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# 12.10: Electricity Problem Set

Difficulty Level: At Grade Created by: CK-12
1. After sliding your feet across the rug, you touch the sink faucet and get shocked. Explain what is happening.
2. What is the net charge of the universe? Of your toaster?
3. As you slide your feet along the carpet, you pick up a net charge of +4mC\begin{align*}+4 \;\mathrm{mC}\end{align*}. Which of the following is true?
1. You have an excess of 2.5×1016\begin{align*}2.5 \times 10^{16}\end{align*} electrons
2. You have an excess of 2.5×1019\begin{align*}2.5 \times 10^{19}\end{align*} electrons
3. You have an excess of 2.5×1016\begin{align*}2.5 \times 10^{16}\end{align*} protons
4. You have an excess of 2.5×1019\begin{align*}2.5 \times 10^{19}\end{align*} protons
4. You rub a glass rod with a piece of fur. If the rod now has a charge of 0.6 μC\begin{align*}-0.6\ \mu C\end{align*}, how many electrons have been added to the rod?
1. 3.75×1018\begin{align*}3.75 \times 10^{18}\end{align*}
2. 3.75×1012\begin{align*}3.75 \times 10^{12}\end{align*}
3. 6000\begin{align*}6000\end{align*}
4. 6.00×1012\begin{align*}6.00 \times 10^{12}\end{align*}
5. Not enough information
5. What is the direction of the electric field if an electron initially at rest begins to move in the North direction as a result of the field?
1. North
2. East
3. West
4. South
5. Not enough information

6. Two metal plates have gained excess electrons in differing amounts through the application of rabbit fur. The arrows indicate the direction of the electric field which has resulted. Three electric potential lines, labeled A,B,\begin{align*}A, B,\end{align*} and C\begin{align*}C\end{align*} are shown. Order them from the greatest electric potential to the least.
1. A,B,C\begin{align*}A, B, C\end{align*}
2. C,B,A\begin{align*}C, B, A\end{align*}
3. B,A,C\begin{align*}B, A, C\end{align*}
4. B,C,A\begin{align*}B, C, A\end{align*}
5. A=B=C\begin{align*}A = B = C \ldots\end{align*} they are all at the same potential

