Physic Ch 15 & 16 conceptual problems

Two charges are arranged as shown on the right. If Q1 is positive, and if the net electric field at point A points up, then

Q2 is negative and |Q1| < |Q2|

Why is it important to avoid sharp edges or points on conductors used in high-voltage equipment?

Sharp corners are where more charge would accumulate, creating larger electric fields. With high voltage equipment, these charges would have greater energy thus making it easier for them to jump off of the corner(s) onto a passing person.

A parallel plate capacitor is charged with a battery. The battery is then disconnected and the capacitor plates are pulled apart. Which of the following is TRUE ?

voltage V increases

5. A parallel-plate capacitor with capacitance C stores charge of the magnitude Q on plates of area A separated by distance d. The potential difference across the plates is V. If the capacitor is attached to a battery and the charge is doubled to 2Q, what are the ratios (a) C new/ C o and (b) V new /V o? A second capacitor is identical to the first capacitor except the plate area is doubled to 2A. If given a charge of Q, what are the ratios (c) C new/ C o and (d) V new/ V o? A third capacitor is identical to the first capacitor, except the distance between the plates is to the first capacitor, except the distance between the plates is doubled to 2d. If the third capacitor is then given a charge of Q o, what are the ratios (e) C new/ C o and (f) V new/ V o?

(a) 1 (b) 2 (c) 2 (d) 0.500 (e) 0.500 (f) 2

Four concentric spheres S1, S2, S3, S4 are arranged as in Figure and each charge in the figure has the same magnitude. What is the ratio of the electric flux through spheres S2, S3, and S4 to the flux through sphere S1:(a) 2/1=(b) 3/1=(c) 4/1=

(a) 1(b) 2(c) -1

1. A proton is released from rest in a uniform electric field. Determine whether the following quantities increase, decrease, or remain unchanged as the proton moves. Indicate your answers with I, D, or U, respectively. (a) the electric potential at the proton's location (b) The proton's associated electric potential energy (c) Its kinetic energy (d) Its total energy

(a) Decreases the proton will move in the same direction as the electric field. electric fields go from high to low potentials. (b) D PE = -qE (change in x)if we increase location then we are increasing it negatively making it have a smaller potential energy. protons want to get rid of potential energy and turn it into kinetic energy. (c) I kinetic energy will increase since potential energy is decreasing. (d) U total energy will stay the same since it is conserved.

7. Choose the words that make each statement correct. (i) After being released from rest in a uniform electric field, a proton will move [(a) in the same direction as; (b) opposite the direction of] the electric field to regions of [(c) higher; (d) lower] electric potential. (ii) After being released from rest in a uniform electric field, an electron will move [(e) in the same direction as; (f) opposite the direction of] the electric field to regions of [(g) higher; (h) lower] electric potential.

(i) a, d(ii) f, g

If you are given three different capacitors C1, C2, and C3, how many different combinations of capacitance can you produce, using all capacitors in your circuits?

All three in series.All three in parallel.C1 and C2 in parallel, with C3 in series.C1 and C3 in parallel, with C2 in series.C2 and C3 in parallel, with C1 in series.C1 and C2 in series, with C3 in parallel.C1 and C3 in series, with C2 in parallel.C2 and C3 in series, with C1 in parallel.Other combinations can be made if only two capacitors are used

Three capacitors have capacitances C1 < C2 < C3. If these capacitors are connected in series, which of the following is true for the resulting equivalent series capacitance?

Cs < C1

13. Rank the electric potentials at the four points shown in figure from largest to smallest

D>C>B>A

A spherical surface surrounds a point charge q. Describe what happens to the total flux through the surface if (a) the charge is tripled, (b) the volume of the sphere is doubled, (c) the surface is changed to a cube, (d) the charge is moved to another location inside the surface, and (e) the charge is moved outside the surface.

