Physics 30 Outcome 1 Lesson 1: Charges

Lesson 1: Charges
Brainiac Electric Fence

Electrostatics
Electro = charges
Static = stationary

Electrostatics is the study of stationary

charges.

Fundamental Matter Particles

Units of charge:

The elementary charge:

In ordinary matter charge is found in the proton (made of the two up quarks and one
down quark) and the electron (which is fundamental and not made of anything
smaller)

Proton: Electron:
Charge = Charge =
Mass = Mass =

Part 1 Conductors and Insulators

The way that charge is able to move in a solid depends on whether the solid is a conductor or an
insulator.
Activity: PHET Balloon and Static Electricity

In this simulation, the balloon and sweater

are made out of insulating materials and
the wall is made out of a conductive
material.

QUESTION 1: At the beginning when you first open the simulation,

all of the objects are neutrally charged. What observations support
this? What does neutrally charged matter mean?

Rub the balloon over the wool sweater. This is a method of charging called “charging by friction”.

QUESTION 2: What do you observe happens to the negative charges as you rub the balloon against the sweater?
Physics 30 Outcome 1 Lesson 1: Charges
QUESTION 3: What happens to the positive charges in the sweater? Explain why this happens?

QUESTION 4: What is the net charge on the balloon and sweater after using friction to charge? Do they have the same
charge or different?

Bring the balloon near the wall and move it away again. Bring the balloon back near the wall and release it. Observe
what happens to the charges in both the wall and the balloon.

QUESTION 5: What happens to the charges in the wall? What happens the charges in the balloon?

QUESTION 6: Based on your observations above, what can you conclude about the difference between conductors and
insulators?

Charging by friction

DEFINITION: Conductor

DEFINITION: Insulator

Charges in solids

Part 2: Charging by Friction

Activity: Friction Rods

1. Rub the ebonite rod with rabbit fur to give it a static
Rod Rubbed with
charge.
2. Verify that the rod is charged by taping the section that Acetate (clear) is an insulator. Cotton, silk, or paper towel
you rubbed on the metal ball of the electroscope. If the Acetate becomes positively which become all negatively
rod is charged, the leaves of the electroscope will charged when rubbed with charged
separate.
3. Remove the charge from the electroscope by grounding it: Ebonite (black) is also an Fur which becomes positively
touch the metal ball of the electroscope with your fingers insulator. Ebonite becomes charged
negatively charged when
and make sure that the leaves return to their neutral
rubbed with
position.
4. Repeat the above steps using the acetate rod and cotton, silk or paper towel.

QUESTION 1: There are two types of charges, positive and negative. Can you tell what kind of charge is on each rod
using the electroscope? What is the electroscope most useful for?
Physics 30 Outcome 1 Lesson 1: Charges
QUESTION 2: There are two kinds of charged particles in atoms. Protons are positively charged and found in the nucleus
of atoms. Electrons are negatively charged and are free to move around outside the nucleus. What do you think is
actually moving when you give an object a positive charge? Explain your reasoning

QUESTION 3: The ebonite rod gains a negative charge when you rub it. Draw a picture of what is happening with the
electrons and protons in the ebonite rod and fur as they are rubbed together.

QUESTION 4: The acetate rod gains a positive charge when you rub it. Draw a picture of what is happening with the
electrons and protons in the acetate rod and cloth as they are rubbed together.

Conductors
A conductor is an object through which charges can easily move. You can imagine charges moving like a gas through a
conductor and spreading out filling all free space within.

1. Rub the metal rod using the rabbit fur

2. Use the electroscope to test the charge on the metal rod

QUESTION 5: Is it possible to charge a metal conductor using friction? Why or why not?

Charge distribution on the surfaces of conductors and insulators:

An irregularly shaped conductor (ex: a cube) will not have a perfectly even spread of charges. Instead the charges tend to
accumulate at corners.

Conductors Insulators
Physics 30 Outcome 1 Lesson 1: Charges
Part 3: The Law of Charges

Activity: Attraction and Repulsion of Charged Rods

Acetate (clear) becomes positive when rubbed with silk, cotton, or paper towel.

Ebonite (black) becomes negative when rubbed with fur.

1) Charge the acetate rod on a string from end to end using friction (cotton,
silk or paper towel) and hang it from the lab bench using tape. Be careful
not to touch the rod as you hand it so it does not lose its charge.
2) Charge up the ebonite rod (no string) using friction (fur). Bring it near but
not touching the hanging acetate rod

QUESTION 1: When you bring the ebonite rod near the hanging acetate rod, do
you observe attractive or repulsive forces?

3) Charge up the other acetate rod (no string) using friction (cotton, silk or
paper towel). Bring it near but not touching the hanging acetate rod.

QUESTION 2: When you bring the acetate rod near the hanging acetate rod, do you observe attractive or repulsive
forces?

