What electricity is
What you’ll learn
- What voltage, current, resistance and power each are — in plain words and in their units.
- Why voltage is measured between two points and current along a path.
- Why a circuit has to be a complete loop, and what “open” and “short” mean.
- The difference between power (watts) and energy (kilowatt-hours).
Electricity on the job comes down to four ideas: voltage is the push, current is the flow, resistance is what gets in the way, and power is how fast the work gets done. Learn to keep those four apart — in your head and in how you talk — and the rest of the trade’s theory is arithmetic on them.
The water picture
Everyone teaches it with water because it works for the first three ideas. A pump makes a pressure difference, water flows round the pipes, and a narrow section limits how much gets through.
The four quantities
| Quantity | Unit | What it is |
|---|---|---|
| Voltage (V or E) | volt, V | The electrical pressure difference between two points — the push. |
| Current (I) | ampere, A | How much charge flows past a point each second — the flow. |
| Resistance (R) | ohm, Ω | How much a material or load opposes the flow. |
| Power (P) | watt, W | How fast energy is turned into heat, light or motion. |
Voltage always has two ends. A conductor is “at 120 V” only compared with something else — usually the neutral or ground. That is why a meter measuring voltage has two leads on two points.
Current is a flow along a path. One amp means about six billion billion (6.24 × 10¹⁸) electrons’ worth of charge passing each second. You measure it by putting the meter in the path, or by clamping around one conductor.
Resistance depends on the material, the length and the thickness. Copper and aluminium have lots of loosely held electrons and let current through easily: conductors. Plastic, rubber and glass hold theirs tightly: insulators. A heater element is a conductor made to have enough resistance to get hot on purpose.
Power is voltage times current: P = V × I. A 120 V circuit carrying 10 A delivers 1,200 W. Leave it on for an hour and it has used 1,200 watt-hours, or 1.2 kilowatt-hours (kWh) — the unit on a power bill.
It has to be a loop
Current only flows around a complete path — out from the source, through the load, and back. Break the loop anywhere and it stops everywhere in that loop: that is an open circuit, and it is what a switch does on purpose. Give the current a path that bypasses the load and it rushes through with nothing to limit it: that is a short circuit, and it is what the breaker is for.
Cut a hose and water sprays out. Cut a wire and nothing “spills” — the loop is simply open and the current stops. And a load does not use up current: every amp that goes out on the hot comes back on the neutral. What the load takes is energy.
Worked examples
1. Power from voltage and current
- A clamp meter reads 8 A on a 120 V circuit feeding a space heater.
- P = V × I = 120 × 8 = 960 W.
2. Power versus energy — what it costs to run
- A 1,500 W heater runs 4 hours a day. Convert watts to kilowatts: 1,500 W = 1.5 kW.
- Energy per day = 1.5 kW × 4 h = 6 kWh.
- At a practice rate of $0.12 per kWh: 6 × 0.12 = $0.72 a day, or about $21.60 over 30 days.
3. Open or short?
- A lamp will not light. Voltage from hot to neutral at the lamp holder reads 0 V (or only a ghost reading on a high-impedance meter), but hot to ground reads 120 V.
- The hot is live, so the break is on the return: the neutral is open somewhere between the lamp and the panel.
- No current flows, so nothing lights — even though the hot conductor is fully live. Treat it as live.
Try one: a 60 W lamp left on for 10 hours
60 W × 10 h = 600 Wh = 0.6 kWh. At $0.12/kWh that is about 7 cents.
En la obra
Tips from the job
- When someone says “there’s no power”, ask the useful question: is there no voltage, or no current? A broken neutral can leave full voltage on the hot and nothing working.
- Think of every load as part of a loop. If one part of the loop is open, nothing in that loop flows.
- Keep units straight when you talk: “it’s pulling 12 amps” and “it’s a 1,500 watt heater” are different facts about the same load.
- Water and electricity mix badly for a reason: wet skin has far less resistance than dry skin.
Common mistakes
- Saying “voltage flows”. Current flows; voltage is the push that makes it.
- Treating watts and kilowatt-hours as the same thing.
- Thinking a load “uses up” current. The current out equals the current back.
- Assuming a conductor with no current on it has no voltage on it. An open switch leg can be fully live.
Check yourself
8 questions. Tap an answer — it is marked straight away and the reason opens under it. Get 80% or better and the lesson is ticked off on the year page.
Not started yet.
-
Which quantity is the “push” that makes current flow?
Answer and why
C. Voltage is the difference in electrical pressure between two points. No difference, no push, no current.
-
Current is measured in…
Answer and why
B. Current is the rate that charge flows past a point, measured in amperes. One amp is one coulomb of charge per second.
-
Why do you read voltage between two points but current through a conductor?
Answer and why
B. Voltage is always “from here to there” — like the pressure difference across a pump. Current is how much flows along the path, so you measure it in line with the path (or around it with a clamp).
-
A lamp is switched on. What happens to the electrons that flow through it?
Answer and why
C. The current coming back on the neutral equals the current going out on the hot. What the lamp takes is energy; the charge keeps circulating. This is why a GFCI can compare the two currents.
-
A heater uses 1,500 W for 4 hours. How much energy is that?
Answer and why
B. Energy = power × time = 1.5 kW × 4 h = 6 kWh. Watts are a rate; kilowatt-hours are the amount — what the utility bills.
-
What does resistance do to current, with the same voltage?
Answer and why
B. Resistance opposes the flow. Same push, more in the way, less flow — that is Ohm’s law, next lesson.
-
Which material is a good insulator?
Answer and why
C. Insulators like PVC, rubber and glass have almost no free electrons, so charge cannot move through them easily. Salt water conducts — which is part of why wet work is dangerous.
-
Where does the water-hose picture of electricity mislead you?
Answer and why
B. Cut a hose and the water pours out. Cut a wire and the current stops, because the loop is open. The picture is good for pressure, flow and restriction; it is wrong about open circuits.
Preguntas frecuentes
Is current the flow of electrons?
In a metal wire, yes — loosely held electrons drift along the conductor. By an old convention, circuit diagrams draw current flowing from positive to negative, the opposite way to the electrons. Both give the same answers; this site uses the conventional direction.
What is the difference between AC and DC?
Direct current flows one way all the time, like a battery. Alternating current reverses direction many times a second — 60 times a second (60 Hz) in North America. Everything in this lesson applies to both; the second-year lessons cover what changes on AC.
Ver también
- Hot, neutral, ground — the three wires by what they do.
- Ohm’s law and power — the next lesson puts numbers on all this.
- Glossary — every term, in plain words.
- Conductors and insulation
- All first year lessons
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