Charge, Current, Voltage, and Resistance
Unit 3: Electricity and Electronics. Lesson 10 of 53 in BMET Academy.
Three quantities explain almost every electrical fault you will ever chase. Learn what each one is, what it is measured in, and how a meter gets at it, and the rest of this unit is arithmetic.
What is actually moving
Everything is made of atoms, and every atom has protons in its core and electrons around the outside. Equal numbers of each make an atom electrically neutral, and electricity is what happens when electrons leave that balance. In some materials those outer electrons are loosely held and can drift from atom to atom. Those materials are conductors: copper and silver are the best common ones, which is why wire is copper and the best contacts are silver plated. In other materials the electrons are locked in place, so nothing drifts. Those are insulators: rubber, glass, plastic, air, and dry skin.
Nothing useful happens until something pushes the electrons along.
The three quantities
- Current, written out as electric current, is the flow: how many charges pass a point each second. It is measured in amps (A) and the symbol in formulas is I. One amp is one coulomb of charge, a fixed and very large number of electrons, passing each second. Small currents are written in mA (thousandths of an amp) and microamps (millionths).
- Voltage is the push: the difference in electrical pressure between two points. It is measured in volts (V) and the symbol is V. Voltage always exists between two places, never at one, which is why a meter reading needs two probes.
- Resistance is the opposition: how hard the material makes it for current to flow. It is measured in ohms and the symbol is R. Large resistances are written in kohm (thousands) and Mohm (millions).
The part that supplies resistance on purpose is a resistor. Fixed resistors carry a printed value, with one band or marking for the resistor tolerance, meaning how far from that value the part is allowed to be. Variable resistors are adjustable: a potentiometer has three legs and works as an adjustable divider, which is what a volume knob and a calibration trimmer are, and a rheostat has two and is wired to set a current.
The water picture
Think of a hose fed by a tank.
voltage the water pressure behind the nozzle current the gallons per minute actually flowing resistance how narrow the hose is
Raise the pressure and more flows. Pinch the hose and less flows. Pressure can sit there with nothing flowing at all, which is exactly what an unplugged outlet does: 120 V present, zero current, nothing happening until a path exists.
How you measure each one
This is the part people get wrong, and the drawing in the lab makes it obvious.
- Volts across. Put the two probes on two points with the power on. You are comparing them.
- Amps in series. Break the circuit and put the meter in the gap so the current has to flow through it, with the power on. The red lead moves to the amps jack.
- Ohms with no power. The meter supplies its own tiny current, so the part must be dead and ideally disconnected at one end. Other parts still wired in parallel pull the reading low.
Ground
Ground is the reference point everything else is measured against, the zero of the system. It is also a safety path: in a grounded device the metal case is wired back to the third pin on the plug, so if a wire inside comes loose and touches the case, current rushes to ground, the breaker trips, and nobody standing beside it becomes the path instead. Those two jobs, zero reference and safety return, are why ground comes up in every chapter after this one.
Charge, Current, Voltage, and Resistance, explained slowly
What is actually going on in the wire
Current is charge in motion. In a metal, the outermost electrons of each atom are barely held, so a push at one end of a copper wire nudges a drift of electrons along the whole length almost instantly. The individual electrons crawl; the effect travels at nearly the speed of light, which is why a light comes on the moment you flip a switch.
Two conventions coexist and both are correct in their own way. Electron flow goes from negative to positive, because electrons are negatively charged and are repelled by the negative terminal. Conventional current, the one every schematic, formula, and diode symbol uses, is drawn from positive to negative. Use conventional current for everything in this course; it is what the arrows mean.
Conductors and insulators
Conductors (electrons move freely) Insulators (electrons are locked) silver, the best rubber copper, the practical choice glass aluminum, light and cheap plastic, including cable insulation gold, for contacts that must not corrode dry air salt water and body fluid ceramic human tissue, poorly but well enough dry, intact skin
Notice the two entries at the bottom left. Body fluid conducts well and tissue conducts adequately, which is exactly why a catheter or a lead that reaches inside the body bypasses the one good insulator a person has.
The three quantities, their units, and their prefixes
Quantity Symbol Unit Common sizes you will read current I amp (A) 1 A = 1,000 mA = 1,000,000 microamps voltage V volt (V) 1 kV = 1,000 V; 1 mV = 0.001 V resistance R ohm 1 kohm = 1,000 ohm; 1 Mohm = 1,000,000 ohm power P watt (W) 1 kW = 1,000 W
Moving between those is just moving a decimal point three places per step. 0.47 A is 470 mA. 19 mA is 0.019 A. 2,200 ohm is 2.2 kohm. Practise that until it is automatic, because every numeric mistake on an electricity question is a decimal mistake, not a concept mistake.
Why each measurement is taken the way it is
- Voltage is a difference, so it needs two points and the circuit must be energized for there to be a difference to see. You put the meter in parallel with the part, across it.
- Current is a flow, so the meter has to be part of the flow. You break the circuit and insert the meter, in series. An ammeter is built to have almost no resistance so it does not change the circuit it is measuring, which is precisely why putting it across a source is a short circuit.
- Resistance is a property of the part, not of the circuit, so the meter supplies its own small test current and the circuit must be dead. If the part is still soldered in, every other path between those two points is in parallel with it and pulls the reading low. A reading that is lower than the marking, in circuit, usually means you are measuring the whole board.
Ground, both meanings
In a circuit, ground is whichever point you have chosen to call zero; every other voltage is stated relative to it. In a building, ground is a physical connection to earth, and the green wire in a cord bonds a device's metal case to it. The second meaning is the safety one: if a hot conductor inside comes loose and touches the chassis, the ground wire gives that current an easy path back, the current rises, and the breaker opens. Without that path the chassis simply sits at 120 V, waiting, and the first person to touch it and something grounded becomes the path. This is why ground continuity, measured chassis to plug pin, is the first test in an electrical safety sequence, with a limit of 0.5 ohm under NFPA 99 for patient-care equipment.