What is Voltage, Current & Resistance?
V-I-R Explained Simply
Three quantities. One equation. The foundation of everything electrical — from a torch battery to a 33 kV overhead crane substation. Let's make this actually make sense.
Photo: Unsplash — Electrical circuit components
Every electrician remembers the moment someone wrote V = I × R on a board in front of them and said: "That's Ohm's Law. Learn it." Some got it immediately. A lot of us nodded politely and spent the next few weeks quietly confused about what exactly each letter meant in practice — and why it mattered beyond passing an exam.
This piece is for everyone who has ever looked at an electrical circuit, a motor nameplate, or a cable sizing chart and thought the foundational concepts were slightly fuzzier than they should be. We're going back to basics. Not dumbed down — just built up properly, with real examples and the kind of explanations that stick.
Voltage
Unit: Volts (V)
Current
Unit: Amperes (A)
Resistance
Unit: Ohms (Ω)
1 Voltage — The Electrical Pressure
Voltage is probably the easiest of the three to misunderstand at first, because it sounds abstract. "Potential difference." "Electromotive force." These phrases don't immediately mean much. But here's what voltage actually is: it is the force that pushes electrical charges through a circuit. Think of it as electrical pressure.
If you've ever used a garden hose, you know that the water only flows because there's pressure behind it — pressure from the water tower or pump. Remove the pressure, and even if the hose is wide open, nothing moves. Voltage is exactly that pressure in an electrical circuit. Without a voltage source pushing charges through a conductor, no current flows, no work gets done, no lights come on, no motor turns.
🚿 The Water Pipe Analogy — How V, I, R Relate to Water Flow
Voltage (V) = Water pressure. A high-pressure pump (like a battery or generator) creates the drive force that makes water (electricity) want to flow. Higher pressure → stronger push.
Current (I) = Flow rate. The actual volume of water flowing through the pipe per second. More pressure and a wider pipe → more flow. In electricity, measured in Amperes (A).
Resistance (R) = A narrowed or partially blocked pipe. It restricts how much can flow for a given pressure. A smaller pipe diameter (higher resistance) means less flow even with the same pressure.
Voltage is always measured between two points — this is why we call it a potential difference. You can't have voltage at a single point in isolation, any more than you can have pressure difference if you're only measuring one end of a pipe. The 240 V across your power socket means there's a 240-volt difference between the live terminal and the neutral terminal. That difference is what drives current through whatever you plug in.
Where Does Voltage Come From?
Voltage is produced by sources that do work on electrical charges — creating a separation between positive and negative charges that naturally want to recombine. A battery does this chemically. A generator does it mechanically, using the principle of electromagnetic induction (a conductor moving through a magnetic field has a voltage induced across it). A solar cell does it optically, with photons knocking electrons loose in a semiconductor material. Different mechanisms, same result: a pressure differential that drives current flow when a circuit is completed.
In a steel plant, you work with voltages across a very wide range. The 1.5 V of a control circuit battery. The 24 V DC of a PLC I/O supply. The 415 V of a three-phase distribution system feeding crane drive panels. The 6.6 kV or 11 kV of the plant HT network feeding transformers. The 33 kV or 132 kV of the incoming grid supply. All of these are voltages — the same fundamental quantity, just at vastly different magnitudes and with proportionately different levels of hazard.
2 Current — The Actual Flow
If voltage is the pressure, current is what actually moves. Current is the flow of electrical charge through a conductor — in practical terms, the movement of electrons through a wire. The unit is the Ampere, named after André-Marie Ampère, and one Ampere means one Coulomb of charge passing a given point in a circuit per second. A Coulomb is approximately 6.24 × 10¹⁸ electrons. So yes — when your crane control panel is drawing 10 A, something in the range of 60 quintillion electrons are streaming past every cross-section of that cable every second.
