Suppose you want to find out the effects of light on the growth of tomatoes. what variables would you need to control in your experiment?

In other words, what you are looking for is what other factors may affect the growth of the tomatoes. You need to get rid of those factors to make sure that the light is the only thing affecting the tomatoes. Other factors that can affect the tomatoes and must be controlled would be:

Food
Water
Temperature

All can affect the experiment. If all tomatoes are given the same food, water, and kept all at the same temperature, then you can ensure that the only thing affecting the tomatoes is the light. 

Hope that helps :)

stands for electromotive force and it is measured in volts. despite its name, it is not actually the force in the sense of being a "pushing" or "pulling" (not measured in newtons).an equation that relates emf,  ℰ,  to a battery's terminal voltage, v, current, i, and internal resistance, r, is    ℰ = v + irin this case, the emf can be interpreted as a voltage—a difference in electric potential.i would not say that it would be considered an energy. if emf can be defined as the energy/work per a unit charge from a battery or other electric source, then it is not strictly energy.