Electric Charge and Electric Force

Georgia Perimeter College

Objectives

  1. Identify the distinguishing properties of positive and negative charges.
  2. Distinguish between conductors and insulators.

  3. Use Coulomb’s Law to determine the net electrostatic force and the electric field intensity on a point electric charge due to known one or more charges within the surrounding vicinity.

  4. Describe the nature of the electric field lines associated with point charges.

This section addresses, in whole or in part, the following Georgia GPS standard(s):
  • S8P2. Students will be familiar with the forms and transformations of energy.
    c. Compare and contrast the different forms of energy (heat, light, electricity, mechanical motion, sound) and their characteristics.
  • S8P5.  Students will recognize characteristics of gravity, electricity, and magnetism as major kinds of forces acting in nature.
         c. Investigate and explain that electric currents and magnets can exert force on each other.

This section addresses, in whole or in part, the following Benchmarks for Science Literacy:
  • There are two kinds of charges-positive and negative. Like charges repel one another, opposite charges attract. In materials, there are almost exactly equal proportions of positive and negative charges, making the materials as a whole electrically neutral. Negative charges, being associated with electrons, are far more mobile in materials than positive charges are. A very small excess or deficit of negative charges in a material produces noticeable electric forces.
  • Different kinds of materials respond differently to electric forces. In conducting materials such as metals, electric charges flow easily, whereas in insulating materials such as glass, they can move hardly at all. At very low temperatures, some materials become superconductors and offer no resistance to the flow of current. In between these extremes, semiconducting materials differ greatly in how well they conduct, depending on their exact composition.
  • Magnetic forces are very closely related to electric forces and can be thought of as different aspects of a single electromagnetic force. Moving electric charges produce magnetic forces and moving magnets produce electric forces. The interplay of electric and magnetic forces is the basis for electric motors, generators, and many other modern technologies, including the production of electromagnetic waves.
  • Without touching them, material that has been electrically charged pulls on all other materials and may either push or pull other charged materials.
     

This section addresses, in whole or in part, the following National Science Education Standards:
  • The electric force is a universal force that exists between any two charged objects. Opposite charges attract while like charges repel. The strength of the force is proportional to the charges, and, as with gravitation, inversely proportional to the square of the distance between them.
  • Between any two charged particles, electric force is vastly greater than the gravitational force. Most observable forces such as those exerted by a coiled spring or friction may be traced to electric forces acting between atoms and molecules.
  • Electricity and magnetism are two aspects of a single electromagnetic force. Moving electric charges produce magnetic forces, and moving magnets produce electric forces. These effects help students to understand electric motors and generators.
  • Energy is a property of many substances and is associated with heat, light, electricity, mechanical motion, sound, nuclei, and the nature of a chemical. Energy is transferred in many ways.
  • Electrical circuits provide a means of transferring electrical energy when heat, light, sound, and chemical changes are produced.
  • In most chemical and nuclear reactions, energy is transferred into or out of a system. Heat, light, mechanical motion, or electricity might all be involved in such transfers.
  • Matter is made of minute particles called atoms, and atoms are composed of even smaller components. These components have measurable properties, such as mass and electrical charge. Each atom has a positively charged nucleus surrounded by negatively charged electrons. The electric force between the nucleus and electrons holds the atom together.
  • The atom's nucleus is composed of protons and neutrons, which are much more massive than electrons. When an element has atoms that differ in the number of neutrons, these atoms are called different isotopes of the element.
  • The nuclear forces that hold the nucleus of an atom together, at nuclear distances, are usually stronger than the electric forces that would make it fly apart. Nuclear reactions convert a fraction of the mass of interacting particles into energy, and they can release much greater amounts of energy than atomic interactions. Fission is the splitting of a large nucleus into smaller pieces. Fusion is the joining of two nuclei at extremely high temperature and pressure, and is the process responsible for the energy of the sun and other stars.
  • In some materials, such as metals, electrons flow easily, whereas in insulating materials such as glass they can hardly flow at all. Semiconducting materials have intermediate behavior. At low temperatures some materials become superconductors and offer no resistance to the flow of electrons. 

Electric Charge and Electric Force

The study of the electric force will begin with a review of the concept of the electric charge and electrostatics. 

Electrostatics is the study of static electric charges and electrical forces (i.e. for charges that are stationary or not in motion). The information derived about the behavior of the electric charge will be used to develop the concepts of electric currents (due to moving charges), the electric circuit, electrical energy and power in another unit yet to be covered.

After reading this content summary, review the relevant chapter and the relevant sections (Chapter 9) in your textbook to learn more about the highlighted topics (in bold face, underline, or italics). Click on the hyperlinked words to review the related information online.

Answer the Review Questions for each section after covering the relevant material. Be sure to complete all online practice exercises and activities to check your understanding before proceeding to the next topic.

Vocabulary

electric force
positive
negative
charge
attract
repel
conductor
insulator
fundamental charge
q
Coulomb's Law
coulomb
electrically polarized
electric field
static electricity
electrostatic charge

Electric Charge

The electric charge is an invisible property acquired by matter that can be observed by the interactions it produces.

