Chapter 16 Electric Forces and Fields

16.1 Electric Charge Properties of Electric Charge Running a plastic comb through your hair sometimes causes the comb to attract the hair strands. If you rub a balloon against your hair it may then stick to the wall. This process works best on dry days (less moisture in the air) Sometimes rubbing your feet across a rug can allow a spark to exit your finger. Electric Charge

There are two kinds of electric charge If you charge two balloons by rubbing them against your hair, they will each have the same charge. Like with magnets, like charges repel, unlike charges attract. With static electricity, we have positive (+) and negative (-) charges. If the object has an equal amount of both changes, there is no net charge. If you charge an object (+) that you are painting and spray (-) charged paint at it, you will waste less paint.

Electric Charge is conserved Protons and neutrons are relatively fixed in the nucleus, but electrons can be easily transferred. If an atom gains an electron it becomes slightly negatively charged. This is called an ion. When a child goes down a slide, it loses electrons. This causes their body to become slightly positive. The positively charged hair repels the other strands.

Electric Charge is quantized Charge is always quantized, or found in factors of “e” which is the charge of one electron. A proton has a charge of +1.602 x 10-19 C. An electron has a charge of -1.602 x 10-19 C. So one (-) coulomb (c) contains 6.2 x 1018 electrons. Millikan’s experiments

Transfer of Electric Charge Materials like copper, aluminum and silver distributes charge easily over their entire surface. They are said to be conductors. Glass, rubber and plastic prevent charges from moving so they are considered insulators.

Semiconductors are a third class of materials. They are somewhere between insulators and conductors. Depending on addition of certain atoms, will increases their ability to conduct. Silicon is a well known substance used in a variety of electronic devices. The fourth class is superconductors. Cooled to certain temperatures, these can conduct electricity indefinitely without heating.

Insulators and conductors can be charged by contact Rubbing a balloon against hair allows it to become charged. This is known as charging by contact. If you try to do the same thing with a copper rod, you will not get the same results. Because you and the copper are both conductors, the charge flows through you and becomes balanced with the earth below you. The earth “grounds” you because it can accept extra electrons.

Conductors can be charged by induction Conductors in contact with earth are said to be grounded. Because of earth’s size, it is an infinite reservoir for electrons. The diagram below shows how a neutral sphere can be charged by a negatively charged object. This process is called induction and the charge is said to be induced on the sphere. Notice that the object never touches the sphere and never lost its negative charge. induction

A surface charge can be induced on insulators by polarization Insulators can also be charged with induction. The presence of the charged object causes surface molecules to realign themselves, causing polarization. This allows objects like plastic and paper to become attracted or repelled.

Questions 1. Like with magnets, like charges repel, unlike charges ______. 2. If an atom gains an electron it becomes slightly negatively charged, this is called an ___. 3. Glass, rubber and plastic prevent charges from moving so they are considered _________. 4. Cooled to certain temperatures, _______________ can conduct electricity indefinitely without heating. 5. The presence of a charged object near an insulator causes surface molecules to realign themselves, causing __________. attract ion insulators superconductors polarization

16.2 Electric Force Coulomb’s Law Charged objects near each other experience an acceleration. This is due to the fact that there is a force between them. This is called electric force. “Like” charges repel, “unlike” attract.

The closer two charges are, the greater the force on them The distance two charges are apart affects the electric force between them. If you double the charge, the force doubles. If you move them twice as far apart, the force becomes 1/4th.

Practice A Coulomb’s Law The electron and proton of a hydrogen atom are separated, on average, by a distance of about 5.3 x 10-11 m. Find the magnitudes of the electric force and the gravitational force that each particle exerts on the other.

Given: r =5.3 ×10-11 m qe = -1.60 x10-19 C qp = +1.60 x10-19 C kC =8.99 ×109 Nm2 /C 2 G = 6.673 x 10-11 Nm2 /kg2 Unknown: Felectric = ? and Fg = ? answer *Note; the proton is about 1800x more massive than the electron. 8.2 x 10-8 N 3.6 x 10-47 N

Resultant force on a charge is the vector sum of the individual forces on that charge Coulomb’s law applies when more than two charges are present. The resulting force on any single charge is equal to the others involved. This is known as the principle of superposition. Superposition

Practice B The Superposition Principle Three point charges q1 q2 and q3, lie along the x-axis at x=0, x=0.04 m, and x=0.06 m. Calculate the magnitude and direction of the electric force on each of the three point charges when q1 = +5.0x10-6 C, q2 = +2.0x10-6 C and q3 = -3.0x10-6 C.

