Electricity

• Electrical circuit symbols

  • Switch
  • Lamp
  • Fixed resistor
  • Variable resistor
  • Thermistor
  • Light-dependent resistor (LDR)
  • Semiconductor diode
• Direct current


  Flow of electrons is consistently in one direction around the circuit
• Alternating current


  Direction of electron flow continually reverses
• Charge


  Property of a body which experiences a force in an electric field, measured in coulombs (C)
• One coulomb of charge is equivalent to 6,250,000,000,000,000,000 electrons
• Current


  Rate of flow of electric charge
• One amp is the current that flows when one coulomb of charge passes a point in a circuit in one second
• Measuring current


  • Using an ammeter placed in series with the component
• Potential difference (voltage)


  Measure of energy, per unit of charge, transferred between two points in a circuit. One volt is the potential difference when one coulomb of charge transfers one joule of energy.
• Measuring potential difference


  • Using a voltmeter placed in parallel with the component
• Resistance


  Conductors have a low resistance, insulators have a high resistance
• Resistance is directly proportional to length
• Investigating current-voltage graphs


  Measure and observe current and potential difference, use appropriate apparatus and methods to measure current and potential difference for a resistor, bulb and diode
• Components investigated


  • Resistor
  • Bulb
  • Diode
• For a fixed resistor, the potential difference is directly proportional to the current (Ohm's Law)
• In a filament bulb, the current does not increase as fast as the potential difference as the resistance of the bulb increases with temperature
• A semiconductor diode only allows current to flow in one direction
• Series circuits


  • Electrical components are connected one after another in a single loop, current is the same through each component, total potential difference is shared between components, total resistance is the sum of individual resistors
• Parallel circuits


  • Electrical components are connected alongside one another, forming extra loops, current splits as it leaves the cell and passes through one or other of the loops, potential difference is the same across each parallel component, overall resistance is reduced as current may follow multiple paths
• Current in parallel


  Since there are different loops, the current will split as it leaves the cell and pass through one or other of the loops
• An ammeter placed in different parts of the circuit will show how the current splits
• Current (I)


  Measured in amps (A)
• Potential difference in parallel

  Since energy has to be conserved, the energy transferred around the circuit by the electrons is the same whichever path the electrons follow. Since potential difference is used to measure changes in energy, the potential difference supplied is equal to the potential differences across each of the parallel components
• Potential difference (V)


  Measured in volts (V)
• Resistance in parallel


  If resistors are connected in parallel the supply current is divided between them. The overall resistance is reduced as the current may follow multiple paths
• Key facts about parallel circuits

  • The total current supplied is split between the components on different loops
  • Potential difference is the same across each loop
  • The total resistance of the circuit is reduced as the current can follow multiple paths
• There are different ways to investigate resistor networks
• In this practical activity, it is important to record potential difference and current accurately and use appropriate apparatus and methods to measure potential difference and current to work out resistance
• The aim of the experiment is to compare the total resistance in series and parallel arrangements
• Method


  1. Set up the circuit as shown in figure 1, turn the power supply on and close the switch

  2. Record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I, where R is resistance, V is potential difference and I is current

  3. Change the resistor and repeat step two to find the resistance of a second resistor

  4. Arrange the two resistors in series as shown in figure 2 and close the switch

  5. Record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I

  6. Arrange the two resistors in parallel as shown in figure 3 and close the switch

  7. Record the voltmeter an ammeter readings once again and calculate the total resistance of both resistors in parallel
• Results


  • Resistor R1: Potential difference (V) 4.00, Current (A) 0.40, Resistance (Ω) 10
  • Resistor R2: Potential difference (V) 4.00, Current (A) 0.40, Resistance (Ω) 10
  • Resistors in series: Potential difference (V) 4.00, Current (A) 0.20, Resistance (Ω) 20
  • Resistors in parallel: Potential difference (V) 4.00, Current (A) 0.80, Resistance (Ω) 5
• Placing the resistors in series causes the resistance to be double that of a single resistor because there is only one path for the electrons to follow - the supply must drive current through one resistor and then the other
• Placing the resistors in parallel causes the resistance to be half that of a single resistor
• As electrons flow through wires, they collide with the ions in the wire which causes the ions to vibrate more. This increased vibration of the ions increases the temperature of the wire. Energy has been transferred from the chemical energy store of the battery into the internal energy store of the wire
• Power (P)


  Measured in watts (W)
• One watt is equal to one joule per second (J/s)
• Resistance (R)


  Measured in ohms (Ω)
• Transmitting energy at a high voltage with a low current will keep the wires cooler and waste less energy
• Direct current


  A direct current flows in only one direction
• Alternating current


  An alternating current regularly changes direction