Understanding Charge at the Atomic Level
Focus Questions: What causes atoms to be charged? How is charge measured?
An understanding of electricity begins with an understanding of the atom. Matter is made of atoms and if any
sample of matter is charged, becomes charged or loses a charge, then the explanation of what happened
demands that one understand matter at the particle level.
ATOMIC CHARGE
Protons and electrons have a basic property known as charge. Protons have what
we call a "positive" (+) charge. Electrons have a "negative" (-) charge. Neutrons
have no charge - they are neutral. The charge of one proton is equal in strength to
the charge of one electron. When the number of protons in an atom equals the
number of electrons, the atom itself has no overall charge - it is neutral.
Label a proton, neutron and electron in the diagram.
ELECTRONS MOVE TO CREATE IONS
The protons and neutrons in the nucleus are held together very tightly. Normally the
nucleus does not change. But some of the outer electrons are held very loosely.
Therefore, they can move from one atom to another. An atom that loses electrons
has more positive charges (protons) than negative charges (electrons). It is positively
charged. An atom that gains electrons has more negative than positive particles. It
has a negative charge. A charged atom is called an ion.
Identify the following particles as being charged (ion) or uncharged (neutral atom).
If charged, indicate whether they are charged positively or negatively.
(n = neutron, p = proton, e = electron)
MEASURING CHARGE
The charge of an object is a measurable quantity. The charge possessed by an object is often expressed using
the scientific unit known as the Coulomb (C). The charge on a single electron is -1.6 x 10 -19 Coulomb. The
charge on a single proton is +1.6 x 10 -19 Coulomb. The quantity of charge on an object reflects the amount of
imbalance between electrons and protons on that object.
Translate that number into a whole number.
Electric “Force” Fields
Focus Questions: How do charged particles interact? What causes charged
particles to interact the way they do?
Electons and protons seem to be surrounded by a kind of invisible force field.
This funky area near any electrically charged object is called an electric field.
Electric field can be represented by lines. A charge is seen to interact with an electric field as opposed to with
another charge. A charged object sends its electric field into space, reaching from the "puller to the pullee."
Each charge or configuration of charges creates an intricate web of influence in the space surrounding it.
Scientifically stated, electric field is the region around a charged particle that can exert a force on another
charged particle. Anything that enters into this area around an object will feel a force (which is a push or pull
on an object). Electric force is a non-contact force (meaning objects do not touch). This is similar to
gravitational force and different from contact forces like frictional force. So an electric force is the push or pull
on an object resulting from its being in an electric field.
To visualize this better think of the Van de Graaff generator. Like any charged object, it alters the space
surrounding it (because it creates an electric field in the space surrounding it). Other charged objects entering
the space feel the strangeness of that space. Electric forces are exerted upon those charged objects when they
enter that space.
Differentiate between electric field and electric force.
Experiment with this using this Electric Force simulation:
http://www.colorado.edu/physics/2000/waves_particles/wavpart2.html
What do you notice about the speed when you change the distance between the proton and the electron?
Can you get the electron to “orbit” the proton? Click the Show Force button to see the electric force.
OBSERVING CHARGE
The only way charge can be “seen” is to observe the effects of the charge. This is done through observing how
the particle interacts with other particles.
Check out this Electric Field of Dreams simulation and use your observations to answer the questions that
follow - http://phet.colorado.edu/simulations/sims.php?sim=Electric_Field_of_Dreams
1. How does a positively charged particle affect a negatively charged particle?
2. How does a positively charged particle affect another positively charged particle?
3. How does a negatively charged particle affect another negatively charged particle?
4. Create a statement that summarizes your answers above: