Static Electricity “Electrostatics”

“Static”- not moving. Electric charges that can be collected an held in one place Examples: sparks on carpet, balloon against hair, lightning, photocopier History: ancient Greeks made little sparks when rubbing amber with fur (Greek word for amber: “elektron”) Electric charge, “q”, is measured in Coulombs, C. One Coulomb is charge is a dangerously high charge. An average lightning bolt has about 10 Coulombs of charge.

Atomic View q = + 1.6 x 10-19 C q = - 1.6 x 10-19 C Proton: in nucleus Positive charge q = + 1.6 x 10-19 C Electron: outside nucleus Negative charge q = - 1.6 x 10-19 C Protons and Electrons have the same amount of charge but a proton has much more mass! Neutron: in nucleus, has no charge Molecules 2 or more atoms bonded together usually atoms and molecules are neutral, but if they have a net charge, they are called IONS

Behavior of charges Unlike charges attract Like charges repel A neutral object will attract both positive and negative charges

Charles Coulomb, mid 1700’s, studied and published papers about the electrostatic force between 2 charged objects.

Hmmm.. + + + - - - Ben Franklin was the first to use the terms “positive” and “negative” to describe electrical charge. Mid 1700’s

Robert Millikan First determined the “elementary charge”- the charge on an electron or proton. (early 1900’s)

Materials Conductors Substances that have easily moveable electric charges Most familiar conductors are metals that have “free electrons” Positive ions may also be mobile Insulators Charges cannot move easily Examples: plastic, wood, glass

Semiconductor: used in computers Conduction is an intermediate magnitude between a conductor and an insulator Superconductor: NO resistance to the flow of electrons. So far, no material is a superconductor except at extremely low temperatures.

Water: insulator or conductor? PURE water does NOT conduct electricity Impurities or ions in water can allow conduction The purer the water, the lower the conductivity (the conduction of electricity is called ELECTROLYTIC behavior- ) Air: insulator or conductor? Usually an insulator, thankfully When strong forces are present, electron’s can be stripped from air molecules, creating ions example: lightning

Lightning An electrical discharge between the clouds and the ground or between two clouds. As the electrons flow through the ionized air, they generate so much heat that a PLASMA is produced. We see that plasma and call it LIGHTNING! The air around the lightning expands so rapidly from the heat that it creates a strong pressure wave of air molecules (that’s sound!) We call that THUNDER!

How many electrons are flowing in a 12 C lightning bolt? How much electrical charge is flowing through a lightning bolt? Typically around 10 Coulombs of charge. How many electrons, each with a negative charge of 1.6 x 10-19 C, does it take to have 10 C of charge? 10 C / 1.6 x 10-19 C = 6.25 x 1019 electrons ! How many electrons are flowing in a 12 C lightning bolt? 7.5 x 1019 electrons

The Earth is able to absorb much electrical charge. Touching a charged object to the Earth in order to discharge it is called GROUNDING

Methods to electrically charge an object Conduction: Direct contact: will transfer electrons, such as touching your car door in the winter Friction: rubbing your feet against carpet, hair against a balloon

Induction: no direct contact Start with a neutral object. Then, bring an electrically charged object near, but not in contact with, a neutral object The charges in the neutral object will be “induced” to separate to get closer or farther from the charged object. If provided a pathway, the separated electrons will leave. The object is now positively charged.

Static devices Electroscope: the separation of metal leaves indicates the presence of static charge Van de Graaff generator: charge is delivered by a rubber belt to a metal dome Electrophorus a device used to transfer electric charge

Coulomb’s Law Calculates the magnitude of the electric force between two charges Each charge experiences equal but opposite forces where k is a constant, k = 9 x 109 N·m2/C2

Coulomb’s Law looks VERY similar to Newton’s Universal Law of Gravitation Differences: Gravitational Force is based on MASS. Coulomb’s law is based on CHARGE. Gravity is ALWAYS an attractive force. The Electric Force can attract and repel. “G” is a tiny number, therefore gravity force is a small force. “k” is a huge number, therefore electric force is a large force.

Both laws are INVERSE SQUARE LAWS “The Force varies with the inverse of the distance squared.” At twice the distance,  d2 = 22 in denominator = ¼ the Force, At three times the distance, 32 in denominator, = 1/9 the Force At half the distance,  (1/2)2 in denominator = 4 times the Force Now if one CHARGE, q, doubles…. The Force doubles since they are directly related.

Remember….Force is a VECTOR!

Electric Fields A gravitational field surrounds all masses. An electric field surrounds all charges. The stronger the electric charge, the stronger the electric field surrounding it.

