Unit 3: Electricity

Grade 9 Academic Science

RH King Academy

Unit Outline

1. Static electricityElectroscope lab

2. Current electricityCircuit lab

3. Electrical energyEnergy ISU

What is electricity?

Recall from chemistry: what is an electron?

An electron is a negatively charged particle in an atom

Electrons can move! They transfer between atoms, giving them positive and negative charges

If a substance gains electrons, what charge does it have? NEGATIVE

If a substance loses electrons, what charge does it have POSITIVE

Electricity is the flow of electrons

Static electricity

• Static charges collect on surfaces

• They remain on surfaces until they are given a path to escape

• Why do you think this girl’s hair sticking to the balloon?

• Read along on p.394

Demonstration of charge

Materials:

Fur

Plastic pen

Bits of paper

Procedure

1. Sprinkle bits of paper on the counter

2. Rub the pen roughly against the fur about 10 times

3. Hold the pen above the bits of paper. What do you observe?

What happened?

You observed static electricity

Recall, what is electricity?

When you rub a balloon against your hair, or you rub plastic against fur, you build up static charges

These charges build because as you rub these two objects together, you are transferring electrons

The fur LOST electrons. It became POSITIVE.

The pen GAINED electrons. It became NEGATIVE.

These charged objects were then able to attract objects with opposite charges.

Transferring electronsHow do atoms lose or gain electrons?

One common cause of electron transfer is friction – when objects rub against each other

The diagram below shows amber and fur being rubbed together. The fur transfers its electrons to the amber.

As electrons are tiny, note that millions of electrons are being transferred, not just a few.

ChargesWhen electrons are transferred, the object that

loses the electrons is positively charged, and the object that gains the electrons is negatively charged.

If the two objects weren’t rubbed together, they would have remained neutral. However, when they were rubbed together, electrons were transferred, so they became charged.

Amber gained electrons

Fur lost electrons

Electron Affinity

How do you know if an object will lose or gain electrons?

Some materials have a tendency to lose electrons, while others have a tendency to gain electrons.

This is called electron affinity.

Refer to the table of Triboelectric series. This will tell you if the material will lose or gain electrons.

Refer to p. 398

1) If you rubbed cotton and Teflon together.

What charge would the cotton be?

What charge would the Teflon be?

2) If you rubbed silk and nylon together.

What charge would the silk be?

What charge would the nylon be?

3) If you petted a cat.

What charge would your hands be?

What charge would the cat be?

4) If the cat rolled around on a cotton blanket

What charge would the cat be?

What charge would the cotton blanket be?

Laws of Attraction and Repulsion

Remember: “opposites attract”

Positive and negative charges attract

Like charges (positive and positive / negative and negative) repel

If you increase the amount of charge, the attraction or repulsion also increases

Example

If you rubbed hair against a nylon balloon, what would the charges be?

The hair would be positive, the balloon would be negative.

Notice that her hairs are spreading apart. That’s because they are positively charged, and they are repelling each other (like charges repel!)

But her hairs are also sticking to the balloon. That’s because her hairs are positively charged, but the balloon is negatively charged (opposite charges attract!).

How do you measure charges?

An electric charge is measured in coulombs (C)

1 coulomb = 6.24 x 1018 electrons added to or removed from a neutral object

Never seen scientific notation before? 1018 means move the decimal 18 places to the right.

Therefore, that’s 6240000000000000000 electrons.

1 coulomb sounds like a lot of electrons! But remember, electrons are tiny.

Insulator and Conductors

Not all materials transfer electrons easily. If that were the case, the electricity running through all these wires around us would flow right into our bodies.

Some materials conduct electricity, while others insulate electricity.

Conductors allow electrons to flow freely. Examples: copper, aluminum, mercury, or any other metal.

Insulators resist the movement of electrons. Examples: rubber, plastic, pure water.

Fair conductors allow some movement of electrons. Examples: carbon, mineral water, human body.

Insulators and Conductors

How insulators and conductors work together

Metals are “conductors”. This means that electrons flow freely through them.

For this reason, electrical wires are made of metal. They conduct electricity (which means they allow electrons to flow through them).

Plastics are “insulators”. This means that they resist the movement of electrons.

For this reason, electrical wires are coated in plastic. They insulate electricity (which means they prevent electrons from flowing)

We rely on metal conductors to bring us electricity, but we also rely on plastic insulators to prevent us from getting shocked by electricity.

Is water a conductor?

Pure water is an insulator

You may have heard that it’s dangerous to be in the water when lighting may strike. This is true. So does that mean that water isn’t an insulator?

Only pure water is an insulator. The water in lakes and rivers is not pure. It has minerals in it. Therefore, it’s a fair conductor.

Lesson Review QuestionsAnswer these questions now in your own words:

1) Define electricity.

2) Explain how objects get charged.

3) How do you know whether an object will lose OR gain electrons?

4) What is the difference between a conductor, fair conductor, and insulator?

Homework

P. 398 #1-5

P. 401 #1-5