Physics 112
Midterm Exam
questions and
answers with
complete solutions
,If two objects both have more of the positive type of charge than negative (usually
referred to as a positive net charge) - ANSWER-then the objects will repel each other.
If two objects both have more of the negative type of charge than positive (referred to as
a negative net charge) - ANSWER-then the objects will repel each other.
If one object has more of the positive type of charge and one object has more of the
negative type of charge - ANSWER-then the objects will attract each other.
If a material gains electrons - ANSWER-the material will have a negative net charge
If a material loses electrons - ANSWER-the material will have a positive net charge.
A particular object has a net 6.5 μC of the positive type of charge. Were electrons
transferred onto or off the object? How many? - ANSWER-Since the object has extra of
the positive type of charge, electrons must have been transferred off the material. Since
electrons carry the negative type of charge, removing electrons would have left behind
more positive than negative.
The total amount of charge moved off the material was 6.5 μC. Each electron moved off
carries 1.6x10-19 C. Convert μC into C and divide.
6.5x10-6 C / 1.6x10-19 C = 4.06x1013 electrons were removed.
Solve for the number of charge carrier particles (protons or electrons) transferred on or
off the object. - ANSWER-The amount of charge carried by one electron is 1.6x10-19 C.
Divide the total charge by the charge of one electron to solve for the number of
electrons.
(3.5x10-6 C) / (1.6x10-19 C) = 2.25x1013 electrons were transferred off the object.
conductors - ANSWER-Materials that allow electrons to move easily through them.
Metals are good conductors
insulators - ANSWER-Materials that do not allow electrons to move freely through them.
Nonmetals are good insulators
Semiconductors - ANSWER-are a class of materials that, as you might expect, share
properties of both.
Conduction - ANSWER-is the movement of charge from one object to another. It is easy
to move charge from metal to metal (conductor to conductor) but much more difficult to
move charge from insulator to insulator.
,The transfer of charge caused by rubbing two objects against each other is a special
type of conduction called triboelectric charging.
An electroscope is a device that has small gold foil strips suspended from a metal rod. If
a negatively charged rod is brought near the top of a neutral electroscope, the gold foil
strips will separate. (See below.) Explain why this happens. - ANSWER-The negative
charge on the rod repels the negative charge already present in the electroscope. Since
the electroscope is made from metals, electrons will be pushed down into the gold
strips. The electroscope is now polarized (the top is more positive, and the bottom is
more negative). However, the relocated negative charge now in the thin gold strips will
repel each other causing the gold strips to separate.
Induction - ANSWER-is a charge transfer process that involves polarizing an object and
then allowing charge to transfer to another object through conduction. While still under
the influence of the dipole, the charge transfer mechanism is removed (the two objects
are moved apart). The two objects are now charged.
Polarization - ANSWER-is a redistribution of charge rather than a transfer of charge; the
total amount of charge on the polarized object does not change.
static electric force - ANSWER-Proportional to the quantity of net charge on both
objects, and proportional to the inverse-square of the distance between the two charged
objects.
In other words:
Fe α q1q2 / r2
Where Fe is the force, q1 and q2 are the net charges on the object, and r is the distance
between the two objects.
The distance d between two positively charged particles is measured. The distance
between the two particles is increased to 2d. If the original electrostatic force between
the objects was F, what is the new force? - ANSWER-Force has an inverse-squared
relationship with distance. If the distance is doubled, then the force will change by a
factor of (1/2)^2 = ¼. In other words, the new force will be four times smaller.
Since the two particles are both positive, the force will be a repelling force.
Fe = kq1q2/r2 - ANSWER-This constant is called the Coulomb's law constant, and has
a value of 9x10^9 N-m2/C2.
What is the static electric force between a -3.0 μC charge and a +6 μC charge if they
are 2.0 cm apart? - ANSWER-F = kqq/r^2
F = (9x10^9 N-m2/C2)(3x10^6 C)(6x10^6 C) / (0.02 m)^2
F = 405 N
What if you have more than two charged particles? - ANSWER-Sketch the system if a
drawing is not provided.
Draw and label a free body diagram.
, Calculate all the forces.
Solve for any x and y force components.
Add all the x direction forces together, and then add all the y direction forces together.
Use the Pythagorean theorem to combine the components into their vector form.
Use arctangent to solve for the direction of the final vector form.
Particle A has a net charge of +4 mC, and is located at coordinate position (0, 2) m.
Particle B has a net charge of -2 mC, and is located at coordinate position (0, 0) m.