Electric Field- the region around every electric charge The electric field around a charge can be represented by Electric field lines Electric fields exist, but electric field lines don’t really exist but provide a model of the electric field.

Electric Field Lines Electric field lines always point OUT of a positive charge and INTO a negative charge

To indicate a stronger electric field, just draw MORE lines. +2q To indicate a stronger electric field, just draw MORE lines. The farther apart the lines, the weaker the field. Since the electric field, E, has both magnitude and direction, it is a vector. - 4q

+ One way to measure the strength of a gravitational field is to release a mass in the field and measure how strength of the force exerted on it. One way to measure the strength of an electrical field is to release a charge in the field and measure the strength of the force exerted on it.

Electric Field = Force ÷ charge So… the strength of the electric field, E, is given by Electric Field = Force ÷ charge E = F ÷ q

For example: A 0.5 C charge experiences a force of 20 N when placed in an electric field. What is the strength of the electric field, E? E = F ÷ q = 20 N ÷ 0.5 C = 40 N/C

The electric field near a charged piece of plastic or styrofoam is around 1000 N/C. The electric field in a television picture tube is around 10,000 N/C. The electric field at the location of the electron in a Hydrogen atom is 500,000,000,000 N/C! The further you go from an electric charge, the weaker the field becomes.

The electric field INSIDE a hollow conductor is ZERO even if there are charges on the OUTSIDE of the conductor!

Electric Shielding There is no way to shield from gravity, but there is a way to shield from an electric field…. Surround yourself or whatever you wish to shield with a conductor (even if it is more like a cage that a solid surface) That’s why certain electric components are enclosed in metal boxes and even certain cables, like coaxial cables have a metal covering. The covering shields them from all outside electrical activity.

Michael Faraday, 1791-1867 Michael Faraday demonstrated that the electrostatic charge only resides on the exterior of a charged conductor, and exterior charge has no influence on anything enclosed within a conductor. This was one of many contributions he made to electromagnetic theory.

“Faraday Cage”

Are you safe from lightning inside your car? Why or why not?

Accelerating Charges A charge placed in an electric field will experience an electric force, F = Eq This force will make the charge accelerate according to Newton’s Second Law F = ma

+ - What direction will a charge accelerate? + + + + + + + + Positive charges will accelerate in the same direction as the electric field. Negative charges will accelerate in the opposite direction of the electric field. + -

The Electric Field can also be determined by using Coulomb’s Law:

Electric Potential Energy d Energy stored up between 2 charges separated by a distance d: Unit: Joules

Changing the Electric Potential Energy If you raise or lower a mass in a gravitational field, you change the gravitational potential energy, UG. If you move a charge in an electric field, you change the electric potential energy, UE.

Move a mass, m Through a gravitational field, g A distance, h Gravitational Potential Energy, mgh Move a charge, q Through an electrical field, E A distance, d Electrical Potential Energy, qEd

The work energy required to move a charge through an electric field is given by W = qEd +++++++++

Two Ways to Find Electric Potential Energy ? Are these the same thing??? Which equation should be used??

Conversion of energy Moving a mass or moving a charge takes work energy that is converted to potential energy Work = mgh Or Work = qEd

If you release an object in a gravitational field, its gravitational potential energy is converted to kinetic energy.

If you RELEASE a charge in an electrical field, its potential energy is converted to kinetic energy! UE = ½ mv2 E -

Examples What is the potential energy stored between 2 charges of 3 C and 4 C separated by 2 m? 5.4 x 1010 J

It takes 2.43 x 10-15 J of work to move an electron as distance of 2 m in an electric field. What is the strength of the field? W = qEd E = 7600 N/C

The electron is then released The electron is then released. What is the maximum velocity it will achieve? 2.43 x 10-15 J = W = qEd = ½ mv2 v = 7.3 x 107 m/s

If two charges are placed close to each other and held in place, there is an electric potential energy stored between them. + +

electric potential energy per charge. Two charges in an electric field at the same location will have twice as much electric potential energy as one charge; Five charges will have five time the potential energy, and so on… It is often convenient to consider the electric potential energy per charge. + + +

Electric Potential = Voltage The concept of the electric potential energy per charge has a special name- Electric Potential Unit: Joule/coulomb. However, it gets its own unit called a volt. 1 volt = 1 joule / coulomb Since electric potential is measured in volts, it is commonly called Voltage. Electric Potential = Voltage

Voltage Voltage can be thought of as a kind of pressure- Electrical Pressure Voltage is also called Electric Potential Think of the water supply at your house- sometimes you have high water pressure-water flows quickly- and sometimes low water pressure- water flows slowly. With Higher Voltage (pressure), charges are able to flow more quickly

Does this mean that there’s a lot of electrical energy? Rub a balloon on your hair and it becomes negatively charged, perhaps to several thousand volts. Does this mean that there’s a lot of electrical energy? Well, the charge transferred to the balloon is typically less than a millionth of a Coulomb (Remember, one Coulomb is charge is a HUGE amount of charge) There’s a LOT of difference between Voltage and Energy! High Voltage does not necessarily mean that there’s a lot of useful energy or that something is dangerous. Voltage = Energy / charge Energy = Voltage x charge Energy = 3000 V x 0.000001 C Energy = 0.003 J That’s not much energy!

