Electric current is a continuous flow of electric charge, measured in amps. Direct current flows in one direction while alternating current regularly reverses direction. Conductors easily allow the flow of charges while insulators do not. Resistance opposes the flow of charges and is affected by a material's thickness, length, and temperature. Voltage provides the potential difference necessary for current to flow in a complete circuit including an energy source. According to Ohm's Law, voltage equals the product of current and resistance, or current equals voltage divided by resistance.