Biophysics Lecture (1) PDF

Biophysics Lecture (1) Basics of electricity  All matter is composed atoms. Atoms have a nucleus with electrons moving around it. The nucleus is composed of protons and neutrons.  Most atoms have an equal number of electrons and protons. Electrons have a negative charge...

Biophysics Lecture (1) Basics of electricity  All matter is composed atoms. Atoms have a nucleus with electrons moving around it. The nucleus is composed of protons and neutrons.  Most atoms have an equal number of electrons and protons. Electrons have a negative charge (-). Protons have a positive charge (+). Neutrons are neutral.  The negative charge of the electrons is balanced by the positive charge of the protons.  Electrons are bound in their orbit by the attraction of the protons. These are referred to as bound electrons.  Electrons in the outer band can become free of their orbit by the application of some external force such as movement through a magnetic field, friction, or chemical action.  These are referred to as free electrons.  A free electron leaves a void which can be filled by an electron forced out of orbit from another atom.  As free electrons move from one atom to the next an electron flow is produced. This is the basis of electricity.  An electric current is produced when free electrons move from one atom to the next.  Materials that permit many electrons to move freely are called conductors.  Copper, gold, silver, and aluminium are examples of materials that are good conductors.  Copper is widely used as a conductor because it is one of the best conductors and is relatively inexpensive.  Materials that allow few free electrons are called insulators. Materials such is plastic, rubber, glass, mica, and ceramic are examples of materials that are good insulators.  An electrical cable is one example of how conductors and insulators are used together. Electrons flow along a copper conductor in a circuit and the insulator around the outside of the copper conductor keeps electrons in the conductor.  Semiconductor materials, such as silicon, can be used to manufacture devices that have characteristics of both conductors and insulators.  Many semiconductor devices act like a conductor when an external force is applied in one direction and like an insulator when an external force is applied in the opposite direction.  This principle is basic to the operation of transistors, diodes, and other solid- state electronic devices.  Elements are defined by the number of electrons in orbit around the nucleus of an atom and by the number of protons in the nucleus.  A hydrogen atom, for example, has only one electron and one proton. An aluminium atom has 13 electrons and 13 protons.  An atom with an equal number of electrons and protons is said to be electrically neutral.  Electrons in the outer band of an atom are easily displaced by the application of some external force.  Electrons which are forced out of their orbits can result in a lack of electrons where they leave and an excess of electrons where they come to rest.  A material with more protons than electrons has a net positive charge, and a material with more electrons than protons has a net negative charge.  A positive or negative charge is caused by an absence or excess of electrons, because the number of protons in an atom remains constant.