High Voltage is not necessarily dangerous- a Van de Graaff generator can have more than 400,000 V, but there’s not much charge that is transferred to you from the globe. Low Voltage is not necessarily safe. Our houses are wired with 120V and you can be killed from that electricity. Voltage (potential) is not the dangerous part of electricity. The dangerous part is how many charges are flowing- the “current”.

The Electric Potential (Voltage), V, changes as you move from one place to another in an electric field The change in Potential (“pressure”), called the “Potential Difference” is given by DV = Ed Electric Field For example, the potential difference between two locations separated by 3 meters in a 4000 N/C electric field is given by DV = Ed = 4000 N/C x 3 m = 12,000 V 3 meters

Sometimes, a charge is said to be located “at ground”. The work energy required to move a charge, q, through an electric field, E, a distance d, is given by W = qEd = qDV Sometimes, a charge is said to be located “at ground”. The potential (voltage) at “ground” is zero. Vground = 0 Volts +++++++++

So the conversion between eV’s and Joules is There is another unit for very tiny amounts of energy associated with atoms and sub-atomic particles. It is called an “electron-Volt” or eV. One electron-Volt is the amount of work energy required to move one electron through 1 Volt of potential difference. In other words, 1 eV = W = qDV = 1.6 x 10-19 C x 1V So the conversion between eV’s and Joules is 1 eV = 1.6 x 10-19 J

The Electric Potential, V, due to a point charge, q, is given by The potential will have the same sign as the charge- there can be a large positive and a large negative potential

At very great distances away from a charge… d is very large… The Potential, V, due to that charge is virtually ZERO.

Potential due to more than one charge Potential is NOT a vector…. (yea!!!)….So The potential due to a group of point charges is given by

Example What is the potential halfway between 2 charges of 3mC and 4mC located 16 cm apart? 787500V What would be the potential if the 4mC charge were negative? -112500 V

Therefore a positive charge will accelerate from high to low V + - Higher V Lower V The potential near a positive charge will be higher (it’s positive!) than the potential near a negative charge (it’s negative!). Therefore a positive charge will accelerate from high to low V A negative charge will accelerate from low to high V

Capacitors: Electric Energy Storage A device consisting of two conductors placed near, but not touching each other in which electric charge and energy can be stored. - + - +

Capacitors are Used in camera flashes defibrillators Computers: tiny capacitors store the 1’s and 0’s for the binary code Many keyboards have a capacitor beneath each key that records every key stroke. Virtually every electronic device

Leyden Jar, the first “capacitor” Dutch physicist Pieter van Musschenbroek of the University of Leyden

The capacitance, C, measures how much charge can be separated in the capacitor per volt and is measured in farads, F. One farad is a very large capacitance, so generally you will see mF, mF, nF, pF Capacitance x Voltage = Charge separated CV = q

Parallel-Plate capacitors the capacitance is increased by placing an insulating material, a dielectric, between the two plates. It is NOT dependent on the voltage of the battery. The capacitance is given by C = A = area of plates eo = permittivity of free space = dielectric constant (air or vacuum, k = 1) d = distance between the plates - + - +

Energy stored in a capacitor Energy U = ½ CV2 Substituting from CV = q will yield other equations for U.

Parallel-Plate capacitors The Electric Field inside a capacitor, - + - +

Move to a sturdy building or car. Do not take shelter in small sheds, under isolated trees, or in convertible automobiles. Get out of boats and away from water. Telephones lines and metal pipes can conduct electricity. Unplug appliances if possible and avoid using the telephone (unless it is an emergency) or any electrical appliances. Do not take a bath or shower.

If you are caught outdoors and unable to find shelter: Find a low spot away from trees, fences and poles. If you are in the woods, take shelter under the shorter trees. If you feel your skin tingle or your hair stand on end, squat low to the ground on the balls of your feet. Place you hands on your knees, put your head down and try to make yourself the smallest target possible while minimizing your contact with the ground.