Ohm's law connects voltage, current, and resistance in a simple DC circuit. It is one of the most useful relationships in electrical engineering because it lets you predict how a circuit will behave before you build it. When you know any two of V, I, and R, you can calculate the third.
This makes it essential for choosing batteries, resistors, wires, and safe operating limits.
Electrical power tells you how quickly electrical energy is converted into heat, light, motion, or another form of energy. In a resistor, power dissipation often appears as heating, so engineers must check that components can handle the required power. The formulas P = VI, P = I^2R, and P = V^2/R are different forms of the same idea when Ohm's law applies.
These relationships help engineers design circuits that work reliably without overheating or wasting energy.
Key Facts
- Ohm's law: V = IR, where V is voltage in volts, I is current in amperes, and R is resistance in ohms.
- Current from Ohm's law: I = V/R.
- Resistance from Ohm's law: R = V/I.
- Electrical power: P = VI, where P is power in watts.
- Power in a resistor: P = I^2R.
- Power using voltage and resistance: P = V^2/R.
Vocabulary
- Voltage
- Voltage is the electric potential difference that pushes charge through a circuit, measured in volts.
- Current
- Current is the rate at which electric charge flows through a circuit, measured in amperes.
- Resistance
- Resistance is a measure of how strongly a component opposes current flow, measured in ohms.
- Power
- Power is the rate at which electrical energy is transferred or converted, measured in watts.
- Conventional current
- Conventional current is the defined direction of positive charge flow, from the positive terminal of a source toward the negative terminal in an external circuit.
Common Mistakes to Avoid
- Using the wrong rearranged form of Ohm's law: V = IR means I = V/R and R = V/I, not I = R/V.
- Mixing units such as milliamps with amps: convert 250 mA to 0.250 A before using formulas so the answer comes out in standard units.
- Assuming power is always equal to V/R: power is P = VI, and V/R gives current, not power.
- Ignoring resistor power ratings: a resistor can have the correct resistance value but still fail if P = I^2R exceeds its wattage rating.
Practice Questions
- 1 A 9 V battery is connected to a 300 ohm resistor. What current flows through the resistor?
- 2 A resistor carries 0.20 A of current when connected to a 12 V source. Find the resistance and the power dissipated by the resistor.
- 3 Two resistors have the same resistance, but one is connected to twice the voltage of the other. Explain how the current and power change, assuming Ohm's law applies.