Physics Test 1

Two tiny particles having charges of +7.00 μC and -9.00 μC are placed along the y-axis. The +7.00-µC particle is at y = 0.00 cm, and the other particle is at y = 40.00 cm. Where must a third charged particle be placed along the y-axis so that it does not experience any net electric force due to the other two particles?

-2.99 m

Which one of the following quantities is equivalent to 1 W?

1 V • A

Which one of the following quantities is equivalent to 1 Ω?

1 V/A

A pair of charged conducting plates produces a uniform field of 12,000 N/C, directed to the right, between the plates. The separation of the plates is 40 mm. An electron is projected from plate A, directly toward plate B, with an initial speed of v0 = 2.0 × 107 m/s. What is the speed of the electron as it strikes plate B? (e = 1.6 × 10-19 C, melectron = 9.11 × 10-31 kg)

1.5 × 107 m/s

If the length and diameter of a wire of circular cross section are both tripled, the resistance will be

1/3 of what it originally was

Two identical small charged spheres are a certain distance apart, and each one initially experiences an electrostatic force of magnitude F due to the other. With time, charge gradually diminishes on both spheres by leaking off. When each of the spheres has lost half its initial charge, what will be the magnitude of the electrostatic force on each one?

1/4 F

An electron is released from rest at a distance of 9.00 cm from a fixed proton. How fast will the electron be moving when it is 3.00 cm from the proton? (k = 1/4πε0 = 8.99 × 109 N • m2/C2, e = 1.60 × 10-19 C, melectron = 9.11 × 10-31 kg, mproton = 1.67 × 10-27 kg)

106 m/s

The potential (relative to infinity) at the midpoint of a square is 3.0 V when a point charge of +Q is located at one of the corners of the square. What is the potential (relative to infinity) at the center when each of the other corners is also contains a point charge of +Q?

12 V

A small object with a 5.0-μC charge is accelerating horizontally on a friction-free surface at 0.0050 m/s2 due only to an electric field. If the object has a mass of 2.0 g, what is the magnitude of the electric field?

2.0 N/C

If an electron is accelerated from rest through a potential difference of 1500 V, what speed does it reach? (e = 1.60 × 10-19 C , melectron = 9.11 × 10-31 kg)

2.3 × 107 m/s

An electric dipole with ±5.0 μC point charges is positioned so that the positive charge is to the right of the origin and the negative charge is at the origin. How much work does it take to bring a point charge from very far away to the point x = 3.0 mm, y = 0.0 mm? (k = 1/4πε0 = 9.0 × 109 N • m2/C2)

23 J

Two ideal parallel-plate capacitors are identical in every respect except that one has twice the plate area of the other. If the smaller capacitor has capacitance C, the larger one has capacitance

2C

You are given a copper bar of dimensions 3 cm × 5 cm × 8 cm and asked to attach leads to it in order to make a resistor. If you want to achieve the largest possible resistance, you should attach the leads to the opposite faces that measure

3 cm × 5 cm

A 4.0-g bead carries a charge of 20 μC. The bead is accelerated from rest through a potential difference V, and afterward the bead is moving at 2.0 m/s. What is the magnitude of the potential difference V?

400 V

The electric potential at a distance of 4 m from a certain point charge is 200 V relative to infinity. What is the potential (relative to infinity) at a distance of 2 m from the same charge?

400 V

If the power rating of a 400-Ω resistor is 0.80 W, what is the maximum current it can safely draw?

45 mA

Two tiny particles having charges of +5.00 μC and +7.00 μC are placed along the x-axis. The +5.00-µC particle is at x = 0.00 cm, and the other particle is at x = 100.00 cm. Where on the x-axis must a third charged particle be placed so that it does not experience any net electrostatic force due to the other two particles?

45.8 cm

An ideal parallel-plate capacitor has a capacitance of C. If the area of the plates is doubled and the distance between the plates is halved, what is the new capacitance?