7. The diagram to the right shows a negatively charged electron. Order the electric potential lines from greatest to least.
1. A,B,C\begin{align*}A, B, C\end{align*}
2. C,B,A\begin{align*}C, B, A\end{align*}
3. B,A,C\begin{align*}B, A, C\end{align*}
4. B,C,A\begin{align*}B, C, A\end{align*}
5. A=B=C\begin{align*}A = B = C \ldots\end{align*} they are all at the same electric potential
8. The three arrows shown here represent the magnitudes of the electric field and the directions at the tail end of each arrow. Consider the distribution of charges which would lead to this arrangement of electric fields. Which of the following is most likely to be the case here?
1. A positive charge is located at point A\begin{align*}A\end{align*}
2. A negative charge is located at point B\begin{align*}B\end{align*}
3. A positive charge is located at point B\begin{align*}B\end{align*} and a negative charge is located at point C\begin{align*}C\end{align*}
4. A positive charge is located at point A\begin{align*}A\end{align*} and a negative charge is located at point C\begin{align*}C\end{align*}
5. Both answers a) and b) are possible
9. Particles A\begin{align*}A\end{align*} and B\begin{align*}B\end{align*} are both positively charged. The arrows shown indicate the direction of the forces acting on them due to an applied electric field (not shown in the picture). For each, draw in the electric field lines that would best match the observed force.
10. To the right are the electric potential lines for a certain arrangement of charges. Draw the direction of the electric field for all the black dots.
11. A suspended pith ball possessing +10 μC\begin{align*}+10 \ \mu \mathrm{C}\end{align*} of charge is placed 0.02m\begin{align*}0.02 \;\mathrm{m}\end{align*} away from a metal plate possessing 6 μC\begin{align*}-6 \ \mu \mathrm{C}\end{align*} of charge.
1. Are these objects attracted or repulsed?
2. What is the force on the negatively charged object?
3. What is the force on the positively charged object?
12. Calculate the electric field a distance of 4.0mm\begin{align*}4.0 \;\mathrm{mm}\end{align*} away from a 2.0 μC\begin{align*}-2.0 \ \mu \mathrm{C}\end{align*} charge. Then, calculate the force on a 8.0 μC\begin{align*}-8.0 \ \mu \mathrm{C}\end{align*} charge placed at this point.
13. Consider the hydrogen atom. Does the electron orbit the proton due to the force of gravity or the electric force? Calculate both forces and compare them. (You may need to look up the properties of the hydrogen atom to complete this problem.)
14. As a great magic trick, you will float your little sister in the air using the force of opposing electric charges. If your sister has 40kg\begin{align*}40 \;\mathrm{kg}\end{align*} of mass and you wish to float her 0.5m\begin{align*}0.5 \;\mathrm{m}\end{align*} in the air, how much charge do you need to deposit both on her and on a metal plate directly below her? Assume an equal amount of charge on both the plate and your sister.
15. Copy the arrangement of charges below. Draw the electric field from the 2C\begin{align*}-2 \;\mathrm{C}\end{align*} charge in one color and the electric field from the +2C\begin{align*}+2 \;\mathrm{C}\end{align*} charge in a different color. Be sure to indicate the directions with arrows. Now take the individual electric field vectors, add them together, and draw the resultant vector. This is the electric field created by the two charges together.
16. A proton traveling to the right moves in between the two large plates. A vertical electric field, pointing downwards with magnitude 3.0N/C\begin{align*}3.0 \;\mathrm{N/C}\end{align*}, is produced by the plates.
1. What is the direction of the force on the proton?
2. Draw the electric field lines on the diagram.
3. If the electric field is 3.0N/C\begin{align*}3.0 \;\mathrm{N/C}\end{align*}, what is the acceleration of the proton in the region of the plates?
4. Pretend the force of gravity doesnot exist; then sketch the path of the proton.
5. We take this whole setup to another planet. If the proton travels straight through the apparatus without deflecting, what is the acceleration of gravity on this planet?
17. A molecule shown by the square object shown below contains an excess of 100\begin{align*}100\end{align*} electrons. (a) What is the direction of the electric field at point A, 2.0×109m\begin{align*}2.0 \times 10^{-9} \;\mathrm{m}\end{align*} away? (b) What is the value of the electric field at point A\begin{align*}A\end{align*}? (c) A molecule of charge 8.0 μC\begin{align*}8.0 \ \mu \mathrm{C}\end{align*} is placed at point A\begin{align*}A\end{align*}. What are the magnitude and direction of the force acting on this molecule?
18. Two negatively charged spheres (one with 12 μC\begin{align*}-12 \ \mu \mathrm{C}\end{align*}; the other with 3 μC\begin{align*}-3 \ \mu \mathrm{C}\end{align*}) are 3m\begin{align*}3 \;\mathrm{m}\end{align*} apart. Where could you place an electron so that it will be suspended in space between them with zero net force?

For problems 19, 20, and 21 assume \begin{align*}3-\end{align*}significant digit accuracy in all numbers and coordinates. All charges are positive.

1. Find the direction and magnitude of the force on the charge at the origin (see picture). The object at the origin has a charge of \begin{align*}8 \ \mu \mathrm{C}\end{align*}, the object at coordinates \begin{align*}(-2 \;\mathrm{m}, \ 0)\end{align*} has a charge of \begin{align*}12 \ \mu \mathrm{C}\end{align*}, and the object at coordinates \begin{align*}(0, -4 \;\mathrm{m})\end{align*} has a charge of \begin{align*}44 \ \mu \mathrm{C}\end{align*}. All distance units are in meters.
2. A \begin{align*}2 \;\mathrm{C}\end{align*} charge is located at the origin and a \begin{align*}7 \;\mathrm{C}\end{align*} charge is located at \begin{align*}(0, 6 \;\mathrm{m})\end{align*}. Find the electric field at the coordinate \begin{align*}(10 \;\mathrm{m}, 0)\end{align*}. It may help to draw a sketch.
3. A metal sphere with a net charge of \begin{align*}+5\ \mu \mathrm{C}\end{align*} and a mass of \begin{align*}400 \;\mathrm{g}\end{align*} is placed at the origin and held fixed there.
1. Find the electric potential at the coordinate \begin{align*}(6 \;\mathrm{m}, 0)\end{align*}.
2. If another metal sphere of \begin{align*}-3 \ \mu \mathrm{C}\end{align*} charge and mass of \begin{align*}20 \;\mathrm{g}\end{align*} is placed at the coordinate \begin{align*}(6 \;\mathrm{m}, 0)\end{align*} and left free to move, what will its speed be just before it collides with the metal sphere at the origin?

