Final answer:
An open switch in a circuit prevents current flow because it acts as a break in the circuit with a high resistance. Power dissipation in an open switch is minimal because there is no current flow, despite the presence of voltage. Conversely, a closed switch has minimal power dissipation due to its very low resistance.
Step-by-step explanation:
If a switch is open in front of a load in a circuit, it means that the circuit is open as well and no current can flow through that part of the circuit. This is because an open switch has a very high, virtually infinite, resistance, preventing electrons from passing through. For example, if there are multiple branches, and the switch in the left branch is open, then all of the current will flow through other branches, if available. If a circuit has only one path and the switch is open, then there will be no current flow in the entire circuit.
In regards to power dissipation, an open switch has voltage across it but dissipates little power because power is a product of current and voltage (P = IV), and since the current is zero, the power dissipated is also zero. Conversely, a closed switch has almost zero resistance, so even though there is current flowing through it, the power dissipated (P = I2R or P = V2/R) is small because the resistance R is very low.
Understanding the role of a switch is crucial when troubleshooting a circuit. For example, in a lab scenario where a light bulb is on with the switch open, and off with it closed, it suggests that there may be incorrect wiring, essentially causing the switch to bypass instead of control the bulb when in the closed position. Always follow safety procedures while working with electric circuits.