Physics _ Chap 12

How will a negatively charged particle, initially at rest in an electric field, tend to move?

toward a region of higher electric potential

The electrical force between two charged objects depends on

the product of the net charge on each of the two objects.

After a glass rod is rubbed with a nylon cloth

the rod will have a positive charge and the cloth negative.

A 10 µC charge is at the origin. A -15 µC charge is on the x-axis 10 cm to the right of the origin. At what point other than at infinity can a 1 µC charge be placed so that there will be no net electrostatic force on it?

At one point on the x-axis to the left of the positive charge

A uniform electric field is directed upward and has a magnitude of 12 N/C. A charge of −6 C is placed in this field. Calculate the magnitude of the force on the charge.

The magnitude of the force on the charge is calculated using the following formula: |Fe|= |qE|=|−6 C×12 N/C|= 72 N

When a glass rod rubbed with silk acquires a positive charge, the silk acquires

a negative charge.

A uniform electric field has a magnitude of 10 N/C and is directed upward. A charge brought into the field experiences a force of 2.0 N upward. The charge must be

+0.2 C.

If a charge of -3 x 10-6 C were allowed to fall through a potential difference of +500 V, the change in potential energy for the charge would be

-1.5 × 10-3 Joules.

If the distance between two point charges is doubled while the size of the charges remains the same, the force between the charges is multiplied by _______.

.25

Refer to the dipole in the z, w coordinate system. Consider the electric field at a point α out on the z-axis, above and to the right of the intersection of the axes. In which direction does the electric field point at point α?

45° up and to the left, parallel to the w-axis:

Consider horizontal parallel plates with a fixed potential difference. The upper plate has a voltage difference of 30 V with the lower plate, and they are separated by 3 cm. You move the two plates carefully to a separation of 4 cm. What is the strength of the electric field between the plates?

750 volts/meter

Two very small spherical metal objects, each with 1 Coulomb of charge, are brought together in a vacuum so that the separation distance between their centers is 1 mm. What is the force of repulsion between the two objects?

9 × 10^12 N

A point charge creates the electric field shown. At which of the points would the potential energy of a positive test charge have the largest value?

D

Two charges are shown. What is the direction of the electric field at the indicated point?

D

You have a pair of horizontal parallel plates, with the positive plate above and the negative plate below. How will the lines of constant potential be arranged in this perspective?

Horizontally

Electrostatic precipitators can effectively remove most of which of the following air pollutants from a smoke stack?

Particulates

Two identical brass balls mounted on wooden posts initially have different amounts of charge: one -15 μC and the other -21 μC. The balls are allowed to touch and then are separated again. What is the final charge on each ball?

The final charge on each ball is calculated as follows: q=q1 + q22 = −15 μC − 21 μC2 = −18 μC

Two identical steel balls mounted on wooden posts initially have different amounts of charge: one -9 μC and the other +28 μC. The balls are allowed to touch and then are separated again. Calculate the final charge on each ball.

The final charge on each ball is calculated as follows: q=q1 + q22 = −9 μC + 28 μC2 = 9.5 μC

A +3.4 × 10−6 C test charge experiences forces from two other nearby charges: a 13-N force due east and an 15-N force due west. Determine the magnitude of the electric field at the location of the test charge.

The magnitude of the electric field at the location of the test charge is calculated using the following formula: E= Feq= ∣∣(13 N− 15N)3.4 × 10−6 C∣∣= 0.5882×106 N/C

Imagine that the charges of an electron and a proton in a hydrogen atom are of magnitude 2.2 × 10−19 C but of opposite signs. The electron and proton are separated by a distance of 5.29 × 10−11 m, and the value of Coulomb's constant is 9 × 109 N·m2/C2. Calculate the magnitude of the electrostatic force exerted on the electron by the proton.

The magnitude of the electrostatic force exerted on the electron by the proton is calculated using the following formula: F= kq1q2r2= (9 × 109 N⋅m2/C2)(2.2 × 10−19C)(2.2 × 10−19C)(5.29 × 10−11 m)2= 15.566×10−8 N

Two charged particles exert an electrostatic force of 24 N on each other. What will the magnitude of the electrostatic force be if the distance between the two charges is reduced to one-third of the original distance?

The magnitude of the electrostatic force is calculated using the following formula: F= kq1q2r2=kq1q2(13r)2= 9(kq1q2r2) F= 9×24 N = 216 N

An electron has a charge of −1.6 × 10−19 C. How many electrons would be needed to produce a net charge of -10.2 × 10−5 C?

The number of electrons required is calculated as follows: Number of electrons = net chargecharge of one electron = −10.2 × 10−5 C−1.6 × 10−19 C Number of electrons = 6.375× 1014

The unit for electric field, N/C, can be written equivalently as

V/m.

The unit for electric potential is

Volt.

A +3.4 × 10−6 C test charge experiences forces from two other nearby charges: a 13-N force due east and an 15-N force due west. What is the direction of the electric field at the location of the test charge?

