2. Electricity

• Circuit components and their symbols


  • Open switch
  • Closed switch
  • Cell
  • Battery
  • Lamp
  • Ammeter
  • Voltmeter
  • Resistor
  • Variable resistor
  • Thermistor
  • Light dependent resistor (LDR)
  • Diode
  • Light emitting diode (LED)
  • Fuse
• Electrical charge (Q)


  A circuit works when charge is allowed to flow through it. Usually these charges are free moving electrons or ions.
• Electric current (I)


  Electric current is the flow of electric charge, quantified by the amount of charge passing a point in the circuit over time. One of the key characteristics of current in a single closed loop is that it has the same value at any point in the loop.
• Potential difference (V)


  Also known as voltage, potential difference is the measure of energy per unit of charge, transferred between two points in a circuit. Simply put, this is the driving force that pushes the current around the circuit.
• Resistance (R)


  Resistance in a circuit slows down the flow of current. The higher the resistance in a circuit, the lower the current, if the potential difference stays the same.
• Ohm's law


  The relationship between current, resistance, and potential difference can be understood by Ohm's law, which states that the current is directly proportional to the potential difference. A conductor that obeys this law is known as an ohmic conductor.
• Calculating resistance


  1. Measures current (I) in amperes (A). Always connect the ammeter in series with the component.

  2. Measures potential difference (V) in volts (V). Always connect the voltmeter in parallel across the component.

  3. Use the formula V = I x R to find resistance.
• Investigating resistance and length of wire


  1. Connect a crocodile clip to the wire at 0 cm on the ruler.

  2. Connect a second crocodile clip at a set length on the wire being investigated, e.g. 5 cm.

  3. Note the current (I) and potential difference (V).

  4. Move the second clip to a new length, e.g., 10 cm, and record again.

  5. Repeat for various lengths and record all measurements.

  6. Use the formula V = I x R to calculate resistance.

  7. Plot a graph of resistance against wire length and draw a line of best fit.

  8. Ensure that the circuit is disconnected between readings, as the wire would warm up and affect the resistance readings.
• Investigating combinations of resistors in series and parallel


  1. For the first circuit, note down the ammeter and voltmeter values, and use the formula to work out the resistance of resistor 1.

  2. Repeat this method for resistor 2.

  3. Connect the second circuit and work out the resistance.

  4. Connect the third circuit and work out the resistance.
• Resistors in series


  The resistance of the combined resistors in series is equal to the sum of the two individual resistances. This is because the electrons flow through just one path through both resistors, so the current does too.
• Resistors in parallel


  The resistance of the combined resistors in parallel is less than the sum of the two individual resistances. This is because the electrons are split between the different paths (or 'loops') but the resistors still have the same potential difference across them.
• Ohmic conductors


  The current through an ohmic conductor (at a constant temperature) is directly proportional to the potential difference across the resistor. This is shown on the graph as a straight line through the origin. This means that the resistance remains constant as the current changes.
• Filament lamp


  At low potential differences, there is a straight portion on the graph which means the resistance is constant; the graph curves at higher potential differences. As the potential difference increases, the gradient (steepness) of line decreases, which shows that resistance is increasing. This is because the filament in the bulb gets hotter, which happens as more current passes through it.
• Diode


  Diodes allow current to flow in only one direction, and have a high resistance in the opposite direction. On an I-V graph, current flowing in the correct direction is shown by a sharp increase in current on the positive axis (Right hand side of the graph). When the diode is connected in the opposite direction (left hand side of the graph), the graph is flat at 0, indicating no current flowing.
• Light-dependent resistors (LDRs)


  An LDR is a resistor whose resistance varies according to the light intensity of the surroundings: In bright light, the resistance of an LDR decreases, allowing more current (I) to pass through. In darkness, the resistance increases, reducing the current flow.
• Thermistors


  A thermistor is a resistor that changes its resistance with temperature.
• ELECTRONS


  INCREASES RESISTANCE
• DIODE


  Allows current to flow in only ONE DIRECTION, and has a HIGH RESISTANCE in the opposite direction
• Current flowing in the CORRECT direction


  Shown by a sharp increase in current on the POSITIVE AXIS (Right hand side of the graph)
• Diode connected in the OPPOSITE DIRECTION (left hand side of the graph)


  Graph is FLAT at 0, indicating NO CURRENT flowing
• LIGHT-DEPENDENT RESISTORS (LDRs)


  A resistor whose resistance varies according to the LIGHT INTENSITY of the surroundings
• In BRIGHT LIGHT


  Resistance of an LDR decreases, allowing more CURRENT (I) to pass through
• In DARKNESS


  Resistance increases, reducing the current flow
• Devices that react to light conditions


  • AUTOMATIC NIGHT LIGHTS, OUTDOOR LIGHTING, and BURGLAR DETECTORS
• THERMISTOR


  A resistor that changes its resistance according to the TEMPERATURE of the surroundings
• When it's HOT


  Resistance of a thermistor decreases, letting more current through
• In COOL environments


  Resistance increases, reducing the current
• Temperature detectors


  • CAR ENGINE SENSORS and ELECTRONIC THERMOSTATS
• 1. V CHARACTERISTICS


  Graphs that show how the CURRENT (I) through a component changes with the POTENTIAL DIFFERENCE (V) applied to it
• LINEAR COMPONENTS


  • Show a STRAIGHT LINE on the graph, indicating that current and potential difference are DIRECTLY PROPORTIONAL
• NON-LINEAR COMPONENTS


  • Show a CURVED LINE, meaning the relationship between current and potential difference is NOT proportional
• Method for I-V Graphs


  1. Build test circuit with variable resistor, ammeter, voltmeter and component

  2. Change variable resistor to alter current and potential difference

  3. Record readings

  4. Swap connections to check behaviour in both directions

  5. Plot graph of current against voltage
• The graph for an OHMIC CONDUCTOR (like a resistor) will be a straight line
• The graph for a FILAMENT LAMP will start to curve as the current increases due to the filament heating up
• The graph for a DIODE will show current flow in one direction and very little in the opposite direction
• SERIES Circuits


  Components are connected in ONE LOOP
• PARALLEL Circuits


  Components are connected in MULTIPLE LOOPS
• In SERIES Circuits


  The CURRENT is the SAME through each component
• In PARALLEL Circuits


  The CURRENT through the whole circuit is the SUM of the currents through the separate components
• In SERIES Circuits


  The POTENTIAL DIFFERENCE supplied by the battery is SHARED between the components