A First Course in Physics · Ch. XIV, §§ 359–368 · Chapter 14: Electricity in Motion
41. Voltage, resistance and Ohm's law
Goal: Your student can explain electromotive force, potential drop along a circuit and resistance using the water analogy, and use Ohm's law (amperes = volts ÷ ohms).
You'll need
- The book, open to these pages
- Two AA batteries in a holder, a flashlight bulb in a holder, and wires
- A soft pencil (graphite) line drawn heavily on paper, or a pencil lead (for a variable resistor)
- A multimeter if you have one (optional)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Voltmeters as high-resistance galvanometers; the water analogy of a cell (a pump and tanks); electromotive force (E.M.F.); the E.M.F. depends only on the materials of the cell; fall of potential along a wire; resistance; specific resistances of metals; resistance proportional to length and inversely to cross section; resistance and temperature; the ohm; Ohm's law.
The lesson, step by step
Hook question
SayWhy do the wires inside the walls of a house stay cool, while the thin wire in an old light bulb glows white hot, with the same current running through both?
Read together
Read §§ 359–368, following the water pictures (Figs. 274 and 278). Ohm's law:
the currents furnished by different galvanic cells or combinations of cells are always directly proportional to the E.M.F.'s existing in the circuits in which the currents flow, and inversely proportional to the total resistances of these circuits
Explain with water
The cell is like a pump (§ 360); voltage is like the difference in water level; current is how much flows per second; resistance is like a narrow, rough pipe. Long thin wires resist more; thick short wires less (§ 365). Copper resists little, German silver (nichrome-like) a lot.
Try it: a pencil dimmer
Make a circuit with the bulb, but instead of connecting the last wire directly, press its bare end on a heavy pencil line (or a pencil lead) and slide it along. A longer stretch of graphite means more resistance: the bulb dims. A short stretch: brighter (§ 365). If you have a multimeter, measure the voltage of one and two batteries and the resistance of the pencil line.
Safety: use only 1.5-volt AA or AAA batteries for every circuit in this course. Never connect anything to a wall outlet, and don't hold a bare wire straight across a battery for more than a few seconds (it gets hot and drains the battery).
Use Ohm's law
amperes = volts ÷ ohms (C = E/R). Practice: 2 volts across 10 ohms gives 0.2 ampere; 0.3 ampere through 100 ohms needs 30 volts; a lamp with 110 volts across it and 0.5 ampere through it has 220 ohms.
What we know now
Ohm's law and these units (volt, ampere, ohm) are exactly the ones used today. The book's "E.M.F." is still used too. Some materials, called superconductors, discovered in 1911, have no resistance at all when very cold.
Say it back
Your student explains, with the water picture, why a voltmeter must let almost no current through it (§ 359).
Afterwards
Read the labels on a few chargers: they often give volts and amperes. Multiply them to find watts (you'll meet that in the next chapter).
Tip
The problems for §§ 359–374 come together at the end of the next lesson.