Electricity

• V I R

  S A S T

  P. E. S. L
• Kirchoffs 1st Law


  The sum of the current entering a junction is the same as the sum of the currents existing a junction. This an example of conservation of charge
• Kirchoffs 2nd Law


  The sum of the EMFs in a closed loop is equal to the sum of the p.ds. This is an example of conservation of energy.

  
• In parallel circuits the current splits at a junction.
• An example of Kirchoffs 2nd Law in parallel circuit.

  
• Proof of deriving equation for combining resistance in series circuit. The current is the same throughout the circuit so they cancel out.

  
• Deriving the equation for combining resistance in parallel circuits. In Kirchoffs 2nd law the p.d is the same so they cancel out.

  
• Charge carriers will lose energy because of collisions with metal ions, this energy will be dissipated as heat.
• Internal resistance is the lost volts per unit current
• Terminal p.d. is the measured p.d. across the terminals of the cell and it is smaller than the EMF
• EMF= terminal p.d. + lost volts
• The lost volts can then be simplified to internal resistance x current
• Potential dividers are used to measure the potential difference across components in a circuit. It is a ratio or share of the voltage across resistors.
• Where the voltage is indicated in the question regarding potential dividers that is the voltage out.
• Formula

  V1/V2=R1/R2
• What will happen to the voltmeter reading if light intensity decreases in series with a fixed resistor?

  The resistance of the LDR increases as high intensity decreases. The resistance in the circuit increases.

  The current in the circuit decreases because I=V/R, V is fixed in the circuit, R increases.

  The p.d. across the fixed resistor decreases because the current decrease V=IR, R is fixed.

  The p.d across the LDR increases because the p.ds have to up to the fixed EMF, Kirchoffs 2nd Law
• What happens to the reading of the voltmeter if the temperature increases in series with a fixed resistor?

  The resistance of the thermistor decrease.

  The total resistance decreases and the current increases.

  The voltage of the fixed resistor increases.

  The voltage of the thermistor decreases.