volt per meter
Electric field strengthexact by definition
One volt of potential difference per metre of separation, the SI unit of electric field strength. It is exact by definition, following from the volt and the metre with no measured factor in between.
Watch out: A field quoted in V/m is the field at a point, not an average across a gap, and the two only agree when the field is uniform. Between large parallel plates it is uniform enough that E = V/d is trustworthy; near an edge, a point or a thin wire it is not, and the real field at the sharp feature can be many times the number the division gives.
| 1 | 1 |
It arrives from two directions that turn out to be one. Field is force per unit charge, N/C; it is also the rate at which potential falls with distance, V/m. Working the algebra through — a newton per coulomb is a joule per metre per coulomb, and a joule per coulomb is a volt — shows the two expressions are identical, which is why this type carries both with a factor of 1.
About the volt per meter
Electric field strength answers a question the voltage alone cannot: not how much energy a charge gains crossing a gap, but how hard it is pushed at any given point along the way. A volt per metre is the gradient of potential — how steeply the voltage falls as you walk across the space between two conductors.
The numbers that matter cluster at the top of the ladder. Dry air at sea level breaks down at roughly 3 MV/m, which is the same thing as 3 kV/mm or 30 kV/cm, and it is the number behind every spark gap, every minimum clearance in a switchgear standard and the fact that a 30 kV Van de Graaff will jump about a centimetre. Transformer oil manages perhaps 10 to 15 MV/m, and a good polyimide or polypropylene film runs into the hundreds.
Down at the other end, the field is what a charge actually feels. run backwards gives the force on a charge in the field, which is how a CRT deflection plate steers a beam, how an electrostatic precipitator drives fly ash onto a collecting plate, and how Millikan weighed an electron against gravity.
One habit is worth building: whenever a field number looks alarming or reassuring, convert it to kV/mm before deciding. Air fails at 3, and almost every insulation question is a comparison against that single figure.
One volt per meter in every electric field strength unit
| 1 | volt per meter (V/m)this unit | |
| 1 | newton per coulomb (N/C) | |
| 0.01 | volt per centimeter (V/cm) | |
| 0.001 | kilovolt per meter (kV/m) | |
| 0.001 | volt per millimeter (V/mm) | |
| 0.000001 | kilovolt per millimeter (kV/mm) | |
| 0.000001 | megavolt per meter (MV/m) | |
| 0.0000254 | volt per mil (V/mil) | |
| 2.54000e-08 | kilovolt per mil (kV/mil) |