What a learner can do afterwards
- Uses E = V over d for the field between plates and states the unit volts per metre
- Calculates the work done moving a charge through a potential difference as QV
- Sketches equipotentials for plates and for a point charge and says how they meet the field lines
1 · Read
Two parallel plates with a voltage across them make a uniform field. Its strength is E equals V over d, with d in metres. The unit is volts per metre, which is the same as newtons per coulomb.
The energy change is Q times the voltage change, sign included. A positive charge moved to higher voltage gains potential energy; an electron moved to higher voltage loses it. An electron released from rest near the negative plate crosses a 200 V gap to the positive plate. Its potential energy falls by Q V = 1.6 times 10 to the minus 19 times 200 = 3.2 times 10 to the minus 17 J, and it gains that much kinetic energy.
An equipotential joins points at one voltage, and moving along it takes no work. Equipotentials always meet field lines at right angles, for plates and point charges alike. Around a point charge Q the potential is V equals k Q over r, and it takes the sign of Q.
Volts per metre is identical to newtons per coulomb, so either unit fits E. Watch the sign: a negative charge moved to higher voltage loses potential energy.
Plates turn voltage into uniform field, and crossing voltage turns into energy.
The rewritten readings were written for this dataset. The reading 'Electric potential, voltage' comes from the supplied Khan Academy article record. License terms for that material have not been verified.
2 · Watch
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Where it sits
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Where this leads
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