1

Electrostatics

Electrostatics

Have you ever been shocked touching a door?Have you ever seen tricks like the ones Mr. Maloney did in class?Have you ever done laundry ….

and noticed that your clothes stick together

Have you noticed that you hair gets frizzy when it is a dry day?Have you ever wondered why???

2

Electrostatics

Static Electricity is the cause of all these occurrences.The Study of Static Electricity is called ELECTROSTATICS.We will first investigate electrostatics, which will enable us to study other areas such as electric current, power, and electricity in general.

What is Static Electricity?

Just like the examples that we did in class, we know objects either are attracted to each other, repelled by each other, or have no effect with respect to each other.

Some objects are able to exert a force on other objects to pull them or push them.

3

Static Charge

Objects we observed that are able to apply a force to other objects are called chargedobjects.

In class, we were able to test a number of charged objects and see how they affect or are affected by other objects.

What were our results?

StaticsWe grouped our objects into three groups.

4

StaticsGroup A objects were attracted to Group B and Group

C, and repelled objects from their own Group.

StaticsGroup B objects were attracted to Group A and Group

C, and repelled objects from their own Group.

5

StaticsGroup C objects were attracted to Group A and Group

B, and felt no effect from objects in their own Group.

StaticsGroup C objects were attracted to Group A and Group

B, and felt no effect from objects in their own Group.

What does this mean?

6

Statics

Something must have happened when the objects were rubbed together because they did not interact this way before.If we think back to Chemistry, we know that objects are made of atoms, which are made of nuclei surrounded by electrons.Nuclei have a (+) charge, and electrons have a (-) charge.

StaticsWhat do you think happens when objects are rubbed together?The electrons may be freed, or pulled away from the nucleus.Unless the objects are identical, one of them will want the electrons more than the other one.The object that does wants the electrons less will transfer some of its free electrons to the other object. (Phet Balloon Simulation)

7

Statics

Before both objects have equal positive and negative charge

Statics

Before both objects have equal positive and negative charge

after …This object has more (-) charge, excess electrons

This object has less (-) charge, less electrons

8

Statics

Before both objects have equal positive and negative charge

after …This object has a net (-) NEGATIVE charge

This object has the opposite, a net POSITIVE charge

Charges

By convention we call the objects with a net (-) charge NEGATIVE.and the objects with the net (+) charge POSITIVE.Notice charge is NOT created or destroyed, it is just transferred.This law is known as CONSERVATION OF ELECTRIC CHARGE.

9

Charged Objects

As for our objects …The objects in Group A like electrons, and acquire a (-) negative chargeThe objects in Group B acquire a (+) positive chargeSo what about Group C?

They are called neutral, they do not seem to acquire any charge at all.

Charged Objects

So what rules can we make about how these charges interact with each other?

A is attracted to B and C, repelled by A

B is attracted to A and C, repelled by B

C is attracted to A and B, nothing with C

(+) are attracted to (-) and neutral, repelled by (+).

(-) are attracted to (+) and neutral, repelled by (-).

neutral is attracted to (+) and (-), and has no effect with other neutrals.

10

Charged Objects

In SummaryObjects with the same charge (+ or -) repel each other.Objects with different charges attract each other.Charged objects (+ or -) and neutral objects are attracted to each other.Neutral objects do not have any effect on each other.

Why the X’s

We now know how objects acquire a charge.

So it seems that all objects should acquire a positive or negative charge when rubbed.

So why do some objects, like the can, act as if they are neutral?

11

2 types of materials

The PVC, fur, and balloons all were able to acquire a charge during the experiment.

The soda can and spoon could not.

Almost all matter can be grouped into two types of materials.

Insulators

Conductors

Insulators

Examples of insulators are the objects that were capable of holding a charge

glass, wood, plastics, cloth, synthetics, air

What do you know about the term insulator?Insulators are used to cover wires

Insulators are used to keep things cold or warm

12

Insulators

Insulators are used to keep things from changingthey keep the direction of electricity from changingthey keep temperature from changing

They also keep charges in place.Insulators do not allow charges to move, so when an area becomes charged, the charge stays there … they are not replaced by charges from other parts of the material, and the object keeps a localized area of (+) or (-) charge.

Conductors

Metals and tap water are examples of conductors.So what do you think happens in conductors?Conductors allow charge to move around and be replaced.So when an area is rubbed and charge is transferred to or from it, the charge quickly moves around throughout the material. The electrons (charge carriers) move freely until they are evened out, and the object remains electrically neutral.

13

Force on the X’sSo if the X’s are conductors and remain electrically neutral, why can we make them move?Why does a balloon attract a metal can that has not been rubbed? And why does your hand or a wall attract a balloon when they have not been rubbed?

What do you think is going on?

