15.1 Electric Charge and Current

15.1 Electric Charge and Currentpp. 340 - 345

Mr. Richter

Agenda

Intro to Electrostatics Notes:

Charge and Net Charge Measuring Charge Electroscopes Charging Objects Three Ways

Objectives: We Will Be Able To… Distinguish between a positive and negative net

charge. Identify the charge of electrons and protons. Explain the meaning of Coulomb’s Law. Describe the different ways of charging an object.

Warm-Up:

Sometimes when you scuff your feet on a carpet and then touch someone, you both experience a small shock. Why do you think this happens?

Discuss at your table, and we will discuss as a class in a few minutes.

Electric Charge

Electricity at an Atomic Level

All matter has electric charge because it contains: protons (positive

charge), and electrons (negative

charge)

When we talk about electricity, we are talking about the flow of electric charge.

Positive and Negative Charge

There are two types of charges, positive and negative.

Like charges will repel each other.

Unlike charges will attract each other.

Like magnets!

Net Charge

Net Charge Just like net force, the net

charge of an object is the sum of the total charges within it.

When there is a perfect cancellation of positive and negative charges, there is a net charge of exactly zero. An object with a net charge of zero is called electrically neutral.

Net Charge

An object is charged if the net charge is not zero. Sometimes called the “excess charge”.

Positively charged if there are more positive charges than negative charges

Negatively charged if there are more negative charges than positive charges

Measuring Charge

Coulomb

The unit of charge is called a Coulomb (C), named for the French physicist who first accurately measured forces between charges.

The coulomb is a HUGE amount of charge. A single proton has a charge of 1.602 × 10-19 coulomb. The charge of an electron is -1.602 × 10-19 coulomb. For example: the amount of charge flowing through one

lightning bolt is about 15 C.

Coulomb’s Law Electric forces (of attraction or

repulsion) are created between charges.

These forces are incredibly strong! If you could separate the positive

and negative charges of the tip of a pencil to about 1 meter apart, the force is about 50,000,000,000,000 Newtons! (The weight of 5 billion cars!)

Coulomb’s Law (continued)

The magnitude of the force between charges depends on: the amount of charge (how many coulombs) the distance between them

The greater the charge, the greater the force The closer the charges are to each other, the greater

the force.

Coulomb’s Law (Continued)

Forces between charges create an action-reaction pair. Equal in magnitude

and opposite in direction.

see p342

Coulomb’s Law (Continued)

…explains the relationship between the amount of each charge, the distance between them, and the electrical force.

k is a constant: k = 9x109 N*m2/C2

This is very similar to Newton’s Law of Universal Gravitation

Your Turn

Two steel marbles are each given a net charge of one thousandth (0.001) of a coulomb.

Calculate the magnitude of the force on the marbles if they are held 2 meters apart.

Given q1 = 0.001 C q2 = 0.001 C r = 2m k = 9x109 N*m2/C2

FE = 2250 N

Warm-Up

When you rub a balloon against your hair for a few seconds, sometimes you can get it to stick to the wall? Why do you think that is?

Discuss at your table, and we will discuss as a class in a few minutes.

The Electroscope

The Electroscope

Because electrons are small, light, and are on the outside edges of atoms, they move easily.

Most electrical effects are caused by moving electrons, not protons.

Because electrons move, we can measure the charge of an object with an electroscope.

The Electroscope

The Electroscope

Electrons in a conductor will move until they are evenly spread around.

If the negatively charged rod is placed near the knob of the electroscope, the electrons move to the leaves.

Now that both leaves are negatively charged, they repel each other.

Using Coulomb’s Law, these forces and charges can be measured.

Three Ways to Charge ObjectsAnd how static electricity works!

Electrostatics and Electroscopes. Charging by friction and charging by contact. https://www.youtube.com/watch?v=yU55lXbrV0U

Charging Three Ways

By Contact If you touch the negatively

charged rod to the electroscope, the leaves separate. Electrons transfer between objects.

When you remove the rod, the leaves remain split.

The rod is now Neutral, and the leaves are negatively charged.

Charging Three Ways

By Friction When one neutral object

is rubbed against another.

Electrons are physically knocked off of one to the other.

One object is now negatively charged and one is positively charged.

Static Electricity

Static electricity (and the shock you feel as a result) is a combination:

First an object is charged by friction (scuffing one’s feet on the carpet).

Then the excess charge is transferred to another object by contact.

The moving charge makes a brief but intense flow of current.

Charging Three WaysBy induction (indirectly) Move a charged object close

(but not touching) to the electroscope, but then ground it (remove the excess negative electrons)…

When you remove your finger the leaves stay separated because the net charge is now positive.

The charge of the original object (balloon) is not changed.

Grounded with your finger p 345

Electroscope: Charging by Induction

https://www.youtube.com/watch?v=cMM6hZiWnig

Polarization

Polarization

The balloon that was charged by friction will stick to a wall (not a conductor) but not a doorknob (conductor).

The atoms in a wall can’t move very well, but they can turn their positive sides toward the balloon.

This is polarization.

A doorknob will absorb some of the negative charge from the balloon.

Because they are both negative, they will repel each other.

Wrap-Up: Did we meet our objectives?

An object has a net negative charge if it has gained electrons, and a net positive charge if it is missing some electrons.

Coulomb’s law relates the amount of charge of two particles, the distance between them, and the electric force between them.

Objects can be charged by contact, by friction, and by induction.

An electroscope will remain charged if charged by induction (grounded).

Homework

p345 #1-4