EA = sigma = Q inside/free space a. if you increase q by 3, then E will increase by 3 b. 4 pi R ^2therefore if we decrease E c. flux does not change when the surface changes d. If charge is moved to another location inside the surface, then the Q will stay the same, so E stays the same e. Q represents the charge on the inside and if you move it then the Q will decrease

In fair weather, there is an electric field at the surface of the Earth, pointing down into the ground. What is the sign of the electric charge on the ground in this situation?

If the field is pointing towards the ground, the ground must have a negative charge. (Electric fields point away from positive charges, toward negative charges.)we are talking about the sky to the ground

9. Explain why, under static conditions, all points in a conductor must be at the same electric potential.

If two points on a conducting object were at different potentials, then free charges in the object would move, and we would not have static conditions in contradiction to the initial assumption. (Free positive charges would migrate from locations of higher to locations of lower potential. Free electrons would rapidly move from locations of lower to locations of higher potential.) All of the charges would continue to move until the potential became equal everywhere in the conductor a) There is no electric field within the conductor, so it takes no energy to move charges around inside it. Thus the potential energy won't change. So the electric potential difference is zero. b) If you move a charge along the surface of the conductor where there might be an electric field, the field must be perpendicular to the surface. Thus it still takes no energy to move the charge and hence no change in electric potential.

If the electrostatic potential at a point in space is zero, then the electric field at that point must be

Impossible to determine based on the information given

15. Is it always possible to reduce a combination of capacitors to one equivalent capacitor with the rules developed in this topic? Explain.

Not all connections are simple combinations of series and parallel circuits. As an example of such a complex circuit, consider the network of five capacitors, C1, C2, C3, C4, and C5 shown in the figure in the next columnThis combination cannot be reduced to a simple equivalent by the techniques of combining series and parallel capacitors

The plates of a capacitor are connected to a battery. (a) What happens to the charge on the plates if the connecting wires are removed from the battery? (b) What happens to the charge if the wires are removed from the battery and connected to each other?

Nothing happens to the charge if the wires are disconnected. If the wires are connected to each other, charges in the single conductor which now exists move between the wires and the plates until the entire conductor is at a single potential and the capacitor is discharged

If a suspended object A is attracted to a charged object B, can we conclude that A is charged? Explain.

Object A could either be uncharged or oppositely charged to object B. If it is uncharged, there will be a shift in the electron cloud to be attracted to object B.One might think that both objects need to be charged, and oppositely charged at that, but that's not quite the case. Bringing a charged object close to a neutral object can attract charges of opposite sign in the neutral object, and at the same time repel charges of like sign. Since the charges of opposite sign in the neutral object are now closer to the charged object than the charges of like sign, there is a net attractive force! Note that it doesn't matter what sign of charge is on the charged object. The force between a charged object and a neutral object, when it exists, is always attractive.The slight separation of charges in the neutral object caused by the charged object is called an "induced charge."

Positive charge Q is located at the center of a hollow, conducting spherical shell. (a) Is the induced charge Qinner on the inner surface of the shell positive or negative? Answer P for positive, or N for negative. (b) Is the induced charge Qouter on the outer surface of the shell positive or negative? Answer P or N. (c) Determine the ratio Qinner/Q and (d) ratio Qouter/Q.

On a conductive sphere electrons are on the outside. The net charge will be what is on the inside surface and the outside surface added together (-Qoutside + Qinside = net charge)(a) Negative;because there is a positive charge on the inside, there will be an induced negative charged on the outside to cancel out and make zero. The inner surface of the outside shell is negative(b) Positive;because of the inner surface of the outer shell is negative, the outer surface will oppose that and be positive.(c) -1(-1/+1)this is the inner surface of the outershell over the center.(d) 1(+1/+1)this is the outer surface over the center

Rank the potential energies of the four systems of particles shown in figure from largest to smallest. Include equalities if appropriate.

PE = k q1q1/r a is the greatest because Q is greater than r and the signs are the same b and d would be the same because we have a q squared on both and they cancel out. c is the smallest.

A student stands on a thick piece of insulating material, places her hand on top of a Van de Graaf generator, and then turns on the generator. Does she receive a shock?

