4.3 Electric ciruits

• (i) The sum of currents entering a junction is equal to the sum of currents leaving a junction

  Charge is conserved

  (ii) The sum of the p.d is equal to the sum of the emf

  Energy is conserved


  Question

  
• Series circuit:

  Current is the same everywhere

  Voltage splits in a manner directly proportional to voltage

  Parallel

  Voltage is the same across each closed circuit

  Current splits in a manner inversely proportional to resistance
• Answer

  R I V P

  31 0.3 9.3 2.8

  12 0.225 2.7 0.62

  36 0.025 2.7 0.29


  Question

  
• Internal Resistance is the lost volts per unit current

  Terminal pd is the measured pd across the terminals of the cell is always smaller than e.m.f
• Internal resistance =

  E = V + I * r

  E = I (R + r)
• Potential divider Equation

  V out = R2/R1 + R2 V in

  V1/V2 = R1/R2
• Light dependant Resistor:

  
• Thermistor:

  
• Resistance across LDR increases therefore total resistance increases so total current in the circuit decreases,

  pd across fixed resistor decreases as current decreases as resistance is fixed

  Due to Kirchhoff's second law pd across the LDR increases


  Practice

  
• As temperature increases Resistance across the thermistor decreases therefore total resistance decreases

  As a result V across the fixed resistor increases as current increases because resistance is fixed

  Due to Kirchhoff's second law voltmeter reading decreases


  Practice