Foundational Physics

Short Answer: Voltage drops across a resistor because moving charge transfers electrical energy to the resistor and its surroundings. The charges keep flowing, but each coulomb has less electric potential energy after moving through the resistor in the direction of conventional current. That loss of energy per unit charge is the voltage drop.

What happens inside the resistor?

Imagine a battery pushing charge through a small resistor. As charge moves through the resistor, the electric field does work on the charge carriers. Their interactions with the material transfer energy into the resistor, which usually warms up.

The moving charges are still there after the resistor. But the electrical energy available per unit charge is lower. That is what a voltage drop tells you.

Think of voltage as energy per coulomb

Suppose the voltage across a resistor is 6 volts. That means 6 joules of electrical energy are transferred for each coulomb of charge that passes through it.

The resistor has not removed a coulomb of charge. It has transferred energy associated with that coulomb’s movement through the electric field.

If 2 coulombs pass through the resistor every second, the energy-transfer rate is 12 joules per second, or 12 watts.

That energy usually becomes thermal energy in the resistor and then spreads into the surroundings.

Why doesn’t the current become smaller afterward?

In a steady series circuit, charge has only one path. If less charge left the resistor each second than entered it, charge would continuously pile up inside the resistor.

That does not happen in normal steady operation. The current entering a resistor equals the current leaving it, even though the electric potential is lower on the side toward which conventional current flows.

Same current, lower electric potential: these statements are completely compatible.

What makes the voltage drop larger?

For an ohmic resistor, a larger current produces a larger voltage drop across that resistor. At a fixed current, a larger resistance also means a larger voltage drop. Ohm’s law summarizes this with V = IR.

But the physical meaning comes first: voltage drop is energy transferred per unit charge.

The takeaway

A resistor transfers electrical energy without consuming the charge passing through it. The voltage difference across the resistor tells you how much energy is transferred per coulomb.

When This Breaks

For a resistor in an ordinary steady DC circuit, electric potential decreases in the direction of conventional current. But a battery or other power source can raise electric potential, and a component’s voltage sign depends on which direction you choose when measuring it. Rapidly changing electromagnetic fields may also require more care than a simple circuit-potential picture.

Keep Exploring: Current, Resistance, and Ohm’s Law

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