How many basic energy sources can detach electrons from their orbits and sustain electric current according to the material?

• A. Four
• B. Five
• C. Six
• D. Seven

Answer: (C)

To sustain electric current, there must be a continuous supply of free electrons, which means energy sources that can overcome the binding of electrons to atoms. The material identifies six basic ways this can happen.

Chemical energy powers devices like batteries and fuel cells, where chemical reactions push electrons out of one material and into the circuit, providing a flow of charge. Light energy can liberate electrons from a surface via the photoelectric effect; photons transfer their energy to an electron enough to overcome its binding, creating a current when those electrons are collected. Thermal energy can cause thermionic emission, where heating a filament or surface gives electrons enough energy to escape into a surrounding medium or vacuum and join the circuit. Mechanical energy, through motion in a magnetic field or by moving conductors, drives electrons via induction and the Lorentz force, turning mechanical work into electrical current. Nuclear energy can liberate electrons through ionizing radiation, which can eject electrons from atoms and contribute to current in certain devices. Finally, strong electric field energy can pull electrons out of atoms directly in a process known as field emission, creating free carriers that can be directed as current.

So six distinct energy sources—chemical, light, heat, mechanical, nuclear, and electric-field-based emission—can detach electrons from their bound states and sustain electric current.