Direct current flows in one direction and, in the steady case, has a constant magnitude. Batteries, photovoltaic modules under steady illumination and regulated DC power supplies are common DC sources. A DC circuit made of resistors obeys Ohm’s law directly, without frequency-dependent effects.
Power in a DC circuit
A 24 V DC load
Problem. A 24 V DC supply is connected across an 8 Ω resistor. Find the current and the power dissipated.
Solution. I = V ÷ R = 24 ÷ 8 = 3 A. P = V × I = 24 × 3 = 72 W. Check with P = I²R = 9 × 8 = 72 W.
Real sources have internal resistance
A real source loses some voltage inside itself as current increases. The terminal voltage therefore falls below the open-circuit value under load. This is modelled as an ideal source in series with an internal resistance, and it explains why a battery that reads a normal open-circuit voltage can still fail to crank an engine.
- DC has a steady polarity and, in the steady case, a steady magnitude.
- P = V × I applies to any DC load.
- For a resistor, P = V²/R = I²R as well.
- A real source has internal resistance and its terminal voltage falls under load.
- DC current flows in one direction.
- Power is P = V × I; for a resistor also V²/R or I²R.
- Real sources have internal resistance.
- Terminal voltage falls as load current rises.
Worked example
A 12 V source with 0.5 Ω internal resistance delivers 6 A to a load. The internal drop is 6 × 0.5 = 3 V, so the terminal voltage is 12 − 3 = 9 V, not 12 V.