All matter (solids, liquids, and gases) are made of atoms.

An atom is composed of electrons, protons and neutrons. The protons and neutrons are tightly bound together to form the nucleus at the center of the atom. The electrons swarm around the nucleus in random directions.

[The strong nuclear force that holds protons and neutrons together inside the nucleus acts only at extremely short distances -- too short to affect these electrons moving around the nucleus].

    

Electrons have a negative charge while protons have a positive charge.

Neutrons have no electric charge.

The amount of negative charge on an electron is exactly equal to the amount of (opposite) positive charge on a proton.

 Since atoms normally have equal numbers of electrons and protons, the total amount of positive charge "balance" the negative charge. Therefore, an atom is described as being electrically neutral, and there is no overall charge on the neutral atom.

Positive and negative charges interact in specific ways.

Charges that are same (or like) repel each other. Charges that are different (or unlike) attract each other.

 

Protons cannot move easily from one object to another since they are held too tightly within the nucleus of an atom.

Conductors and Insulators

The electrons moving around the nucleus can be moved from an atom to another atom, and from object to object. These electrons will move depending on whether the material is a conductor or an insulator.

Some of the electrons in a conductor are held loosely by the atom. Such electrons move freely from atom to atom within the material.

In insulators, the electrons are held tightly to the atom and are not able to move freely within the material.

Static Electricity or Electrostatic Charge

The stationary electric charge that may accumulate on an object is called an electrostatic charge or static electricity.

Whenever an on object is charged, charges can only be transferred from one object to another. No electrons are created or destroyed. The total amount of charge remains constant. Thus, charge is always conserved!

When an atom gains or loses electrons, it becomes charged and the "charged" atom is called an ion. An atom may gain excess electrons and becomes a negatively charged ion. If the atom loses one or more electrons, it becomes a positively charged ion -- in this case, the charge balance is altered due to the deficiency of electrons.

In solid objects, static charges are due to the gain or loss of electrons only. In solutions, the flow of charges from point to point can be caused by the movement of ions instead of electrons. In solution, the positive and negative ions from the dissolved salts separate and move freely.

 


Static electricity makes your hair stand up.
Van de Graff generator (left).  Image courtesy of Brookhaven National Laboratory.
Balloon and static electricity (right).  Image courtesy of NASA.

Hands-On Activity

Static electricity from a balloon really makes an empty soda can move! Credit: NASA.

You will need:

Balloons
Empty aluminum soda can
String (optional)

Procedure:

1. Blow up a balloon.
2. Rub the balloon on your hair. This makes an electrical charge.
3. Lay the soda can horizontally on a smooth floor.
4. Bring the balloon close to the can.
5. Look at what happens.
6. Have a race with a friend with two cans and two balloons. See who can move the can across the room first, without touching it.
7. Try rubbing the balloon on your hair and then sticking it to a wall.
8. Tie string to the ends of two balloons that you have blown up.
9. Rub the two balloons together.
10. Hold them by the strings right next to each other.
11. Watch what happens.
 

Review Questions

1.      What types of electric charges exist?

2.      How do electric charges affect other electric charges?

3.      How does an object become electrically charged?

4.      Explain the charge on an atom that has lost two electrons.

5.      Can you describe three methods of charging an object?

Activity/Assignments

a.      Click on this simulation: Interaction of Charges and wait for the applet to load completely.

b.      Next, click the button to read the "Objectives".

c.      Open and run simulation to learn how to describe the motion of an object using vector diagrams.

d.      Record and submit your answers for Questions 1, 2, and 3.

 

Electric Force - Measuring Quantitative Interactions Between Electric Charges

A positive electric charge exerts an attractive electric force on a negative charge, but a repulsive electric force on another positive charge.

Measuring Electric Charge

The quantity of charge (q) on an object is related to the (unbalanced) number of electrons that have been either gained or lost by the object.

The unit of charge is the coulomb (C).

The charge on one electron is the smallest charge known to exist independently and has the value of 1.60 x 10-19 coulomb. This value of the electron charge is known as the fundamental charge, e = 1.60 x 10-19 C. Therefore every electron has a charge of -e and every proton (or positive charge due to the loss of one electron) has a charge of +e.

A rubber balloon becomes negatively charged after you rub the balloon with a wool cloth. The quantity of charge due to the excess electrons on the balloon can be found according to the following general relationship:

Quantity of (positive or negative) charge, q = (number of electrons, n) x (fundamental (electron) charge, +e or - e)

So, q = (n)(e)

Measuring Electric Forces

The size of the attractive or repulsive force between two charged objects acts along a straight line drawn from one charged object to the other, depends on the amount of charge on each object (q1 and q2), and on the distance (d) between these objects. This relationship, known as Coulomb's Law, is given as:



 

where k is the proportionality constant and has a value of nearly 9.0 x 109 newton-meters2/coulomb2 or (9.0 x 109 N.m2/C2).

It does not matter whether this coulomb force is attractive (for unlike charges) or repulsive (for like charges). Each charged object feels the same amount of the mutual force in a direction towards or away from each other, respectively, according to Newton's Third law.

