Electric Charge

If you rub two materials together, sometimes the materials get opposite charges. This effect is longer lasting in dry weather because the water in the air allows the charges to return to equilibrium.

Experiment: Try rubbing a balloon on your hair and then putting the balloon near an empty soda can. You are strongly encouraged to setup a race between two balloon powered soda cans.

Experiment: Try sticking two strips of scotch tape together and quickly pealing them apart. The strips should now be oppositely charged. As you bring them near each other, they should be attracted.

Try repeating the experiment again to make 4 strips of tape. Put the newly charged tape near the old and some will repel instead of attract.

Question: After playing with the tape what factors determine the strength of the force?
answer

The force between the tape strips is strong when they are closer.

The force also varies with how much charges is separated. Separating more charge is easier when the humidity is lower.

Some particles have an electric charge. The rules that govern these charges can explain electricity, chemical bonds, magnetism, and light!

Electric charge determines the magnitude and direction of the electrostatic force. Charge has a role in the electrostatic force similar to mass's role in gravity.


opposite charges have an attractive force



negative charges have a repulsive force



positive charges have a repulsive force



neutral charges have no force



Charge can be neutralized. If a positive and negative charge are close together the attractive and repulsive forces mostly cancel each other out. This explains why we don't notice the electrostatic forces from neutral atoms.

Electric charge can't be created or destroyed. Like energy and momentum, electric charge is conserved.

The evidence for this conservation law is based on repeated experiments. No one has ever documented the total charge of a system increasing or decreasing. This law is important in particle physics where charged particles can be created and destroyed, but only when another particle is created or destroyed to balance out the total charge.

Elementary Electric Charge

In 1909 Robert Millikan and Harvey Fletcher performed the oil drop experiment to investigate electric charge. They sprayed a fine mist of oil into a uniform electric field. The electric field produced a force on some of the oil droplets. Based on that force, they found that charge only came in even multiples of about 1.6 × 10−19 C.

This evidence helped build our modern model of the atom: negative electrons bound to a nucleus of positive protons and neutral neutrons. Most charge comes from electrons or protons, but there are also more exotic particles.

- electron:
charge = −1.602 × 10−19 C
mass = 9.109 × 10−31 kg
+ proton:
charge = +1.602 × 10−19 C
mass = 1.672 × 10−27 kg
Example: These 6 values were recorded for electric charge. Which of the measurements are probably inaccurate? Why?
+3.2 × 10−19 C
−2.4 × 10−19 C
−8.0 × 10−19 C
+1.6 × 10−19 C
+5.2 × 10−19 C
−0.4 × 10−19 C
solution

Charge has only been observed in packets of 1.602 × 10−19 C. Any recorded charge must be a multiple of this value.

$$\frac{3.2 \times 10^{−19} \, \mathrm{C}}{1.6 \times 10^{−19} \, \mathrm{C}} = 2.0$$
$$\frac{2.4 \times 10^{−19} \, \mathrm{C}}{1.6 \times 10^{−19} \, \mathrm{C}} = \cancel{1.5}$$
$$\frac{8.0 \times 10^{−19} \, \mathrm{C}}{1.6 \times 10^{−19} \, \mathrm{C}} = 5.0$$
$$\frac{1.6 \times 10^{−19} \, \mathrm{C}}{1.6 \times 10^{−19} \, \mathrm{C}} = 1.0$$
$$\frac{5.2 \times 10^{−19} \, \mathrm{C}}{1.6 \times 10^{−19} \, \mathrm{C}} = \cancel{3.25}$$
$$\frac{0.4 \times 10^{−19} \, \mathrm{C}}{1.6 \times 10^{−19} \, \mathrm{C}} = \cancel{0.25$$

Conductivity

Conductive materials allow electric charges to easily move through them. If a charge is applied to one part of a conductive material the charge will quickly spread out because like charges repel.

These simulations don't include the nuances of quantum mechanics, they only show a cartoony approximation of conductivity.

In some materials, current is understood as the flow of an electron hole. This model of current helps us understand the semiconductors used in solar panels, LEDs, and transistors.

In chemistry, elements are roughly divided into metals, metalloids and nonmetals. Metals are held together by loosely sharing their outer valence electrons. The cloud of free flowing electrons give metals most of their shared characteristics, like conductivity.

low high conductivity high low resistance

insulators
vacuum, nonmetals: gases, plastics, silk, fur

semi-conductors
metalloids: carbon and silicon

electrolytes
solvents with dissolved ions:
salt water, tap water, soda water

conductors
metals, plasma

superconductors
certain low temperature ceramics

Question: Rank these substances by conductivity:

air, coca cola, copper, carbon, plastic fork

answer

high conductivity
copper (conductor)
coca cola (electrolyte)
carbon (semi-conductor)
plastic fork (insulator)
air (insulator)
low conductivity

Question: Why are metals more conductive than nonmetals?
answer

In metals some electrons are free to move between atoms. In nonmetals the electrons are locked up in covalent bonds so they resist the electrostatic force.

Another way to make a substance conductive is to heat it up so much that electrons can leave the nucleus. We call this state of matter a plasma.

The positive charges on the left polarize the "atoms" on the right, but not enough to get the negative charges to jump over.


Question: Why can't the negative charges spread out into the positive charge group on the left?
answer

The electrostatic force follows a 1/r² rule, so it is weaker with distance. The negative charges want to spread out into the positive charge, but the short distance attraction to the nearby positive charge is a stronger force.

Add a conductive for the negative charges.

Static Electricity

It's easy to separate a couple trillion electrons from their protons by walking with socks on a carpet. Lightning is produced in a similar way when a cloud with rising air ends up with an unbalanced distribution of charge.

Static electricity occurs when there is an imbalance of electrons and protons. A lasting charge separation can only occur in insulating materials, because in conductors positive and negative charges quickly pair up.

Static electricity effects are much stronger and longer lasting in low humidity. This is because water molecules increase the conductivity of air, allowing more separated charges to return.

Play around with this PhET simulation for static electricity.


Question: Which are conductors and which are insulators:
(balloons, sweater, wall, air)
answer

conductors: nothing in this simulation
insulators: balloon, sweater, wall, air

Question: Why do some electrons jump to the balloon from the sweater?
Why not the other way around?
answer

Some materials are better at holding onto extra electrons for complex quantum mechanical reasons.

Question: Why is the balloon attracted to the sweater after it gains some electrons?
answer

After rubbing, the balloon has unpaired negative charge, and the sweater has unpaired positive charge. Opposite charges attract.

Question: Why is the balloon attracted to the wall after it gains some electrons?
answer

After rubbing, the balloon has unpaired negative charge. When charge is near another insulator it repels the electrons enough that they are slightly farther away, but not enough to cause them to leave the nucleus. This charge separation is called polarization.

The polarization of the positive and negative pairs creates an induced charge. The charged balloon causes the wall to polarize, which increases the attraction and decreases the repulsion between the balloon and wall.

A TriboElectric Series lists which materials will become electrically charged after they are rubbed together.

Question: If you rubbed polystyrene foam (styrofoam) on a cat, static electricity would cause them stick together. Which would gain a positive charge?
answer

The cat would end up with a positive charge. This means the cat would lose electrons to the styrofoam.

Question: Rubbing glass on plastic wrap can get super staticky. Use the TriboElectric series to decide which gets the positive charge.
answer

The glass would get the positive charge.