Electrostatics

Electrostatic Also called Static Electricity The structure of matter is the atom. All matter is made of atoms. Atoms are known as elements Elements can combine to form compounds. Because everything is made of atoms, everything has a an electrical property.

What does an Atom look like?

What are the parts of an atom? Protons In the nucleus Strong bond positive Neutrons No charge Electrons Outside nucleus Weak bond Negative charge

Charge There are two kinds of charges Positive Negative

Charge Electrons are negatively charged Protons are positively charged

Charge Like charges repel Example: protons repel protons Unlike Charges attract Example: Protons attract electrons

What causes something to be charged? The number of electrons dictate if something has a charge. If P=E, then it is electrically neutral. If P=E, the it is electrically charged. It is now called an ion and not an atom. If P>E, positively charged. If P<E, negatively charged

Charge Electric Charge is quantized (it has a value). An electron has a charge of -1.6 x 10-19 C C=Coulomb.

Charge Electric charge is conserved Charges can be transferred, but or not created nor destroyed

Charged + Neutral A charged object (Positive or negative) will attract a neutral object. Positive and neutral attract each other. Negative and neutral attract each other.

Transfer of Charge Conductors Conductor: material that transfers charge easily. Electrons are not held tightly to the nucleus so they are distributed across the surface of the object.

Examples of Conductors Metals Pots and pans when you conduct move heat energy Copper wire allows movement of electrons to charge your phone Human body

Transfer of Charge Insulators Insulators: a material that does not transfer charge easily. Insulators hold their electrons more tightly. A poor conductor of electricity. If charge is transferred to an insulator, the excess charge will remain at initial location of charging.

Examples of Insulators Plastic, Styrofoam Paper Rubber Glass Dry air

One is copper and one is rubber. Which is which?

Transfer of Charge Polarization When a charged object is brought next to a neutrally charged insulator, polarization occurs.

Transfer of Charge Polarization Polarization: The presence of a charged object will cause another object’s charges to realign. One side of the object will be more positive and the other more negative.

Which diagram correctly depicts polarization?

In this situation, the conducting sphere is….. Charged Uncharged Polarized

Law of conservation of Charge Electrons always come from somewhere. They are not created from nothing. Electrons are acquired from somewhere. Charge is conserved. The electrons move somewhere else

Transfer of Charge Charging by Contact Also called charging by friction. The transfer of electrical charges by objects in contact.

A plastic strip was rubbed with cotton and became positively charged A plastic strip was rubbed with cotton and became positively charged. Why? A. Plastic strip acquired extra protons from the cotton. B. The plastic strip acquired extra protons during the charging process. C. Protons were created as the result of the charging process. D. The plastic strip lost electrons to the cotton during the charging process.

Saran Wrap has a larger electron affinity than Nylon Saran Wrap has a larger electron affinity than Nylon. If nylon is rubbed against Saran Wrap, which would end up with the excess negative charge?

Transfer of Charge Induction Induction: The charging of an object without direct contact from another object.

Transfer of Charge Induction Example of induction: A negatively charged rod is moved close to a metal sphere of neutral charge that is attached to Earth. The negative rod will repel the electrons in the sphere.

Transfer of Charge Induction The electrons in the sphere will be repelled by the negatively charged rod and will move to the Earth. The sphere will have uniformly distributed positive charges on its surface.

Induction Example

Grounding An object is considered to be grounded if it is attached to the Earth. Grounded: attaching an object to the Earth to eliminate excess charge.

Coulomb’s Law F = k q1q2 r2 Where k = 8.99 x 109 N m2 / C2

Coulomb’s Law Example Problem A large harvester ant has accumulated 6 C of charge. A large atta major has accumulated 10 C. The atta is to the left of the harvester. The two ants are separated by 2 cm. What is the force on the atta ant?