from the source
• Practical EMF sources have internal resistance, which causes the voltage at the terminals to reduce when current is taken from the device