What does electrophoresis primarily use to separate molecules?

Electrophoresis is a method primarily used to separate molecules, such as nucleic acids or proteins, based on their charge and size. The technique relies on an electric current that is applied across a gel or another medium containing the mixture of molecules. When the electric current is applied, charged molecules migrate towards the electrode of the opposite charge; for example, negatively charged molecules will move towards the positive electrode, while positively charged ones will migrate towards the negative electrode. The speed and distance that each molecule travels depend on its charge and size, allowing for effective separation.

In this technique, factors such as gravity, magnetic fields, or heat do not play a primary role. Gravity would cause all molecules to settle down but lacks the specificity and control offered by electric current. Magnetic fields can influence charged particles but are not used in the separation process of electrophoresis. Heat may affect the reactions or stability of the molecules involved but does not inherently separate them based on charge or size. Thus, the application of electric current is the foundational principle behind electrophoresis, making it the correct answer.