check the "Summary" and "Key Terms"
KEY TERMS: electric charge, electrostatics, electron, proton, neutron, positive and negative ions, principle of conservation of electric charge, conductor and insulator, induced charge, point charge, Coulomb's law, coulomb, principle of superposition of forces, electric field, test charge, vector field, source point, field point.
Before we can discuss DC circuits we need to understand the concept of
ELECTRIC CHARGE (Chapter 21)
For a clear picture of electric charge go to an informative website with interactive pictures that will quickly show you pictures that will help give you a better idea of what an electric charge is:
then click on "Electricity
and Magnetism" button,
You can then click anyplace on the interactive page to get an explanations of the many new ideas in this chapter. You will find this very helpful to have a picture of some of these abstract and complicated concepts.
The electrons found "orbiting" the atoms in the universe have various physical properties. Last semester we studied the property of mass and the (gravitational) forces between all masses. This semester we will study the electron's property called "electric charge" and the (electrical) forces between all electric charges. Unfortunately we cannot see electric charge the way we can see masses. There are two kinds of charge, negative and positive. The electric charge on an electron is very small and it is NEGATIVE: -1.6(10)-19 Coulombs. The protons found at the center of atoms have the same amount of charge as the electron except it is a POSITIVE electric charge: +1.6(10)-19 Coulombs. We will find later that two charges of opposite sign attract each other and two charges of similar sign repel each other.
If an object has the same amount of positive and negative charge the NET CHARGE on the object is the sum of the two kinds of charge and adds to zero, and we say the object is electrically neutral. So if an object has a number of protons equal to the number of electrons its net charge is zero. (All atoms have a net charge of zero, but ions can have a positive or negative charge depending on whether they have more protons or electrons.) Since the electrons are more mobile than the protons (which are buried in the nucleus of the atom) we transfer electrons to a neutral object to make it have a net negative charge. When we do that the neutral object from which we transferred the electrons becomes positively charged. Note that the total electric charge of the two bodies together does not change - this is known as the conservation of charge. We cannot create or destroy electric charge, we simply transfer charges (electrons) from one object to another to change the NET charge of objects.
The ancient Greeks discovered that electric charge could be transferred between two objects by rubbing them together. In the diagram below, rubbing plastic and fur together results in electrons from the fur being rubbed off onto the plastic, leaving the fur positively charged and the plastic negatively charged. When glass and silk are rubbed together which one has the electrons rubbed off?
21.1 Plastic rubbed with fur becomes negatively charged,
The diagram below shows a lithium atom with its 3 electrons and 3 protons (and three uncharged neutrons). Also shown is a positive lithium ion, positively charged because it is missing one of its electrons, and a negative lithium ion, negatively charged because it has an extra electron.
The diagram below shows a process called "charging by induction" in which a neutral metal sphere is supported on an insulating stand. Materials which are called electrical "INSULATORS" have electrons strongly attached to the nucleus of the atoms in the material so that no flow of electrons (or current) can take place in the material.
Materials which are called electrical "CONDUCTORS" have electrons which can leave the atoms and migrate freely through the material. Most metals are good electrical conductors because they only have a few electrons in the outer shell of the atom which can be easily detached and moved through the material,becoming an electric current.
When a negatively charged rod is placed close to the neutral metal sphere the (negatively) charged electrons in the sphere are repelled to the far side of the sphere, leaving the atoms on the near side positively charged owing to their missing electrons. If we then connect a copper wire to the negative side of the sphere and an electrical ground some of the free electrons will flow into the ground. When we then remove the copper wire and the negatively charged rod what remains is a metal sphere with a uniform distributed positively charge.
In the first laboratory session this semester you will measure the magnitude and direction of the electric field, a vector quantity. The electric field (E, a vector) is an abstract quantity which we can not touch, feel, or see; we can only experience the effects of the field. Although this sounds complicated, it's very similar to the gravitational field most of us have been living in for the past few decades, and even though no one has ever seen the gravitational field, no one will ever deny the existence of the field (sometimes we simply call it gravity).
semester you measured the value of the gravitational field near the surface
of the Earth. We called it g= 9.8 m/sec2. It has the same units of an
acceleration, so sometimes we called it the acceleration due to gravity.
If the magnitude of the gravitational field vector is 9.8 m/sec2, what
is the direction of the field vector? You know how to determine the direction
- just place a mass in the field and see which direction it is forced
to move. The direction is always "down" or toward the center
of the Earth. Newton's second law gives us the vector equation
a similar way, an electric field will exert a force on an electric charge
(in blue) for various voltages (0, 30, 50, and 70 Volts) and electric
field lines (in red) which are always perpendicular to the equipotential
lines and lie along the
Basic elements of
a cathode-ray tube used in a TV set or computer monitor. The electron
beam (in green) is deflected horizontally and then vertically so that
it hits a specific spot on the phosphor coating on the inside of the screen
where a bright spot will appear (either
Vertical deflection of an electron beam in a cathode-ray tube, F = QE
© 2009 J. F. Becker