<p align="right">Last Update: <font color="#4f81bd">July, 17, 2024</font></p> ## BIG IDEAS - Electric charges are conserved. - Electric charges are quantized. - Insulators and conductors can be charged by contact. Rub a balloon back and forth across your hair. Try to stick the balloon to the wall. When materials behave this way, they are called _electrically charged_. Rub two balloons back and forth across your hair. In this case, the two balloons push each other apart. **There are two kinds of electric charges** [](https://lh6.googleusercontent.com/0jdnrzBUO3QelCLF6KPUaXGJ-7RT_ZsE3YDKF-39Tz37nahmpHIm4XW5JXA3Gi8gZ5Pk4feYXAOUFXM_tKZ2eBRdDj6grRP0T5V9LrrCcNuap8HU3Qn-XYtlT7TTv59uu-6vv9K8) A general statement is that _like charges repel_. Conversely, oppositely charged bodies attract. **Benjamin Franklin** named the two charges _positive_ and _negative_. The charge on your hair is positive and the charge on the balloon is negative. **Electric charge is conserved** The _principle of conservation of charge_ is one of the fundamental laws of nature. Protons and neutrons are relatively fixed in the nucleus of the atom, but electrons are easily transferred from one atom to another. When the electrons in an atom are balanced by an equal number of protons, the atom has no net charge. If an electron is transferred from one neutral atom to another, the second atom gains a negative charge and the first atom loses a negative charge thereby becoming positive. Atoms that are positively or negatively charged are called _ions_. **Electric charge is quantized** **Robert Millikan** observed the motion of tiny oil droplets between two parallel metal plates. The droplets were charged by friction in an atomizer and pass through a hole in the top plate. Initially, the droplets fell due to their weight. The top was given a positive charge as the droplets fell, and the droplets with a negative charge were attracted back upward toward the positively charged plate. Millikan turned on and off the plate and the oil droplets would drop or rise to the positive plate. The unit of charge is symbolized by the letter _e_. The value of _e_ is 1.602 x 10-19 C. TRANSFER OF ELECTRIC CHARGE Materials in which electric charges move freely (metals) are called **electrical conductors**. Materials in which electric charges do not move freely (glass, rubber, silk, and plastic) are called **electrical insulators**. _Semiconductors_ are a third class of materials characterized by electrical properties that are somewhere between those of insulators and conductors. _Superconductors_ have zero electrical resistance when they are at or below a certain temperature. **Insulators and conductors can be charged by contact** A common experiment to show charging by contact is to use a glass rod rubbed with silk and a rubber rod rubbed with wool or fur. The two become oppositely charged and are attracted to each other. The glass rods will repel each other. If you try the same experiment with a copper rod, the rod does not attract or repel; however, if you hold the copper rod with an insulating handle then the copper rod will attract the glass rod and repel a charged rubber rod. In the first case, the charge moves through the copper and into the human body because human bodies are also conductors. In the second case, the rod remains charged because of the insulating handle. **Conductors can be charged by induction** If a conducting wire is connected to earth by a copper pipe, the conductor is said to be _grounded_. Charging without contact is said to be **induction**. **A surface charge can be induced on insulators by polarization** A realignment of charge within individual molecules produces an induced charge on the surface of an insulator. A polarized object has no net charge but can still attract or repel objects. There is no physical contact. --- Return [[Home|Home]] | [[Notes Vault/Physics Notes Vault/Physics Skills/Physics|Physics]] | [[Electricity]] | [[Charges]] | [[Coulomb's law]] | [[Electric Field]]