Particle C has a net charge of +6 mC, and is located at position (3, 0) m.
What is the net charge on particle B? Ignore any gravity effects. - ANSWER-F = kqq/r2F
a on b = (9x109 N-m2/C2)(4x10-3 C)(2x10-3 C) / (2 m)2F
a on b = 18,000 N
F c on b = (9x109 N-m2/C2)(6x10-3 C)(2x10-3 C) / (3 m)2F
c on b = 12,000 N
Since these forces don't have any angles, you don't need to solve for their x and y
components. Solve for the net force and the direction of the net force.
ΣF2 = Fy2 + Fx2
ΣF2 = (18,000 N)2 + (12,000 N)2
ΣF = 21,633 N
tanθ = (18,000 N) / (12,000 N)
θ = tan-1 (1.5)
θ = 56.3°
A neutral pith ball (a small Styrofoam ball) coated with graphite is suspended between a
negatively charged metal sphere and a neutral metal sphere. The pith ball will bounce
back and forth between the two metal spheres for a while until it eventually comes to a
stop suspended midway between the two. Explain this behavior. - ANSWER-The
negatively charged sphere will polarize the pith ball pushing the negative charge to the
left side of the pith ball. Coulomb's law shows the attractive static force between the
negative sphere and the closer positive side of the pith ball will be larger than the
repelling force between the negative sphere and the further away negative side of the
pith ball. The pith ball will move into contact with the negative sphere.
Once the pith ball is in physical contact with the metal sphere, negative charge will
conduct onto the pith ball giving the pith ball a net negative charge. The negatively
charged sphere and the negatively charged pith ball will now repel.
The pith ball will repel into the neutral sphere, and will conduct its excess negative
charge onto the neutral sphere. The pith ball will now be neutrally charged, and the
process begins again.
Unless the neutral sphere is grounded, the negative charge between the two metal
spheres will eventually balance, and the pith ball will experience the same static electric
forces from both sides causing it to remain in equilibrium in the middle between the two.
Explain why a charged balloon will electrostatically stick to a neutral ceiling. - ANSWER-
The balloon will polarize the ceiling. If the balloon is negatively charged, then the ceiling
will polarize such that the positive pole of the ceiling is closer to the balloon than the
Midterm Exam
questions and
answers with
complete solutions
,If two objects both have more of the positive type of charge than negative (usually
referred to as a positive net charge) - ANSWER-then the objects will repel each other.
If two objects both have more of the negative type of charge than positive (referred to as
a negative net charge) - ANSWER-then the objects will repel each other.
If one object has more of the positive type of charge and one object has more of the
negative type of charge - ANSWER-then the objects will attract each other.
If a material gains electrons - ANSWER-the material will have a negative net charge
If a material loses electrons - ANSWER-the material will have a positive net charge.
A particular object has a net 6.5 μC of the positive type of charge. Were electrons
transferred onto or off the object? How many? - ANSWER-Since the object has extra of
the positive type of charge, electrons must have been transferred off the material. Since
electrons carry the negative type of charge, removing electrons would have left behind
more positive than negative.
The total amount of charge moved off the material was 6.5 μC. Each electron moved off
carries 1.6x10-19 C. Convert μC into C and divide.
6.5x10-6 C / 1.6x10-19 C = 4.06x1013 electrons were removed.
Solve for the number of charge carrier particles (protons or electrons) transferred on or
off the object. - ANSWER-The amount of charge carried by one electron is 1.6x10-19 C.
Divide the total charge by the charge of one electron to solve for the number of
electrons.
(3.5x10-6 C) / (1.6x10-19 C) = 2.25x1013 electrons were transferred off the object.
conductors - ANSWER-Materials that allow electrons to move easily through them.
Metals are good conductors
insulators - ANSWER-Materials that do not allow electrons to move freely through them.
Nonmetals are good insulators
Semiconductors - ANSWER-are a class of materials that, as you might expect, share
properties of both.
Conduction - ANSWER-is the movement of charge from one object to another. It is easy
to move charge from metal to metal (conductor to conductor) but much more difficult to
move charge from insulator to insulator.
,The transfer of charge caused by rubbing two objects against each other is a special
type of conduction called triboelectric charging.
