Q: A proton is released from rest at the midpoint between two parallel plates separated by $$2.0\ \mathrm{mm}$$ and charged to $$\pm 1.0\ \mathrm{nC}. $$What is the change in electric potential energy $$\Delta U_{\text{elec}} of $$the proton as it moves to the negative plate?

$$\Delta U_{\text{elec}} is $$negative because the proton moves in the direction of the electric field.

Q: A portable defibrillator moves $$-145\ \mu\mathrm{C} of $$electrons through a potential difference of $$2.3\ \mathrm{kV}. $$What is the change in energy of the charge?

$$\Delta U = q \Delta V = (-1.45 \times 10$$^{-4}$$\ \mathrm{C})(2.3 \times 10^3$\ \mathrm{V})$$

Question 1

A proton is released from rest at the midpoint between two parallel plates separated by $$2.0\ \mathrm{mm}$$ and charged to $$\pm 1.0\ \mathrm{nC}. $$What is the change in electric potential energy $$\Delta U_{	ext{elec}} of $$the proton as it moves to the negative plate?

Accepted Answer

$$\Delta U_{	ext{elec}} is $$negative because the proton moves in the direction of the electric field.

Question 2

Which of the following best describes the work done by the electric field when a negative charge is moved perpendicular to a uniform electric field?

Accepted Answer

The electric field does zero work.

Question 3

A water molecule with a dipole moment $$p = 6.2 	imes 10$$^{-30}$$\ \mathrm{C\cdot m} is $$aligned with an electric field $$E = 1.0 	imes 10^7$\ \mathrm{N/C}. $$How much energy is required to rotate the molecule by $$90^\circ$$?

Accepted Answer

$$\Delta U = pE(1 - 0)$$

Question 4

Which statement about the electric potential $$V$$ and electric field $$E of a $$point charge is correct?

Accepted Answer

$$V \propto 1/r$$ and $$E \propto 1/r^2$

Question 5

A portable defibrillator moves $$-145\ \mu\mathrm{C} of $$electrons through a potential difference of $$2.3\ \mathrm{kV}. $$What is the change in energy of the charge?

Accepted Answer

$$\Delta U = q \Delta V = (-1.45 	imes 10$$^{-4}$$\ \mathrm{C})(2.3 	imes 10^3$\ \mathrm{V})$$

Question 6

Which of the following is true about equipotential surfaces?

Accepted Answer

The electric field is perpendicular to equipotential surfaces.

Question 7

For two like charges, what happens to the system's electric potential energy as the separation increases?

Accepted Answer

It decreases but remains positive.

Question 8

A proton is fired from far away at a glass sphere of radius $$R$$ and charge $$+Q. $$What is the minimum initial speed $$v_i$$ required for the proton to just reach the surface?

Accepted Answer

$$\frac{1}{2}mv_i^2$ = \frac{kQe}{R}$$

Question 9

Which of the following is a correct statement about the zero of electric potential energy?

Accepted Answer

Only changes in potential energy $$\Delta U$$ have physical consequences.

Question 10

A negative charge is moved from a point of higher potential to a point of lower potential. What happens to its electric potential energy?

Accepted Answer

It increases.

Question 11

What is the relationship between the work $$W$$ done by you to assemble a system of point charges and the change in electric potential energy $$\Delta U$$?

Accepted Answer

$$W = \Delta U$$

Question 12

A proton is released from rest at point B, where the potential is $$0\ \mathrm{V}. $$What happens to the proton?

Accepted Answer

It moves toward A with increasing speed.

Question 13

Which of the following is true about the electric potential $$V in a $$region of space?

Accepted Answer

It is a scalar quantity determined by the source charges.

Question 14

What is the electric field strength $$E$$ between the paddles of a defibrillator if the potential difference is $$2.3\ \mathrm{kV}$$ and the separation is $$25.4\ \mathrm{cm}$$?

Accepted Answer

$$E = \frac{2.3 	imes 10^3$\ \mathrm{V}}{0.254\ \mathrm{m}}$$

Question 15

Which of the following is NOT a property of equipotential surfaces?

Accepted Answer

The electric field is zero everywhere on an equipotential surface.

Question 16

A system consists of three point charges: $$q_1 = 1$$\ \mu\mathrm{C}$$, q_2$ = 2\ \mu\mathrm{C}$$, $$q_3 = 3$$\ \mu\mathrm{C}$$, each separated by 5$$\ \mathrm{m}$$. What is the total work required to assemble the system?

Accepted Answer

$$W_{	ext{total}} = 19.8\ \mathrm{mJ}$$

Question 17

What is the potential energy $$U_{	ext{elec}} of $$two point charges $$q_1$$ and $$q_2$$ separated by a distance $$r$$?

Accepted Answer

$$U_{	ext{elec}} = \frac{k q_1$ q_2$}{r}$$

Question 18

A positive charge moves through a negative potential difference $$\Delta V \u003c 0. $$What happens to its kinetic energy?

Accepted Answer

It increases.

Question 19

Which of the following best describes the energy of a bound system of two opposite charges?

Accepted Answer

The total energy is negative and increases (approaches zero) with separation.

Chapter 25: Electric Potential Energy – Structured Study Notes

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