When you heat the air inside a hot air balloon, it rises. why does this occur? ?

Describe the path of a ray that approaches a mirror parallel to the principal axis.

According to social exchange theory, altruistic behaviors benefit the individual who performs them.

When applying kirchhoff's rules, one of the essential steps is to mark each resistor with plus and minus signs to label how the electric potential changes from one end of the resistor to the other. the circuit in the drawing contains four resistors, each marked with the associated plus and minus signs. however, one resistor is marked incorrectly. which one is it?