Electric Charge


 

ALWAYS check the "Summary" and "Key Terms"
at the end of the chapters. 

 

 

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:

http://230nsc1.phy-astr.gsu.edu/hbase/hframe.html

then click on "Electricity and Magnetism" button,
then "electric charge" button and scroll down to a description of conventional electric current that we will discuss later.

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?

 

 

 

 

 

 

 

 

 

 

 

 

 

Fig. 21.1 Plastic rubbed with fur becomes negatively charged,
glass rubbed with silk becomes positively 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.

Fig. 21.4 The element Lithium and its ions.

 


 

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.

 

Fig. 21.7 Charging a metal sphere by the process of induction.

 


 

Although the charges (on the electrons) are tightly bound to the atoms in an insulator they are free to move slightly within the atom. This is called polarization. If a plastic comb is rubbed on fur (or your dry hair) electrons will be rubbed off the hair onto the plastic comb. The plastic comb becomes charged negatively. If the comb is brought close to a neutral insulator, like a piece of dry paper, it will repel the negatively charged electrons in the atoms causing them to moving away slightly, leaving the protons without an electron closer to the comb. Since opposite charges attract and the positive charges are closer to the comb than the negative charges, the piece of paper is attracted to the comb. This effect is used to remove soot and ashes from smoke going up industrial chimneys. The inventor became rich as a result of his patent!

Part (b) of the diagram shows the same result (attraction of the paper) even if we had a positively charged comb.

Fig. 21.8 A charged comb
attracts a piece of paper.

 


 

NOTE:

Coulomb's Law enables us to calculate the magnitude of the force vector between charges. This is covered in Sections 21.3 and 21.4 in the text. We will skip these sections for now and return to them after we cover DC circuits.

In the first laboratory experiment (Lab #1) you will measure and plot the electric field lines caused by various electric charge configurations. So let's cover some of that material now from section 21.6 and from sections 23.4 and 23.5 of the text.

 


 

 

ELECTRIC FIELD
(PREVIEW - See Sections 21.4, 23.4 and 23.5 of text)

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).

Last 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
F = mg.

In a similar way, an electric field will exert a force on an electric charge according to
F
= qE,
where q is the electric charge. If we place an electric charge in an electric field, the direction of the force on the charge can indicate the direction of the field. Just as the gravitational field lines start at a mass, the Earth in this case, the electric field lines start at the electric charges.

It gets more complicated...

We will find that the electric field points in a direction away from a positive electric charge and toward a negative charge. As we know there are two kinds of electric charge: positive and negative. Protons have the positive charge the electrons have the negative charge. If a positive charge is placed in an electric field the force on it is in the same direction as the electric field vector (F = +QE), and if a negative charge is placed in an electric field the force on it is in the opposite direction as the electric field (F = QE). This is consistent with the fact that like charges repel and opposites attract each other. Although all of this is true it is impossible to see electric charges. So you will have to resort to more complicated methods to measure the electric field vectors in the lab. Consider the following:

 

 

 

 

 

 

 

 

 

 

 

Using our gravitational field analogy, the contour lines on a topographic map are lines of constant elevation above sea level and hence of constant gravitational potential energy. If we let a ball roll down a mountain we would see that the ball would roll down a path perpendicular to the contour lines, i.e., down the steepest path or gradient. Sop if we could first measure the contour lines we could then predict the path of a ball rolling down the mountain. In the lab we do something analogous. We (you) will measure the lines of equipotential (potential energy per unit charge - a whole chapter on this later in Chapter 23 Electric Potential) and from a plot of the equipotential lines you can determine the direction and magnitude of the electric field. The field lies along the path of steepest (negative) slope or gradient.

 
Fig. 23.22 Contour lines on a topographic map.

 


 

 

Equipotential lines (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
steepest (negative) gradient. (a) a single positive charge, (b) an electric dipole, i.e., two equal charges of opposite sign, (c) two equal positive charges.

Fig. 23.24 Equipotential lines (blue) and electric field lines (red) for various charges.

 


 

Equipotential lines (blue) for various voltages and electric field lines (red) which are always perpendicular to the equipotential lines and lie along the steep-est (negative) gradient. The electric field lines are always perpendicular to the surface of a conductor. Why? Draw the equipotential line at the surface of the conductor.

Fig. 23.25 Equipotential lines and electric field lines for a charge near a conductor.

 


 

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
red, green, or blue in a color CRT tube).

 

See Fig. 23.34 for the basic elements of a cathode ray (or TV) tube.

 

 


 

 

Vertical deflection of an electron beam in a cathode-ray tube, F = QE

Similar to Fig. 21.21 Electrostatic deflection of an electron beam
in a TV tube.

 

© 2009 J. F. Becker