If a single-load series circuit has one 20 ohm bulb, and a
second, identical, 20 ohm bulb is added in parallel with
the first, what will be the effect on the brightness of the
bulbs?

At the instant the second bulb is connected ...

... the brightness of the first bulb doesn't change

... the brightness of the second bulb changes from dark (no brightness) to equal to the first bulb

thermal energy is the answer

the order for strong to weak forces such as

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force

the fundamental forces are those forces that act between to objects or element to make interactions. there are four types of fundamental forces such as strong and weak nuclear forces (due to protons and electrons), electromagnetic force (due to the motion of electrons) and gravitational force (due to orbital movement of the planet). if these forces are arranged on the basis of their maximum strength then the order is

strong nuclear force → electromegnetic force → weak nuclear force → gravitational force