Electricity
Charge, current, voltage and resistance, then Ohm's law, series and parallel circuits, power, the electricity bill and staying safe. The circuit lab lets you build circuits from batteries, bulbs, resistors, switches and fuses and works out every current as you go.
About this course
Benjamin Franklin could see what electric charge did without knowing what caused it. In the 1700s he chose which kind of charge to call positive, and he had no way to see electrons. His choice later turned out to make electrons negative. That old choice is still with us: the direction we draw current in a circuit is the way positive charge would move, which is opposite to the way the electrons in a wire actually go. One electron carries 1.602176634 × 10⁻¹⁹ coulombs, so one coulomb is about 6.24 billion billion electrons.
The seven units start with charge, then current, voltage and resistance. A truck battery sending 720 coulombs through its starter in 4 seconds delivers 180 amperes. Ohm's law follows, then series circuits, where two 12 Ω bulbs on a 6 V battery each get 3 V, and parallel circuits, where the same bulbs each take 0.5 A so the battery supplies 1 A. Power and the electricity bill come next: a 60 W bulb left on for 1,000 hours uses 60 kWh, or $7.20 at 12 cents per kWh. The final unit on safety notes that 10 to 20 mA through the body can lock the hand onto a wire. In the circuit lab you build circuits from batteries, bulbs, resistors, switches and fuses and see every current worked out. You can open a switch to see one bulb go out in series and not in parallel, and close a loop to make a short circuit.
Ohm ran his experiments in 1825 and 1826 with a magnetic needle hung near the wire, since ammeters had not been invented, and published the law in 1827. The Smithsonian Libraries note that his work was ridiculed at first.
Before you start
Ohm's law is a one-line equation, and the series and parallel units add and divide resistances, so the algebra in Linear equations covers what you need. A quick look at Atoms helps with the opening unit on electrons and charge.
When you finish
Afterwards you can calculate current from charge and time, use Ohm's law to find a resistance or current, and work out totals for resistors in series and in parallel. You can also turn watts and hours into kilowatt-hours and a bill, say why two bulbs in series glow dimmer than one, and explain what a fuse does in a circuit.
- 1Electric chargeAn electron's charge has a magnitude of 1.602176634 × 10⁻¹⁹ coulombs, so a single coulomb corresponds to the charge magnitude of about 6.24 billion billion electrons.
- 2Current, voltage and resistanceA truck battery that sends 720 coulombs through the starter motor in 4 seconds is delivering a current of 180 amperes.
- 3Ohm's lawA car headlight drawing 2.50 A from a 12.0 V battery has a resistance of 12.0 ÷ 2.50 = 4.80 Ω.
- 4Series circuitsTwo 12 Ω bulbs in series on a 6 V battery each get 3 V and 0.25 A, and each glows with a quarter of the power one bulb alone would have.
- 5Parallel circuitsTwo 12 Ω bulbs side by side on 6 V each take 0.5 A, so the battery supplies 1 A: twice as much as for one bulb.
- 6Electric power and the electricity billA 60 W bulb left on for 1,000 hours uses 60 kWh; at 12 cents per kWh that is $7.20.
- 7Electrical safetyA current of about 10 to 20 mA through the body can lock the hand onto a wire, and a person soaked with water may have only 10 kΩ of resistance.
Sources for this course (14)
- OpenStax College Physics 2e (openstax.org/books/college-physics-2e): 18.1 Static electricity and charge (amber and the Greek word elektron, like charges repel, glass and silk, Franklin)
- 20.1 Current (I = ΔQ/Δt, 1 A = 1 C/s, truck battery 720 C in 4.00 s, calculator 0.300 mA, conventional current traced to Franklin, drift speed of order 10⁻⁴ m/s, 12-gauge copper wire 2.053 mm, copper density 8.80 × 10³ kg/m³)
- 20.2 Ohm's law (Ohm first showed I ∝ V by experiment, ohmic materials, headlight 12.0 V and 2.50 A = 4.80 Ω, dry person hand to foot about 10⁵ Ω)
- 20.3 Resistance and resistivity (Table 20.1: silver 1.59, copper 1.72, gold 2.44, aluminium 2.65, tungsten 5.6, iron 9.71, nichrome 100, all × 10⁻⁸ Ω·m; headlight cold 0.350 Ω)
- 20.4 Electric power and energy (P = IV = V²/R = I²R, kWh, 1 kWh = 3.6 × 10⁶ J, 60 W bulb for 1000 h at 12 cents per kWh = $7.20, 15 W CFL)
- 20.6 Electric hazards and the human body (thermal and shock hazards, short circuit 120 V across 0.100 Ω = 144 kW, worn cord 2.00 Ω at 10.0 A = 200 W, fuses and circuit breakers, Table 20.3 shock effects, dry skin 200 kΩ, wet 10.0 kΩ)
- 21.1 Resistors in series and parallel
- 21.2 Electromotive force and internal resistance
- 21.3 Kirchhoff's rules
- 23.8 Electrical safety (three-wire system, earth/ground wire, double insulation, GFI trips above 5 mA)
- NIST CODATA elementary charge 1.602176634 × 10⁻¹⁹ C exact (physics.nist.gov/cgi-bin/cuu/Value?e)
- Safety text is general information, not advice for working on mains wiring. Linda Hall Library, Scientist of the Day: Georg Ohm (lindahall.org/about/news/scientist-of-the-day/georg-ohm, experiments 1825 and 1826 with a compass needle, Die galvanische Kette 1827)
- Smithsonian Libraries, Die galvanische Kette (library.si.edu/digital-library/book/diegalvanischeke00ohmg, first publication of Ohm's law, ridiculed at first)
- IEC 60038:2009 standard voltages (230 V grew out of 220 V and 240 V systems; 220 V still used).
Checked: 2026-10-17