Volts, Amps, Ohms, and Watts

Amps: how much flows

Electric current is charge moving through a wire. Its unit, the ampere (amp, A), is one coulomb of charge passing a point each second ¹. In copper, loosely held outer electrons do the moving.

They move surprisingly slowly. With 1 amp of direct current in 14 AWG wire, a single electron drifts about 1.8 meters per minute, roughly ten seconds to travel a foot. The light comes on instantly because all the free electrons along the wire start moving at once, like water in a full hose ¹.

Current always flows in a loop. It leaves its source and returns to the same source; break the loop anywhere and it stops ¹. That one idea explains switches, breakers, and every shock.

Volts: the push

Voltage is usually described as pressure or push. Strictly, it’s the energy carried by each bit of charge: one volt is one joule per coulomb ¹. The charges pick up energy at the source and give it up in the loads, as heat in a heater or light in a lamp.

Household circuits are 120 volts and 240 volts. Anything over about 15 volts can be dangerous ².

Ohms: resistance

Resistance, measured in ohms (Ω), is how strongly a material opposes current. It depends on the material and its shape: double a wire’s length and you double its resistance; double its cross-sectional area and you halve it ¹. As an example, 200 feet of 12 AWG copper has about 0.36 ohm ¹.

Metals are conductors; glass, plastic, porcelain, and dry wood are insulators. Impurities in water make it conduct, so wet wood and wet skin become conductors ³.

Ohm’s law

The three are tied together by Ohm’s law: volts = amps × ohms. Draw a triangle with V on top and A and Ω on the bottom, cover the one you want, and what’s left tells you whether to multiply or divide ⁴.

A practical use is voltage drop. If 15 amps flows through wire with 2 ohms of resistance, 15 × 2 = 30 volts is lost along the wire before it reaches the load ⁴. That’s why long runs to a shop or barn need larger wire.

Watts: volts times amps

Power is the rate of using energy, measured in watts: watts = volts × amps ¹. It uses its own triangle, W over V and A.

Work it for a 1,500-watt space heater on 120 volts:

• Amps = 1,500 ÷ 120 = 12.5 A.
• Ohms = 120 ÷ 12.5 = 9.6 Ω for the heating element.
• Running it 4 hours uses 1.5 kW × 4 h = 6 kilowatt-hours, the unit on your electric bill.

Two notes. The simple watts = volts × amps works directly for resistive loads like heaters and incandescent bulbs; motors need a correction called power factor ⁴. And motors are rated in horsepower: 1 horsepower = 746 watts ¹.

Milliamps and your body

A shock is current through your body, and it takes very little. For current passing hand to foot for one second, OSHA gives these effects ³:

• Under 1 mA: not felt.
• 1 mA: faint tingle.
• 5 mA: slight shock, not painful but disturbing.
• 6–30 mA: painful; the “let-go” range, where muscles may clamp onto the conductor.
• 50–150 mA: extreme pain, breathing stops; death possible.
• 1,000–4,300 mA: the heart stops pumping normally; death likely.
• 10,000 mA: cardiac arrest and severe burns.

Your body completes a circuit when it touches both wires, a hot wire and ground, or a metal part that has become energized and ground ³. Most shock deaths are reported to come from ordinary 120-volt contact ⁵. A 15-amp breaker trips at 15,000 milliamps, hundreds of times more than it takes to kill. Breakers protect wire. People are protected by GFCIs, safe practice, and shutting the power off.

What to take from this

Amps are charge per second, volts are energy per charge, ohms are resistance (double the length, double the ohms), and watts are volts times amps. Ohm’s law (V = A × Ω) and the power formula (W = V × A) solve almost every everyday problem: a 1,500 W heater draws 12.5 A at 120 V. Current only flows in a complete loop. A few tens of milliamps through the body can kill, far below what any breaker notices.