Which of the following devices is used to detect electric charge?

Friction can exist between two surfaces when there is no relative motion between them.

Two glass rods separately rubbed with a piece of silk cloth attract each other.

It is not safe to carry an umbrella during a thunderstorm.

If two forces acting on an object are equal but not acting opposite too each other, then the forces are _____. (balanced forces/unbalanced forces)

The term used for the perception of the frequency of sound is _____. (pitch/loudness)