electrostatics - asd-ssussexhigh.nbed.nb.ca/jgaunce/grade 9 science/electrostatics 9... ·...

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Electrostatics Before scientists discovered how electricity works they had to make discoveries using static electricity. The study of static electricity, or stationary electrical carges, is called electrostatics. We all have experience with static electricity: 1. you may have rubbed a balloon on your head and stuck it to a wall 2. you may have walked across a carpet and got a shock on a door knob 3. if you have long hair you have had your hair stick up or out 4. you may have slid down a slide at a playground as a child and had your hair stick out 5. you have likely worn some type of clothing that has clung or stuck to you So why does this happen? Lets review what we know about opposite and like charges based on our experiment and our experiences. The following statements are called the Law of Electric Charges 1. Like charges - 2. Opposite charges - 3. Both positively and negatively charged objects will attract most neutral objects - including liquids and gases In order to determine if something has a positive or negative charge we would need to observe what happens when

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Page 1: Electrostatics - ASD-Ssussexhigh.nbed.nb.ca/jgaunce/Grade 9 Science/Electrostatics 9... · Electrostatics Before scientists discovered how electricity works they had to make discoveries

Electrostatics

Before scientists discovered how electricity works they had to make discoveries using static

electricity. The study of static electricity, or stationary electrical carges, is called electrostatics.

We all have experience with static electricity:

1. you may have rubbed a balloon on your head and stuck it to a wall

2. you may have walked across a carpet and got a shock on a door knob

3. if you have long hair you have had your hair stick up or out

4. you may have slid down a slide at a playground as a child and had your hair stick out

5. you have likely worn some type of clothing that has clung or stuck to you

So why does this happen?

Let’s review what we know about opposite and like charges based on our experiment and our

experiences.

The following statements are called the Law of Electric Charges

1. Like charges -

2. Opposite charges -

3. Both positively and negatively charged objects will attract most neutral objects - including

liquids and gases

In order to determine if something has a positive or negative charge we would need to observe

what happens when

Page 2: Electrostatics - ASD-Ssussexhigh.nbed.nb.ca/jgaunce/Grade 9 Science/Electrostatics 9... · Electrostatics Before scientists discovered how electricity works they had to make discoveries

Can we predict how any particular model will behave when it is charged? Scientists believe that

all matter is made of atoms. Although atoms can't be seen, they are believed to be made up

based on a theory about the atom called the Bohr-Rutherford model shown below.

Based on this model scientists created and agreed upon the Model for Electrical Nature of

Matter

1. All matter is made up of submicroscopic particles called atoms

2. At the center of each atom is the nucleus. The nucleus is made up of protons (positively

charged) and neutrons (uncharged). Protons and neutrons do not leave the nucleus of the

atom and are what defines the particular atom. (Nitrogen has 7 protons and oxygen has 8

protons)

3. Atoms have a cloud of negatively charged particles called electrons around the nucleus.

One proton has the same charge as one electron except the electron is negatively charged.

If an atom becomes charged it is due to the movement of electons.

4. Like charges attract, opposite repel

5. Some atoms have a strong attraction between the nucleus and its electrons (ex, sulfur) and

some have a weak attraction (ex. copper)

6. In each atom, the number of electrons around the nucleus equals the number of protons.

One single atom is always neutrally charged.

7. If an atom gains an electron, the net charge is negative and it is called a negative ion.

If an atom loses an electron, the net charge is positive, and it is called a positive ion.

If something is positively charged it means there are MORE protons than electrons.

If something is negatively charged it means there are LESS protons than electrons.

Page 3: Electrostatics - ASD-Ssussexhigh.nbed.nb.ca/jgaunce/Grade 9 Science/Electrostatics 9... · Electrostatics Before scientists discovered how electricity works they had to make discoveries

So how do objects become charged? There are 3 ways:

1. Charge by Friction

2. Charge by Contact

3. Charge by Induction

Charging By Friction

It is the charging of materials when they rub against another material.

Examples:

- Clothes in the dryer become charged as they move past each other

- You become electrically charged when you walk across a carpet due to your feet rubbing on the

floor.

- Plastic wrap (Saran wrap) becomes charged and sticks to a bowl when it is rubbed against the

bowl

- Combing your hair - the plastic comb easily attracts electrons and your hair loses them. Your

hair may stick out and the comb is also charged.

