ELECTRICITY

• Electricity


  The flow of charge or charges like electrons, they carry energy from a source of energy to a component where the energy is released as another type of energy
• Simple circuit


  • Cell
  • Battery (several cells connected in line)
  • Wires (drawn as straight lines)
  • Lamp/Light bulb
  • Complete loops of components and wires required for charges to flow
• How a circuit works

  1. Battery has a store of chemical potential energy

  2. Energy transferred to electrons which move through wires

  3. Movement of charge is called current, always flows from positive to negative terminal

  4. Electrons' energy converted to light and heat in bulb

  5. Electrons pushed back round to battery to be refilled with energy
• Coulomb


  Unit used to measure charge, similar to moles in chemistry
• Potential difference (PD) / Voltage

  Measure of how much energy is transferred per coulomb of electrons
• Measuring PD/Voltage


  1. Use a voltmeter, connected in parallel to the component

  2. Voltmeter measures the PD across the component
• Current


  Rate of flow of charges, measured in amps
• Measuring current


  1. Use an ammeter, connected in series with the component

  2. Ammeter measures the current flowing through the component
• Resistance


  Components resist the flow of charge/current through them
• Increasing PD


  Results in greater current, PD and current are directly proportional
• Ohmic


  Resistors have constant resistance, shown by a straight line graph of PD vs current
• Non-ohmic


  Components like light bulbs have changing resistance, shown by a curved graph of PD vs current
• Resistance of metals increases with higher PD and current due to increased collisions between electrons and ions</b>
• Diode


  Only allows current to flow in one direction, has very low resistance in one direction and very high resistance in the other
• LED (Light Emitting Diode)


  Similar to a diode but also emits light
• Measuring resistance of a wire

  1. Measure V and I, then use Ohm's law to calculate resistance

  2. Vary length of wire to see how resistance changes
• Series circuits


  • Total PD shared between components
  • Current is the same for all components
  • Total resistance is the sum of all resistances
• Parallel circuits


  • PD is the same for each branch
  • Current is shared between branches
  • Total resistance decreases as more resistors are added in parallel
• Resistor using 4 volts


  The other resistor must be using up 2 volts
• Using Ohm's law


  1. Find the resistance of the other resistor

  2. Total PD, total current, total resistance
• Rule of thumb for resistors

  • The greater the resistance, the greater the share of the total PD it gets
• Parallel circuits


  • PD is the same for every branch
  • Current is shared between each branch
  • The more resistors you add in parallel, the lower the total resistance
• The two resistors are connected to the 6 volt battery in parallel
• The PD for both resistors has to be 6 volts in a parallel circuit
• Calculating current in parallel circuit

  1. Total current is 0.5 amps

  2. Top resistor has 0.2 amps

  3. Bottom resistor has 0.3 amps
• Thermistor


  Resistance decreases if the temperature increases
• LDR (light dependent resistor)


  Resistance goes down with increased light intensity
• Power


  The rate of energy transferred, can be calculated as P=VI or P=I^2R
• DC (direct current)


  Current that only flows in one direction
• AC (alternating current)


  Current that flows back and forth, resulting from an alternating PD
• In a socket, the blue wire is the neutral, the brown wire is the live, and the yellow/green wire is the earth
• Fuse


  Designed to melt or blow if the current exceeds a certain number of amps, usually 3, 5 or 13
• Calculating fuse size for an appliance

  Use P=VI to find the current, then choose the next fuse size up from the calculated current
• Transformers


  • Step-up transformers increase the transmission voltage to over 100,000 volts
  • Step-down transformers reduce the voltage back down to 230 volts for homes
• Transformer efficiency


  In an ideal world, the power in and out of a transformer should be the same, meaning 100% efficiency
• How transformers work


  1. The alternating current in the primary coil produces a magnetic field

  2. The magnetic field induces a voltage and current in the secondary coil

  3. The ratio of turns in the coils is equal to the ratio of the voltages
• Electrostatic charge


  When insulating materials are rubbed together, electrons are transferred, leaving one object positively charged and the other negatively charged
• Electric field


  The space between two objects with different charges, represented by field lines pointing from positive to negative
• A single charged object creates a radial electric field around it