The direction of an electrical field at a point is the same as the direction of the electrical force acting on a positive test charge at that point. For example if you place a positive test charge in an electric field and the charge moves to the right you know the direction of the electric field in that region points to the right. Created by David SantoPietro.

Sal shows that there will be a net torque on a loop of current in a wire. Sal shows that this net torque will cause the loop to rotate. Created by Sal Khan.

Sal shows that a commutator can be used in order to keep the loop of wire rotating in the magnetic field. Created by Sal Khan.

Sal finishes the explanation of how a commutator will allow a loop of wire to continue spinning in a magnetic field, thereby allowing it to work as an electric motor. Created by Sal Khan.

In this video David explains how to find the electric potential energy for a system of charges and solves an example problem to find the speed of moving charges. To see the calculus derivation of the formula watch this video. Created by David SantoPietro.

In this video David shows how to find the total electric potential at a point in space due to multiple charges. Created by David SantoPietro.

In this video David derives the formula to find the power used by a resistor. Created by David SantoPietro.

How a cell can use a molecule's electrochemical gradient to power secondary active transport.

So we know that all matter is made up of atoms, but what is an atom made out of? Chemists describe the structure of atoms using models. This section will cover the Bohr model, photoelectric effect, absorption and emission spectra, quantum numbers, and electron configurations.

Common elemental building blocks of biological molecules: Carbon, Oxygen, Hydrogen, Nitrogen and Phosphorus.

How elements relate to atoms. The basics of how protons, electrons, and neutrons make up an atom. Created by Sal Khan.

An emf induced by motion relative to a magnetic field is called a motional emf. This is represented by the equation emf = LvB, where L is length of the object moving at speed v relative to the strength of the magnetic field B.

Starting with the reign of Augustus and ending with the reign of Marcus Aurelius, the Pax Romana is a relatively stable 200 year period. The first five emperors are of the Julio-Claudia Dynasty. The last five of the Pax Romana are considered the "good" emperors by Machiavelli.

Sometimes we use reflexive pronouns like "myself" and "ourselves" for emphasis in a sentence, like, "If you won't help me, then I'll do it myself!" KA's Grammar Fellow, David, explains this usage.

An overview of the membrane-bound structures that form the endomembrane system in eukaryotic cells.

Structure of the endoplasmic reticulum and Golgi body (Golgi apparatus). Their role in protein secretion. Created by Sal Khan.