Topic 2 – Electricity

• Current


  The rate of flow of charge
• Amperes (A) / I


  Unit of current
• Voltage (p.d.)


  The driving force that pushes charge
• Volts (V)


  Unit of voltage
• Resistance


  The force opposing the flow of charge
• Ohms (Ω) / R

  Unit of resistance
• Charge


  Energy carried by electrons
• Coulombs (C) / Q


  Unit of charge
• Investigating how length of wire affects resistance

  1. Attach crocodile clips to test wire 0cm apart

  2. Voltmeter (Parallel) - to measure voltage

  3. Ammeter (Series) - to measure current

  4. Turn on switch and measure Voltage and Current

  5. Calculate resistance

  6. Turn off switch and move crocodile clips apart e.g 2cm

  7. Repeat steps 3-5 until sufficient wire has been used

  8. Plot Length (cm) vs. Resistance (Ω)

  9. Should be directly proportional (assuming temp is constant)
• Reliable results


  • Wire may heat up (affects resistance) - leave switch until wire cools
  • Keep thickness of wire constant
  • If plot doesn't go through origin means clips weren't 0cm apart at start

  
• Ohmic conductors


  Constant resistance. In both directions

  
• Filament Lamp


  Resistance increases at higher voltages. In both directions
• Diode


  One side low resistance Other side high
• As temperature increases


  Resistance increases
• Investigating IV Characteristics


  1. Set up a standard test circuit for the component you wish to investigate

  2. Voltmeter (parallel) - to measure voltage

  3. Ammeter (series) - to measure current

  4. Place a variable resistor and change its resistance to change current in circuit and voltage across component

  5. Record current and voltage and calculate resistance using R = V/I

  6. Keep changing resistance of variable resistor to get sufficient readings

  7. Plot V/I on graph

  8. Resistance = 1/Gradient à (Steeper gradient → lower resistance)

  
• LDR


  Dark (scared) can't move = High R
• Thermistor


  Cold (freezing) can't move = High R
• Sensing Circuits


  • Automatic night lights
  • Temperature sensors
• Mechanism (to understand)


  1. In Darkness → LDRs R^

  2. Total Circuit R^

  3. Total V is constant and since V = IR, Total I must decrease

  4. Fixed resistors I will decrease

  5. Fixed resistor R is constant, since V=IR, Fixed resistors V will decrease

  6. LDRs V will increase

  7. Bulbs V will increase (parallel)

  8. Bulb turns ON/brightens.

  9. In Heat → Thermistors R decrease

  10. Total Circuit R decrease

  11. Total V is constant and since V = IR, Total I must increase

  12. Fixed resistor I will increase

  13. Fixed resistor R is constant, since V=IR, Fixed resistors V will increase

  14. Fans V will increase (parallel)

  15. Fan turns ON/speeds up.
• Shortened version - What to write in EXAM

  • In Darkness → LDRs R^
  • LDR takes greater share of total V
  • Bulb V also increase because in parallel
  • In Heat → Thermistors R decrease
  • F.R takes greater share of total V
  • Fan V also increase because in parallel
• Series (end-to-end)


  Adds up V, Same everywhere I, Adds up R, Gets smaller with more loops
• Parallel (side-by-side)


  Same everywhere V, Adds up I, Adds up R, Gets smaller with more loops
• Practical - Investigating adding resistor to series or parallel on total resistance

  1. Get a few IDENTICAL resistors

  2. Build a circuit (include Ammeter - Current)

  3. Make note of the voltage of battery

  4. Measure current and calculate resistance using R=V/I

  5. Add more resistors in series or parallel (depending on type of circuit)

  6. Measure current and calculate resistance

  7. Plot (no. of resistors vs. resistance)

  
• UK Mains = AC (230V and 50Hz)
• Batteries are DC
• Devices use DC, which is why we need adaptors to change AC (Mains) into DC
• Blue (left)


  Neutral (0V)
• Green/Yellow (top)


  Earth (0V)
• Brown (right)


  Live (230V)
• When there is a fault (e.g Brown touches metal casing of toaster)

  1. Metal casing becomes live - if you touch toaster you get shocked

  2. Earth wire will absorb the current from casing and send away (to earth)

  3. Casing has low resistance so brown current might also increase in brown

  4. Fuse over brown wire will detect high current and blow - stops brown
• Earth wire and fuse

  • Both safety mechanisms during a fault

  
• Memorise ALL formulae for Topic
• National Grid


  System of Cables and Transformers
• High Current


  • Too much energy lost by heating to thermal stores
• High Voltage


  • More efficient way (less current, less energy lost)
• Transformers


  1. Step UP (increase voltage)

  2. Step DOWN (reduce to safe levels)
• Static Electricity


  • Electrons move and keep building up on an insulating material
  • Object become negatively charged
  • If large p.d develops then e- jump to earth/earthed conductor = Spark/Lightning
• When rubbed with a cloth:

  • Acetate Rod (loses e-)
  • Polystyrene (gains e-)
• Charged materials create electric fields around them

  • Fields point from Positive → Negative
  • The closer the lines the stronger the field.
  • 90o to surface
• Sparks/Lightning


  1. High p.d between charged object and earth

  2. Strong electric fields are created

  3. Electric fields pull off e- from atoms in the air (ionisation)

  4. Air becomes a conductor and carries a current (spark)