18 Introduction to Electric Current, Resistance, and Ohm’s Law

The flicker of numbers on a handheld calculator, nerve impulses carrying signals of vision to the brain, an ultrasound device sending a signal to a computer screen, the brain sending a message for a baby to twitch its toes, an electric train pulling its load over a mountain pass, and a hydroelectric plant sending energy to metropolitan and rural users—these and many other examples of electricity involve electric current, the movement of electric charge.

In earlier chapters, we studied static electricity, where electric charges are present but remain at rest. We examined electric forces and electric fields and learned how charges interact. In this chapter, we shift our attention to moving charge. This transition—from charge at rest to charge in motion—marks the beginning of our study of circuits, energy transfer, and electromagnetism.

What Is Electric Current?

Electric current is defined as the rate at which electric charge flows through a material. In metallic conductors, such as copper wires in a hospital power system, this flow typically consists of moving electrons. In biological systems, however, current is often carried by ions such as Na⁺, K⁺, and Ca²⁺ moving across cell membranes.

Although the scale and materials differ, the underlying physics is identical: current is charge in motion. Whether powering an MRI scanner or generating an action potential in a neuron, electric current represents organized charge flow driven by an electric field.

Energy Transport by Moving Charge

Figure 18.1 illustrates a hydroelectric facility where falling water spins turbines connected to generators. Mechanical energy from the moving water is converted into electrical energy. This electrical energy is then transmitted over long distances as electric current in high-voltage power lines.

Inside the human body, similar energy transfer occurs at microscopic scales. Instead of turbines and copper wires:

• Ion channels open and close in cell membranes.
• Concentration gradients create electric potentials.
• Charged particles move across membranes, producing electrical signals.

In both cases—power grids and physiology—energy is transported by moving charge.

A Unifying Insight: Electricity and Magnetism

As we progress through this unit, we will encounter one of the most important ideas in physics:

All magnetism arises from moving electric charge.

Magnetic fields are not separate from electricity; they are a direct consequence of electric current. This principle explains how electric generators operate, how MRI machines function, and how electromagnetic waves propagate through space.

For students in the life and health sciences, this connection is especially significant. Technologies such as electrocardiography (ECG), electroencephalography (EEG), pacemakers, neural stimulators, and ultrasound imaging all depend on controlled electric currents and their associated magnetic effects.

Why This Matters for Health and Biosciences

Electric current underlies many physiological and clinical processes, including:

• Electrical conduction in the heart
• Neural signaling and synaptic transmission
• Muscle contraction
• Defibrillation therapy
• Medical imaging and monitoring devices

Understanding current allows us to connect physics principles directly to biological function and medical technology. Electricity is not merely an engineering concept—it is fundamental to life itself.

In the sections that follow, we will define electric current quantitatively, examine how it relates to voltage and resistance, and explore how electrical energy is delivered and controlled in both technological and biological systems.