Chapter 9 Notes
9.1 Investigating Electrical Charges - Demo
What have we learned about electrical charges.
- Charged and uncharged attract
- Similar charged repel
9.2 The Electrical Nature of Matter
Static electricity is a charge on a substance that stays in the same place.
Ex – clothes from the dryer
Electric charges are a negative or positive quantity of electricity that builds up on an object.
You do not create electric charges, they are there all the time, but when materials are rubbed together they
become charged.
Electrostatics is the study of static electric charge.
Items are uncharged or neutral until they are rubbed together. Then one becomes positively charged and the
other negatively charged. Electrons move from one item to the other.
Negative charge has more electrons.
Positive charge has less electrons.
Law of electric charges – the same charges repel one another, and different charges attract one another.
+ ←→
+
- ←→ -
→ + - ←
Neutral objects are attracted to both positive and negative charges.
How the Bohr – Rutherford model of the atom relates to the Electrical Nature of Matter.
1. All matter is made of atoms.
2. The atom has a nucleus with + protons and uncharged neutrons. Protons do not leave the nucleus.
3. Electrons surround the nucleus. Electrons have a – charge and they can move from atom to atom.
4. The same charges go away from each other. Different charges go toward each other.
5. Some elements are weak and e- can move easily, some elements are strong and e- can not move easily.
6. The number of electrons equals the number of protons in each atom, each atom is neutral.
7. If an atom gains an e- it is negative and is called a negative ion. If an atom loses and e- it is positive and
is called a positive ion.
9.3 Charging by Friction
There are 3 ways an object can become electrically charged.
1. by Friction
2. by Contact
3. by Induction
Charging by Friction
-
Transferring an electric charge from one substance to another by a rubbing action.
o Ex. Plastic food wrap on a bowl
-
This can even happen when air rubs over a fast moving car.
o Combing hair to explain the model for the electrical nature of matter.
o The + charged nucleus of the molecules in the comb attract the electrons from the hair
molecules.
o The hair molecules do not attract electrons as strongly.
o Touching the comb to the hair transfers only a few electrons. Combing the hair transfers many
electrons.
o Comb becomes – charge.
o Hair becomes + charge
Electrostatic Series
- A list of substances listed in order of increasing ability to gain electrons, See Table 1 pg. 275
Static charges do not build up on items made of the same material.
Ex. Dryer load of cotton towels.
Items further apart on the electrostatic series will build up a stronger charge
Ex. plastic and wool – strong charge
plastic and cotton – weak charge
9.4 Charging by Contact & 9.5 Transferring Charge by Contact
Transferring an electric charge from one substance to another by touching
Ex. touching someone and getting a shock.
You can neutralize some things by touching them with your fingertips. The electrons will leave the object or
your fingers to neutralize the object.
Charges by contact can transfer in the form of a spark even before you touch an object.
Ways to reduce the build-up of static electric charges
- increase humidity (moisture in air)
- wear clothes all made from the same material
- use a product made to reduce static charge
o static guard – bounce
9.8 Induction
The third way object become electrically charged
Charging by induction – transferring an electric charge from one substance to another without direct contact.
After a TV has been on for a while the screen becomes negatively charged. Dust in the air is neutral. When dust
gets close to the TV the - charge of the screen causes the + charge in the dust to move to one side, closer to the
TV. Then the + and – charges are attracted to each other and the dust sticks to the TV. The dust is still neutral.
Induced charge separation – a slight shift in position of electrons that produces opposite charges on the two
sides of a particle.
Using Static Electricity
- cleaning the air – electrostatic air cleaners collect dust and smoke.
-
spray painting – paint is charged as it passes through the nozzle and it is attracted to a surface.
9.7 Discharging Electrically Charged Objects
It is very important to be able to remove charges. Ex. Airplane, gas pump
Discharge – neutralize, or remove all electrical charges.
Grounding – the simplest way to discharge an object is to connect it to the earth.
- the damp soil is a good conductor and the earth is so big it removes all the extra charge from an
object
Discharge at a point
- Removing an electric charge by repelling electrons off the tip of a conductor that is pointed at the end.
- Smooth spherical shapes keep charges because they spread evenly over the surface
- Objects pointed at the ends lose charges quickly
- Planes use “ static wicks” – pointed metal rods that stick out from the plane. They continuously get rid
of static electricity.
Other ways to Discharge Objects
- exposure to moist air, light, radioactivity
9.10 Lightning
-
Right now there are 2000 thunderstorms happening around the world
Just now, in the past 1 second, lightning struck the earth 100 times.
Today lightning will hit the earth 8 million times.
How does all that electricity being grounded into the earth go away?
Evaporation of water is one way
In a thunderstorm negative charges collect at the bottom of the cloud. Sometimes those charges are close
enough to buildings, objects, or the earth to move to them in a huge spark - lightning.
Lighting looks like it is jagged. This is because it is looking for the easiest path, away from moisture in the
air. The lightning will hit tall objects because they are closer, especially if they are metal conductors.
Lightning rods – protects a house in 2 ways.
1. Prevents lightning from hitting the house by induction
2. If it is hit, the rod is highest so it is hit and it carries the charge to the earth where it is absorbed.
9.11 Insulators and conductors
Fabric Softener Sheets
 Dryer is a great place to create static electricity. Rubbing, different materials, dry
 The molecules from the sheets coat the clothes making them all seem the same, so the electrons do not
move to certain materials
 The sheets act as conductors collecting electrons
Photocopier
 Uses element # 34 Selenium – Se
Photoconductive
 Insulator in dark, conductor in light
Insulator – prevents the passage of electrons
Conductor – allows electrons to pass along
A photocopier has a metal plate covered in Se in it. The Se has a positive charge. When you run the copier the
image is projected on the plate with light. The light gets rid of the Positive charge. Negative charged powdered
ink is sprinkled on the plate and stays where it is positive. Paper that is positively charged is put next to the
plate and attracts the negative ink. Then a heat lamp fuses the ink to the paper.
Cell Phone and Car Radios
 The signal your cell phone and radio pick up are by induction
 Electromagnetic energy travels in the air and reacts with the antenna ( a conductor) on your car or cell
phone. The charge moves through the device making a voice