Chapter 17- Electric potential Flashcards
Why can we define potential energy for electrostatic force?
The electrostatic force between any two charges (Eq. 16–1, F = kQ1 Q2/r^2) is conservative because the dependence on position is just like the gravitational force , which is conservative.
What happens when p.e decreases? KE?
the potential energy decreases (∆pe is negative); and as the charged particle accelerates from point a to point b, the particle’s kinetic energy KE increases—by an equal amount
Suppose a negative charge, such as an electron, is placed near the negative plate in Fig. 17–1, at point b, shown here in Fig. 17–2. If the electron is free to move, will its electric potential energy increase or decrease? How will the electric potential change?
An electron released at point b will be attracted to the positive plate. As the electron accelerates toward the positive plate, its kinetic energy increases, so its potential energy decreases: pea 6 peb and ¢pe = pea - peb 6 0. But note that the electron moves from point b at low potential to point a at higher potential: ¢V = Va - Vb 7 0. (Potentials Va and Vb are due to the charges on the plates, not due to the electron.) The signs of ¢pe and ¢V are opposite because of the negative charge of the electron.
What is the relationship between E and V?
it is possible to show that the electric field in a given direction at any point in space is equal to the rate at which the electric potential decreases over distance in that direction.
What is a capacitor?
device that can store electric charge, and normally consists of two conducting objects (usually plates or sheets) placed near each other but not touching.
A parallel-plate capacitor carries charge Q and is then disconnected from a battery. The two plates are initially separated by a distance d. Suppose the plates are pulled apart until the separation is 2d. How has the energy stored in this capacitor changed?
If we increase the plate separation d, we decrease the capacitance, C = EoA/d, by a factor of 2. The charge Q hasn’t
changed. So according to Eq. 17–10, where we choose the form pe = 1 Q2C 2
because we know Q is the same and C has been halved, the reduced C means the PE stored increases by a factor of 2.