***When two materials are rubbed together, the item higher on the list always loses its electrons

and one lower on the list gains those electrons.***

Page 4: Electrostatics - ASD-Ssussexhigh.nbed.nb.ca/jgaunce/Grade 9 Science/Electrostatics 9... · Electrostatics Before scientists discovered how electricity works they had to make discoveries

Transferring Charge by Contact

- it is exactly what it says. One object or both are charged. Upon contact the charge is

transferred from one object to the other.

- this is what happens when you get a shock when you walk across a carpet and touch a

doorknob or when you get out of a car and get a shock when you touch the body of the car.

Care is required in areas in which a single spark from charge transfer could cause an explosion.

Special clothing, gloves or boots may produce sparks that could cause explosions in coal mines,

grain elevators and hospital operating rooms.

Page 279 - 2-5

Insulators and Conductors

Insulator -

- important because

Conductor -

- important because

Using Table 1 on Page 280 fill in the table below with at least 6 items for each column

Page 5: Electrostatics - ASD-Ssussexhigh.nbed.nb.ca/jgaunce/Grade 9 Science/Electrostatics 9... · Electrostatics Before scientists discovered how electricity works they had to make discoveries

Static Electricity - Summer vs. Winter

Issues with Static Electricity are greater in the

In the winter the air is drier and contains fewer water molecules. Dry air acts as an insulator and

allows charges to build up on objects easier. In the summer the air has more water molecules

that pick up the charge keeping objects from building up charge.

Pg 281 - questions 2, 4, 6

Discharging Electrically Charged Objects

When charges on an object build up to much there is a large chance for a shock. Objects moving

through the air build up charge on the outside of the object. Another example is gasoline flowing

through a nozzle generates large static electricity.

For safety purposes the static electricity must be discharged. To do so the objects can be

discharged by:

Grounded - means an object is connected to a large surface, often Earth, which allows the

charge to spread across the surface to a harmless level.

Discharge at a point - large objects sometimes have a rod that comes to a sharp point. The

static charges are pushed to the end and then repel each other and are pushed off the tip. (On a

plane these are called static wicks and are attached to the movable surfaces on the wings of the

plane)

Pg 283 - 3,5,6

Page 6: Electrostatics - ASD-Ssussexhigh.nbed.nb.ca/jgaunce/Grade 9 Science/Electrostatics 9... · Electrostatics Before scientists discovered how electricity works they had to make discoveries

Charging by Induction

This occurs when a charge is transferred without contact.

Example : a TV or computer screen will build up a negative charge after it has been on for

awhile. Dust which is neutrally charged will have its positive charges attracted to the surface of

the screen.

It is called induction because it induces (causes) the positive charges to come closer to the

negative charge.

Positive Uses of Static Electricity and Induction

1. Pollution and Dust Control - dirty air is sprayed with positive ions. The positive ions stick to

the dust particles. The air then flows through negatively charged plates. The dust is then

attracted to the plates and the air comes out "clean"

2. Coating surfaces with Particles (aka powder coating) - paint powder is given a charge as it

leaves the paint gun. The object being painted is given the opposite charge and the paint is then

attracted to the object. Any particles that miss can be collected and reused unlike wet paint

particles.

Pg 283 - 3,5,6

Pg 287 1, 3, 4, 6

Homework - 7,8

Lightning

Did you know that there are about 100 lightning strikes every second around the world and about

8 million daily!

-Natural process of the atmosphere exchanging electric charges with the Earth.

-Electrons are continuously being removed from the Earth by evaporation of water, gases from

automobiles and industrial processes.

-Eventually large numbers of these electrons build up in the bottom of a cloud. When the cloud is

close enough to the ground or a large building the charge will go to the ground.

- This is lightning - a large spark.

- It appears jagged because it is taking the path of least resistance to the ground by following

drops of moisture in the air or by concentration of positive ions.

- It is likely to hit tall structures because they are

Page 7: Electrostatics - ASD-Ssussexhigh.nbed.nb.ca/jgaunce/Grade 9 Science/Electrostatics 9... · Electrostatics Before scientists discovered how electricity works they had to make discoveries

Lightning Rods

Serve two purposes

1. Help to prevent strikes. The rod has a

positive charge and repels other positive ions

that are higher in the atmosphere. The electrons

in the cloud are more attracted to the positive

ions in the air.

2. If lightning does strike, the charge will be

transferred through a cable and disperses safely

in the ground.

Cars and Lightning

In a lightning storm if a car is struck the charge will go around the outside of the car and to the

ground. The occupants will not be affected.

Pg 291 questions 2,5,6, 7&8

Chapter Review Questions

Page 294-295 2, 6, 7, 11, 12, 20, 21, 24, 26