For an electric current to flow there has to be a source of electrons, ie, a battery. Closing the switch completes the circuit. This electron flow causes the lamp to light. If the switch is opened the electron movement ceases and the lamp is therefore turned off. Electrons are released from the negative battery terminal (-), and attracted by the positive terminal (+) through the circuit. The amount of electron movement being limited by the resistance of the lamp, the filament of which is made of tungsten. Filament resistance causes more work (power) in moving the electrons and heat is generated, up to as much as 1000°C which is white hot and emits visible light. The direction of current flow can be considered in two ways, either as conventional current when the flow is from positive (+) to negative (-) or as electron flow, where electrons are released from the negative battery terminal and attracted by positive terminal. Both are valid. In simple DC circuits conventional current is appropriate. However, to explain the operation of a semiconductor, which is a transistor or diode, then electron flow must be considered as we are then primarily interested in the combination of negative electrons and positive holes. TEST YOUR UNDERSTANDING: Here's a really simple assignment to get you started. Select the topic for Measuring an Electric Current from the drop-down menu. The circuit load resistance R and the current meter are connected in series. What is the voltage applied across this series circuit for three clicks of the switch?
