Electrical circuits.

• Kirchoff’s second law the conservation of energy. In any circuit, the sum of the emf (energy given out by the source) is equal to the sum of the pd(s) in a closed loop
• Kirchhoff’s first law states that the total current entering a junction is equal to the total current leaving the junction
• To calculate resistance in series you add them up R1 + R2 = Rn
• To calculate resistance in parallel, use formula 1/R1 + 1/R2. The total resistance decreases because there’s more pathways.
• EMF = when charged particles gain energy from a source (a cell). The greater the EMF = more energy per coulomb. When voltage increase current decreases (Only with emf)
• Internal resistance = the resistance within a cell. E.g. car batteries have a low voltage + internal resistance but high current to turn over the car
• Terminal PD = voltage measured at the terminals of the cell. This is the usable PD. Therefore, terminal pd + lost volts (Energy lost due to work done when charges move through power source)= emf
• In a potential divider, if resistance of resistors are same, pd is equally shared. It’s proportional e.g. 1/4 ; 3/4.
• Temperature sensing circuits - use thermistor

  As temps increase, resistance of thermistor decreases therefore V out decreases.
• Light sending circuits - LDR

  As light intensity increases, resistance of ldr falls so pd across decreases when in position one of two resistors.

  The second resistor would have a higher voltage entering so therefore has a higher v out.
• EMF = energy per unit charge transferred from chemical to electrical

  PD = energy per unit charge transferred from electrical to other means
• To determine internal resistance:Use variable resistor to change resistance of circuit, changing the current in doing so.

  By using y =mx + c (V = -ir + emf) from equation (emf = v +ir)(Ir = lost volts)

  As current increases, terminal pd drops and lost volts increases. When current = 0 terminal pd will = emf. if current is too high, temps increase which increases internal resistance.

  
• Potential divider (read)

  
• Thermistor

  • Small change in temp = large change in resistance
  • Due to charge carriers being released when temperatures increase
  • Resistance decreases (see below) when temps increase

  
• LDR

  • As light intensity increases, resistance decreases
  • this is due to more charge carriers being releases