Given: r = 0.04 m q1 = +5.0x10-6 C q2 = +2.0x10-6 C kC = 8.99 ×109 Nm2 /C 2 Answer   56 N 

Given: r = 0.02 m q3 = -3.0x10-6 C q2 = +2.0x10-6 C kC = 8.99 ×109 Nm2 /C 2 Answer   135 N 

Given: r = 0.06 m q3 = -3.0x10-6 C q1 = +5.0x10-6 C kC = 8.99 ×109 Nm2 /C 2 Answer   37 N 

Final solution

If an object is in equilibrium, all net external forces acting on it must equal zero. To find the equilibrium position of an electrical charge, you must find where the charges balance or are equal and opposite the other charges. http://www.classzone.com/books/earth_science/terc/content/investigations/es0501/es0501page04.cfm?chapter_no=investigation

Practice C Equilibrium Three charges lie along the x - axis. One positive charge, q1 = 15 uC, is at x = 2.0 m, and another positive charge, q2 = 6.0 uC, is at the origin. At what point on the x–axis must a negative charge, q3, be placed so that the resultant force on it is zero?



Answer d = 0.77 m

Electric force is a field force A field force is one in which there is no physical contact. Electric force, like gravity, are field forces. They are both proportional to the square of the distance of separation. Important differences are that electrical force can attract or can repel. Electrical force is many times stronger than gravity.

Coulomb quantified electric force with a torsion balance Charles Coulomb was the first to measure the force between charged objects. He used the device shown here to measure the attractive force between two charges. Now we use his constant… KC = 8.99 x 109 Nm2 / C2

Questions 1. Charged objects near each other experience an acceleration, this is due to the fact that there is a _____ between them. 2. If you move two charges ____ as far apart, the force becomes 1/4th. 3. If an object is in equilibrium, all net external forces acting on it must equal ____. 4. A field force is one in which there is no _______ contact. 5. Electrical force is many times ________ than gravity. force twice zero physical stronger

16.3 The Electric Field Electric Field Strength A charged object sets up an electric field around it. As another charge is brought closer, they begin to interact with one another. Electric Field Strength

The strength of the electric field is “E”. Electric field strength is a ratio of force to charge, so “E” is N/C. Note direction of the force arrow. The electric field is also a vector quantity. “E” can be defined as the direction of the electric force exerted in a small positive charge. This is call a “test charge”.

Remember, the test charge is always positive. To find the electric field, you need to find the electric force on this charge, then divide by the force of the test charge. A strong test charge will cause a rearrangement of the charges on the sphere. This is why we use a small test charge.

Electric Field Strength depends on charge and distance Remember field strength depends on charge and distance. By substituting this equation with… We get electric field strength due to a point charge. If more than one charge is present, we apply the principle of superposition.

To find the force at this point, we need to remember if q is positive, the forces repel. If q is negative, the field is directed toward q. As the equation shows, an electric field at a given point depends only on the charge (q), and the distance(r2) to a specific point in space.

Practice D Electric Field Strength A charge q = +7.00x10-6 C is at the origin. Find the electric field strength at a point “P”, which is on the x - axis 0.400 m away. kc = 8.99 x 109 N*m2/C2 Answer 3.93 x 105 N/C

Electric Field Lines A way of visualizing electric field patterns is to draw lines pointing in the direction of the electric field. These are called electric field lines. They do not really exist but they offer a useful means of analyzing fields with strength and direction.

The number of field lines is proportional to the electric field strength The electric field vector “E” is tangent to the lines at each point. The number of lines is proportional to the strength. “E” is stronger where the field lines are closer together. Electric Field Lines

This is an example of two point charges of equal strength but opposite charge. Here we have two equal negative charges.

The remaining half terminate at infinity. In this example, the number of lines leaving +2q is twice the number terminating on charge -q. The result is only half the lines that leave the positive charge end at the negative charge. The remaining half terminate at infinity. Lines drawn proportional to the field strength

Conductors in Electrostatic Equilibrium A good electric conductor, like copper, contains charges (electrons) that are only weakly bound to the atom. When no net motion of charge is occurring within a conductor, it is said to be in electrostatic equilibrium. When there is excess charge, it moves to the outer surface. This is due to the repulsive nature pushing charges away from one another.

Excess charges placed on an object moves to its surface. At the sharp end of an object, repulsion between charges are directed perpendicular to the surface. For this reason, the sharp end produces a larger electric field directed away from the surface.

Questions 1. Electric field strength “E” is a ratio of force to charge, or newtons per ________. 2. T / F A test charge is always negative. 3. Field strength depends on ______ and distance. 4. The number of field lines is proportional to the strength “E” and are also stronger where the field lines are _____ together. 5. At the “sharp” end of an object, repulsion between charges are directed ____________ to the surface and produce a large electric field. coulomb false charge closer perpendicular

End