4C

You are given a copper bar of dimensions 3 cm × 5 cm × 8 cm and asked to attach leads to it in order to make a resistor. If you want to achieve the smallest possible resistance, you should attach the leads to the opposite faces that measure

5 cm × 8 cm

A proton with a speed of 2.0 x 105 m/s accelerates through a potential difference and thereby increases its speed to 4.0 x 105 m/s. Through what magnitude potential difference did the proton accelerate? (e = 1.60 × 10-19 C , mproton = 1.67 × 10-27 kg)

630 V

A proton is placed in an electric field of intensity 800 N/C. What are the magnitude and direction of the acceleration of the proton due to this field? (e = 1.60 × 10-19 C, mproton = 1.67 × 10-27 kg)

7.66 × 1010 m/s2 in the direction of the electric field

A +3.0-µC point charge is initially extremely far from a positive point charge Q. You find that it takes 41 J of work to bring the +3.0-µC charge to the point x = 3.0 mm, y = 0.0 mm and the point charge Q to the point x = -3.0 mm, y = 0.00 mm. What is Q? (k = ...)

9.1 µC

An ideal parallel-plate capacitor having circular plates of diameter D that are a distance d apart stores energy U when it is connected across a fixed potential difference. If you want to triple the amount of energy stored in this capacitor by changing only the size of its plates, the diameter should be changed to

D square root 3

A battery charges a parallel-plate capacitor fully and then is removed. The plates are then slowly pulled apart. What happens to the potential difference between the plates as they are being separated?

It increases

The length of a certain wire is doubled while its radius is kept constant. What is the new resistance of this wire?

It is 2 times as large

The length of a certain wire is kept same while its radius is doubled. What is the new resistance of this wire?

It is reduced by a factor of 4

The length of a certain wire is doubled and at the same time its radius is also doubled. What is the new resistance of this wire?

It is ½ as large

The length of a certain wire is doubled and at the same time its radius is reduced by a factor of 2. What is the new resistance of this wire?

It it 8 times as large

Two point charges, Q1 and Q2, are separated by a distance R. If the magnitudes of both charges are halved and their separation is also halved, what happens to the electrical force that each charge exerts on the other one?

It remains the same

Two point charges, Q1 and Q2, are separated by a distance R. If the magnitudes of both charges are doubled and their separation is also doubled, what happens to the electrical force that each charge exerts on the other one?

It remains the same.

In outer space, a positive charge q is released near a positive fixed charge Q, as shown in the figure. As q moves away from Q, what is true about the motion of q? (There may be more than one correct choice.)

It will move with increasing speed.

Two very small plastic balls of equal mass are released from rest. One of them carries +10 µC of excess charge and the other one carries +1µC of charge. No other charges or fields are present. Which of the following statements are true about them as they move away from each other? (There may be more than one correct choice.)

The balls always have accelerations of equal magnitude.

A conductor is placed in a steady external electric field. Which of the following statements are correct for this situation? (There could be more than one correct choice.)

The electric field is zero inside the conductor.

Which statements must be true about the surface of a charged conductor in which no charge is moving? (There could be more than one correct choice.)

The electric potential is constant over the surface.

A negative point charge is in an electric field created by a positive point charge.

The field points away from the positive charge, and the force on the negative charge is in the opposite direction to the field.

Two tiny beads are 25 cm apart with no other charges or fields present. Bead A carries 10 µC of charge and bead B carries 1 µC. Which one of the following statements is true about the magnitudes of the electric forces on these beads?

The force on A is exactly equal to the force on B

Consider two copper wires of equal cross-sectional area. One wire has 3 times the length of the other. How do the resistances of these two wires compare?

The longer wire has 3 times the resistance of the shorter wire

A glass marble is rubbed against a piece of silk. As a result the piece of fabric acquires extra electrons. What happens to the glass marble?

The marble has lost the same number of electrons acquired by the piece of silk The marble acquires a positive charge and attracts the piece of silk.

Copper wire #1 has a length L and a radius b. Copper wire #2 has a length 2L and a radius 2b. Which statement about the resistance across the ends of the wires is true?

The resistance of wire #1 is twice as high as that of wire #2

A negatively-charged rod is brought close to (but does not touch) two neutral spheres that are in contact with each other but insulated from the ground. If the two spheres are then separated, what kind of charge will be on the spheres?

The sphere near the charged rod becomes positive and the other one becomes negative

Consider two copper wires with circular cross-sections and equal lengths. One wire has 3 times the diameter of the other. How do the resistances of these two wires compare?

The thicker wire has 1/9 the resistance of the thinner wire

Consider two copper wires of equal length. One wire has twice the cross-sectional area of the other. How do the resistances of these two wires compare?

