Electrostatics ConcepTests

In The Beginning Amber: Greeks noticed that when rubbed, attracts small things (amber stuff, or electricity). Rubbed Items Attract New Force Force Agent  Electromagnetic Charge

Electromagnetism Electromagnetism Electrostatics Electrical Currents Electromagnetism (Currents & Magnetic Fields)

Types of Charge French Botanist C. Dufay found 2 different types of charge Franklin arbitrarily called charges + & - + & - Charges Can Add to Zero

Charge is Quantized Charge is Conserved Charge Is What It Does Charges Charge is Quantized qe= -1.6x10-19 Charge is Conserved Charge Is What It Does

Conductors Conductors: electric charges move freely due to an electric force (or field) Ex: Copper, aluminum and silver Charge a Conductor: the charge distributes itself over the entire surface of the material

Insulators Insulators: e charges do not move freely Ex: Glass and rubber Charge insulators by rubbing: only the rubbed area becomes charged

Charging by Conduction Charged rod contacts the sphere Some electrons on the rod can move to the sphere When the rod is removed, the sphere is left with a charge The object being charged is always left with a charge having the same sign as the object doing the charging

Charging by Induction When an object is connected to a conducting wire or pipe buried in the earth, it is said to be grounded A negatively charged rubber rod is brought near an uncharged sphere

Charging by Induction, 2 The charges in the sphere are redistributed Some of the electrons in the sphere are repelled from the electrons in the rod

Charging by Induction, 3 The region of the sphere nearest the negatively charged rod has an excess of positive charge because of the migration of electrons away from this location A grounded conducting wire is connected to the sphere Allows some of the electrons to move from the sphere to the ground

Charging by Induction, final The wire to ground is removed, the sphere is left with an excess of induced positive charge The positive charge on the sphere is evenly distributed due to the repulsion between the positive charges Charging by induction requires no contact with the object inducing the charge

A positively charged object is placed close to a conducting object attached to an insulating glass pedestal (a). After the opposite side of the conductor is grounded for a short time interval (b), the conductor becomes negatively charged (c). Based on this information, we can conclude that within the conductor both positive and negative charges move freely. only negative charges move freely. only positive charges move freely. We can't really conclude anything about the sign of the charges that move. ANSWER: D. The same result is achieved regardless of whether the charge carriers are positive or negative. So, it could be either that positive charges left or additional negative charges entered the conductor.

Three pithballs are suspended from thin threads Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (rubber against fur, glass against silk, a plastic comb against hair, etc.) and each of the pithballs may have been charged by touching it with one of these objects. It is found that pithballs 1 and 2 repel each other and that pithballs 2 and 3 repel each other. From this we can conclude that 1 and 3 carry charges of opposite sign. 1 and 3 carry charges of equal sign. all three carry the charges of the same sign. one of the objects carries no charge. we need to do more experiments to determine the sign of the charges. Charges of equal sign repel, so 1 and 2 carry charges of equal sign and so, too, do 2 and 3.

Three pithballs are suspended from thin threads Three pithballs are suspended from thin threads. Various objects are then rubbed against other objects (ribber against fur, glass against silk, etc.) and each of the pithballs may have been charged by touching it with one of these objects. It is found that pithballs 1 and 2 attract each other and that pithballs 2 and 3 repel each other. From this we can conclude that 1 and 3 carry charges of opposite sign. 1 and 3 carry charges of equal sign. all three carry the charges of the same sign. one of the objects carries no charge. we need to do more experiments to determine the sign of the charges. Answer: 5. Charges of opposite sign attract, charges of equal sign repel, and any type of charge attracts a neutral object. So 1 and 2 either carry charges of opposite sign or one of the two is neutral and the other charged. Since 2 and 3 repel, however, we know that 2 and 3 carry charges of equal sign. So there are two possibilities: 2 and 3 carry charges of equal sign and (i) 1 is neutral, or (ii) 1 carries a charge of the opposite sign to that of 2 and 3.

Two uniformly charged spheres are firmly fastened to and electrically insulated from frictionless pucks on an air table. The charge on sphere 2 is three times the charge on sphere 1. Which force diagram correctly shows the magnitude and direction of the electrostatic forces: Answer: 5. The magnitude of the electrostatic force exerted by 2 on 1 is equal to the magnitude of the electrostatic force exerted by 1 on 2. If the charges are of the same sign, the forces are repulsive; if the charges are of opposite sign, the forces are attractive.

Consider the four field patterns shown Consider the four field patterns shown. Assuming there are no charges in the regions shown, which of the patterns represent(s) a possible electrostatic field: (a) (b) (b) and (d) (a) and (c) (b) and (c) some other combination None of the above. Answer B. Pattern (a) can be eliminated because field lines cannot simultaneously emanate from and converge at a single point; (c) can be eliminated because there are no charges in the region, and so there are no sources of field lines; (d) can be eliminated because electrostatic field lines do not close on themselves.

Charles Coulomb 1736 – 1806 Studied electrostatics and magnetism Investigated strengths of materials Identified forces acting on beams

Vector Nature of Electric Forces Two point charges are separated by a distance r Like charges produce a repulsive force The force on q1 = - q2 This is a N3L deal

Electrical Forces are Field Forces This is the second example of a field force Gravity was the first For field forces, the force is exerted by one object on another object without physical contact similarities and differences between electrical and gravitational forces Similarities: Differences: ? ?

The Superposition Principle Fnet (resultant force) on any one charge equals the vector sum of the forces exerted by the other individual charges that are present. Huh?