UCSB Science Line
 How can lights turn on and off when you flip a light switch? Question Date: 2019-04-18 Answer 1:A light is "on" when an electric current (electricity) is flowing through it. For electricity to flow, it needs a complete circuit. Flipping a light switch to turn on a light means that the circuit with the light is being closed. When the switch is flipped the opposite way, to turn the light off, the circuit is being opened. Closed circuits being associated with on and open circuits with off may be confusing; thinking of these as complete (=closed) and broken (=open) may help. This is similar to how one pass cross a castle's moat when the drawbridge is lowered (i.e., the path is complete), but not when the bridge is raised (i.e, the path is broken). To understand why a complete circuit is necessary, a consideration of electricity at a microscopic scale is helpful. Electrical current is the movement of charge carriers. (In metals, these carriers are electrons.) These charge carriers are being pushed by some driving force, which results from electrical potential energy. By moving in the direction dictated by the force, the charge carriers become lower in energy. This is analogous to any other form of potential energy driving a change, such as gravitational potential energy which provides a driving force for objects to fall (or, really, to move toward one another). In a battery, a reaction releases many electrons at the anode and a depletion at the cathode*. Charges of the same sign repel each other, so the anode is a region of high potential energy for electrons (many of the same charge -> large electric field (and hence large repulsion) -> large energy). This drives the electrons toward the cathode, where fewer electrons means less repulsion. However, if the circuit is open (broken), then the electrons cannot reach the cathode. Instead, they will accumulate wherever their energy is minimized (e.g., as far down the circuit from the anode as possible). While they are accumulating, current is flowing (i.e., charges are moving) [so I suppose a light between the battery and the break could be on at this point]. Now it is important to realize that each electron has an electric field, and as they accumulate, the electric field from those electrons increases in magnitude. But, it will have a direction opposite that which is driving the electrons along the circuit. Eventually these two fields become equal in strength but opposite in direction, and thus cancel. At this point no current flows because there is nothing causing the electrons to move. If the circuit is then closed (completed), the electrons can further lower their energy by continuing to the cathode; current flows and the light turns on. *Other circuits there is still one location with more electrons than another, but not necessarily from a chemical reaction like in a battery. Answer 2:A switch is something that opens and closes a circuit. A circuit is a 'circle' that the electricity can go around in. It's like a flashlight. YOu can turn it on and off with a switch. Or, you can take out the batteries and the flashlight is Off even if you turn it to On. Imagine using wires to connect a battery and a light bulb from a flashlight. If the wires are connected to the 2 places on the battery, and the wires are connected to the 2 places on the light bulb, the light will light. But if you take the end of any wire away from what it's touching, the light goes off. So a light switch is like that - in one direction, the wires are connected and there is light. In the other direction, the wires are not connected, and there is no light. Answer 3:Flipping the light switch moves a piece of electrical wiring into place that completes a circuit that electricity can flow through. The electrical current flowing through the lamp creates light as it dissipates energy. Answer 4:Hi Tyler, all light switches do the same thing: connect/disconnect the wires allowing electric current to flow from the battery to the lightbulb. When you turn the light off, the switch is in the open position, meaning the wire is disconnected and no current can flow. If the switch is in the closed position, the wire is connected and current can flow from battery to lightbulb, and it lights up. Click Here to return to the search form.