What is the difference between voltage and current?
Current is how much charge flows past a point each second, measured in amperes (1 A is 1 coulomb per second). Voltage is the energy each coulomb gains or loses between two points, measured in volts (1 V is 1 joule per coulomb), and the current a voltage drives depends on the resistance in its way.
Two different questions about a circuit
Current answers "how much charge is going by?" Voltage answers "how much energy does each bit of charge carry from here to there?" A common picture is water in pipes: voltage is like the pressure difference between two points, current is like the flow rate. The picture breaks down eventually, but it gets the main point right: the push and the flow are different things.
That difference shows up in how you measure them. Voltage is always between two points, so a voltmeter goes in parallel, across a component. Current is the flow through a component, so an ammeter goes in series, in the path of the charge.
Ohm's law and power with real numbers
For a resistor, V = IR, and the power it turns into heat is P = IV. Put a 9 V battery across a 450 Ω resistor: I = 9/450 = 0.02 A, or 20 mA, and P = 9 × 0.02 = 0.18 W.
Now put 100 Ω and 200 Ω resistors in series across 6 V. The total resistance is 300 Ω, so the current is 6/300 = 0.02 A, the same through both. The voltage splits: 0.02 × 100 = 2 V across the first and 4 V across the second, adding back to 6 V. In parallel the rule flips: each branch gets the full voltage, and the branch currents add.
Mistakes to watch for
- "Current gets used up." It does not: the current into a resistor equals the current out. What the resistor takes is energy, which is why the voltage drops across it.
- "Voltage flows through the wire." Charge flows; voltage is a difference between two points.
- Assuming a high voltage is always dangerous. A static shock can be thousands of volts but moves very little charge; what hurts you is current through your body.
- Applying V = IR to everything. A bulb filament's resistance rises as it heats up, so its current is not proportional to its voltage.
Seeing charge and energy separately
Ask Isotoon "What is the difference between voltage and current?" and you get a short narrated video. The useful version shows charges moving around a loop, counts how many pass one point per second, then shows each charge's energy stepping down as it crosses each resistor. For where this electricity comes from at scale, see how nuclear power works, and for the rest of the subject, physics.
Questions
- Can you have voltage without current?
- Yes. A battery sitting on a shelf has a voltage across its terminals, but no current flows until a complete circuit connects them.
- Which is more dangerous, voltage or current?
- Current through the body does the harm, but voltage is what drives that current through the body's resistance. A high voltage that can only supply a tiny charge, like a static shock, is unpleasant but usually harmless.
- Why is the current the same everywhere in a series circuit?
- Charge cannot pile up or vanish in the wire, so every coulomb that enters one component has to leave it and enter the next. With only one path, the flow rate is the same at every point.