A First Course in Physics · Ch. XIII, §§ 340–350 · Chapter 13: Static Electricity
39. Potential, the volt, condensers and electrical machines
Goal: Your student can explain electrical potential by comparison with water pressure, define the volt and capacity, describe a Leyden jar, and explain how an electrophorus and an influence machine make charge.
You'll need
- The book, open to these pages
- Two jugs or bottles and a tube (to show the water-pressure comparison)
- A 1.5-volt AA battery and a 9-volt battery (just to read the labels)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Potential difference compared with water pressure; the earth as zero potential; the volt; measuring potential with an electroscope or by spark length; condensers and the Leyden jar; the electrophorus; the Toepler-Holtz and Wimshurst machines. Nine questions follow.
The lesson, step by step
Hook question
SayA battery is labeled 1.5 volts. What does a volt measure?
Read together
Read §§ 340–350, skimming the machine descriptions (§§ 348–350). The analogy:
just as water tends to flow from points of higher hydrostatic pressure to points of lower hydrostatic pressure, so electricity tends to flow from points of higher electrical pressure or potential to points of lower electrical pressure or potential.
Show the analogy
Connect two partly filled bottles with a tube (or simply describe Fig. 249). Water flows from the higher level to the lower until they are even. Charge does the same between two conductors until their potentials are equal (§ 340).
Explain the idea
Potential difference is the work needed to carry a unit charge from one body to another. The volt is the practical unit (§ 342): a flashlight battery gives 1.5 volts, a house outlet in the United States about 120. A spark jumping 1 cm of air means about 30,000 volts (§ 343).
A condenser (today called a capacitor) is two conductors close together with an insulator between; it can hold far more charge at the same potential (§ 344). Your phone and computer contain millions of tiny ones.
Watch only: Leyden jars and static machines
Watch only: don't try this at home. Leyden jars, the electrophorus sparks and the Toepler-Holtz and Wimshurst machines (§§ 345–350) make high voltages, and a charged Leyden jar can give a painful, dangerous shock. Look up a video of a teacher demonstrating a Wimshurst machine or a Leyden jar. Never open or play with real capacitors from electronics either; some hold a charge long after unplugging.
Energy in a spark
Read § 347: the energy in the electrophorus's spark comes from the work of lifting the plate against the attraction of the charges. Conservation of energy again.
Say it back
Your student explains why touching only the knob of an insulated Leyden jar gives almost no spark, but connecting both coats gives a big one.
Questions
Work questions 1–9 on book page 260.
Afterwards
Read the voltage labels on batteries, chargers and appliances around the house (read only; never open them).
Answers to the book's problems
1. The clapper is attracted to one bell, takes its charge, is repelled to the other bell, gives up the charge there, and so carries charge back and forth. 2. No; charge lives only on the outside surface. 3. Surround it with a metal screen or cage connected to the earth. 4. 3 × (12 − (−8)) = 60 ergs (in the book's absolute units). 5. The cloud induces positive electricity in the rod, which escapes from the point into the cloud. 6. Opposite electricity is induced on the nearby grounded conductor and "binds" the charge, so more can be put on before the potential rises. 7. The leaves stand farther apart now; grounding the outer coat binds the inner charge and lowers its potential. 8. The opposite charge cannot flow onto the outer coat to bind the inner charge. 9. The outer coats then supply each other with the opposite charges needed.
Worked out for this site from the scan. If a number in your copy differs, trust the book.