WEST

If the potential at point A is one volt it means that

a charge at A has potential energy one joule for each coulomb of charge.

n the diagram shown, which point will have the higher electric potential?

both will have the same electric potential (B and C)

Materials that allow electric charges to move freely are called

conductors.

How does the electric potential change as we move towards a negative charge?

decreases

If we move a positive charge towards a second negative charge, how does the potential energy of the charge change?

decreases

A positive charge is fixed at the origin. A negative charge is moved along the x-axis from x = 50 cm to x = 10 cm. The potential energy of the system

decreases.

The electrical force between any two charged objects is

dependent on the sign of the charge on each object.

If the plates of a charged parallel plate capacitor were moved further apart, while the charge on the plates is kept constant, the potential energy of each of the charges on the capacitor would

increase.

If a negative charge is moved in the same direction as the electric field lines in some region of space, how does the potential energy of the negative charge change?

increases

If we move a negative charge towards a second negative charge, how does the potential energy of the charge change?

increases

If the distance between two spherical charged objects decreases by a factor of two, then the electrical force between the objects

increases by a factor of four.

Materials that do not allow electric charges to move freely are called

insulators.

A positively charged rod is brought close to an uncharged electroscope. While the rod is close, one's finger touches the far side of the metal ball on the electroscope. The finger is removed and then the rod is taken away. The electroscope is

negatively charged.

If you touch the metal ball of a charged electroscope with an uncharged glass rod, what will happen to the electroscope?

nothing will happen

A metallic object has a net charge on it. For steady state conditions, the excess charge is

only on the surface.

A rubber rod is rubbed with fur, so that the fur becomes positive. The rod is then used to charge an electroscope. The charge on the electroscope will be

positive if charged by induction.

Our model for explaining the electrical interaction between charged objects

postulates that an electric field surrounds each charged object.

Electric field is calculated as

the Coulomb force divided by the size of the test charge.

The change in electric potential is equal to the

the change in electrostatic energy divided by the size of the charge.

Two pith balls are charged by touching one to a glass rod that has been rubbed with a nylon cloth and the other to the cloth itself. How will the two pith ball react with one another?

they will attract each other

How will bits of paper act near a charged rod even when they are uncharged?

they will be attracted to the rod

You charge an electroscope with a plastic rod that has been rubbed with cat fur. What will the metal-foil leaves do when you approach the ball of the electroscope?

they will move closer together

Two equal charges repel one another with a force of 4.0 × 10-4 N when they are 10 cm apart. (Coulomb's constant is k=9.0 × 109 N.m2/C2.) If they are moved until the separation is 5 cm, the repulsive force will be

16 × 10-4 N

If the distance between two point charges remains constant while the size of one of the charges is doubled, the force between the charges is multiplied by ___________.

2

A capacitor consisting of two parallel plates separated by 2.0 cm has a potential of 40 V on the top plate and a potential of 0 V on the bottom plate. The electric field in the middle is

2000 N/C, downward.

The electric potential increases from 52 V to 331 V from the bottom plate to the top plate of a parallel-plate capacitor. A −4 ×10-3 C charge is moved from the bottom plate to the top plate. What is the magnitude of the change in potential energy of the charge?

The change in potential energy of the charge is calculated using the following formula: |ΔPE|= |qΔV|=∣∣(−4×10−3 C)×(331 V−52 V)∣∣ = 1.116 J

Two charged particles exert an electrostatic force of 36 N on each other. What will the magnitude of the force be if the distance between the two particles is increased to twice the original distance?

The magnitude of the electrostatic force is calculated using the following formula: F= kq1q2r2=kq1q2(2r)2= 14(kq1q2r2) F= 14(36 N)= 9N

Two charges of equal magnitude but opposite sign are 10 cm apart. The potential is largest

close to the positive charge.

As a given thundercloud's base elevation gets lower and lower, the possibility of lightning strike

is greater, because the electric field strength increases.

What will happen if you touch the ball of a charged electroscope with your finger?

it will discharge

If you have four identical metal balls mounted on insulating stands, what sequence of touches would obtain a quantity of charge on one ball that is four times as large as the quantity on another ball? The first ball has a charge of 16 units.

1 touches 2, 2 touches 3, and 3 touches 4

Which is better during a thunderstorm: A. being in your car B. standing under a tree on the golf course

A. being in your car

The potential energy of a +8 × 10−6 C charge decreases from 0.7 J to 0.34 J when it is moved from point A to point B. What is the magnitude of the change in electric potential between these two points?

The change in electric potential between the two points is calculated using the following formula: ΔV= ΔPEq= (0.7 J− 0.34 J)8 × 10−6 C =45000 V

10.5 coulombs of positive charge flows from the +12 V positive terminal of a battery to the negative terminal at 0 V. What is the change in potential energy of the charge?

The change in potential energy of the charge is calculated using the following formula: ΔPE= qΔV= (10.5 C)×(0V−12V) = -126 J

A charge of +0.38 C is moved from a position where the electric potential is 20 V to a position where the electric potential is 50 V. What is the change in potential energy of the charge associated with the change in position?

