Exam 1 Problems
Three identical metal balls A, B, and C are mounted on insulating rods. Ball A has a charge +q, and balls B and C are initially uncharged. Ball A is touched first to ball B and then Ball A is separately touched to ball C. At the end of the experiment, what is the charge on A?
+q/4
Two objects are supplied with equal amounts of heat. They don't end up with the same temperature. Which of the following statements is a possible explanation why?
- the objects have the same mass, but different specific heat capacities - vice versa
What is the magnitude and direction of the electric field at a point located 0.11mm directly below an electron?
0.12 N/C upward
You are sitting a certain distance from a point charge, and you measure an electric field of E0. If the charge is doubled and your distance from the charge is also doubled, what is the electric field strength now?
1/2 E0
The force between two charges separated by a distance d is F. If the charges are pulled apart to a distance 3d, what is the force on each charge?
1/9F
What is the change in electric potential in moving from point A to point B? (see image)
108 V
What is the electric potential at point A due to the two charges in the figure shown (assuming that V=0 at infinity)?
180V
In a uniform electric field in empty space, a 4C charge is placed and it feels an electric force of 12N. If this charge is removed and a 6C charge is placed at that point instead, what force will it feel?
18N
What is the total electric potential energy for the three-charge system shown?
2.1 x 10⁻⁸ J
A metal plate that is 10cm by 10cm contains a total charge of 4.42 nC uniformly distributed on the surface. What is the value of the electric field 4cm from the surface?
2.5 x 10⁴ N/C
A charge Q generates an electric field that has a magnitude E at a distance r from the charge. If the charge is cut in half and the distance is also cut in half, what is the magnitude of the electric field at the new field point?
2E
What is the magnitude of the force F2, according to the diagram? If we increase one charge to 4Q, what is the magnitude of F1?
3.0 N 12 N
The rails on a railroad track are made of steel and are 10m long. If the max gap between the rails is 1.0cm, what is the max temp change the rails can undergo without the rails beginning to buckle? coefficient of thermal expansion for steel is 12x10⁻⁶ C⁻¹
83C
A charge Q generates an electric field that has a magnitude E at a distance r from the charge. If charge is changed to 2Q, what is the magnitude of the electric field at a point a distance 4r from the charge?
E/8
A particle of charge q, exerts a force F on charge q2. The charges are separated by a distance r. If the magnitudes of both charges are cut in half, and the distance between the charges is also cut in half, what is the new force on charge q2?
F
Two containers A and B are filled with gas with the same volume and the same pressure. If there are twice as many molecule of gas in container B as in container A, how are the absolute temperatures of the gases related?
Ta = 2Tb
What is the direction of the electric field at the center of the square?
a
In the diagram shown, the potential difference across the entire circuit is 20V. a) Find the equivalent capacitance of this circuit b) Find the charge on each capacitor
answer for B: - 10pF → 100pC - 1pF → 10pC - 36pF → 90pC - 12pF → 90pC
When the temperature of a metal ring increases, the hole in the ring gets:
bigger
A proton and an electron are in a constant electric field created by oppositely charged plates. You release the proton from the positive side and the electron from the negative side. When it strikes the opposite plate, which one has more KE?
both acquire the same KE Since U=qEd and the proton and electron have the same charge in magnitude, they both have the same change in electric potential energy. Because energy is conserved, they both must have the same kinetic energy after they reach the opposite plate.
Two neutral conductors are connected by a wire. A negatively charged rod is brought near one of the conductors and touches it. The rod is moved away and the wire is removed. What are the charges on the conductors?
both conductors are negatively charged
What is the direction of the electric field at the center of the diamond? Assume all sides have equal length
c
What is the electric field at the center of the square?
c
Two charges are fixed along the x-axis. They produce an electric field E directed along the negative y-axis at the indicated point. What is the true about the charges?
charges are equal and negative
Which of the arrows best represents the direction of the net force on charge -3Q due to the other two charges?
d
What is the electric field at the center of the square?
e
Which of the arrows best represents the direction of the net force on charge +3Q due to the other two charges?
e
A proton and an electron are in a constant electric field created by oppositely charged plates You release the proton from the positive side and the electron from the negative side. Which has the larger acceleration?
electron Since F=ma and the electron is much less massive than the proton, the electron experiences the larger acceleration
A proton and an electron are held apart a distance of 1m and then released. Which particle has the larger acceleration at any one moment?
electron; smaller mass, larger acceleration! (F=ma)
A proton and an electron are held apart a distance of 1m and then released. As they approach each other, what happens to the force between them?
it gets bigger
When a charge is moved along an equipotential line, what happens to the potential energy?
it remains constant
Two neutrally-charged metal spheres A and B are brought into contact. A negatively charged plastic rod is brought near to, but doesn't touch, sphere A. The spheres are then separated and the rod is removed. What is the charge of sphere B?
negative
A metal sphere is grounded through a switch, and a positively charged balloon is brought near it. The switch is opened and the balloon is taken away. What is the charge of the sphere?
negatively charged
Two neutral conductors are connected by a wire. A positively charged rod is brought near one of the conductors but doesn't touch it. The wire is removed and then the rod is moved away. What are the charges on the conductors?
one conductor is negative and the other is positive
An electron is placed at the center of the gap between the two charged plates and a net force causes it to move to the right. Which plate is the positively charged plate?
right
Two drinking glasses are stuck, one inside the other. Which of the following methods could help you get them unstuck?
run hot water over the outer glass only
A parallel-plate capacitor initially has a voltage of 24V and stays connected to the battery. If the spacing between the plates is then halved, what happens?
the charge on the plates increases by a factor of 2
The electric field between two charged parallel plates is shown. An electron is placed at the center of the gap between the plates and released. What is true about the electron and its potential energy?
the electron moves to the left and its potential energy decreases
The value of a capacitor depends on?
the geometry of the capacitor
You may notice that if a mercury-in-glass thermometer is inserted into a hot liquid, the mercury column first drops, and then later starts to rise (as you expect) How do you explain this drop?
the glass expands before the mercury expands
Two different liquids with the same mass, initially at the same temperature, are heated for the same time over the same stove. You measure the temperatures and find that one liquid has a higher temperature than the other. Which liquid has a lower specific heat?
the hotter one
You take heat away from a liquid and find that the temp doesn't change. What can you conclude?
the temp of the liquid is at the melting point
A proton and an electron are in a constant electric field created by oppositely charged plates You release the proton from the positive side and the electron from the negative side. Which feels the larger electric force?
they feel the same magnitude force but opposite direction Since F=qE and the proton and electron have the same charge in magnitude, they both experience the same force. However, the forces point in opposite directions because the proton and electron are oppositely charged.
You are stranded on a desert highway when your radiator loses all of its water. You are carrying the following 3 liquids with you in your car: mercury (c=140 J/kgC), vegetable oil (c=2000 J/kgC), and carbon tetrachloride (c=861 J/kgC). Which liquid would work the best to keep your engine cool? Why?
vegetable oil; has largest specific heat
Two balls with charges +Q and +4Q are fixed at a separation distance of 3R. Is it possible to place another charged ball Q0 on the line between the two charges such that the net force on Q0 will be zero?
yes, independent of the sign (or value) of Q0
A positively charged particle is brought near a neutron. What is the nature of the electric force between the two particles?
zero
