# Equipotential Surfaces and Capacitors

• All points on an equipotential surface have the same electric potential (i.e. the same voltage).
• The electric force neither helps nor hinders motion of an electric charge along an equipotential surface.
• Electric field lines are always perpendicular to an equipotential surface.
• Electric potential is analogous to altitude; one can make maps of each in very similar ways.
• The electric field strength can be determined by looking at the local spatial gradient of the electric potential:
```                              change in potential
electric field = --------------------
distance
```
• A capacitor is a device which stores positive and negative charges in separate places.
• The capacitance of a device tells how much charge it can store for a given voltage across it:
```                             charge            coulombs
capacitance = -----------         -------- = farads
voltage             volts
```
• The energy stored in a capacitor is
```
energy stored = 0.5 * (capacitance) * (voltage)^2
```

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