40 terms

# Physics Chapter 16 Electric Forces and Fields

Chapter 16- electric forces and fields

#### Terms in this set (...)

When a rubber rod is rubbed with wool, the rod becomes negatively charged. What can you conclude about the magnitude of the wool's charge after the rubbing process? Why?
When the rod becomes negatively charged, you can conclude that the magnitude of the wool's charge is going to be equal to that of the plastic rod, because when the rod dups against the wool, some of the wool's electrons are transferred to the rod, giving the rod a certain amount of negative charge, and the wool loses the same amount of negative charge, thus making it positive and equal to the amount of negative charge the rod gained.
What did Millikan's oil drop experiment reveal about the nature of electric charge?
Millikan's experiment proved that when an object is charged, its charge is always a multiple of a fundamental unit of charge (e), which means that charge is quantized This means that charge occurs as integer multiples of e in nature
If you stick a piece of transparent tape on your desk and then quickly pull it off, you will find that the tape is attracted to the other areas of your desk that are not charged. Why does this happen?
The tape becomes negatively charged when it is pulled off the desk, and attracts other areas of the desk because it induced an opposite surface charge on the desk
Metals, such as copper and silver, can become charged by induction, while plastic materials cannot. Explain why.
Most plastics are electrical insulators so charge cannot move through them while metals such as copper and silver are electrical conductors so charges can move through them
Why is an electrostatic spray gun more efficient than an ordinary spray gun?
More paint hits the object being painted because of an electrical attraction between the charged droplets and the oppositely charged object
Explain why you're more likely to get a shock from static electricity by touching a metal object with your finger instead of with your entire hand.
the less surface area that comes into contact with the metal, the more concentrated the shock can be, therefore it will be greater
How are conductors different from insulators?
conductors transfer charges easily while insulators do not
When a conductor is charged by induction, is the induced surface charge on the conductor the same or opposite the charge of the object including the surface charge?
opposite
Would life be different if the electron were positively charged and the proton were negatively charged? Explain.
No, because the names would change but the properties would stay the same
explain from an atomic viewpoint why charge is usually transferred by electrons
electrons are less compact compared to the protons which are tightly bonded to the nucleus. Electrons are found in the outermost layer of the electron cloud and have a very small mass
Because of a higher moisture content, air is a better conductor of charge in the summer than in the winter. Would you expect the shocks from static electricity to be more severe in the summer or winter? Why?
in the winter because the dryer the air, the more severe the shock will be
A balloon clings to a wall after it is negatively charged by rubbing then clings to a wall. Does this mean the the wall is positively charged?
Not necessarily, the wall could be neutral, which is an application of the induction phenomenon, where the balloon induces a local surface charge on the wall but the wall as a whole is not charged
Which proves more conclusively that an object is charged, attraction or repulsion from an object? Explian
repulsion because attraction can be the result of an induced charge. Repulsion occurs when there are 2 net charges
What determines the direction of the electric force between two charges?
the sign of the charge
The gravitational force is always attractive, while the electric force is both attractive and repulsive. What accounts for this difference?
in newton's equation for the force of gravity, the masses can only be positive, so the force of gravity can only be positive. In coulombs' equation for the electric force, the charge could be positive or negative.
When more than one charged object is present in an area, how can the total electric force on one of the charged objects be predicted?
Each force exerted on an object is found and then you add them to find the net force
Identify examples of electric forces in everyday life.
static electricity, a lightbulb, rubbing a balloon against your hair, making it stand up.
According to Newton's third law, every action has an equal and opposite reaction. When a comb is charged and held near small pieces of paper, the comb exerts an electric force on the paper pieces and pulls them toward it. Why don't you observe the comb moving toward the paper pieces as well?
the comb is too heavy to be attracted to the small pieces of paper
What is an electric field?
a region where an electric force on a test charge can be detected
As you increase the potential on an irregularly shaped conductor, a bluish purple glow called a corona forms around a sharp end sooner than around a smoother end. Explain why.
on an irregular shaped conductor, charge tends to accumulate where the radius of the curvature of the surface is smallest at sharp points.
When electric field lines are being drawn, what determines the number of lines originating from a charge? What determines whether the lines originate from or terminate on a charge?