Current is what actually does the work and causes the effects we care about. It's current that heats a resistor. It's current flowing through a motor winding that creates the magnetic field that produces torque. It's current that creates the arc in arc welding. And critically from a safety perspective — it's current, not voltage alone, that determines how dangerous an electrical shock is to the human body. Voltage creates the driving force, but current is what flows through tissue and causes physiological harm.
⚡ Animated: Current Flowing in a Simple Circuit
AC vs DC — Two Types of Current
There are two types of electrical current, and both appear in industrial environments. Direct current (DC) flows in one direction only — from negative terminal to positive terminal continuously. Batteries produce DC. Control circuits in crane panels typically operate on DC (24 V or 110 V DC is common for safety relay circuits and PLC systems). Crane drive systems — especially older Ward-Leonard systems and many modern variable frequency drive DC bus circuits — also involve DC at various stages.
Alternating current (AC) reverses direction periodically — in India's grid, 50 times per second (50 Hz). This is what comes out of your wall socket and what the 415 V three-phase distribution system in your plant delivers to motor control centres. AC dominates power distribution because it can be transformed efficiently to different voltages — critical for transmitting power over long distances without excessive losses.
3 Resistance — The Opposition to Flow
Resistance is the property of a material that opposes the flow of electrical current. Every material has some resistance — the question is how much. Pure copper has very low resistance per unit length, which is why it's used for electrical conductors. Nichrome wire has high resistance, which is why it heats up when current flows through it — used in industrial heaters and resistance welding. Air has extremely high resistance (effectively infinite under normal conditions), which is why your bare live cable doesn't shock you from across the room.
The unit of resistance is the Ohm, named after Georg Simon Ohm who discovered the relationship between voltage, current, and resistance. The symbol is the Greek letter Omega (Ω). One Ohm is defined as the resistance that allows exactly one Ampere of current to flow when one Volt is applied across it.
Four factors determine the resistance of a conductor: the material it's made from (its resistivity), its length (longer = more resistance), its cross-sectional area (thicker = less resistance), and its temperature (for most metals, resistance increases with temperature). These relationships matter directly in cable selection for industrial power systems — choosing a cable that's too thin for the load current means higher resistance, more voltage drop, more heat generation, and ultimately a safety and reliability problem.
Resistance vs Impedance — A Note for AC Circuits
In DC circuits, resistance is the only opposition to current flow. In AC circuits, two additional effects come into play: inductance (associated with coils and motors — it resists changes in current) and capacitance (associated with capacitors — it resists changes in voltage). The combined opposition to AC current flow from all three effects is called impedance, measured in Ohms but represented by the symbol Z. For practical purposes in basic circuit understanding, resistance is the concept to grasp first; impedance builds on it.
Voltage
The Push
The force that drives current. Created by batteries, generators, solar cells. Measured between two points. The "why" electrons want to move.
Symbol: V | Unit: Volt (V)Current
The Flow
The actual movement of charge. Does the work — creates heat, magnetic fields, light. What makes things happen in a circuit.
Symbol: I | Unit: Ampere (A)Resistance
The Opposition
How much a component or conductor restricts current flow. Converts electrical energy to heat. Controlled in circuit design to limit or direct current.
Symbol: R | Unit: Ohm (Ω)4 Ohm's Law — The Relationship That Ties It Together
Georg Simon Ohm published his findings in 1827 after painstaking experiments with different conductors and circuit configurations. What he established — and what has been verified countless millions of times since — is beautifully simple: for a conductor at constant temperature, the current flowing through it is directly proportional to the voltage applied across it.
Written as an equation, Ohm's Law is:
Ohm's Law — The V-I-R Triangle
The triangle is a memory aid used by electricians everywhere: cover the quantity you want to find, and what remains shows you how to calculate it. Cover V → you see I × R. Cover I → you see V ÷ R. Cover R → you see V ÷ I. Three transformations, one relationship.
What this means physically is equally worth understanding. If you increase the voltage applied to a circuit while resistance stays constant, more current flows — proportionally. Double the voltage, double the current. If you increase resistance while voltage stays constant, less current flows. Double the resistance, halve the current. Voltage and current move together; resistance works against current. This is not just algebra — it describes how real circuits behave.
