Final answer:
By using the power formula P = IV and rearranging for voltage (V = P/I), we can calculate that the voltage across an iron with a current of 5.0A and power dissipation of 390W is 78V.
Step-by-step explanation:
To find the voltage across the iron when the current in the heating element of an iron is 5.0A and the iron dissipates 390W of power, we can use the power formula P = IV, where P is power in watts, I is current in amperes, and V is voltage in volts.
Rearranging this equation to solve for voltage gives us V = P/I. Substituting in the given values, V = 390W / 5.0A = 78V. Therefore, the voltage across the iron is 78 volts.