PHYS 245- Exam 1
What is electric field produced by?
Charges.
Which can electrons move freely through, Conductors, or Insulators?
Conductors.
When does potential exist?
Potential always exists, but it only becomes Potential Energy when a charge is present.
What does Polarized mean?
Something is overall neutral, but the +'s are closer to one side, and the -'s are closer to another.
What is potential?
The electric potential energy per unit charge. V= PE/q, PE =qV
How are lines configured in a Field Line Diagram with a positive and a negative?
The lines will leave the positive pole and enter the negative pole.
How can you tell the strength of a charge on a Field Line Diagram?
The number of lines leaving it. Higher density means higher charge.
Two pieces of tape are stuck together. The overall net charge of the two tapes is zero. You then pull the pieces of tape apart. If the charge on one piece of tape is +Q Coulombs, what can be said about the charge on the other piece of tape? a) More information is needed. b)The other tape also has charge +Q. c) The other tape has charge -Q. d) The other tape has a charge of zero.
The other tape has charge -Q. Te two pieces started out with 0 total charge, so the pieces must add up to 0 after being pulled apart.
If you have a positive charge and a negative charge, which direction will the charge flow to?
The positive charge will flow towards the negative charge.
What is the superposition principle?
When more than one wave occupies the same space at the same time, the displacements add at every point
Are electric force and charge directly/inversely proportional?
Yes, more charge, more force.
Is charge discrete?
Yes. All charges are some multiple of the electron/proton charge.
Are neutral objects still attracted to charge?
Yes. The strong attraction between the opposite forces will push through the weak repulsion of the negative charges.
A orange balloon with a negative charge of 4.34*10-6 C and a green balloon with a negative charge of 7.18*10-6 C are separated by 0.17 m. a) What is the magnitude of the force of the green balloon on the orange balloon? b) What is the magnitude of the force of the orange balloon on the green balloon? c) Is the force attractive or repulsive?
You use Coulomb's Law, F=k*((q1*q2)/r^2) to find the two forces, which will be equal to each other. a) 9.704 N b) 9.704 N c) Because both forces are negatively charged they will be repulsive.
Two charged balls are repelling each other as they hang from the ceiling. What can you say about their charged? a) One is positive, the other is negative. b) Both are positive c) Both are negative d) Both are positive or both are negative.
d) Both are positive or both are negative. Same forces repel.
What does it mean for something to be discrete?
Defined for a finite or countable set of values; not continuous.
Punch Points
E points from hi-V to low-V In the direction of E, V < 0
What is Coulomb's Law?
like charges repel, unlike charges attract
Question: Two negative charges q1 and q2 are a distance r apart from each other. If r is increased to 14 times its original amount then the force between them will become (x) times as strong as it was before. What is the value of x?
0.005. The force is inversely proportional to distance between two objects squared. F= 1/r^2
What is the ΔPE of a +1 uC charge that moves from the left plate (+) to the right plate (-)?
-10^-4 J. ΔPE=qΔV ΔPE=q(Vf-Vi)=(10^-6)(0-100)
A rod and a piece of cloth are rubbed together. If the rod acquires a charge of 7*10^-6, the cloth gets a charge of what?
-7.10^-6. C This is because you reverse the charge.
What is the charge of one electron?
1.6 x 10^-19 C
What is the charge of an electron?
1.6*10^-19C
A sphere has a negative charge of 3.04*10^-18C. About how many electrons must be removed to make the sphere neutral?
19. Divide the charge by the charge of an electron, which is constant qe.
Two negative charges q1 and q2 are a distance r apart from each other. If q1 is increased to 20 times its original amount then the force between them will become (x) times as strong as it was before. What is the value of x?
20 times. If force is increasing, exact number, if distance is increased, inversely proportional.
What kind of field do Charged parallel plates cause?
A uniform E-field.
Two parallel conducting plates are connected to a constant voltage source. The magnitude of the electric field between the plates is 1700 N/C. If the voltage is doubled and the distance between the plates is reduced to 1/5 the original distance, what is the magnitude of the new electric field?
E= 17000 N/C The formula you use is Ed=ΔV. You need to rearrange it to be E=ΔV/d. Then, you solve. E=2/(1/5) E=10 10 * 1700= 17000
In which direction does the electric field due to a negative charge face?
