Fig 3Voltage, current, resistancedrag the sliders6 V across 3 Ω
The battery's voltage pushes charge around the loop, and the resistor resists it. The current, the amount of charge passing any point each second, is the voltage divided by the resistance: double the voltage and twice the current flows, double the resistance and half as much. Every bit of charge that passes through the resistor gives up its energy there as heat, so the heat goes up with both the voltage and the current.