What is it called if there are no particles present?

Three point charges, two positive and one negative, each having a magnitude of 20 μc are placed at the vertices of an equilateral triangle (30 cm on a side). what is the magnitude of the electrostatic force on the negative charge

Look at the potential energy diagram for a chemical reaction. which statement correctly describes the energy changes that occur in the forward reaction?

The passage that directs air to the lungs is called a. diaphragm. b. larynx. c. bronchi. d. pharynx.