5) Charge the ebonite rod on a string from end to end using friction (fur) and hang it from the lab bench using tape. Be
careful not to touch the rod as you hand it so it does not lose its charge.
6) Charge up the other ebonite rod (no string) using friction (cotton, silk or paper towel). Bring it near but not touching
the hanging ebonite rod

QUESTION 3: When you bring the ebonite rod near the hanging ebonite rod, do you observe attractive or repulsive
forces?

7) Charge up the acetate rod (no string) using friction (cotton, silk or paper towel). Bring it near but not touching the
hanging ebonite rod.

QUESTION 4: When you bring the acetate rod near the hanging ebonite rod, do you observe attractive or repulsive
forces?

The Law of charges:

LIKE CHARGES

UNLIKE CHARGES
Physics 30 Outcome 1 Lesson 1: Charges
Part 4: Induced Charge Separation
An Electrostatics Puzzler
http://serc.carleton.edu/sp/compadre/demonstrations/examples/48756.html

Describe the “puzzle” shown in this demonstration

Activity: The Physics Classroom Aluminum Can Interactive

After exploring the Aluminum Can Interactive, can you explain the pop can puzzle?

DEFINITION: Induced Charge Separation

Part 5: Grounding
Demonstration: Van de Graaff generator and pith ball

Grounding is when a charged object is neutralized by an object of substantial size. A

ground is simply an object that serves as a seemingly infinite reservoir of electrons; the
ground is capable of transferring electrons to or receiving electrons from a charged
object in order to neutralize that object.

Use the definition of grounding to explain the behaviour of the pith ball in the Van de
Graaff generator demonstration.
Alternate Video
Physics 30 Outcome 1 Lesson 1: Charges
Grounding a negatively charged object —> draw which way the charges move

Grounding a positively charged object —> draw which way the charges move

Activity: The Physics Classroom Charging Interactive

Part 6: Charging by Conduction

Conduction is a transfer of charge by contact. When a charged object touches a neutral conductor, the conductor gains a
charge by conduction.

Activity: Charging pith balls using friction rods

Acetate (clear) becomes positive when rubbed with silk, cotton, or paper towel.

Ebonite (black) becomes negative when rubbed with fur.

1. Charge up the ebonite rod using friction (fur).
2. Touch the ebonite rod to the hanging pith ball. The pith ball has now gained a charge through conduction.
3. Increase the charge on the ebonite rod by continuing to rub it with fur.
4. Bring the ebonite rod near BUT NOT TOUCHING the charged pith ball.
5.Through the next couple steps be careful not to touch the pith ball so it doesn’t lose its charge!

QUESTION 1: Is the pith ball attracted to or repelled from the negatively charged ebonite rod?
Physics 30 Outcome 1 Lesson 1: Charges
6. Charge up the acetate rod using friction (cotton, silk or paper towel).
7. Bring it near BUT NOT TOUCHING the charged pith ball.

QUESTION 2: Is the pith ball attracted to or repelled from the positively charged acetate rod?

QUESTION 3: Based on your observations from questions 1 and 2, what is the charge on the pith ball after being touched
by the negative rod?

8. Touch your fingers to the pith ball to neutralize it.

9. Charge up the acetate rod using friction (cotton, silk or paper towel).
10. Touch the acetate rod to the hanging pith ball.
11. Increase the charge on the acetate rod by continuing to rub it with cotton, silk or paper towel.
12. Bring the acetate rod near BUT NOT TOUCHING the charged pith ball.
13. Through the next couple steps be careful not to touch the pith ball so it doesn’t lose its charge!

QUESTION 4: Is the pith ball attracted to or repelled from the positively charged acetate rod?

14. Charge up the ebonite rod using friction (fur).

15. Bring it near BUT NOT TOUCHING the charged pith ball.

QUESTION 5: Is the pith ball attracted to or repelled from the negatively charged ebonite rod?

QUESTION 6: Based on your observations from questions 4 and 5, what is the charge on the pith ball after being touched
by the positive rod?

QUESTION 7: When an object is charged using conduction, does it gain a charge that is the same or opposite to the
object that touched it? How do you know this?

QUESTION 8: A negatively charged rod is brought to touch a neutral charged conductor. After the contact, the conductor
gains a charge. Draw a diagram of the charge distribution (positive and negative charges) in the rod and conductor
before, during, and after contact.
Before During After
Physics 30 Outcome 1 Lesson 1: Charges
QUESTION 9: A positively charged rod is brought to touch a neutral charged conductor. After the contact, the conductor
gains a charge. Draw a diagram of the charge distribution (positive and negative charges) in the rod and conductor
before, during, and after contact.

Before During After

DEFINITION: Conduction

Charging by conduction

Part 7: Conservation of Charge

When two charged conductors touch and are separated:

Physics 30 Outcome 1 Lesson 1: Charges

Part 8: Charging by Induction

Recall from part 1 that the negative charges in a conductor are free to move anywhere inside it. In a
neutral conductor, the negative charges are spread out evenly in an equilibrium configuration. There
are the same number of positive and negative charges in the conductor.