The change in potential energy of the charge is calculated using the following formula: ΔPE= qΔV= 0.38 C×(50V−20V) = 11.4 J

A charge of −5.2 × 10−6 C is placed at a point in space where the electric field is directed toward the right and has a magnitude of 6.4 × 105 N/C. What is the magnitude of the electrostatic force on the charge?

The magnitude of the electrostatic force on the charge is calculated using the following formula: |Fe|= |qE|= ∣∣(−5.2×10−6 C)×(6.4×105 N/C)∣∣ = 3.328 N

Two negative charges, each of magnitude 17 × 10−6 C, are located at a distance of 12 cm from each other. What is the magnitude of the force exerted on each charge? Take the value of Coulomb's constant as 9 × 109 N·m2/C2.

The magnitude of the force exerted on each charge is calculated using the following formula: F= kq1q2r2= (9 × 109 N⋅m2/C2)(17 × 10−6 C)(17 × 10−6 C)(0.12 m)2 F =180.625 N

A charge of 18 × 10−6 C is located 21 cm from a charge of −7 × 10−6 C. Determine the magnitude of the force exerted on each charge. Take the value of Coulomb's constant as 9 × 109 N·m2/C2.

The magnitude of the force exerted on each charge is calculated using the following formula: |F|= ∣∣kq1q2r2∣∣= ∣∣∣(9 × 109 N⋅m2/C2)(18 × 10−6 C)(−7 × 10−6 C)(0.21 m)2∣∣∣ = 25.7143 N

How does Benjamin Franklin's single fluid model explain the different charges that emerge when we charge a glass rod by rubbing it with a nylon cloth?

a surplus or deficit of fluid accounts for the positive or negative charge

When you shuffle your feet on a carpet on a dry day, you can accumulate a charge. The fact that you accumulate a charge means that the carpet is

an insulator, because if it weren't, the charge would immediately leave you.

A capacitor consisting of two separated parallel horizontal plates has a uniform electric field directed upward. If a negative charge is placed exactly midway between the two plates, it will

be accelerated downward.

Comparing the electrostatic force and the gravitational force we can say that

both have the same dependence on distance, the electrostatic force can be either attractive or repulsive while the gravitational force is only attractive, and the electrostatic force is stronger.

How will the force between two charged objects change if the both charges are doubled in magnitude?

the force will be four times larger

How will the electrostatic force on two charged objects change if the distance is doubled?

the force will be four times smaller

To bring a negative charge from an infinitely great distance away into the presence of a positive charge would require a ____________________ amount of work to be done.

negative

Two pith balls are both charged by contact with a plastic rod that has been rubbed by cat fur. What sign will the charges on the pith balls have?

negative

Which of the following statements is true?

only the electric force can be both attractive and repulsive, while gravity can only be attractive

The electric field created by a moving negative charge

points toward the charge.

If in a typical thundercloud the bottom of the cloud is negatively charged, what is the sign of the excess charge on the ground?

positive

When a metal ball is charged by induction using a negatively charged plastic rod, what sign is the charge acquired by the metal ball?

positive

During a thunderstorm, why can a much greater flow of charge proceed along the path of the leader (after the initial flow of charge)?

the atoms along the path are ionized

If, when charging by induction, you remove the charged rod from the vicinity of the metal ball before moving your finger from the ball, what will happen to the charge of the ball?

the ball will not charge

Two equal positive charges are near each other. If we increase the amount of charge on just one of them, then

the electric field at the position of the smaller charge increases.

The most common form of lightning strike from a cloud to the ground involves negative charge moving from the cloud to the ground. Just before a lightning strike,

the electric field must point toward the sky.

Why are pith balls initially attracted to a charged rod and later repelled by the same rod after being touched, even though they have not touched any other object?

the initial attraction is due to charge flow, but after the rod is touched the charge of the two are the same

Two pith balls are charged by touching one to a glass rod that has been rubbed with a nylon cloth and the other to the cloth itself. What sign will the charge on each pith ball have?

the pith ball that touched the cloth will be negative and the pith ball which touched the rod will be positive

The increase in electrical potential energy is calculated as

the product of the electric field times the charge times the distance the charge moves in the electric field.

When an electroscope is charged what is the charge of the metal-foil leaves?

the same for both leaves

Two pith balls are both charged by contact with a plastic rod that has been rubbed by cat fur. How will the two pith balls react to each other?

they will repel each other

Which of the following concepts are involved in the operation of Coulomb's torsion balance?

torque

A negatively charged rod is brought close to an uncharged electroscope. While the rod is close one's finger touches the far side of the metal ball on the electroscope. After the charged rod has been removed the finger is removed. The electroscope is

uncharged

A rod with a positive charge is brought near a positively charged pith ball. The pith ball is suspended from an insulating string. The pith ball

will be repelled by the rod.

Are electrostatic precipitators effective for removing pollutants such as sulfur and mercury from exhaust gases?

yes, in conjunction with a dry scrubber

Imagine four equal positive charges, q, placed on points of a circle of radius r at the 3, 6, 9, and 12 o'clock positions. Coulomb's constant is k. The magnitude of the electric field at the center of the circle is given by

zero.