Electrostatics

Review

Charges

e-either electrons or charged compounds such as O-2

p+ protons or charged compounds such as K+1

What type of medians do anions and cations move in, how about electrons?

Charges

• In a liquid or gas both positive and negative charges may move

• In a solid only electrons can move

Electric Field

• Both positive and negative charges have an invisible magnetic field around them. We symbolize this field with lines radiating from the center of the charge

The reason the force lines go out of the positive and into the negative is based on placing a small positive test charge in the field and recording the direction of the force on it

Static ElectricityStatic electricity, or I think of as stationary

electricity, occurs when there is an imbalance of positive or negative electrons on a surface and they cannot move anywhere

Opposites attract, likes repel

• And of course we know that by definition and experience that opposite charges (+ vs. -) attract and that like charges ( + and +) or (- and -) repel one another.

Measuring electric charge

• The forces acting between repelling and attracting charges was first described by Charles Coulomb.

• He described the force between two charges, referred to as charge 1 (q1) and charge 2(q2) as being proportional to the charges and inversely proportional to the distance squared between them when this force was measured

Coulomb’s Law

• Since this force was described by Charle’s Coulomb it is appropriate to understand why the units for the charges were called the coulomb (C).

• And since the force of electricity’s formula is similar to the force of gravity’s formula it is also understandable why the unit for electrical force is also called the Newton

• Fe = k x q1 x q2/r2

So what happens to the force as charge doubles, as distance doubles, etc.

Coulomb’s constant

• Cavendish actually measured the force between charges and came up with the inverse square proportion law. But he did not publish it, so Coulomb’s later discovery begat Coulomb the credit. The constant, 8.989E9 Nm2/C2 has to do with the permittivity of space (how easily it allows the permeability of a charge) compared to other mediums. It also tells us we must use charges in coulombs and distance in meters.

Go, or no go• Elements, like metals, whose primary characteristic is to

allow it outermost electrons, valence electrons, to move freely between the atoms making up the compound, are conductors. When electrons are added to the conductor the already free moving electrons are allowed to continue moving . This is what conduction is all about.

Insulators• Materials, like non metals, that do not allow

their valence electrons to move freely will not allow added electrons to move around either. These materials may build up a charge, but the charge stays stationary (static)

Semiconductors

Have a few free electrons with the remaining electrons bound as in insulators

Charging by friction?• Electrons rubbed off of one insulator (now

electron deficient or positively charged) are picked by the other insulator who is now negatively charged, and they attract

• If we place the charged surfaces near a neutral surface the electrons in that neutral surface will be repelled or attracted by induction or transferred by conduction, and there you have a generator of electrical current

• A current of electricity is defined as the number of electrons passing a point in a wire each second and is measured in amperes

Electric Fields• Coulomb’s law measures the force on a point charge a

given distance from another charge.• However each charge has an invisible electric field

around it that has equal force at equal distance from it’s center.

• Lines drawn radiating from the center of the charge can be connected via lines perpendicular to the radiating lines showing equal force at all points along the perpendicular lines (HUH???)

Electric Field Strength• When a small positive test charge, qo, is placed

near a larger charge, Q, the electric field strength of Q on qo can be measured as well as the direction. If Q is positive the qo will be repelled by E, if Q is negative qo will be attracted by E.

• E = Felectric/qo

• The voltage between two points is a short name for the electrical driving force (the concept of driving force is not a force measured in newtons) that could determine an electric current between those points. It is used interchangebly with electric potențial difference

• An electric field can be produced by maintaining a voltage difference across any insulating space

• E = F/q

Electric Potential Energy = Voltage

• As two like charges are pushed closer together the greater the energy stored in them grows

• The Work it takes to push the charges together is equal to the Electrical Potential Energy (UE) the charge gains

• UE is equal to ke q1 q2/r• So where is the greatest potential energy for

two like charges and two unlike charges? Closer, further away, does it matter

Q = Ne

• If the number of electrons is known you can calculate the total charge in coulombs using the formula above and e= 1.6 x 10-19 C

Alas, there is capacitance

• Two conducting plates separated by an insulator will hold a charge when touched to a power source. The separation distance, type of insulation between them, total voltage difference, etc., determine the amount of voltage they will build up. Different substances between the plates have varying permittivity that prevent transfer between plates and build up a larger potential difference, called voltage

Capacitance = q/V

• You can calculate the capacitance by dividing the total charge on the plates by the voltage difference of the plates

Ex: a parallel plate capacitor has a voltage difference of 50 volts and holds 300,000 C of charge. It’s capacitance is 300,000C/50V or

6000 C/V

Electric Field = V/d

The strength of the electric field on a capacitor can be determined by diving the voltage on the capacitor plates by the distance separating them

So a 60,000 volt capacitor on your television with a separation between the plates of .1mm or .0001m would have an electric field strength of 600000000 farads or 6x108 farads