E & M Exam #1

A particle with a charge of 5 × 10⁻⁶ C and a mass of 20 g moves uniformly with a speed of 7m/s in a circular orbit around a stationary particle with a charge of −5×10⁻⁶ C. The radius of the orbit is:

0.23m (use the ratio of the electrostatic force to the gravitational force between electron and proton which is kQq/GMm and solve for the missing variable {this case was G, since the forces were the same on both particles I assumed the mass was the same as well} by having Fk/Fg = and the answer I found for G = the radius since F = 1.)

Two thin spherical shells, one with radius R and the other with radius 2R, surround an isolated charged point particle. The ratio of the number of field lines through the larger sphere to the number through the smaller is:

1

The magnitude of charge on an electron is approximately:

1 x 10⁻¹⁹ C

A particle with charge 2-μC is placed at the origin, an identical particle, with the same charge, is placed 2m from the origin on the x axis, and a third identical particle, with the same charge, is placed 2m from the origin on the y axis. The magnitude of the force on the particle at the origin is:

1.3 × 10⁻² N

An electric dipole consists of a particle with a charge of +6×10^−6 C at the origin and a particle with a charge of −6 × 10^−6C on the x axis at x = 3× 10^−3 m. Its dipole moment is:

1.8 × 10^−8 C · m, in the negative x direction

The total negative charge on the electrons in 1 mol of helium (atomic number 2, molar mass 4) is:

1.9 x 10⁵ C

The electric field at a distance of 10 cm from an isolated point particle with a charge of 2×10^−9 C is

1800N/C

A wire contains a steady current of 2 A. The number of electrons that pass a cross section in 2 s is:

2.5 × 10¹⁹

Two point particles, one with charge +8 × 10^−9 C and the other with charge −2 × 10^−9 C, are separated by 4m. The electric field in N/C midway between them is:

22.5

A 200-N/C electric field is in the positive x direction. The force on an electron in this field is:

3.2 × 10^−17 N in the negative x direction

Two identical conducting spheres A and B carry equal charge. They are separated by a distance much larger than their diameters. A third identical conducting sphere C is uncharged. Sphere C is first touched to A, then to B, and finally removed. As a result, the electrostatic force between A and B, which was originally F, becomes:

3F/8

A wire carries a stead current of 2A. The charge that passes a cross section in 2s is:

4 C (think that a Coulomb is amperes-second so 2A x 2s= 4C)

Consider the following procedural steps: 1. ground an electroscope 2. remove the ground from the electroscope 3. touch a charged rod to the electroscope 4. bring a charged rod near, but not touching, the electroscope 5. remove the charged rod To charge an electroscope by induction, use the sequence:

4, 1, 2, 5 4. bring a charged rod near, but not touching, the electroscope 1. ground an electroscope 2. remove the ground from the electroscope 5. remove the charged rod

Two identical charges, 2.0m apart, exert forces of magnitude 4.0N on each other. The value of either charge is:

4.2 × 10⁻⁵ C

The total negative charge on the electrons in 1 kg of helium (atomic number 2, molar mass 4) is:

4.8 x 10⁷ C

An isolated charged point particle produces an electric field with magnitude E at a point 2m away. At a point 1m from the particle the magnitude of the field is:

4E

An isolated charged point particle produces an electric field with magnitude E at a point 2m away from the charge. A point at which the field magnitude is E/4 is:

4m away from the particle

Particles 1, with charge q1, and 2, with charge q2, are on the x axis, with particle 1 at x = a and particle 2 at x = −2a. For the net force on a third charged particle, at the origin, to be zero, q1 and q2 must be related by q2 =:

4q1

The magnitude of the force of a 400-N/C electric field on a 0.02-C point charge is:

8.0N

A 5.0-C charge is 10m from a −2.0-C charge. The electrostatic force on the positive charge is:

9.0 × 10⁸ N toward the negative charge

A charged oil drop with a mass of 2 × 10^−4 kg is held suspended by a downward electric field of 300N/C. The charge on the drop is:

A. +1.5 × 10^−6 C B. −1.5 × 10^−6 C C. +6.5 × 10^−6 C D. −6.5 × 10^−6 C E. 0

A uniform electric field of 300N/C makes an angle of 25degrees with the dipole moment of an electric dipole. If the torque exerted by the field has a magnitude of 2.5×10^−7 N·m, the dipole moment must be:

A. 8.3 × 10^−10 C · m B. 9.2 × 10^−10 C · m C. 2.0 × 10^−9 C · m D. 8.3 × 10^−5 C · m E. 1.8 × 10^−4 C · m

When the dipole moment of a dipole in a uniform electric field rotates to become more nearly aligned with the field:

A. the field does positive work and the potential energy increases B. the field does positive work and the potential energy decreases C. the field does negative work and the potential energy increases D. the field does negative work and the potential energy decreases E. the field does no work

The purpose of Milliken's oil drop experiment was to determine:

A. the mass of an electron B. the charge of an electron C. the ratio of charge to mass for an electron D. the sign of the charge on an electron E. viscosity

The dipole moment of a dipole in a 300-N/C electric field is initially perpendicular to the field, but it rotates so it is in the same direction as the field. If the moment has a magnitude of 2 × 10^−9 C · m, the work done by the field is:

A. −12 × 10^−7 J B. −6 × 10^−7 J C. 0 D. 6 × 10^−7 J E. 12 × 10^−7 J

A positively charged metal sphere A is brought into contact with an uncharged metal sphere B. As a result:

Both spheres are positively charged

Two particles, each with charge Q, and a third particle, with charge q, are placed at the vertices of an equilateral triangle as shown (q top of triangle + charge, both Qs on bottom + charges) . The total force on the particle with charge q is:

D. perpendicular to the bottom side of the triangle

A positively charged insulating rod is brought close to an object that is suspended by a string. If the object is attracted toward the rod we can conclude:

E) none of the above (the object is positively charged, the object is negatively charged, the object is an insulator, the object is a conductor)

Two small charged objects attract each other with a force, F, when separated by a distance, d. If the charge on each object is reduced to one-fourth of its original value and the distance between them is reduced to d/2 the force becomes:

F/4 (bc orig.: F= k (q1xq2 / d^2) then: F= k [(q1/4)(q2/4) / (d/2)^2] => F= k [(q1q2/16) / (d^2/4)] => F= k [(q1q2/16) x (4/d^2)] => F= k [(q1q2 x 4) / 16d^2] => F= k (q1q2 / 4)

Two charged particles are arranged as shown. In which region could a third particle, with charge +1 C, be placed so that the net electrostatic force on it is zero? I II III --(+)----------(-)-- 2C −4C

I only

The units of the electric field are:

J/(C·m)

Two uncharged metal spheres, L and M, are in contact. A negatively charged rod is brought close to L, but not touching it, as shown. The two spheres are slightly separated and the rod is then withdrawn. As a result:

L is negative and M is positive

The units of 1/4πε₀ are:

N ∙ m² / C²

The units of the electric field are:

N/C

A small object has charge Q. Charge q is removed from it and placed on a second small object. The two objects are placed 1m apart. For the force that each object exerts on the other to be a maximum, q should be:

Q/2

A particle with charge Q is on the y axis a distance a from the origin and a particle with charge q is on the x axis a distance d from the origin. The value of d for which the x component of the force on the second particle is the greatest is:

a/√2

To make an uncharged object have a negative charge we must:

add some electrons

A coulomb is the same as:

an ampere-second

Electric field lines:

are none of the above (are trajectories of a test charge, are vectors in the direction of the electric field, form closed loops, cross each other in the region between two point charges)

Two charged point particles are located at two vertices of an equilateral triangle and the electric field is zero at the third vertex. We conclude:

at least one other charged particle is present

A charged particle is placed in an electric field that varies with location. No force is exerted on this charge:

at locations where the electric field is zero

Two particles have charges Q and −Q (equal magnitude and opposite sign). For a net force of zero to be exerted on a third charge it must be placed:

at none of these places (there is no placed {bc charges on the other two particles are opp. of each other}) (A. midway between Q and −Q, B. on the perpendicular bisector of the line joining Q and −Q, but not on that line itself, C. on the line joining Q and −Q, to the side of Q opposite −Q, D. on the line joining Q and −Q, to the side of −Q opposite Q)

Two point particles, with the same charge, are located at two vertices of an equilateral triangle. A third charged particle is placed so the electric field at the third vertex is zero. The third particle must:

be on the perpendicular bisector of the line joining the first two charges

The charge on a glass rod that has been rubbed with silk is called positive:

by arbitrary convention

a kiloampere-hour is a unit of:

charge

A charged insulator can be discharged by passing it just above a flame. This is because the flame:

contains ions

Choose the correct statement concerning electric field lines:

field lines are close together where the field is large

An electroscope is charged by induction using a glass rod that has been made positive by rubbing it with silk. The electroscope leaves:

gain electrons

Two particles, X and Y, are 4m apart. X has a charge of 2Q and Y has a charge of Q. The force of X on Y:

has the same magnitude as the force of Y on X

Charge is distributed uniformly on the surface of a spherical balloon (an insulator). A point particle with charge q is inside. The electrical force on the particle is greatest when:

it is anywhere inside (the force is zero everywhere)