(Back to Phet Balloon Simulation)

Separation of Charge

Think of bringing a charged balloon near a neutral can?The electrons in the can are repelled by the balloon and since the can is a conductor, they move away.The can does not gain or lose charge, but they become separated.The electrical force, like gravity, gets weaker as distance increases. Now the electrons are farther from the balloon than the positive charges are, so the repelling force they feel is less than the attractive force the (+) feel.Therefore the can moves towards the balloon.

SKIP?

14

How to charge

We already demonstrated how we can charge objects by rubbing them together, but there are other ways to do this.

To investigate this we will use an object known as an electroscope.

The Electroscope

The electroscope consists of a metal knobattached to a metal rodattached to 2 thin pieces of metal foil called leaves.

15

Charging by Conduction

One way to charge an object is through conduction, or touching a charged object to an uncharged object.Using the electroscope as an example, when a negatively charged object is touched to the knob, what happens?Since the electroscope is made of metal, the charge spreads all throughout the metal.The leaves are so thin that although they want to distribute their charge they cannot, and each develops a net negative charge.This can be seen as they push each other apart.

What’s

happening

here?

Show electric

ping

pong

16

Charging by Induction

Separation of charge can be used to charge an object without touching it.Suppose a (-) charged object is brought near two metal balls that are touching each other. What will happen?Electrons from the closer sphere will separate into the farther sphere.If the spheres are then moved apart while the charged object is near, each will retain their charge separation.This is called charging by induction.Let’s see it with the electroscope.

Electric Force

We now know all about how charged particles interact with each other.Objects of like charge apply a repulsive force towards each other.Objects of opposite charge apply an attractive force on each other. Sometimes this force is so great, that the electrons jump through the air. (Travoltage)But how big is this force, and what does it depend on?

17

Electric Force

Let’s think about the can and balloon again.

As I move a charged balloon near a can, the can is attracted and begins to move towards the balloon.

What happens as I move the balloon closer?Right, the can moves faster.

So the attractive force must have increased as the distance between the two decreased.

Force Dependence on r

As the separation of two charged particles decreases, the force between the two increases.

Separation and Force have an inverse relationship, as one goes up the other goes down, and vice versa.

18

Electric Force

Back to the balloon and can.Lets compare what happens when I try to move the can with a balloon rubbed a little, and then with a balloon rubbed a lot.Which one gets the balloon moving faster?

Right the one rubbed more, the one that has more charge build up on it.This one applies a greater force.

Dependence on charge

As the charge of the objects increases, the force between them also increases.

The force and charge size have a directly proportional relationship

As one goes up, so does the other.

19

Putting it all Together

So lets put it all together.

As separation increases, force decreases

As charge increases force increases

We need a formula that relates this all together.

The formula is not straight forward, but was discovered by Charles Coulomb.

Coulomb’s Law

After much testing, Coulomb was able to determine the relationship between charge, separation and force.

It is summarized as Coulomb’s Law

F = forceq1 = charge of particle 1q2 = charge of particle 2r = separationk = 8.99 x 109 N*m2/C2

221

rqqkFe =

20

Coulomb’s Law

Notice that force depends on the charges of both of the objects (q1 and q2).The force also depends on the separation (r2) squared, not just r.k is a proportionality constant that makes the equation true.Also notice the emergence of a unit C.This is the unit of charge the Coulomb.So for this equation to work, the charge must be measured in coulombs, the separation in meters, and the Force will be given in Newtons.

221

rqqkFe =

the Coulomb

The coulomb is the charge of 6.25 x 1018 electrons or protons.So one electron has a charge of

1.6 x 10-19 C.

We know that normal objects have millions of electrons and protons. Since the protons and electrons usually balance each other out, we often see no net effect. But even a small amount of charge imbalance can cause huge forces to exist.

21

Direction of Force

The direction of the electric force can be determined based on the charge of the particles. They follow our same old laws.

Opposites attract

Likes repel

The electric force is always in a straight line connecting the two charges, and points in a direction determined by the charges.

Example 1

Two charges (2 C and 3 C) are positioned 0.5 meters from each other. What force does the 2 C charge feel as a result of the 3 C charge?

F = (8.99 x 109 N*m2/C2)*(2 C)*(3 C)

(0.5)2

F = (5.34 x 1010 N*m2) = 2.1 x 1011 N (repulsive)

(0.25 m2)

221

rqqkFe =

22

Example 2

Two other charges (2 C and -1 C) are positioned 2 meters from each other. What force does the 2 C charge feel as a result of the -1 C charge?

F = (8.99 x 109 N*m2/C2)*(2 C)*(-1 C)

(2)2

F = (-1.8 x 1010 N*m2) = -4.5 x 109 N

(4 m2) or 4.5 x 109 N (attractive)

221

rqqkFe =