The force experienced by a unit test charge is a measure of the strength of an electric:

an electric field is the answer

The force experienced by a unit test charge is a measure of the strength of an electric field

Electric field strength is the intensity of an electric field at a particular location. The standard unit is the volt per meter (v/m or v. m -1). A field strength of 1 v/m represents a potential difference of one volt between points separated by one meter. A test charge shows the direction of the field. But the "test" charge should be small, since charges generate their own fields, hence they interfere with the external field.

thermal energy is the answer