Static Electricity Atoms (and groups of atoms) have an electric charge when they have an unequal number of electrons and protons Recall that atoms are made up of three particles: protons have +1 charge (located in nucleus) neutrons have no charge (located in nucleus) electrons have -1 charge (located around nucleus) Because protons are locked tightly in the nucleus, objects become charged when they gain or lose electrons

Charge is Conserved Charge – like momentum, energy, and mass – is conserved – it is neither created or destroyed The total charge of a system after an interaction is the same as the total charge of the system before the interaction The Conservation of Charge is important when looking at two ways that objects become charged – friction and conduction

Charging by Friction When two objects rub together one of the objects will have a greater “affinity” for the electrons – it will gain electrons that object will become negatively charged and the other object, which lost electrons, will become positively charged If both objects started out neutral (no charge), then the magnitude of the negative charge on the one object will be equal to the magnitude of the positive charge on the other

Charging by Conduction When a charged object comes into contact with another object – charged or neutral – the total charge on all of the objects becomes evenly distributed among the objects This happens because the electrons move to be as far away from other electrons as possible This process – the redistribution of charge among objects in physical contact – is called “conduction”

What’s My Charge? (Part 1) A glass rod is rubbed with a piece of silk. If the silk acquires a charge of -10, what is the charge on the glass rod? The glass rod is then touched to a neutral aluminum can. What is the final charge on the glass rod? On the aluminum can? A piece of rabbit fur is used to rub a rubber rod. The rubber rod is then simultaneously touched to two steel balls. If the charge on one of the steel balls is -2, what is the charge on the fur?

Conductors & Insulators All materials can gain or lose electrons and become electrically charged Even so, it is useful to consider two categories of materials based on their electron mobility Conductors are materials with highly mobile electrons that distribute themselves as evenly as possible in/on the material (e.g., metals) Insulators are materials with relatively immobile electrons – electric charge tends to remain in the region where it originated (e.g., plastic, glass)

Electrostatic Force Electrically charged objects exert forces on each other that depend on the magnitude and sign (+/-) of the charges, and the “inverse square” of the distance between them The direction of the force depends on the sign “like charges repel” (negative-negative, positive-positive) “unlike charges attract” (negative-positive) any charge attracts a neutral object! (positive-neutral, negative-neutral)

What’s My Charge? (Part 2) There are five balloons. Balloon A is attracted to Balloon B and Balloon C Balloon A is repelled by Balloon D Balloon D is negatively charged Balloon E is repelled by Balloon B Balloon E is attracted to Balloon C What are the charges on all five balloons?

Electrostatic Fields The ability of a charged object to cause an electrostatic force at various locations around itself is called its “electrostatic field” Electrostatic fields can be thought of as maps of the force exerted on a very small positively charged object electric field line point away from positively charged objects and toward negatively charged objects the more closely the electric field lines are spaced, the stronger the electric field and the electric force

Electrostatic Field Diagram