Enter values
Use signed charges for attraction or repulsion. Select a medium to account for screening effects.
Example data table
These sample scenarios show how sign and medium affect the interaction energy.
| q1 (µC) | q2 (µC) | r (m) | εr | Energy sign | Typical use |
|---|---|---|---|---|---|
| +2 | +3 | 0.20 | 1 | Positive | Repulsive point charges |
| +2 | −3 | 0.20 | 1 | Negative | Attraction and binding |
| +1 | +1 | 0.10 | 78 | Positive | Screened interaction in water |
| −5 | +2 | 0.05 | 2.2 | Negative | Charges in insulating oil |
Formula used
For two point charges separated by distance r in a uniform medium:
- U is interaction energy in joules.
- q1 and q2 are signed charges in coulombs.
- εr is relative permittivity of the medium.
- Positive U indicates repulsion; negative indicates attraction.
How to use this calculator
- Enter q1 and q2 with correct signs.
- Select the charge unit, then enter the separation distance.
- Set εr for the medium, or use a preset.
- Open advanced options for output units and precision.
- Click Compute to show results above the form.
- Use CSV or PDF buttons to export the report.
FAQs
1) Why can the energy be negative?
Energy is referenced to infinite separation. Opposite charges lower system energy when brought closer, giving a negative interaction energy and indicating attraction.
2) Does εr change the force and energy?
Yes. A higher relative permittivity reduces the effective constant, lowering both the force and energy magnitude for the same charges and distance.
3) When should I use electronvolts instead of joules?
Electronvolts are convenient for atomic and molecular scales. If charges are near elementary charge levels and distances are nanometers, eV values are easier to interpret.
4) Is this valid for extended charge distributions?
This calculator assumes point charges. For extended objects, integrate over the distribution or approximate with effective charges when separation is large.
5) What happens if the distance is extremely small?
The point-charge model may break down, and values can become huge. Use realistic limits and consider finite size, quantum effects, or material breakdown.
6) Can I enter charges in microcoulombs with negatives?
Yes. Enter signed numbers like −3 in the charge fields and keep the unit as microcoulombs. The calculator converts to coulombs automatically before computing.
7) Why include a thermal scale check?
Comparing |U| to kBT helps judge whether thermal motion can overcome electrostatic binding. If |U| is much smaller than kBT, interactions are easily disrupted.
8) Which constant mode should I choose?
Use vacuum with εr for most cases. Use derived mode when you want k to include εr automatically. Use custom mode for specialized conventions or literature values.