Materials Needed (for each group of 2-3 students):

  • Multimeter with leads
  • 9v battery
  • 12v automotive brake light (2 filaments)
  • 9-12v DC motor
  • 2 alligator clip jumper wires
  • Lemon
  • Zinc coated nail
  • Copper wire
  • Wire stripper/cutter

Additional material shared with larger group

  • 12v SLA robot battery
  • a few 1.5v AA batteries
  1. Voltage as electrical pressure.
    • Battery concepts: two terminals, one wanting to get rid of electrons, the other wanting to gain them (redox chemical reaction)
    • Discuss Volts as a measure of the intensity of the push to get rid of electrons/pull to acquire them
    • Measuring voltage with a multimeter using COTS 9v and 1.5v batteries (mostly 9v)
    • Taste a 9v battery challenge
    • Measure a big 12v robot SLA battery (supervised)
    • Make cells with a zinc-coated nail, copper wire, and lemon and measured voltage.
    • Introduce alligator clip jumper wires, use them to place cells in series and measure voltage
  2. Current as volume of electrons flowing through a circuit
    • Used alligator clip wires to connect automotive incandescent brake light bulbs to a 9v battery, showing bright filament and less bright filament. Discuss bulb and filament structure
    • Point out how warm wires and bulb get as current flows and how it the brighter filament causes more heat.
    • Configured multimeters for 10A current measurement and inserted in series with bulb to measure current flow with dim and bright filaments. Note more light/heat/work = more current.
  3. Introduce motors
    • Used alligator clips to connect DC motors to 9v battery, observe direction motor spins
    • Reverse polarity and observe motor spins in other direction. Note that direction of motor rotation is caused by direction of current flow.
    • Insert multimeter in series with motor and measure current flow with motor spinning.
    • Slow down shaft with your hand (make motor work harder). Observe current drawn rising with amount of work motor is doing.
    • Note wires getting warm if motor is stalled.
    • Introduce concept of work/power: watts = amps*volts
  4. Resistance, Ohms law
    • Discuss concept of resistance - everything can conduct, but some things conduct better than others
    • Configure multimeter for resistance measurement and measure your own resistance (grab probes)
    • Use pencil to make a heavy line of graphite on paper, place probes about 1" apart on graphite line and measure resistance. Slide probes closer and further apart, watching resistance change as electrons pass through more or less resistive material
    • Introduce Ohm's Law: E=I * R
    • Discuss what happens to the voltage dropped across a resistor (conversion to heat)
    • Introduce concept of wires having resistance which, while small, can still generate significant heat if current is large.
    • Discuss how much current robot motors can draw if stalled (hundreds of amps) and implications with respect to melting insulation, fire, hazards.
  5. Electrical safety
    • Using thicker (lower resistance) wire to carry higher currents (e.g. to motors)
    • Introduce fuses (hands on examination of a glass fuse) and circuit breakers for over-current protection
    • Discuss fuses/breakers in your home: purpose to protect house from burning down if over-current through wires in walls.
    • Discuss voltage/shock hazards (mainly voltages > 48vdc)
    • Discuss short-circuit/overcurrent burn/fire/explosion hazards wrt Robot SLA batteries
Last modified 4 years ago Last modified on Oct 8, 2017, 12:55:29 PM