the number of lines originating from a charge is determined by the magnitude of the charge. the sign of the charge determines whether the lines originate from or terminate on a charge
Do electric field lines actually exist?
No, they are ficticious lines used to visualize and analyze the electric field
When defining the electric field,why must the magnitude of the test charge be very small?
so the test charge won't affect the original charge
Why can't two field lines from the same field cross one another?
the field lines must be perpendicular to equipotential lines
A "free" electron and "free" proton are placed in an identical electric field. Compare the electric force on each particle. How do their accelerations compare?
the force on both the "free" electrons and the force on the "free" proton will be equal and opposite. The acceleration of the electron will be greater because it has a smaller mass.
In which way is electric force similar and different to the gravitational force?
similarities: fundamental or field force (does not need an applied force), follow inverse squared law.
differences: gravitational force is only attractive while electric force can be attractive or repulsive, electric force is stronger than gravitational force.
A negatively charged object is brought close to the surface of a conductor, whose opposite side is then grounded.
What is this process of charging called?
B charging by induction
A negatively charged object is brought close to the surface of a conductor, whose opposite side is then grounded.
What kind of charge is left on the conductor's surface?
H positive
three identical charges (q = +5.0 mC) are along a circle with a radius of 2.0 m at angles of 30°, 150°, and 270°, as shown in the figure.What is the resultant electric field at the center?
o
If a suspended object is attracted to another object that is charged, can you conclude that the suspended object is charged? Briefly explain your answer.
No, you cannot definitively prove it because the object may be charged by induction, rather than conduct.
Suppose that you have an electrically charged stick. If you divide the stick in half, each half will have half the original charge. If you split each of these halves, each piece will have a quarter of the original charge. Can you keep on dividing the charge in this manner forever? If not, why not?
you can't keep dividing the charge in half because charge can only be divided until you get to the fundamental unit of charge (e)
A Ping-Pong ball contains an enormous number of electrically charged particles. Why don't two Ping-Pong balls normally exert electrostatic forces on each other?
a normal pingpong ball is electrically neutral. Thus the attractive and repulsive forces between the balls cancel
In industrial settings, neutral metal objects are often coated by spraying them with electrically charged paint or powder particles. How does placing charge on the particles help them to stick to an object's surface?
the charged paint particles electrically polarize the surface being coated, so that the paint particles are attracted to it and stick
The paint or powder particles discussed in Exercise 5 are all given the same electric charge. Why does this type of charging ensure that the coating will be highly uniform?
The charged particles repel one another and tend not to accumulate in clumps. The forces they exert on one another tend to spread out uniformly.
If two objects repel one another, you know they have like charges on them. But how would you determine whether they were both positive or both negative?
you'd have to compare the objects with a reference. The object that repelled the reference would have its charge
You have an electrically neutral toy that you divide into two pieces. You notice that at least one of those pieces has an electric charge. Do the two pieces attract or repel one another, or neither.
they attract because it is polarized and then and then the positive charge goes into one and the negative to the other
rules of drawing electric field lines
must begin at positive charge or infinity
must terminate at negative charge or infinity
no lines can cross
electric field is zero everywhere inside the conductor
on an irregular shaped conductor, charge tends to accumulate where the radius of the curvature of the surface is smallest at sharp points
field lines should be perpendicular to equipotential lines
why does a girls hair stand when she touches a Van de Graaff generator?
the charges are being transferred form the generator to the had by conduction, and there is an excess of positive charges going to her hair
By how much does the electric force between two charges change when the distance between them is doubled
J 1/4
describe Millikan's oil drop experiment
he observed the motion of tiny oil droplets between two parallel metal plates. The oil droplets were charged by friction in an atomizer and allowed to pass through a hole in the top plate. Initially the oil droplets fell because of their weight. Then, the top plate was given positive charges as the droplets fell, and the droplets with a negative charge were attracted to the top positive charge. By switching it on and off, he was able to watch a droplet for many hours as it rose and fell. After repeating this process, Millikan found that a charge is always a multiple of a fundamental unit of charge (e).