Ch 20 Static Electricity

Have you ever…  Shocked someone when you touched them?  Had your hair stick to your hairbrush?  Seen sparks in a wooly blanket in the dark?  Hmmm… Good questions, eh

How it all began…  Ben Franklin was the first to see the link between lightning and electricity  During his famous kite and key experiment, realized that the strands of the strings were standing up and repelling each other. He then put his hand near the key and received a shock. This was the proof he needed to know that “electrical fire” was produced in the clouds.

20.1 Electrical Charge

Electrostatics  The study of electrical charges that can be collected and held in one place  Also think of the word static as meaning not moving (the player was very static)  These charges do move some, but do not flow like current

Charged Objects  Ancient Greeks noticed some effects when they rubbed amber (“elektron”)  They noticed that other items might be attracted to the amber  Like charges – objects repel  Tape side by side on desk  Opposite charges – objects attract  Tape on top of each other on desk

Types of Charges  Positive charge – when an object has extra protons  Items like glass and wool  Negative charge – when an object has extra electrons  Items like hard rubber and plastic  Neutral – no charge / when an object’s protons and electrons are equal  Atoms want to be neutral so they need to have the same number of electrons as they do protons in their nucleus

Charge is conserved!  If you rub two materials together, like rubber and wool, then…  The rubber collects electrons from the wool and becomes negatively charged  The wool now has extra protons so it is positively charged  The total charge in the system is the same as before the contact  No charge was created or destroyed so the system still has the same total charge

Conductors and Insulators  Insulators:  Materials in which charge will not easily move  Glass, dry wood, most plastics, cloth, and dry air  Conductors:  Materials that allow charges to move about easily  Metals (such as copper and aluminum because of metallic bonding), plasma (highly ionized gas), and graphite  Air does temporarily become a conductor during lightning and when sparks leap between objects

20.2 Electrical Force

 Always known to be stronger than the force of gravity because it produces larger accelerations than gravity  Daniel Bernoulli did a little preliminary work in 1760  In 1770, Henry Cavendish did show that electrical forces must obey an inverse square force law, but being shy, he did not publish his work and his notebooks were not discovered for another century, long after others had already duplicated his findings

Forces on Charged Bodies  Demo of one balloon hanging and one brought next to it  This shows:  There are two kinds of electrical charges: positive and negative  Charges exert force on other charges over a distance  The force is stronger when the charges are closer together  Like charges repel; opposite charges attract

Charging by Conduction  Charging a neutral body by touching it with a charged body

Charging by Induction  Charging a neutral body by bringing a charged body close to the neutral body but not touching it

Coulomb’s Law  The magnitude of the force between charge A and charge B, separated by a distance of d, is proportional to the magnitude of the charges and inversely proportional to the square of the distance

Coulomb’s Law  Coulomb: the SI standard unit of charge, C  One coulomb is the charge of 6.25 x electrons or protons  The magnitude of the charge of an individual electron is called the elementary charge an is 1.60 x C  K is a constant:

Homework  P477: 1 – 5, 7, 9 – 13, 17 – 29