Which equation is Ohm's law?
A waffle iron's heating element has a resistance of 12 Ω, and the outlet puts 120 V across it. What is the element's power? You could find the current first with I = V/R, then use P = IV. Two steps, every time. One substitution folds the two steps into a single formula.
Here is the two-step route, run for the waffle iron.
The two steps land on 1200 W. Now run the same two steps with symbols instead of numbers.
The current through the element is I = V/R.
Substitute V/R for I in P = IV: P = (V/R) × V.
V × V is V². So P = V²/R.
Substituting I = V/R into P = IV gives...
The element transfers the energy it receives away as thermal energy. Power transferred away as thermal energy is called the power dissipated.
The voltage appears twice in P = V²/R: V drives the current, and V also sets the energy each coulomb transfers.
So doubling the voltage across a fixed resistance quadruples the power dissipated.
The R sits underneath: more resistance at the same voltage means less current, so less power dissipated.
The voltage across a fixed heating element falls from 120 V to 60 V. The power it dissipates...
A resistor transfers electrical energy away as thermal energy. What is the power transferred this way called?
Which substitution into P = IV produces P = V²/R?
Two elements sit across the same 120 V supply. Element A's resistance is half of element B's. How does A's power dissipated compare with B's?
P = V²/R gives the power a resistance transfers away as...