How many watts of electrical power are consumed if 400 VDC is supplied to an 800-ohm load?
Why Power can be found from voltage and resistance with P = E squared divided by R. Here 400 V squared is 160,000, and 160,000 divided by 800 ohms gives 200 watts. Equivalently, the current is 400/800 = 0.5 A, and P = E x I = 400 x 0.5 = 200 watts.
Watch out The choice of 0.5 is the current in amperes (400 V / 800 ohms), not the power; you still have to multiply that current by the voltage.
P = E squared / R. Square the volts first, then divide by ohms.
Easy path: 400 over 800 is half an amp, and half of 400 volts is 200 watts. The answer is half the voltage number.