Electric Quantities Table
Reference list (Q, I, V, R) and Ohm's Law Calculator.
Ohm's Law Solver
Symbols & Units
| Quantity | Sym | Unit |
|---|---|---|
Charge | Q | CoulombC |
Current | I | AmpereA |
Voltage (PD) | V | VoltV |
Resistance | R | OhmΩ |
Power | P | WattW |
Resistivity | ρ (rho) | Ohm-meterΩ·m |
Conductance | G | SiemensS |
Capacitance | C | FaradF |
Inductance | L | HenryH |
Speak Electricity Fluently.
To dominate Physics, you must know your $Q$ from your $I$. Every quantity tells a story about how energy flows, pushes, and resists.
The "Water Pipe" Analogy
Voltage (V)
Water Pressure
The "Push" from the pump. High voltage means a deeper tank or stronger pump pushing the electrons harder.
Current (I)
Water Flow Rate
The actual movement of water. Describes how many "gallons" (Coulombs) pass a point per second.
Resistance (R)
Pipe Narrowness
A narrow pipe or a clogged filter. It fights against the flow. Thin wires have higher resistance than thick ones.
Units Cheat Sheet
| Quantity | Base Definition |
|---|---|
| Volt ($V$) | Joules per Coulomb ($J/C$) |
| Ampere ($A$) | Coulombs per Second ($C/s$) |
| Watt ($W$) | Joules per Second ($J/s$) |
| Coulomb ($C$) | Ampere-Seconds ($A \cdot s$) |
Caution: The "High Voltage" Myth
A Van de Graaff generator can charge to 100,000 Volts, but it won't kill you because the current ($I$) is tiny. Conversely, a car battery is only 12 Volts but can deliver huge current (Amps) that can melt a wrench. It's the combination of Voltage AND Current (Power) that does the damage!
Frequently Asked Questions
What is Ohm's Law?
Ohm's Law states that Voltage ($V$) is directly proportional to Current ($I$) for a given Resistance ($R$). Formula: $V = I \times R$. If you increase voltage, current increases. If you increase resistance, current decreases.
What implies the symbol "I" for Current?
The symbol "I" stands for "Intensité" (Intensity), from the French phrase "Intensité du courant" (Intensity of current). It was established by André-Marie Ampère long before "C" for current could be considered.
What is the difference between Voltage and Current?
Current (Amps) is the flow rate of charge (how many electrons pass per second). Voltage (Volts) is the pressure or force pushing those electrons. You can have high voltage with no current (like static electricity), but you need voltage to drive a current.
Which direction does Current flow?
By historic convention, Current flows from Positive (+) to Negative (-). However, physical electrons actually flow from **Negative (-) to Positive (+) **. We still use "Conventional Current" for all circuit diagrams.
What is Power (P)?
Electrical Power is the rate at which energy is used. Formula: $P = V \times I$. It is measured in Watts ($W$). A 100W bulb uses more energy per second than a 60W bulb.
What is Resistivity (ρ)?
Resistivity (Rho, $\rho$) is a property of the material itself, unlike Resistance which depends on shape and size. Copper has low resistivity; rubber has high resistivity. $R = \rho L / A$.
What is AC vs DC?
DC (Direct Current) flows in one constant direction (e.g., Batteries). AC (Alternating Current) changes direction back and forth periodically (e.g., Wall outlets). AC is better for transmitting power over long distances.
What is EMF?
EMF (Electromotive Force) is the maximum potential difference a battery can provide when no current is flowing. It's the total energy given to each coulomb of charge.
Why is Resistance measured in Ohms?
It is named after Georg Ohm, a German physicist who discovered the relationship between V, I, and R. The symbol $\Omega$ (Omega) is used because "Ohm" sounds like "Omega".
How do I calculate charge (Q)?
Charge is Current multiplied by Time. Formula: $Q = I \times t$. If 2 Amps flow for 10 seconds, the total charge moved is $2 \times 10 = 20$ Coulombs.