4. Collisions of electrons with the surface of your television set give rise to the images you see. How are the electrons accelerated to high speed? Consider the following: two metal plates (The right hand one has small holes allow electrons to pass through to the surface of the screen.), separated by \begin{align*}30 \;\mathrm{cm}\end{align*}, have a uniform electric field between them of \begin{align*}400 \;\mathrm{N/C}\end{align*}.
1. Find the force on an electron located at a point midway between the plates
2. Find the voltage difference between the two plates
3. Find the change in electric potential energy of the electron when it travels from the back plate to the front plate
4. Find the speed of the electron just before striking the front plate (the screen of your TV)

5. Two pith balls of equal and like charges are repulsed from each other as shown in the figure below. They both have a mass of \begin{align*}2 \;\mathrm{g}\end{align*} and are separated by \begin{align*}30^\circ\end{align*}. One is hanging freely from a \begin{align*}0.5 \;\mathrm{m}\end{align*} string, while the other, also hanging from a \begin{align*}0.5\;\mathrm{m}\end{align*} string, is stuck like putty to the wall.
1. Draw the free body diagram for the hanging pith ball
2. Find the distance between the leftmost pith ball and the wall (this will involve working a geometry problem)
3. Find the tension in the string (Hint: use \begin{align*}y-\end{align*}direction force balance)
4. Find the amount of charge on the pith balls (Hint: use \begin{align*}x-\end{align*}direction force balance)

## Answers to Selected Problems

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11. b. \begin{align*}1350 \;\mathrm{N}\end{align*} c. \begin{align*}1350 \;\mathrm{N}\end{align*}
1. \begin{align*}1.1 \times 10^9 \;\mathrm{N/C}\end{align*}
2. \begin{align*}9000 \;\mathrm{N}\end{align*}
12. \begin{align*}F_g = 1.0 \times 10^{-47} \;\mathrm{N}\end{align*} and \begin{align*}F_e = 2.3 \times 10^{-8} \;\mathrm{N}\end{align*}. The electric force is \begin{align*}39\end{align*} orders of magnitudes bigger.
13. \begin{align*}1.0 \times 10^{-4} C\end{align*}
14. .
15. a. down b. Up \begin{align*}16c, 5.5 \times 10^{11} \;\mathrm{m/s}^2\end{align*} e. \begin{align*}2.9 \times 10^8 \;\mathrm{m/s}^2\end{align*}
1. Toward the object
2. \begin{align*}3.6 \times 10^4 \;\mathrm{N/C}\end{align*} to the left with a force of \begin{align*}2.8 \times 10^{-7} \;\mathrm{N}\end{align*}
16. Twice as close to the smaller charge, so \begin{align*}2 \;\mathrm{m}\end{align*} from \begin{align*}12\mu \mathrm{C}\end{align*} charge and \begin{align*}1 \;\mathrm{m}\end{align*} from \begin{align*}3\mu \mathrm{C}\end{align*} charge.
17. \begin{align*}0.293 \;\mathrm{N}\end{align*} and at \begin{align*}42.5^\circ\end{align*}
18. \begin{align*}624 \;\mathrm{N/C}\end{align*} and at an angle of \begin{align*}-22.4^\circ\end{align*} from the \begin{align*}+x-\end{align*}axis.
1. \begin{align*}7500\mathrm{V}\end{align*}
2. \begin{align*}1.5 \;\mathrm{m/s}\end{align*}
1. \begin{align*}6.4 \times 10^{-17}\;\mathrm{N}\end{align*}
2. \begin{align*}1300\mathrm{V}\end{align*}
3. \begin{align*}2.1 \times 10 ^{-16} \;\mathrm{J}\end{align*}
4. \begin{align*}2.2 \times 10^7 \;\mathrm{m/s}\end{align*}
19. b. \begin{align*}0.25\mathrm{m}\end{align*} c. \begin{align*}F_T = 0.022\;\mathrm{N}\end{align*} d. \begin{align*}0.37 \mu \mathrm{C}\end{align*}

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