Electricity

• Series circuit


  Only one path for the current to follow
• Parallel circuit


  Current can split at a junction
• In a series circuit, the current is the same at every point
• In a series circuit, the voltage is shared between the components</b>
• Kirchhoff's first law


  • The sum of the currents entering a junction is equal to the sum of the currents exiting a junction
• Kirchhoff's second law


  • The sum of the EMFs in a closed loop is equal to the sum of the potential differences
• In a parallel circuit, the potential difference is the same across each individual loop
• Adding resistors in series: the total resistance is the sum of the individual resistances
• Adding resistors in parallel: the total resistance is given by 1/Rtotal = 1/R1 + 1/R2
• Adding resistors in parallel decreases the total resistance
• Adding resistors in series and parallel

  1. Find total resistance of parallel part

  2. Add series and parallel parts to get total resistance
• Finding current and voltage in a circuit

  1. Find total resistance

  2. Use V=IR to find current

  3. Find voltage across each resistor
• Analyzing a more complex circuit


  1. Find EMF using Kirchhoff's 2nd law

  2. Find resistance of each resistor using Ohm's law

  3. Find current in each branch using Kirchhoff's 1st law

  4. Find voltage across each resistor
• Internal resistance


  Energy lost due to collisions with ions in power supply, modeled as a small resistor in series with the power supply
• Terminal voltage


  Measured voltage across power supply terminals, less than EMF due to internal resistance
• Equation for internal resistance in a circuit with external resistor


  e = v + i * r
• Experiment to determine internal resistance


  1. Connect cell to variable resistor, ammeter in series, voltmeter in parallel

  2. Vary resistance, measure current and terminal pd

  3. Plot graph of v against i

  4. Intercept = emf, gradient = -internal resistance
• Potential divider


  Circuit which divides the voltage
• Output voltage is across r2, can choose any component for r2
• LDR


  • Resistance decreases as light intensity increases
• Thermistor


  • Resistance decreases as temperature increases
• Light intensity decreases


  Resistance of LDR increases, total resistance increases, current decreases, voltage across fixed resistor decreases, voltage across LDR increases
• Temperature increases


  Resistance of thermistor decreases, total resistance decreases, current increases, voltage across fixed resistor increases, voltage across thermistor decreases