The thicker wire has one-half the resistance of the thinner wire

An electron and a proton are released simultaneously from rest and start moving toward each other due to their electrostatic attraction, with no other forces present. Which of the following statements are true just before they are about to collide? (There could be more than one correct choice.)

They are closer to the to the initial position of the proton that to the initial position of the electron.

X and Y are two initially uncharged metal spheres on insulating stands, and they are in contact with each other. A positively charged rod R is brought close to X as shown in part (a) of the figure. Sphere Y is now moved away from X, as shown in part (b). What are the final charge states of X and Y?

X is negative and Y is positive.

Which of the following will increase the capacitance of a parallel-plate capacitor? (There could be more than one correct choice.)

an increase in the plate area and a decrease in the plate separation

An ideal parallel-plate capacitor consists of two parallel plates of area A separated by a distance d. This capacitor is connected across a battery that maintains a constant potential difference between the plates. If the separation between the plates is now doubled, the magnitude of the charge on the plates will

be cut in half

An ideal parallel-plate capacitor consists of two parallel plates of area A separated by a distance d. This capacitor is connected to a battery that maintains a constant potential difference across the plates. If the separation between the plates is now doubled, the amount of electrical energy stored on the capacitor will

be cut in half

If the voltage across a circuit of constant resistance is doubled, the power dissipated by that circuit will

be four times as large

If we double the resistance in a circuit but keep the current in it constant, the power dissipated by that circuit will

be two times as great

A wire of resistivity ρ must be replaced in a circuit by a wire of the same material but four times as long. If, however, the total resistance is to remain as before, the diameter of the new wire must

be two times the original diameter.

As a proton moves in a direction perpendicular to the electric field lines

both its electric potential and electric potential energy remain constant

If a quantity you calculated has units of A • s, what is that quantity?

charge

Doubling the capacitance of a capacitor that is holding a constant charge causes the energy stored in that capacitor to

decrease to one half

If the resistance in a constant voltage circuit is doubled, the power dissipated by that circuit will

decrease to one-half its original value

An capacitor consists of two large parallel plates of area A separated by a very small distance d. This capacitor is connected to a battery and charged until its plates carry charges +Q and -Q, and then disconnected from the battery. If the separation between the plates is now doubled, the potential difference between the plates will

double

The electric field at point P due to a point charge Q a distance R away from P has magnitude E. In order to double the magnitude of the field at P, you could

double the charge to 2Q

The electron-volt is a unit of

energy

A parallel-plate capacitor is connected to a battery and becomes fully charged. The capacitor is then disconnected, and the separation between the plates is increased in such a way that no charge leaks off. As the plates are being separated, the energy stored in this capacitor

increases

When the current through a resistor is increased by a factor of 4, the power dissipated by the resistor

increases by a factor of 16

A plastic rod is charged up by rubbing a wool cloth, and brought to an initially neutral metallic sphere that is insulated from ground. It is allowed to touch the sphere for a few seconds, and then is separated from the sphere by a small distance. After the rod is separated, the rod

is repelled by the sphere

The plates of a parallel-plate capacitor are maintained with constant voltage by a battery as they are pulled apart. What happens to the strength of the electric field between the plates during this process?

it decreases

As an electron moves in the direction the electric field lines

it is moving from high potential to low potential and gaining electric potential energy

As a proton moves in the direction the electric field lines

it is moving from high potential to low potential and losing electric potential energy.

The plates of a parallel-plate capacitor are maintained with constant potential by a battery as they are pulled apart. During this process, the amount of charge on the plates

must decrease

A small metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positively charged rod held near the ball.

negative or neutral

A negatively-charged plastic rod is brought close to (but does not touch) a neutral metal sphere that is connected to ground. After waiting a few seconds, the ground connection is removed (without touching the sphere), and after that the rod is also removed. The sphere is now

positively charged.

Doubling the potential across a given capacitor causes the energy stored in that capacitor to

quadruple

If the current flowing through a circuit of constant resistance is doubled, the power dissipated by that circuit will

quadruple in magnitude

Two uncharged metal spheres, #1 and #2, are mounted on insulating support rods. A third metal sphere, carrying a positive charge, is then placed near #2. Now a copper wire is momentarily connected between #1 and #2 and then removed. Finally, sphere #3 is removed. In this final state

sphere #1 carries positive charge and #2 carries negative charge