1.2 The quantities
Outcome 2 of 9 in domain 1 · Foundations, direct current
Charge, current, voltage, resistance, power, energy; units, prefixes, orders of magnitude.
My targets
I declare:
Exercises
By target, three levels: the bare notion, the application, the reasoning. Calculations, true-or-false and choices are checked here; for an essay, you read the answer and judge yourself.
Target 10 of 9
Name the six quantities (charge, current, voltage, resistance, power, energy), their unit and their symbol, unaided.
Target 20 of 9
Explain the difference between power and energy with a tool battery: the Wh on the label, the W of the motor, how long it lasts.
Target 30 of 9
Convert between prefixes (mA ↔ A, kΩ ↔ Ω, mAh ↔ Ah ↔ Wh at a given voltage) without error.
Target 40 of 9
Give the order of magnitude of an LED, a Raspberry Pi, an EGO chainsaw and a 100 W panel, in current and in power.
Target 50 of 7
Diagnosis: a "5 Ah" pack that doesn’t last the advertised time, say which quantities to measure to decide between pack and tool.
Read
Kuphaldt, Lessons in Electric Circuits, volume I (DC), chapter 2: Ohm’s Law, "How voltage, current, and resistance relate" for the table of quantities, their symbols and their units, and "Power in electric circuits" for the steam-engine makers’ horsepower, 550 foot-pounds per second, 745.7 W; he does not tell Watt’s story, that is done here. Chapter 4: Scientific notation and metric prefixes, the prefix number line. Chapter 11: Batteries and power systems, "Battery ratings", the amp-hour and what it is worth when the current strays from the test current; its summary calls it an energy capacity, and it is a charge.