When a charged object is brought near (but not touching) a neutral conductor, the negative charges in the conductor
move towards or away from the object, depending on the charge. As a result the conductor becomes polarized, one side
becomes more negative than the other. The net charge on the whole conductor is still zero, but the two sides have
overall charge. This is called an induced charge separation and the process is called induction. Induction is when you
bring a charged object near but not touching a neutral object.

Question 1: A negatively charged rod is brought near but not touching a neutral conductive sphere. As a result of this
action, there is an induced charge separation on the sphere. Draw a picture of the rod and sphere before and during the
induction process.

Before During

Question 2: A positively charged rod is brought near but not touching a neutral conductive sphere. As a result of this
action, there is an induced charge separation on the sphere. Draw a picture of the rod and sphere before and during the
induction process.

Before During
Physics 30 Outcome 1 Lesson 1: Charges

Activity: Charging by Induction (with grounding)

You can give a conductor a permanent, or at least a long lasting, charge using a process that involves induction and
grounding.

Charging using a negative rod:

1. Charge up the ebonite rod (negative) using friction (fur)
2. Bring the section of the rod that you gave the most charge near but not
touching the conductive sphere on the top of the electroscope.
3. Touch the opposite side of the electroscope with your finger.
4. Remove your finger
5. Remove the rod
6. Repeat process until the leaves are very far apart.

Question 3: Explain how the electroscope is able to gain a net charge even though it
was never touched by the negatively charged rod.

Question 4: What is the charge on the electroscope after the induction process is completed?

Charging using a positive rod:

1. Ground the electroscope by touching it with your fingers.
2. Charge up the acetate rod (positive) using friction (paper towel, silk or cotton)
3. Bring the section of the rod that you gave the most charge near but not touching the conductive sphere on the
top of the electroscope.
4. Touch the opposite side of the electroscope with your finger.
5. Remove your finger
6. Remove the rod
7. Repeat process until the leaves are very far apart.

Question 5: What is the charge on the electroscope after the induction process using a positive rod is completed?

Question 6: When an object is charged using induction, does it gain a charge that is the same or opposite to the rod?
Explain.

Methods of charging:

Friction

Conduction

Induction (with grounding)

Physics 30 Outcome 1 Lesson 1: Charges

Lesson 1 Hand in Assignment

1. As a charged rod is brought near to the knob of the metal-leaf electroscope, what happens to the leaves? Why? Draw
sketches to show the distribution of charge on the knob and leaves of the electroscope, in each case. What happens to the
leaves when the charged rods are removed? Why? What is the net charge on the electroscope?
2. Why does touching a conductor with your hand ensure that it is neutral? Be sure your explanation covers both cases: when
the object has an excess of electrons, as well as when it has a deficit.
3. When a nearby charged object causes a change in the distribution of charge on a neutral object, this is called an induced
charge separation. Is it possible to create an induced charge separation on an insulator? On a conductor? Explain your
answers.

4. When a negatively charged rod touches the pith ball, what charge did the pith ball acquire? How do you know? Which way
did the electrons move? Illustrate your answers with sketches labelled "before contact", "during contact", and "after
contact”.
5. Two identical metal spheres have different charges. One has a charge of +10 C and the other has a charge of –20 C. If the
spheres are allowed to touch each other for a time and then are moved away from each other, what is the final charge on
each sphere? Explain

6. A glass rod is charged to +5 nC by rubbing.

a. Have electrons been removed from the rod or protons added? Explain
b. How many electrons have been removed or protons added?

7. The leaves of an electroscope will repel each other if you touch the electroscope with a positively charged glass rod. Use a
series of charge diagrams to explain what happens and why the leaves repel each other.

8. A plastic balloon that has been rubbed with wool will stick to a wall. Can you conclude that the wall is charged? If not, why
not? If so, where does the charge come from? Draw a series of diagrams to show how the balloon is held to the wall.

9. Two neutral metal spheres on wood stands are touching. A negatively charged rod is held directly above the top of the left
sphere, not quite touching it. While the rod is there, the right sphere is moved so that the spheres no longer touch. Then the
rod is withdrawn. Afterward, what is the charge state of each sphere? Use charge diagrams to explain your answer.

10. You have two neutral metal spheres on wood stands. Devise a procedure for charging the spheres so that they will have
opposite charges of exactly equal magnitude. Use charge diagrams to explain your procedure.

11. You have two neutral metal spheres on wood stands. Devise a procedure for charging the spheres so that they will have like
charges of exactly equal magnitude. Use charge diagrams to explain your procedure.

12. A plastic rod is charged to -20nC by rubbing.

a. Have electrons been added to the rod or protons removed? Explain
b. How many electrons added or protons removed?

14. A 1.8 g (0.0018 kg) pith ball is suspended from a thread and placed in an electric field.
When placed in the electric field, the pith ball is deflected to the left so that the thread
makes a 9.5o angle with the vertical. Calculate the electrostatic force on the pith ball.
(0.0030 N)