This Core Resource focuses on developing the idea of voltage as an easy-to measure, though abstract idea. Students first see that equal currents do not light bulbs equally when the bulbs have different resistances. Through a lab, they find that the sum of the voltages across all elements in a circuit loop adds to zero (conservation of energy). The voltage tells how much energy is transferred to a circuit element for every Coulomb of charge that passes through the circuit element. They also note that elements with larger resistances require greater fractions of the battery’s voltage, setting up the definition of Ohm’s law. Finally, students conclude that both current and voltage together determine the power delivered to a circuit element.