Away from the charge.
O An apple is thrown upwards. Air-drag is negligible, so the only force is gravity downward. What can be said about the direction of the gravitational force F on the apple during its flight? A) F points in the direction of increasing PE the whole time B) F points in the direction of decreasing PE the whole time C) F points in the direction of increasing PE, then points in the direction of decreasing PE D) F points in the direction of decreasing PE, then points in the direction of increasing PE E) F points wherever it feels like, because the Earth is big and doesn't take orders from nobody.
B) F points in the direction of decreasing PE the whole time The direction of velocity can be up or down, but the direction of the ACCELERATION of the apple is always down, pointing in the direction of the gravitational force. So force always points down, the direction along which PE would decrease.
What is the unit for charge?
C (Coulomb)
Why is there a blank space in-between two positive charges in a Field Line diagram?
Each charge contributes an equal and opposite vector to the field at the middle, and they cancel, adding to zero.
What is the force per unit charge?
Electric Field.
What does this statement mean: "Each charge produces an electric field of magnitude kq/r2 at each point in space. This field is present even if no charge is there. Once a charge shows up, you have a force"
Electric field magnitude always exists, force only exists when you have a charge in an existing electric field.
A conducting sphere may become charged by losing or gaining what?
Electrons. Only Electrons
What are Field Lines always Right-Angles to?
Equipotential Lines
What is Coulomb's Law?
F=kq1q2/r^2 directly proportional to the charge, inversely proportional to the square of the distance
In which direction do Electric Fields point?
From high-V to low-V
What way does an Electric Field always point?
From high-V to low-V
What is the difference between an Insulator vs a Conductor?
In an insulator, the valence shell has distance from the valance electrons, keeping them inside. In a conductor, the shell is touching the valence electrons, giving them more freedom.
When in a region of constant electric field, what is Energy?
It is the absolute value of the change in Voltage divided by the distance along the field line between the two points in question.
A solid conducting sphere is given a positive charge. How is the charge distributed in or on the sphere?
It is uniformly distributed on the surface of the sphere only.
What is contact charging?
Objects come into direct contact with each other, allowing the transfer of electrons from one object to another.
What does it mean if q is +?
PE is higher at A than B F is to the right
What does it mean if q is -?
PE is lower at A than B. F is to the left
Consider a spherical conductor sitting 3m from a charge of 9C. What is the electric field at the center of the sphere?
The correct answer is: b) The electric field in a conducting material is zero in electrostatic situations. In electrostatic situations, a conductor in equilibrium redistributes its charges such that the electric field inside the conductor is zero. This is known as electrostatic shielding. Charges will redistribute themselves on the surface of the conductor in such a way that the net electric field inside the conductor becomes zero. In this scenario, since the spherical conductor is in equilibrium and there are no other external factors mentioned, the electric field at the center of the sphere will indeed be zero. Therefore, option (b) is the correct choice.
Which of the following is true about equipotential surfaces/lines?
The electric field is always zero along an equipotential surface/line The electric field is always parallel to an equipotential surface/line The electric field is always perpendicular to an equipotential surface/line
What does it mean for a Field Line Diagram when field lines are closer together?
The field is stronger, as they are more dense.
What is the inverse square law?
The intensity of radiation is inversely proportional to the square of the distance from the source of radiation. I= K/r^2. r= distance.
1. Two neutral conductors are brought near each other. 2. They are connected by a wire 3. A positively charged rod is brought near the left charge, but does not touch. 4. The wire is removed. 5. The charged rod is removed. What are the charges on the conductors now?
The left charge conductor is negative, and the right charge conductor is positive. In step 3, electrons are pulled from magenta to cyan (right to left) through the wire towards the positive rod. When the wire is removed, the electrons are stranded on the cyan, and this is true even if the rod is removed.
Which principle do Electric forces/fields follow?
The superposition principle.
Where does excess charge go?
The surface of a conductor.
What is induction charging?
The use of electromagnetic fields to charge an object without direct contact.
What is electric potential negative of?
The work of the field as a charge moves between two points.
What ways do electric fields point?
