Chapter 5: Electric Forces

Question 1

What are the two types of electric charge?

Answer 1

Positive and negative.

Question 2

What does it mean for an object to be charged?

Answer 2

It has more of one type of charge than the other type.

Question 3

What does it mean for an object to be neutral?

Answer 3

It has the same amount of positive charges as negative charges.

Question 4

Can an object have no charge at all?

Answer 4

Only a neutron (tiny particle) carries no charge. All objects made up of atoms include both positive charges (from the protons in the atomic nucleus) and negative charges (from electrons around the nucleus).

Question 5

How would two negative charges interact?

Answer 5

Like charges repel.

Question 6

How would two positive charges interact?

Answer 6

Like charges repel.

Question 7

How would two neutrons interact?

Answer 7

They would have no interaction through the electromagnetic force because they have no charge.

Question 8

How would a positively charged object and a negatively charged object interact?

Answer 8

Opposite charges attract.

Question 9

How would a positively charged object and a neutral object interact?

Answer 9

The positively-charged object would attract the electrons in the neutral object, causing the neutral object to become polarized. Because the side of the neutral object closest to the positively-charged object becomes negatively charged, and the positively-charged side of the neutral object is slightly further away, the neutral object would be slightly attracted to the positively-charged object.

Question 10

How would a negatively charged object and a neutral object interact?

Answer 10

The negatively-charged object would repel the electrons in the neutral object, causing the neutral object to become polarized. Because the side of the neutral object closest to the negatively-charged object becomes positively charged as the electrons leave it, and the negatively-charged side of the neutral object is slightly further away, the neutral object would be slightly attracted to the negatively-charged object.

Question 11

What does it mean for an object to be polarized?

Answer 11

The electrons move to one side of the object, making that side more negatively charged and the side they left more positively charged.

Question 12

When you transfer charge to or from an object, what is actually moving and why?

Answer 12

The electrons move. Electrons are only loosely bound to atoms (not tightly bound to the nucleus like protons) and weigh far less than a proton, so when charges move around, it’s the electrons moving, not the protons.

Question 13

What is the smallest amount of charge you can have?

Answer 13

The smallest unit of charge is the amount of charge on a single electron or proton, equal to 1.602 x 10^-19 Coulombs (C).

Question 14

What is the unit of charge?

Answer 14

Coulombs (C)

Question 15

Is one Coulomb a large or small amount of charge?

Answer 15

A large amount of charge. To have one Coulomb of charge, you need 1.602 x 10^19 electrons or protons.

Question 16

What kinds of materials are conductors?

Answer 16

Conductors include metal, water (except deionized water), and the human body.

Question 17

What kinds of materials are insulators?

Answer 17

Insulators include air, wood, plastic, ceramics, silicone, and deionized water.

Question 18

What can a conductor do that an insulator does not do?

Answer 18

Conductors allow the free flow of electrons across the material (outer electron(s) are not tightly bound to the atoms).

Question 19

What can an insulator do that a conductor does not do?

Answer 19

An insulator prevents electrons from moving through the material (electrons are tightly bound to the atoms).

Question 20

Can you have any amount of charge? Why or why not?

Answer 20

No; charge is quantized. It comes in discrete amounts, like money. An electron or proton is the smallest amount of charge you can have, just like a penny is the smallest amount of money you can have.

Question 21

What is the electric force (also known as the Coulomb Force)? Use a description, not an equation.

Answer 21

The electric or Coulomb force is the force between two charged particles and depends on a constant, the charge of both particles, and the distance between the particles.

Question 22

What does it mean for charge to be quantized?

Answer 22

Charge comes in discrete bits, not any random amount you want. An electron or proton is the smallest amount of charge you can have. You can have one electron, or two, or three, but not one and a half.

Question 23

When you put an electron and proton together (like in an atom), do they cancel each other out when adding up the total charge?

Answer 23

Yes, positive charges count as positive amounts and negative charges count as negative amounts, so they add to zero if you have the same amount of both.

Question 24

When you put an electron and proton together (like in an atom), do they destroy each other?

Answer 24

No. Charge is conserved; the universe has the same amount of negative and positive charges today as it has from (almost) the very beginning. We don’t create or destroy them, we just move them around.

Question 25

How many microCoulombs (uC) are in a Coulomb of charge?

Answer 25

There are 10^6 uC (microCoulombs) in a Coulomb (1 C) of charge.

Question 26

How many milliCoulombs (mC) are in a Coulomb of charge?

Answer 26

There are 10^3 uC (milliCoulombs) in a Coulomb (1 C) of charge.

Question 27

Is the electric force between two charged objects attractive, repulsive, or does it depend on the objects?

Answer 27

It depends on the objects, so you need to draw a free body diagram (FBD) for each interaction to determine where the force points.

Question 28

What are the units of force?

Answer 28

Newtons (N)

Question 29

What does k stand for?

Answer 29

k is the electric constant and is equal to 8.99 x 10^9 N m^2/C^2.

Question 30

Does the electric force increase or decrease as the distance between the objects increases?

Answer 30

The electric force is inversely proportional to the square of the distance between the objects, so as the distance gets larger, the electric force decreases.

Question 31

Does the electric force increase or decrease as the distance between the objects decreases?

Answer 31

The electric force is inversely proportional to the square of the distance between the objects, so as the distance gets smaller, the electric force increases.

Question 32

What does it mean for an object to have excess charge?

Answer 32

Most objects are neutral (equal amount of positively and negatively-charged particles). Excess charge is the amount of unmatched additional charge on the object.

For example, if an object contains 3 electrons and 5 protons, it has an excess charge equal to the charge on two protons (-3 + 5 = +2).

Question 33

Object B has twice as much excess charge as Object A. How does the force of object A on object B compare to the force of object B on object A?

Answer 33

The forces are equal and point in opposite directions.

Why are the forces equal? Two reasons.

1. Newton’s Third Law said that objects exert equal and opposite forces on each other.
2. The equation for the Coulomb Force (the electric force) between two objects includes the charge of both objects, so it doesn’t matter which one you’re talking about - both charges are in the equation.

Question 34

How do you find the electric force due to more than two particles?

Answer 34

Draw a free body diagram (FBD) for all the forces acting on a specific particle. Add these forces, recalling that forces are vectors (calculate the force vector using the electric force equation, break the force into x and y components, add the x and y components separately, then combine them using Pythagorean’s Theorem and find the angle using trig).

Question 35

If you put a bunch of electrons on the surface of a conductor, what will they do and why?

Answer 35

The electrons will spread out on the surface of the conductor in order to get as far away from each other as possible. They will not penetrate the surface of the conductor.

Question 36

If you put a small, charged, conducting object into physical contract with a large, neutral, conducting object, what will happen and why?

What happens when you separate the objects again and why?

Answer 36

When put into contract, some of the excess electrons will flow from the smaller object to the larger object in order to maximize the space between the electrons (because they repel each other).

If you then separate the objects, they will each retain whatever charge they were carrying after they were put into contact (the electrons do not return to the smaller object). Both objects are now charged, but each carries a fraction of the original charge carried by the charged object.

This is called charging by conduction.