STATIC ELECTRICITY Pre AP Physics

STATIC ELECTRICITY Electrostatic – the study of electrical charges that can be collected and held in one area. They flow in no particular direction (trapped in a body)

Electricity of the Atom They are ordinarily neutral in charge # of protons = # of electrons Neutrons = no charge Protons = + 1 charge Electrons = -1 charge

The actual charge of an electron -1.60217657 X 10-19 coulombs

Little History: Ben Franklin Proved lightning was static electricity Invented lightning rod Built a static electricity generator Coined the terms positive and negative for electrical charge

Atoms become charged by gaining or losing electrons Charged objects Atoms become charged by gaining or losing electrons An object that exhibits electrical interaction after rubbing is said to be charged

Opposite will attract Like will repel Rules of Attraction Only the electrons will move Like will repel Protons will move but they don’t leave the nucleus.

Interaction between a charged object and a neutral object: Any charged object (positive or negative) will have an attraction interaction with a neutral object.

Why are protons never lost or gained? Protons are held in the nucleus by the strongest forces in the universe.

Ions Are electrically charged particles Charge is unbalanced Negative Charge # e > # p Positive charge # p > # e

Where do charges come from? Rubbing materials does NOT create electric charges. It just transfers electrons from one material to the other.

Transfer of charge Charge can be transferred between neutral objects by FRICTION. Ex: Rubber rod rubbed with fur or wool will cause electrons to transfer to the rod Rubber Rod gains electrons and becomes negative Fur/wool loses electrons and becomes positive

Here’s my silly demo. The balloon will rub against the head and then pull away with the “hair” standing on end. You might need to explain what it’s supposed to be. The next few slides show what is happening to the charges. These should run automatically.

- + - + + - - + - - + + + - + - - + - +

- - + - + + - - + - - + + + - + - + - +

- - + - + + - - + - + + + - + - + - +

- - + - + + - - + - + + + - + - + +

- - + - + + - - + - + + + + - + +

- - + - + + - - + - + + + + - + + Re iterate that the head an balloon each started off with the same number of positive and negative charges but the rubbing moved charges onto the balloon and off the head. The balloon is now negatively charged and the head positively charged so that because opposites attract the hairs are attracted to the balloon And because like charges repel the airs are pushed apart from each other.

The Electroscope A device used to detect electric charge

Charging by Induction Isolating charge without physical contact between objects Once charged rod is removed, the electroscope stays neutrally charged

Charging by Conduction Isolating electric charge by physical contact Once rod is removed, the electroscope remains charged

Permanently Charging by Induction

Neutral State Charged objects will eventually return to their NEUTRAL state What/How does this happen? The charge “leaks off” onto the water molecules in the air. On humid/rain days it would be difficult to hold a charge for long.

Grounded - neutral Charged objects lose their charge when grounded.  Being positively charged, the electroscope attracts some electrons from the conducting material (in this case, a person). The negatively charged electrons enter the electroscope and neutralize the positive charge. As the electroscope loses its charge, the needle relaxes back to its naturally upright position.

"What is the bottom prong of a plug called?" The bottom prong of a plug is called the ground. It discharges any excess charge that may develop in an electrical device

When grounded: Negatively charged objects Positively charged objects .... Lose electrons and become neutral Positively charged objects ... Gain electrons and become neutral

Electricity, Height, and Women https://www.youtube.com/watch?v=FGoaXZwF lJ4&nohtml5=False When a bird (or any similarly small animal) lands on a single power line or wire between two electrical poles, the wire temporarily charges the perimeter of the bird's body at the same voltage as the wire. However, the electric current doesn't enter the bird's body because unlike wire, the bird's body isn't a good conductor of electricity, meaning it is not an easy path for the electrons to flow through. The bird's body gives too much resistance. While on the wire, if the bird touches anything "grounded," meaning anything with no voltage, such as the earth's surface or the wooden pole (which is also grounded), the current would instantly flow through the bird's body to reach the ground, and the bird would be immediately electrocuted and killed. Only birds that are light enough to impact just one wire are safe from electrocution. Larger birds can be electrocuted when they sit on wires because their bodies are so heavy they often end up touching two wires at once. Increasing the space between electrical wires can help prevent this from happening.

Interesting facts – read, you don’t need to write… How hot is a lightning bolt? Only about 54,000 degrees Fahrenheit — roughly five times hotter than the surface of the sun! Lightning flashes more than 3 million times a day worldwide — that’s about 40 times a second. Not all those flashes hit the ground — some happen between or inside clouds. An average lightning bolt can release enough energy to operate a 100-watt light bulb for more than three months straight (about 250 kilowatt-hours of energy). Lightning starts in cumulonimbus clouds — aka thunderheads — which have a positive charge up top and a negative charge below. We don’t know how the charges start, but water droplets and ice crystals carry them.

Coulomb’s Law There exists an electrostatic force between charged objects that is directly proportional to the magnitude of the charges and indirectly proportional to the square of the distance between them. This force could be attractive or repulsive

Coulomb’s Law

Coulomb’s Law – Gives the electric force between two point charges. Inverse Square Law k = Coulomb’s Constant = 9.0x109 Nm2/C2 q1 = charge on mass 1 q2 = charge on mass 2 r = the distance between the two charges The electric force is much stronger than the gravitational force.

Coulomb’s Constant

Coulomb’s Law F21 indicates force on q1 due to q2

Example 1 Two charges are separated by a distance r and have a force F on each other. q1 q2 r F If r is doubled then F is : ¼ of F If q1 is doubled then F is : 2F If q1 and q2 are doubled and r is halved then F is : 16F

Example 2 Two 40 gram masses each with a charge of 3μC are placed 50cm apart. Compare the gravitational force between the two masses to the electric force between the two masses. (Ignore the force of the earth on the two masses) 3μC 40g 50cm

The electric force is much greater than the gravitational force

Electric Fields An electric field is an invisible force field that affects electric charges The field is defined based on how it affects a positive test-charge All charged objects are assumed to have an electric field associated with them

Electric Field Mapping

Electric Field Around a Positively Charged Object

Electric Field Around Charged Pairs

Electric Field Mapping Rules E-Field lines must never touch or cross themselves or other field lines E-Field lines run out of a positive charge and into a negative charge Number and density of lines indicate field strength or intensity

The Van de Graaff Generator Generates large static charges that build up on dome Some generators can charge to thousands of volts They are generally safe, but caution should be heeded