They always point in the direction of decreasing potential.
How does electric force get weaker the farther apart the two charges are?
They operate on the inverse-square law. If they move 3 times farther apart, force gets 3^2=9 times weaker.
What is the Conservation of Charge?
Total charge of the universe is conserved. Like energy, Charge can move from one place to another, but isn't created or destroyed.
In which direction does the electric field due to a positive charge face?
Towards the charge.
The contour lines in the image below are close together on the left side of the image and farther apart on the right side. What do you think contour line spacing tells you? Which of the following apply? a) Farther spaced lines represent flatter (less steep) parts of the hill. b) Closely spaced lines represent flatter (less steep) parts of the hill. c) Line spacing provides no information about the slope/steepness of the hill. d) Farther spaced lines represent steeper parts of the hill. e) Closely spaced lines represent steeper parts of the hill.
a and e a) Farther spaced lines represent flatter (less steep) parts of the hill. e) Closely spaced lines represent steeper parts of the hill.
What is a dipole?
a molecule that has two poles with opposite charges
Two large, flat, parallel, conducting plates are 0.04 m apart, as shown above. The lower plate is at a potential of 2 V with respect to ground. The upper plate is at a potential of 10 V with respect to ground. Point P is located 0.01 m above the lower plate. a) What is the electric potential at point p? b) The magnitude of the electric field at point P?
a) 4 b) 200 V/m
a) What is the magnitude of the electric field at a distance 56 centimeters from a point charge of 8.2 μC? Remember to convert units. b) What magnitude of force would an electron feel at that point?
a) E=K*(C/r^2) b) F=qE
When a copper strip becomes negatively charged, it: a) gains electrons b) loses electrons c) gains protons d) loses protons
a) gains electrons Electrons have a negative charge, gaining more means the charge goes down.
Two objects on a horizontal frictionless surface each have a charge +Q and each are fixed in place on the x-axis at the same distance d from the origin as shown in the figure above. A particle of charge constrained to move along the y-axis is released from rest. After release the particle will: a) stay where it is b) exhibit oscillatory motion c) move in the direction of decreasing and keep going till negative infinity d) move in the direction of increasing e) move in direction of decreasing and stop at the origin
b)exhibit oscillatory motion. The force will push it up until it reaches the bounds of its strength, at which point gravity will pull it back down until the strength of the electric force is enough that it pushes it back up.
If a negatively charged rod is held next to one end of a neutral, insulated nail, (but does NOT contact it), what happens? a. the far end of the nail becomes positively charged b. the far end of the nail becomes negatively charged c. the entire nail becomes negatively charged d. the entire nail becomes positively charged
b. the far end of the nail becomes negatively charged. This is because the positive charge will be drawn to the negatively charged rod, while the negatively charge will be pushed to the end of the nail.
The diagram above shows electric field lines in an isolated region of space containing two small charged spheres, Y and Z. Which of the following statements is true? a) Both charged spheres and carry charge of the same sign. b) The charge on is negative and the charge on is positive. c) A small negatively charged object placed at point would tend to move toward the right. d) The electric field is strongest midway between Y and Z. e) The strength of the electric field is the same everywhere.
c) A small negatively charged object placed at point would tend to move toward the right.
An apple is thrown upwards. Air-drag is negligible, so the only force is gravity downward. What can be said about the gravitational potential energy of the apple during its flight? a) PE increases the whole time b) PE decreases the whole time c) PE increases then decreases d) PE decreases then increases e) PE doesn't change due to energy.
c) PE increases then decreases. As the apple moves upward, height y increases, so PE=mgh increases. The apple reaches the top of its trajectory and then starts to fall, so now PE=mgh decreases.
A metal ball hangs from the ceiling by an insulating thread. The ball is attracted to a positive-charged rod held near the ball. The charge of the ball must be: a) positive b) negative c) neutral d) positive or neutral e) negative or neutral
c) negative or neutral.
What is the potential energy per unit charge?
electrical potential
What is Coulomb's Constant represented by?
k
What is Coulomb's Constant?
k= 8.99x10^9 Nm^2/C^2
How do you find the direction of the total force if you have two vectors?
tan^-1(F_y/F_x)
