Chapter 27 Current and Resistance

A photograph of a magnified glowing tungsten filament.

A lightbulb filament is a very thin tungsten wire that is heated until it glows by passing a current through it.

In This Chapter, you will learn how and why charge moves through a wire as a current.

What is current?

Current is the flow of charge through a conductor. We can’t see charge moving, but two indicators of current are:

  • A nearby compass needle is deflected.

  • A wire with a current gets warm.

Current I is measured in amperes, a charge flow rate of one coulomb per second. You know this informally as “amps.”

The figure illustrates a compass placed on top of current carrying wire. The current passes from the left to the right. As current passes the needle of the compass deflects and the wire gets warm.

《 LOOKING BACK Section 23.6 The motion of charges in electric fields

How does current flow?

We’ll develop a model of conduction:

  • Connecting a wire to a battery causes a nonuniform surface charge distribution.

  • The surface charges create an electric field inside the wire.

  • The electric field pushes the sea of electrons through the metal.

  • Electrons are the charge carriers in metals, but it is customary to treat current as the motion of positive charges.

Current is I. Current density J=I/A is the amount of current per square meter.

The figure illustrates the non uniform charge distribution when current passes through a wire.

What law governs current?

Current is governed by Kirchhoff’s junction law.

  • The current is the same everywhere in a circuit with no junctions.

  • The sum of currents entering a junction equals the sum leaving.

The figure depicts Kirchhoff’s junction law. Two wire with and without junctions are shown in two parts.

What are resistivity and resistance?

Collisions of electrons with atoms cause a conductor to resist the motion of charges.

  • Resistivity is an electrical property of a material, such as copper.

  • Resistance is a property of a specific wire or circuit element based on the material of which it is made and its size and shape.

The figure illustrates a uniform positive charge distribution and motion of electrons in a plane. The electric field vector arrows labeled vector E point toward the left.

What is Ohm’s law?

Ohm’s law says that the current flowing through a wire or circuit element depends on both the potential difference across it and the element’s resistance:

I=ΔV/R

《 LOOKING BACK Section 26.4 Sources of potential

The figure illustrates Ohm's Law with the help of a circuit that consists of a battery labeled delta V and a resistor labeled R. A current labeled I flows across the circuit.

Learning Objectives

After studying this chapter, you should be able to

  1. 27.1 Explain the motion of charged particles through a conductor.

  2. 27.2 Apply a statistical model of conduction.

  3. 27.3 Calculate current as the rate of flow of charge.

  4. 27.4 Use conductivity and resistivity to relate current and current density.

  5. 27.5 Apply Ohm’s law to simple circuits with one resistor.