🔧 Worked Example: Crane Control Circuit Fault Finding
The situation: A 24 V DC crane control circuit has a resistive heating element used to keep the pendant control box free of condensation. The element is rated for a 2 A operating current. You need to verify the correct resistance to specify a replacement element.
Applying Ohm's Law: R = V ÷ I → R = 24 V ÷ 2 A
The verification check: If a 12 Ω element is fitted instead, what current will flow? I = V ÷ R = 24 ÷ 12 = 2 A. Correct. If someone accidentally fitted an 8 Ω element: I = 24 ÷ 8 = 3 A — 50% overcurrent, risk of overheating.
5 V, I, and R in Real Industrial Equipment
The three quantities show up everywhere in industrial electrical work — often in combinations that become second nature once you've worked with them for a while. Here's how they appear in the specific context of steel plant cranes and electrical systems.
| Equipment / Situation | Quantity | Typical Value | Why It Matters |
|---|---|---|---|
| Three-phase crane hoist motor supply | V | 415 V AC (L-L) | Supply voltage determines motor speed characteristics and insulation requirements |
| Hoist motor full-load current | I | Nameplate FLA (e.g., 45 A) | Used to select cable size, fuse rating, overload relay setting |
| Motor winding insulation test (Megger) | R | >1 MΩ at 500 V DC (healthy) | Low insulation resistance indicates moisture ingress or winding degradation |
| 24 V DC PLC control supply | V | 24 V DC ±10% | Out-of-tolerance supply voltage causes erratic PLC I/O behaviour |
| Resistor braking unit (crane deceleration) | R | Calculated for duty cycle | Converts kinetic energy to heat during braking — must be sized for thermal duty |
| Earth continuity resistance (LOTO check) | R | <1 Ω (steel plant requirement typically) | High earth resistance impedes fault current — reducing protective device response speed |
| Conductor rail voltage drop (long runway) | V | Should be <5% of supply | Excessive drop from conductor resistance causes undervoltage at crane end of runway |
6 Common Misconceptions — Set Straight
A few misunderstandings about V, I, and R persist even among people who've been in the trade for years. These are worth clearing up explicitly.
"High voltage is always more dangerous than low voltage."
Body resistance and current path matter. 240 V AC through a wet hand can be lethal. The danger depends on the current that flows through tissue — determined by voltage AND the body's resistance at that moment. Extra-low voltage DC can kill under the right conditions.
"Voltage kills you — just avoid the high-voltage parts."
Current is the direct physiological agent of harm. Ventricular fibrillation can occur at currents as low as 50–100 mA AC through the chest. Voltage matters because it determines current flow given the circuit's resistance — but the current is what causes harm.
"Adding more resistance always wastes energy and should be avoided."
Resistance has essential design roles. Braking resistors in crane VFD systems deliberately dissipate energy. Current-limiting resistors protect sensitive components. Earthing resistors control fault current magnitudes. Resistance is a tool, not just a loss mechanism.
"Ohm's Law applies to all components and materials."
Ohm's Law strictly applies to ohmic conductors at constant temperature — materials where V and I maintain a linear relationship. Diodes, transistors, thermistors, and fluorescent lamp electrodes are non-ohmic: their resistance changes with voltage, current, or temperature.
7 Why This Actually Matters Every Day on the Job
There's a practical reason that every electrical course starts here, and it isn't just academic thoroughness. The decisions an electrical maintenance person makes — fuse selection, cable sizing, relay setting, insulation testing interpretation, fault diagnosis — are all, at their foundation, applications of V-I-R relationships. When you understand what the three quantities mean and how they relate, your instrument readings stop being numbers and start being a story about what's happening in the circuit.
