Need help preparing for the MCAT physics section? MedSchoolCoach expert, Ken Tao, will teach you about electric potential and electric potential energy in electrostatic force. Watch this video to learn how to do well on the physics section of the MCAT exam!
Electric potential (Φ) is given by the equation below, where k is Coulomb's constant, Q is a charge, and r is the distance from the charge. The gravitational analogy for electric potential is height. Electric potential is higher closer to sources of positive charge, and lower closer to sources of negative charge. If you drop an object from some height, the object will spontaneously fall to a lower height. Similarly, if you place a charge at some electric potential, it will spontaneously move in one direction, although the direction depends on whether a charge is positive or negative.
The unit of electric potential is the volt (V), which is equivalent to a J/C. Next, the equation for finding electric potential energy is found by multiplying electric potential by another charge placed at some radius within an electric field. Therefore, multiplying J/C by C, the unit of electric potential energy is the joule (J).
When an object falls from one height to a lower height, it experiences a change in gravitational potential energy. Similarly, as a charge moves spontaneously through an electric field, it experiences a change in electric potential energy. The change in potential energy (ΔPE) is given by qΔΦ, which can also be written as qV. The term ΔΦ is the change in electric potential, or potential difference, given in units of voltage (V). Remember that when an object is falling, it experiences a negative change in gravitational potential energy. In the same way, an electric charge will move spontaneously if it is experiencing a decrease in potential energy (ΔPE is less than 0). Conversely, if we move a charge manually against the direction it naturally wants to travel, so that it’s not moving spontaneously, it will experience an increase in potential energy (ΔPE is greater than 0).
We’ve discussed the idea that a charge will spontaneously move in the direction in which it will experience a negative , but how do we know what direction this is? Recall that electric potential is higher closer to sources of positive charge (Φ is greater than 0) and lower closer to sources of negative charge (Φ is less than 0).
1. If a positive charge (q is greater than 0) moves from high to low electric potential (ΔΦ is less than 0), PE will be negative. Therefore, positive charges spontaneously move from high to low electric potential, away from other positive charges/towards negative charges.
2. If a negative charge (q is less than 0) moves from low to high electric potential (ΔΦ is greater than 0), PE will be negative. Therefore, negative charges spontaneously move from low to high electric potential, away from other negative charges/towards positive charges.
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