She is not shocked. She becomes part of the dome of the Van de Graaff, and charges flow onto her body. They do not jump to her body via a spark, however, so she is not shocked.

A charged comb often attracts small bits of dry paper that then fly away when they touch the comb. Explain why that occurs.

The dry paper is initially neutral. The comb attracts the paper because its electric field causes the molecules of the paper to become polarized - the paper as a whole cannot be polarized because it is an insulator. Each molecule is polarized so that its unlike-charged side is closer to the charged comb than its like-charged side, so the molecule experiences a net attractive force toward the comb. Once the paper comes in contact with the comb, like charge can be transferred, the like-charged paper is then repelled by the like-charged comb.The charge in the comb induces dipoles in the paper. The dipoles are attracted to the charge on the comb. When the paper touches the comb, some charge is transferred from the comb making the paper now charged with charges of the same sign. Thus they now repel and fly away from the comb.

There are great similarities between electric and gravitational fields. A room can be electrically shielded so that there are no electric fields in the room by surrounding it with a conductor. Can a room be gravitationally shielded? Explain.

The electric shielding effect of conductors depends on the fact that there are two kinds of charge; positive and negative. As a result, charges can move within the conductor so that the combination of positive and negative charges establishes an electric field that exactly cancels the external field within the conductor and any cavities inside the conductor. There is only one type of gravitation charge, however, because there is no negative mass. As a result, gravitational shielding is not possible. A room cannot be gravitationally shielded because mass is always positive or zero, never negative.

Explain why a dielectric increases the maximum operating voltage of a capacitor even though the physical size of the capacitor doesn't change.

The material of the dielectric may be able to withstand a larger electric field than air can withstand before breaking down to pass a spark between the capacitor plates.

If you were asked to design a capacitor in which small size and large capacitance were required, what would be the two most important factors in your design?

The plates should be close together, have a large surface area, and have a dielectric between them. (C = A/d --> if the area is bigger and distance is smaller than the C is bigger)

Suppose a region of space has a uniform electric field, directed toward the right, as shown in the figure. Which statement is true?

The potential (voltage) at points A and B are equal, and the potential at point C is lower than the potential at point A

When charging two objects by rubbing them together...

They must be made of different materials.

Why should a ground wire be connected to the metal support rod for a television antenna?

This gives any electrical charge that gets to the antenna a path to follow other than into your TV. For example, a lightning bolt.

Wires A and B are both made of copper. Wire A has twice the length and twice the radius of wire B. If you measure the electrical resistance of the two wires you will find:

Wire A has half the resistance of B

If more electric field lines leave a Gaussian surface than enter it, what can you conclude about about the net charge enclosed by that surface?

You can only conclude that the net charge inside the Gaussian surface is positive.Electric fields point towards negative charge and move away from positive charge.

An electron is released from rest in a uniform electric field. Determine whether the following quantities increase, decrease or remain unchanged as the electron moves. Indicate your answers with I, D, or U. (a) The electric potential at the electron's location (b) The electron's associated electric potential energy (c) It's kinetic energy (d) Its total energy

a) the electric potential will increase since an electron goes the opposite way as the proton. b) It's electric potential energy will decrease since it is going in the opposite direction as the electric field lines. c) It's kinetic energy will decrease since potential energy is increasing d) its total energy is staying the same.

The fundamental charge is e=1.6 x 10^-19 C. Identify whether each of the following statements is true or false. (a) It's possible to transfer electric charge to an object so that its net electric charge is 7.5 times the fundamental electric charge, e. (b) All protons have a charge of +e. (c) Electrons in a conductor have a charge of -e while electrons in an insulator have no charge.

a. false; yes it is possible to transfer electric charge but you can't transfer a fraction of a charge. you cannot divide an electron. b. protons and electrons have the same charge except the opposite. So yes. true c. false. in a conductor, you have a lot of free electrons. In an insulator, electrons are bound.

