Grade 10

Electric Circuits

Six lessons that build a circuit from the charge upward: current, then voltage, then resistance, then the two ways to wire components together, the power they draw and the real battery that cannot quite deliver it.

6 lessons

  1. 1If electrons crawl, why does the lamp light up instantly?Because the electrons are already spread along the whole wire before the circuit is closed. Closing the switch does not send an electron from the battery to the lamp — it pushes every electron at once, like water in a full pipe. The push travels at close to the speed of light; the electrons themselves drift at millimetres per second.35m
  2. 2What is actually being measured in volts?Energy per unit of charge. One volt means one joule of energy for every coulomb that passes. So a nine-volt battery gives each coulomb nine times the energy a one-volt battery does, and the voltage across a component tells you how much energy that component takes from every coulomb going through it.35m
  3. 3What decides how much current goes through a component?The voltage across it and its resistance. Ohm's law says the current equals the voltage divided by the resistance, so the same voltage drives a larger current through a smaller resistor. On a graph of voltage against current an ordinary resistor gives a straight line, and the resistance is that line's gradient.40m
  4. 4Why do the other lights stay on when one bulb blows at home?Because a house is wired in parallel. In a series circuit there is only one path, and a blown bulb breaks it for everything. In parallel each bulb has its own branch, so one going out does not touch the rest. It is also why every socket in the house receives the full voltage.40m
  5. 5What is an electricity bill counting — watts or kilowatt hours?Kilowatt hours, which is a unit of energy and not of power. Power is a rate — how much energy per second — and is measured in watts. Energy is power times time. An appliance of one kilowatt running for an hour has used one kilowatt hour, and that is what you pay for.35m
  6. 6Why do the lights dim for a moment when a large motor starts?Because every source has internal resistance. The motor draws a large current as it starts, and that current loses voltage inside the source and in the wiring, so the voltage reaching the rest of the house drops briefly. Once the motor is up to speed the current falls and the voltage recovers.40m