 

 

Polarization of Charge

Normally, in an isolated atom, the center of the negatively-charged electron cloud within an atom is centered on the positively-charged nucleus. 

However, when an external charged object is brought close to an object, the coulomb's force of attraction or repulsion between the charged objects distorts the coincidence of the center of the positive and the center of the negative charges.

The atom or molecule or uncharged object, though still electrically neutral, becomes electrically polarized when the charge "centers" are slightly separated, as shown below:

Click on the hyperlink to read more about the force between charges.

Review Questions

1.      A rubber balloon rubbed with wool cloth became negatively charged and the charge on the wool is measured as +1.0 x 10-8 C.
According to this charge,
(a) how many excess electrons has the balloon gained?
(b) How many electrons were "rubbed" out of the wool due to friction?
 

2.      Why would charged rubber balloon stick to a wooden wall? [Hint: refer to the textbook]

3.      One piece of packing peanut (Styrofoam) is given a negative charge of 3.0 x 10-10 C and suspended by a light string at the edge of a table. Another similar peanut with a negative charge of 2.0 x 10-10 C hangs 2.0 cm from the first peanut.
(a) What is the direction of the force between the peanuts?
(b) Calculate the magnitude of the force exerted by the peanuts on each other?

Electric Fields - Force fields around electric charges

Electric Field

An electric charge alters the condition of the space around the charge. The presence of the charge creates a force field within the space that surrounds the charge. This field around the electric charges is known as an electric field.

An electric charge exerts a force within the region of its electric field. When a charged object, with its surrounding electric field, is in the electric field of another charged object, the overlap (or superposition) of the two fields results in the coulomb force of attraction or repulsion between the charges, though the charged objects are separated by some distance.

Electric Field Lines

Though the electric field is invisible, it can be visualized by making a map of the field.

Mapping of the electric field is done by drawing electric field lines (or lines of force) that point in the direction of the force on a positive (test) charge. (By convention, the test charge is always a small positive charge).

These electric field lines are drawn closer together in regions where the electric field is stronger. The electric fields become stronger as you move towards a single charge and weaker as you move away from a single charge.

Electric field lines of a positive charge.
The given positive charge repels a positive test charge. The electric field lines (in the direction of the force on the test charge) point outward from the charge.

 Electric field lines of a negative charge.
The given negative charge attracts a positive test charge. The electric field lines point inward to the charge.

 

When there are two or more charges, the resulting electric field becomes a combination of the electric fields due to the individual charges. The electric field lines are always drawn away from positive charges and toward the negative charges.

 

Any (positive) test charge that is placed in the space around the given charges will experience electric forces in the directions of the arrows of the field lines as drawn. This test charge will be attracted (pulled) toward the negative charge but repelled (pushed) away from the positive charge.

Measuring Electric Fields

Electric field is a vector quantity.

The magnitude of the electric field (E) at any point in space is equal to the electric force (F) per unit charge (q).

The direction of the electric field is the direction of the force on a positive charge placed at the point.

 


 
 

Charge placed in an external electric field

If a charge were placed in an external electric field (not its own), the electric force on a positive charge points in the same direction as the electric field. The electric force on a negative charge in the an electric field points opposite direction to the direction of the electric field.

 

Force on a positive charge points in the same direction as the external electric field.

 

 

Force on a negative charge points in opposite  direction to the external electric field.

 

Electric field due to a point charge

When we apply Coulomb's law to find the electric force between a given charge q (as q1) and a test charge (as q2), separated apart by a distance d, the electric field due to charge q is the force per test charge (placed at the position of the test charge).

Therefore, the magnitude of the electric field (E) due to charge (q) at the position of the test charge (a distance d from the charge) is given as:



 

where the electrostatic proportionality constant, k = 9.0 x 109 N.m2/C2.

This is the value of the electric field at that position in space due to charge q, regardless of whether there is any other charge at the same position or not.

Review Questions

1.      In the study of the gravitational force, all masses are considered to be surrounded by a gravitational field. In what ways are the gravitational force and the electric force similar?

2.      How are the interactions between electric charges different from the interactions between two masses under gravity?

3.      Can you sketch the electric field lines due to two positive charges placed 2.0 cm apart?

4.      While combing your hair, 62.5 billion electrons (i.e. 6.25 x 1010 electrons) were transferred from your hair to the comb. By finding the charge on the comb due to this charge transfer, determine the magnitude and direction of the electric field of the comb at a distance of 20 cm from the comb.

Activity/Assignments

a.      Click on this simulation: Electric Field due to point charges and wait for the applet to load completely.

b.      Next, click the button to read the "Objectives".

c.      Open and run simulation to learn how to describe the motion of an object using vector diagrams.

d.      Record and submit your answers for Questions 1, 2, and 3.

 

Return to Physical Science page

Return to Georgia Geoscience Online

Content provided by Mr. Martin O. Okafor, Georgia Perimeter College

Page created by Pamela J.W. Gore
Georgia Perimeter College,
Clarkston, GA

Page created March 7-9, 2007
Modified May 18. 2007