The tendency for a material to oppose the flow of electrons, converting electrical energy into heat or light, is called

• A. Current
• B. Resistance
• C. Voltage
• D. Power

Answer: (B)

Resistance is the property of a material that opposes the flow of electric charges. When current moves through a material, electrons collide with atoms, and some of the electrical energy becomes heat; in some devices, energy may also be emitted as light. This opposition to flow is what we call resistance, measured in ohms. Ohm's law links these ideas: for a given voltage, higher resistance reduces the current. The energy turned into heat or light at a given moment is the power, which can be written as P = I^2R or P = V^2/R. Current is the flow itself, voltage is the driving push, and power is the rate of energy transfer—resistance is what causes the energy to be dissipated as heat or light.