Two uncharged, conducting spheres are separated by a distance d. When charge -Q is moved from sphere A to sphere B, the Coulomb force between them has magnitude Fo. (a) Is the Coulomb force attractive or repulsive? (b) If an additional charge -Q is moved from A to B, what is the ratio of the new Coulomb force to the original Coulomb force, Fnew/Fo? (c) If sphere B is neutralized so it has no net charge, what is the ratio of the new to the original Coulomb force, Fnew/Fo?

a. they both start out neutral and so when you take a negative charge from A and put on B, A will be + and B will be -. This will cause them to have an attractive force between sphere A and B.b. so if we take another charge it will make +2Q and -2q. The first one is is just 1q^2 and the second one is 4Q^2 so it is going 4:1.(F = kQq/r^2)c. If they take out a charge then it will be 0:1 aka it is just zero.F = kQq/d^2

What happens when a charged insulator is placed near an uncharged metallic object? (a) They repel each other. (b) They attracted each other. (c) They may attract or repel each other, depending on whether the charge on the insulator is positive or negative. (d) They exert no electrostatic force on each other. (e) The charged insulator always spontaneously discharges.

b.

A parallel plate capacitor containing a paper dielectric is charged with a battery. Which of the following is TRUE if the dielectric is removed while the battery remains connected?

charge Q decreases

A positive test charge q is released near a positive fixed charge Q. As q moves away from Q, it will move with

decreasing acceleration

a glass object receives a positive charge of +3 nC by rubbing it with a silk cloth. in the rubbing process, have protons been added to the object or have electrons been removed?

electrons are removed

Consider point A in Figure located an arbitrary distance from two point charges in otherwise empty space. (a) Is it possible for an electric field to exist at point A in empty space? (b) Does charge exist at this point? (c) Does a force exist at this point?

field lines go whichever way the charges will make them want to go. because they are two positively charged points they will go in opposite directions.a) yes it is possible for point A to exist in that empty space. The field is everywhere.b) in this diagram, no. a charge does not exist at this point.c) we have an electric field there, but NO charge.E = F/qthis is because we have an electric field with no charge. Aka F = qE. At point A there is no charge, therefore q=0 and so F=0. Electric field times zero charge equals zero force.No charge exists at point A. All of the charge is at q1 or q2.No force exists there since there is nothing on which to exert a force.But yes, an electric field does exist there. The electric field embodies the capability to exert a force. And certainly if a charge was placed as A, the other two would exert a force on it. So the electric field does exist there

Each of the following statements is related to conductors in electrostatic equilibrium. Choose the words that make each statement correct. (i) The net charge is always zero [(a) inside; (b) on] the surface of an isolated conductor. (ii) The electric field is always zero [(c) inside; (d) just outside] a perfect conductor. (iii) The charge density on the surface of an isolated, charged conductor is highest where the surface is [(e) sharpest; (f) smoothest].

i) the charge inside the isolated conductor is zero ii)The electric field inside the spherical charged conductor is zero iii) The charge density on the surface of an isolated charged conductor is highest where the surface is sharpest.

An air-filled parallel-plate capacitor with capacitance C stores charge Q on plates separated by distance d. The potential difference across the plates is V and the energy stored is PE. If the capacitor is disconnected from its voltage source and the space between the plates is then filled with a dielectric of constant k = 2.00, evaluate the ratios (a) C new/ C o (b) V new/ V o and (c) PE cnew/PE co

if connected to a batteryC = eA/d if distance increases, capacitance will decrease Q = CV so then if capacitance decreases the overall charge will increase if not connected to a battery. since it is air filled, the k is going to be 1 SO, a) 2:1 because of the new k b) 1:2 because if charge stays the same, C increases, V will decrease by 2 c) PE= .5CV^2 since v increases by exponent 2 and C decreases by half, you get four/two so the ratio is 2:1

you cannot determine an el field from the...

potential at a single point

Cs is always bigger or smaller than the smallest Ci?

smaller