Introduction

Concept First

In the previous chapter, we studied charges that were essentially motionless — whether a charge sat freely on a conductor or was bound within an atom or molecule, it was treated as being at rest. Once charges begin to move, they constitute what we call an electric current, and this motion occurs naturally in many everyday and natural situations.

Lightning is a striking natural example: charges flow from storm clouds down to the earth through the atmosphere, sometimes causing serious, even disastrous, damage. But this flow is far from steady — it is a brief, irregular burst rather than a continuous stream.

By contrast, many devices we rely on every day carry charge in a smooth, uninterrupted way — much like water flowing steadily through a river. A torch and a cell-driven clock are everyday examples of this steady current. This chapter develops the basic laws that describe such steady electric currents.

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Key Ideas from the Introduction

• Charges in motion constitute an electric current; earlier, both free and bound charges were treated as being at rest.
• Currents can be unsteady/irregular — lightning is a natural example, where charge flows from clouds to the earth through the atmosphere, sometimes with disastrous effects.
• Currents can also be steady, flowing continuously like water in a river — as in a torch or a cell-driven clock.
• This chapter studies the fundamental laws governing steady electric currents.

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