Ch 25 – Electric Potential A difference in electrical potential between the upper atmosphere and the ground can cause electrical discharge (motion of charge).

Ch 25 – Electric Potential So far, we’ve discussed electric force and fields. Now, we associate a potential energy function with electric force. This is identical to what we did with gravity last semester. gravity electricity ?

Ch 25.1 – Electric Potential and Potential Difference Place a test charge, q0, into an E-field. The charge will experience a force: This force is a conservative force. Pretend an external agent does work to move the charge through the E-field. The work done by the external agent equals at least the negative of the work done by the E-field.

Ch 25.1 – Electric Potential and Potential Difference Let’s introduce a new symbol: We’re talking about moving charges through some displacement. The “ds” vector is a little tiny step of displacement along a charge’s path.

Ch 25.1 – Electric Potential and Potential Difference If q0 moves through the E-field by a little step ds, the E-field does some work: As the E-field performs this work, we say that the potential energy of the charge-field system changes by this amount. This is the basis for our definition of the potential energy function.

Ch 25.1 – Electric Potential and Potential Difference If q0 moves through the E-field by a little step ds, the E-field does some work:

Ch 25.1 – Electric Potential and Potential Difference The change in electrical potential energy of a charge-field system as the charge moves from A to B in the field. The integral accounts for the motion of the charge through a 1-D path. It’s called a “path” or “line” integral.

Ch 25.1 – Electric Potential and Potential Difference The change in electrical potential energy of a charge-field system as the charge moves from A to B in the field. Because electric force is conservative, the value of the integral does not depend on the path taken between A and B.

Ch 25.1 – Electric Potential and Potential Difference Potential Energy refresher: Potential Energy measures the energy a system has due to it’s configuration. We always care about changes in potential energy – not the instantaneous value of the PE. The zero-point for PE is relative. You get to choose what configuration of the system corresponds to PE = 0. The change in electrical potential energy of a charge-field system as the charge moves from A to B in the field.

Ch 25.1 – Electric Potential and Potential Difference What we’re about to do is different than anything you saw in gravitation. In electricity, we choose to divide q0 out of the equation. We call this new function, ΔV, the “electric potential difference.”

Ch 25.1 – Electric Potential and Potential Difference Potential difference between two points in an Electric Field. This physical quantity only depends on the electric field. Potential Difference – the change in potential energy per unit charge between two points in an electric field. Units: Volts, [V] = [J/C]

Ch 25.1 – Electric Potential and Potential Difference Potential difference between two points in an Electric Field. Do not confuse “potential difference” with a change in “electric potential energy.” A potential difference can exist in an E-field regardless the presence of a test charge. A change in electric potential energy can only occur if a test charge actually moves through the E-field.

Ch 25.1 – Electric Potential and Potential Difference Pretend an external agent moves a charge, q, from A to B without changing its speed. Then: But:

Ch 25.1 – Electric Potential and Potential Difference Units of the potential difference are Volts: 1 J of work must be done to move 1 C of charge through a potential difference of 1 V.

Ch 25.1 – Electric Potential and Potential Difference We now redefine the units of the electric field in terms of volts. E-field units in terms of volts per meter

Ch 25.1 – Electric Potential and Potential Difference Another useful unit (in atomic physics) is the electron-volt. One electron-volt is the energy required to move one electron worth of charge through a potential difference of 1 volt. If a 1 volt potential difference accelerates an electron, the electron acquires 1 electron-volt worth of kinetic energy. The electron-volt

Quick Quiz 25.1 Points A and B are located in a region where there is an electric field. How would you describe the potential difference between A and B? Is it negative, positive or zero? Pretend you move a negative charge from A to B. How does the potential energy of the system change? Is it negative, positive or zero?

Ch 25.2 – Potential Difference in a Uniform E-Field Let’s calculate the potential difference between A and B separated by a distance d. Assume the E-field is uniform, and the path, s, between A and B is parallel to the field.

Ch 25.2 – Potential Difference in a Uniform E-Field Let’s calculate the potential difference between A and B separated by a distance d. Assume the E-field is uniform, and the displacement, s, between A and B is parallel to the field. 1

Ch 25.2 – Potential Difference in a Uniform E-Field The negative sign tells you the potential at B is lower than the potential at A. VB < VA Electric field lines always point in the direction of decreasing electric potential.

Ch 25.2 – Potential Difference in a Uniform E-Field Now, pretend a charge q0 moves from A to B. The change in the charge-field PE is: If q0 is a positive charge, then ΔU is negative. When a positive charge moves down field, the charge-field system loses potential energy.

Ch 25.2 – Potential Difference in a Uniform E-Field Electric fields accelerate charges… that’s what they do. What we’re saying here is that as the E-field accelerates a positive charge, the charge-field system picks up kinetic energy. At the same time, the charge-field system loses an equal amount of potential energy. Why? Because in an isolated system without friction, mechanical energy must always be conserved.

Ch 25.2 – Potential Difference in a Uniform E-Field If q0 is negative then ΔU is positive as it moves from A to B. When a negative charge moves down field, the charge-field system gains potential energy. If a negative charge is released from rest in an electric field, it will accelerate against the field.

Ch 25.2 – Potential Difference in a Uniform E-Field Consider a more general case. Assume the E-field is uniform, but the path, s, between A and B is not parallel to the field.

Ch 25.2 – Potential Difference in a Uniform E-Field Consider a more general case. Assume the E-field is uniform, but the path, s, between A and B is not parallel to the field.

Ch 25.2 – Potential Difference in a Uniform E-Field If s is perpendicular to E (path C-B), the electric potential does not change. Any surface oriented perpendicular to the electric field is thus called a surface of equipotential, or an equipotential surface.

Quick Quiz 25.2 The labeled points are on a series of equipotential surfaces associated with an electric field. Rank (from greatest to least) the work done by the electric field on a positive charge that moves from A to B, from B to C, from C to D, and from D to E.

EG 25.1 – E-field between to plates of charge A battery has a specified potential difference ΔV between its terminals and establishes that potential difference between conductors attached to the terminals. This is what batteries do. A 12-V battery is connected between two plates as shown. The separation distance is d = 0.30 cm, and we assume the E-field between the plates is uniform. Find the magnitude of the E-field between the plates.

EG 25.1 – Proton in a Uniform E-field A proton is released from rest at A in a uniform E-field of magnitude 8.0 x 104 V/m. The proton displaces through 0.50 m to point B, in the same direction as the E-field. Find the speed of the proton after completing the 0.50 m displacement.