An electroscope is a device that has small gold foil strips suspended from a metal rod. If
a negatively charged rod is brought near the top of a neutral electroscope, the gold foil
strips will separate. (See below.) Explain why this happens. - ANSWER-The negative
charge on the rod repels the negative charge already present in the electroscope. Since
the electroscope is made from metals, electrons will be pushed down into the gold
strips. The electroscope is now polarized (the top is more positive, and the bottom is
more negative). However, the relocated negative charge now in the thin gold strips will
repel each other causing the gold strips to separate.
Induction - ANSWER-is a charge transfer process that involves polarizing an object and
then allowing charge to transfer to another object through conduction. While still under
the influence of the dipole, the charge transfer mechanism is removed (the two objects
are moved apart). The two objects are now charged.
Polarization - ANSWER-is a redistribution of charge rather than a transfer of charge; the
total amount of charge on the polarized object does not change.
static electric force - ANSWER-Proportional to the quantity of net charge on both
objects, and proportional to the inverse-square of the distance between the two charged
objects.
In other words:
Fe α q1q2 / r2
Where Fe is the force, q1 and q2 are the net charges on the object, and r is the distance
between the two objects.
The distance d between two positively charged particles is measured. The distance
between the two particles is increased to 2d. If the original electrostatic force between
the objects was F, what is the new force? - ANSWER-Force has an inverse-squared
relationship with distance. If the distance is doubled, then the force will change by a
factor of (1/2)^2 = ¼. In other words, the new force will be four times smaller.
Since the two particles are both positive, the force will be a repelling force.
Fe = kq1q2/r2 - ANSWER-This constant is called the Coulomb's law constant, and has
a value of 9x10^9 N-m2/C2.
What is the static electric force between a -3.0 μC charge and a +6 μC charge if they
are 2.0 cm apart? - ANSWER-F = kqq/r^2
F = (9x10^9 N-m2/C2)(3x10^6 C)(6x10^6 C) / (0.02 m)^2
F = 405 N
What if you have more than two charged particles? - ANSWER-Sketch the system if a
drawing is not provided.
Draw and label a free body diagram.
, Calculate all the forces.
Solve for any x and y force components.
Add all the x direction forces together, and then add all the y direction forces together.
Use the Pythagorean theorem to combine the components into their vector form.
Use arctangent to solve for the direction of the final vector form.
Particle A has a net charge of +4 mC, and is located at coordinate position (0, 2) m.
Particle B has a net charge of -2 mC, and is located at coordinate position (0, 0) m.
Particle C has a net charge of +6 mC, and is located at position (3, 0) m.
What is the net charge on particle B? Ignore any gravity effects. - ANSWER-F = kqq/r2F
a on b = (9x109 N-m2/C2)(4x10-3 C)(2x10-3 C) / (2 m)2F
a on b = 18,000 N
F c on b = (9x109 N-m2/C2)(6x10-3 C)(2x10-3 C) / (3 m)2F
c on b = 12,000 N
Since these forces don't have any angles, you don't need to solve for their x and y
components. Solve for the net force and the direction of the net force.
ΣF2 = Fy2 + Fx2
ΣF2 = (18,000 N)2 + (12,000 N)2
ΣF = 21,633 N
tanθ = (18,000 N) / (12,000 N)
θ = tan-1 (1.5)
θ = 56.3°
A neutral pith ball (a small Styrofoam ball) coated with graphite is suspended between a
negatively charged metal sphere and a neutral metal sphere. The pith ball will bounce
back and forth between the two metal spheres for a while until it eventually comes to a
stop suspended midway between the two. Explain this behavior. - ANSWER-The
negatively charged sphere will polarize the pith ball pushing the negative charge to the
left side of the pith ball. Coulomb's law shows the attractive static force between the
negative sphere and the closer positive side of the pith ball will be larger than the
repelling force between the negative sphere and the further away negative side of the
pith ball. The pith ball will move into contact with the negative sphere.
Once the pith ball is in physical contact with the metal sphere, negative charge will
conduct onto the pith ball giving the pith ball a net negative charge. The negatively
charged sphere and the negatively charged pith ball will now repel.
The pith ball will repel into the neutral sphere, and will conduct its excess negative
charge onto the neutral sphere. The pith ball will now be neutrally charged, and the
process begins again.
Unless the neutral sphere is grounded, the negative charge between the two metal
spheres will eventually balance, and the pith ball will experience the same static electric
forces from both sides causing it to remain in equilibrium in the middle between the two.
Explain why a charged balloon will electrostatically stick to a neutral ceiling. - ANSWER-
The balloon will polarize the ceiling. If the balloon is negatively charged, then the ceiling
will polarize such that the positive pole of the ceiling is closer to the balloon than the
