When the computer receives a rich signal (high voltage) from the O2 sensor, it leans the fuel mixture to reduce the sensor's reading. When the O2 sensor reading goes lean (low voltage), the computer reverses again making the fuel mixture go rich.
Chemistry and physics are branches of science that both study matter. The difference between the two lies in their scope and approach. Chemists and physicists are trained differently, and they have different professional roles, even when working in a team.
Physics is the study of the basic elements of nature dealing with the key components that govern it, energy, matter and the their interaction in time and space. As such physics provides an important anchor to the to study all higher order scientific studies such us Biology and Chemistry. Without space, matter, energy and time the living organisms will not exist, and the ability to create a science that would study living things, Biology, would not exist either.