Topic 10 - Electricity

• Atom


  Consists of a dense central nucleus surrounded by a cloud of negatively charged electrons, the nucleus is tiny more than ten thousand times smaller than the atom itself so the atom is mostly empty space
• Nucleus

  Consists of nucleons - positively charged protons and uncharged neutrons
• Particles that make up atoms

  • Protons
  • Neutrons
  • Electrons
• Symbols used to represent components of a circuit


  • Battery
  • Voltmeter
  • Ammeter
  • Lamp
• Series circuit


  Each component follows on in the same path, one item after another after another
• Parallel circuit


  The two paths run adjacent to each other, so current can go through either the first lamp or the second lamp
• Total current going into a parallel circuit

  Is shared between the two branches of the circuit, so a2 plus a3 is equal to a4
• Total current going out of a parallel circuit

  Is equal to the total current coming in, so a4 equals a1
• Potential difference (voltage)


  The energy transferred per unit charge, hence the definition of the volt as a joule per coulomb
• Joule


  Is equal to coulomb times volt
• Three things all small electrical devices have in common


  • A source of electrical energy
  • A path for the electric current
  • A component that uses electricity to work
• Electric current


  A flow of electrons, electrons are negatively charged and travel away from the negative terminal and towards the positive terminal
• Escalator


  Represents the source of electrical energy, like a battery, it gives the charges potential energy by lifting them to a higher level
• Paddle wheel


  Represents the electrical component, like a light bulb, as the charges pass the wheel they turn it and fall to a lower level, losing potential energy
• Current

  The amount of charge passing a point in one second, measured with an ammeter
• Voltage

  The amount of potential energy the electrons are carrying, measured by a voltmeter
• Series circuit

  Bulbs connected one after the other to form a continuous loop, old-fashioned festive lights were wired in series
• Parallel circuit


  Each component directly connected to the power supply, modern low voltage halogen lamps are connected in parallel powered from a transformer
• Brightness of bulbs in a series circuit

  Decreases as more bulbs are added, because the current is reduced
• Brightness of bulbs in a parallel circuit

  Remains equally bright as more bulbs are added, because each bulb has its own path to take current directly from the supply
• Voltage in a series circuit

  The total voltage of the supply is split between the bulbs, the voltages across each component add up to equal the total voltage of the supply
• Voltage in a parallel circuit

  The voltage supplied by the battery is the same as the voltage across each component, each component is directly connected to the battery
• Current (symbol i)


  Measured in amps, defined as the rate of flow of charge, charges (symbol q) measured in coulombs
• The total current going into any junction is equal to the total current leaving that same junction
• In metals, electrons flow from the negative to the positive, but by convention we do it the other way around
• Potential difference (symbol v)

  Measured in volts, defined as a joule per coulomb
• Energy


  Is equal to charge times potential difference
• Ohm's law


  States that the potential difference is equal to the current times resistance, v = ir
• As resistance increases


  The current will decrease for the same potential difference
• Experiment to investigate i-v characteristics of a lamp

  1. Connect lamp to power supply

  2. Measure current with ammeter in series

  3. Measure potential difference with voltmeter in parallel

  4. Vary potential difference using power supply dial

  5. Record potential difference and current values

  6. Plot graph of i vs v
• Experiment to investigate i-v characteristics of resistors

  1. Connect resistor to power supply

  2. Measure current with ammeter in series

  3. Measure potential difference with voltmeter in parallel

  4. Vary potential difference using power supply dial

  5. Record potential difference and current values

  6. Plot graph of i vs v
• Experiment to investigate i-v characteristics of a diode

  1. Set up potential divider circuit

  2. Measure current with milliammeter in series

  3. Measure potential difference with voltmeter in parallel

  4. Vary potential difference using slider

  5. Record potential difference and current values

  6. Plot graph of i vs v
• Gradient of i-v graph for a resistor

  Is equal to 1/resistance
• 1. v characteristic of a lamp

  Is a curve, because resistance increases with increasing current and temperature
• 1. v characteristic of a resistor


  Is a straight line through the origin, current is directly proportional to potential difference
• 1. v characteristic of a diode


  Only allows current to flow in one direction, once a positive threshold is reached the voltage increases proportionally with the current
• Light dependent resistor (LDR)

  Resistance decreases as light intensity increases
• Thermistor


  Resistance decreases as temperature increases
• Electrons traveling in a wire

  Collide with metal ions in a lattice, converting their kinetic energy into thermal energy which heats the wire
• Low resistance wires


  Have fewer collisions, therefore get less hot