A multimeter reading of 230 V at a terminal block is only useful if you know what voltage should be there, and you can only know what should be there if you understand the circuit. Similarly, an insulation resistance reading of 50 kΩ between a motor winding and earth is only alarming if you know that healthy insulation should typically read in the megohms — and that a reduced reading means current is finding a path through degraded insulation rather than staying within the conductor where it belongs.
Ohm's Law underlies the calculation behind every fuse and circuit breaker selection: if the prospective fault current (I = V/Z, where Z is the impedance of the fault path) doesn't reach a sufficient level, the protective device may not operate fast enough to prevent a fire or equipment damage. It underlies voltage drop calculations that determine whether your crane runway conductor rail can supply the end-of-runway crane with adequate voltage under full load. It underlies motor starting current analysis, which determines whether your transformer can handle the inrush without the voltage dipping enough to affect other connected equipment.
Perhaps most importantly in a maintenance context: understanding V, I, and R makes you a better fault diagnoster. When an overhead crane motor trips on overload, you're dealing with an I (current) problem — but the cause could be a V problem (supply voltage too low, causing higher current draw to maintain torque) or an R problem (increased winding resistance from a partial fault, affecting current distribution), or neither (mechanical overload from a mechanical issue). Knowing which quantity to measure first — and what to do with the reading — separates systematic diagnosis from guesswork.
8 Bringing It Together
Three quantities. One equation. They don't exist independently of each other — change one, and at least one of the others responds. Voltage is the pressure that drives the system. Current is the flow that does the work. Resistance is the opposition that shapes and controls that flow. Together, they describe every electrical circuit that has ever existed, from a child's science experiment to a 132 kV transmission system.
Ohm's Law — V = I × R — is not just something to memorise for a test. It's a lens through which every electrical observation becomes interpretable. The motor that draws more current than it should. The voltage that drops at the end of a long cable run. The insulation resistance that's declining month by month. Each of these is a V-I-R story waiting to be read by someone who understands the language.
If you're starting out in electrical maintenance in a steel plant or heavy industry, time spent really understanding these fundamentals — not just the formula, but the physical meaning — will repay you many times over across a career. If you've been in the trade for years and some of this felt like revision, the best thing you can do with it is take the next opportunity to explain it to someone who's earlier in the journey than you are. The clearest sign that you truly understand something is the ability to make someone else understand it too.
R Sources & References
- Ohm, G.S. (1827). Die galvanische Kette, mathematisch bearbeitet. T.H. Riemann, Berlin. [Original publication of Ohm's Law]
- Bureau of Indian Standards. IS 732:2019 — Code of Practice for Electrical Wiring Installations. BIS, New Delhi.
- Bureau of Indian Standards. IS 3043:2018 — Code of Practice for Earthing. BIS, New Delhi.
- Central Electricity Authority. CEA (Measures Relating to Safety and Electric Supply) Regulations, 2010 (amended 2023). Government of India.
- Hughes, E., Hiley, J., Brown, K. & McKenzie-Smith, I. (2012). Electrical Technology. 10th ed. Pearson Education. [Standard UK electrical engineering textbook]
- Theraja, B.L. & Theraja, A.K. (2005). A Textbook of Electrical Technology, Vol. 1. S. Chand & Company, New Delhi. [Widely used Indian electrical engineering reference]
- IEC 60364. Low-Voltage Electrical Installations. International Electrotechnical Commission. [International standard for LV electrical design]
- IEC 60038:2009. IEC Standard Voltages. International Electrotechnical Commission.
- IEEE Std 241-1990. IEEE Recommended Practice for Electric Power Systems in Commercial Buildings (Gray Book). [Practical guidance on V-I-R in power systems]
- Reeves, E.A. (Ed.). (2003). Newnes Electrical Pocket Book. 23rd ed. Newnes / Butterworth-Heinemann.
- National Safety Council. (2022). Electrical Safety Fundamentals. NSC, Itasca IL. nsc.org
- Bureau of Indian Standards. IS 8884:1978 — Code of Practice for Installation and Maintenance of HT Switchgear. BIS, New Delhi.
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