Electric Potential Energy and the Chapter 19 Electric Potential Energy and the Electric Potential

1) Electric Potential Energy

2) Electric Potential

3) Point Charges If V = 0 at r = ∞, then V r q Superposition: potentials add as scalars q1 q2

4) Equipotential surfaces Definition Surface with constant potential e.g. For a point charge, equipotential surfaces are spheres

e.g. parallel plates E is uniform, and W = qEs, so equipotential surfaces are planes

b) Work along an equipotential surface

E is perpendicular to equipotential surfaces c) Electric field direction and equipotential surfaces If W = 0, and E ≠ 0, then E is perpendicular to equipotential surfaces

Electric field lines Equipotential lines

d) Electric field as a potential gradient units: V/m = J/(Cm) = N/C

In general, Electric field points in the direction of maximum change of the potential

5) Biomedical examples a) Conduction of electrical signals in neurons Resting state (selective permeability) V=-70mV

Stimulated cell Signal travels at ~ 50 m/s

b) Medical diagnostics - body is not an equipotential surface - Flow of Na+, K+, Cl- ions; potential differences ~ 30 - 500 µV - depend on stimuli and can form regular patterns

- heartbeat (electrocardiography, ECT, EKG)

- brain waves (electroencephalography, EEG)

- response to light (electroretinography, ERG)