Electricity

• A voltage against time graph of a charging capacitor. The voltage will reach a maximum value which is equal to the supply voltage. The maximum voltage does not change with the introduction of a resistor into the circuit.

  
• A voltage against time graph of a discharging capacitor. Once the capacitor is fully charged, it will discharge and the voltage across it will decrease to zero.

  
• A current against time graph of a charging capacitor. When the capacitor is fully charged the current through it is zero amps. If a resistor is introduced into the circuit, it will take longer for the current to reach zero.

  
• A current against time graph of a discharging capacitor (blue). When a capacitor is discharging the current moves in the opposite direction to when the capacitor was charging, this is represented by the negative current.
• What does this graph represent?

  Voltage across a charging capacitor

  
• What does this graph represent?

  Voltage across a discharging capacitor

  
• What does this graph represent?

  Current through a charging capacitor

  
• Why is current negative in a discharging capacitor?

  It is moving in the opposite direction
• When is a capacitor considered to be fully charged?


  When its voltage is equal to the supply
• What does this graph represent?

  The total energy transferred to a capacitor

  
• What does the area under this graph calculate?

  The total energy stored in a capacitor

  
• What does the gradient of this graph calculate?

  The capacitance of the capacitor

  
• Is the LED is reverse or forward bias?

  Forward

  
• An LED is an example of a p-n junction, meaning it is a diode. As diodes only work in one direction, they can be placed in forward or reverse bias: where forward bias allows conduction and reverse does not.
• Some applications of capacitors are:

  • Smoothing AC current
  • Camera flashes
  • Touch screens
  • Energy storage
• Potential difference (V) is the same as voltage and is the measure of the amount of energy per unit charge is transferred between two points.
• What is work done?


  The amount of energy used to move a charge between two points
• $V=$$IR$What is this equation?


  Ohm's law
• What happens when two resistors are place in series?

  Their resistances are combined/added
• $1/R=$$1/R1+$$1/R2$ When is this equation used?


  When calculating the total load resistance of multiple resistors in parallel
• Current (I) is the rate of flow of charge through a circuit or component
• What is the name given to this type of circuit?

  Potential divider

  
• In parallel circuits, the current is split between each branch but the voltage is the same for each branch.
• In series circuits, the current is the same everywhere but the voltage is split between components depending on their resistance.
• Power (P) is the amount of energy required each second by a component. There are four equations to calculate power, three of which are derived from Ohm's Law (V=IR).
• Direct current (DC) is when charges drift in one direction only, and the voltage is constant.
• Alternating current (AC) is when the direction of charge drift changes over time, and voltage is constantly changing.
• Vrms represents the DC current or voltage that dissipates the same amount of power as the average AC current or voltage.
• Does this graph show DC or AC current/voltage?

  DC

  
• Internal resistance (r) refers to the resistance within a supply that causes a loss of volts in each charge as they pass through due to the material that composes the supply component.
• Electromotive force or EMF (E) is the total energy supplied to each coulomb of charge when passing through a supply. EMF does not take into account the loss of volts in each charge due to internal resistance.
• What does the y-intercept represent?

  EMF

  
• What does the x-intercept represent?

  Short circuit current

  
• What is calculated through the gradient?

  Negative the internal resistance

  
• Terminal p.d. (Vtpd) is the voltage across the terminals of a cell and the total amount of voltage available to a circuit. Vtpd is the EMF of the cell minus the loss of volts due to internal resistance.
• $E=$$Vtpd+$$Ir$ What does this equation calculate?


  EMF
• What is the SI unit for capacitance?

  Farad (F)
• In insulators, the band between the conduction and valence band is very large which prevents electrons from moving from the valence to the conduction band. This means that the valence band is full.
• In conductors, the conduction and valence band overlap so electrons freely cross between them, allowing for constant conduction. This means the valence band is partially filled.
• In semi-conductors, the conduction and valence band are separated by a small band gap. As the gap is so narrow, when the semi-conductor is at room temperature, the electrons will have enough energy to cross the gap and move into the conduction band. This means that the valence band is partially filled at room temp.