Charge is distributed on the surface of a spherical conducting shell. A point particle with charge q is inside. If polarization effects are negligible the electrical force on the particle is greatest when:

it is near the inside surface of the balloon

In the Rutherford model of the hydrogen atom, a proton (mass M, charge Q) is the nucleus and an electron (mass m, charge q) moves around the proton in a circle of radius r. Let k denote the Coulomb force constant (1/4πε₀ = 8.99x10⁹ N∙m²/C²) and G the universal gravitational constant. The ratio of the electrostatic force to the gravitational force between electron and proton is:

kQq/GMm r² cancels out in both

Let k denote 1/4π0. The magnitude of the electric field at a distance r from an isolated point particle with charge q is:

kq/r^2

Two particles A and B have identical charge Q. For a net force of zero to be exerted on a third charged particle it must be placed:

midway between A and B (bc charges are same on particles A and B)

A conductor is distinguished from an insulator with the same number of atoms by the number of:

nearly free electrons

When a hard rubber rod is given a negative charge by rubbing it with wool:

negative charges are transferred from wool to rod

As used in the definition of electric field, a "test charge":

none of the above (has zero charge, has charge of magnitude 1C, has charge of magnitude 1.6 × 10^−19 C, must be an electron)

The electric field due to a uniform distribution of charge on a spherical shell is zero:

only inside the shell

The torque exerted by an electric field on a dipole is:

perpendicular to both the field and dipole moment

The force exerted by a uniform electric field on a dipole is:

perpendicular to the electric field

The leaves of a positively charged electroscope diverge more when an object is brought near the knob of the electroscope. The object must be:

positively charged

Two point particles, with a charges of q1 and q2, are placed a distance r apart. The electric field is zero at a point P between the particles on the line segment connecting them. We conclude that:

q1 and q2 must have the same sign but may have different magnitudes

To make an uncharged object have a positive charge:

remove some electrons

An electron traveling north enters a region where the electric field is uniform and points north. The electron:

slows down

A negatively charged rubber rod is brought near the knob of a positively charged electroscope. The result is that:

the electroscope leaves will tend to collapse

An electric field exerts a torque on a dipole only if:

the field is not parallel to the dipole moment

An electric field is most directly related to:

the force acting on a test charge

A positively charged insulating rod is brought close to an object that is suspended by a string. If the object is repelled away from the rod we can conclude:

the object is positively charged

Charge Q is spread uniformly along the circumference of a circle of radius R. A point particle with charge q is placed at the center of this circle. The total force exerted on the particle can be calculated by Coulomb's law:

the result of the calculation is zero

Experimenter A uses a test charge q0 and experimenter B uses a test charge −2q0 to measure an electric field produced by stationary charges. A finds a field that is:

the same in both magnitude and direction as the field found by B

A neutral metal ball is suspended by a string. A positively charged insulating rod is placed near the ball, which is observed to be attracted to the rod. This is because:

there is a rearrangement of the electrons in the ball

An electrical insulator is a material:

through which electrons do not flow easily

An electron traveling north enters a region where the electric field is uniform and points west. The electron:

veers east

Two protons (p1 and p2) are on the x axis, as shown below. The directions of the electric field at points 1, 2, and 3, respectively, are: --1---p1-----2--p2---3--

←−, ←−, −→

Two protons (p1 and p2) and an electron (e) lie on a straight line, as shown. The directions of the force of p1 on e, the force of p2 on e, and the total force on e, respectively, are: ---•(p1)--•(e)------•(p2)---

←−, −→, ←− total force on e is in <-- direction because the distance between e and p2 is larger than the distance between e and p1 and since Coulomb's law is an inverse squared function, the force is therefore much smaller between e and p2 than e and p1.

Two electrons (e1 and e2) and a proton (p) lie on a straight line, as shown. The directions of the force of e2 on e1, the force of p on e1, and the total force on e1, respectively, are: ----∙(e1)---∙(e2)---∙(p)----

←−, −→, ←− total force on e1 is in <-- direction bc the force on e1 by p is 1/4 (due to twice distance) the force on e1 by e2 so it is in the (-) direction.

Positive charge Q is uniformly distributed on a semicircular rod. What is the direction of the electric field at point P, the center of the semicircle? +Q ( •P

→

Positive charge +Q is uniformly distributed on the upper half a rod and negative charge −Q is uniformly distributed on the lower half. What is the direction of the electric field at point P, on the perpendicular bisector of the rod? +Q |--------- •P -Q

↓

Positive charge +Q is uniformly distributed on the upper half a semicircular rod and negative charge −Q is uniformly distributed on the lower half. What is the direction of the electric field at point P, the center of the semicircle? +Q ( •P −Q

↓