Chapter 34 Review Answers: "Wetness" lowers the resistance of your skin in a roundabout way. Both your skin and pure water are poor conductors (good insulators) of electricity, but your skin generally has trace amounts of salts on it due to perspiration. These salts dissolve readily in water, and provide an excellent conducting path for electric current. A bird can perch without harm on a high voltage wire because both of its feet are at the same potential. If there is no potential difference between two points, no current will flow. Household appliances are designed so that the case of the appliance shields you from all of the electrical potentials inside. However, if an electrical connection inside the appliance should break and accidentally contact the case of the appliance (this is often called a "short" or "short circuit"), when you touch the appliance there will be a potential difference between the electrical appliance and ground - across you. This means that a large current could flow - through you. That would be bad.
The third prong in a household electric plug connects the case of the appliance directly to ground. If there is an accidental electrical connection to the appliance's case in this situation, there will be a large current that flows to ground, and a circuit breaker will "trip" or a fuse will "blow".
The smart thing to do: Identify the appliance that is causing the problem, unplug it, and have it repaired by a qualified service person or discard it.
Two really dumb things to do:
I. NEVER, NEVER, NEVER remove the third prong from an appliance - especially if you suspect it is causing fuses to blow! If a "short" exists to the case of the appliance, anyone who touches the appliance is in great danger of a potentially lethal shock.
II. NEVER, NEVER, NEVER put a penny (or anything else) under a fuse to keep it from blowing, or try to impede the motion of a circuit breaker to keep it from tripping. These devices are cutting off the current because something is causing an unsafe amount of current to flow in a circuit. If you try to defeat these safety devices, there is a very great danger of an electrical fire! (a) DC is "Direct Current" - the potential difference is constant (more or less) across the circuit, and the current flows in one direction. AC is "Alternating Current" - the potential difference across the circuit switches direction periodically, and the current flows back and forth in the circuit.
(b) A battery produces a constant potential difference - DC
A generator generally produces a flip-flopping potential difference - AC A capacitor is used to smooth the pulsed DC produced by the diode. It does this by storing energy for half of the cycle, and releasing it during the other half. In an AC/DC converter, diodes are used to convert AC to DC (by blocking the current flow for half of the AC cycle), and the capacitors are used to smooth the pulsing DC voltage (by storing energy during half the AC cycle and releasing it during the other half). (a) The "drift speed" of electrons in a DC circuit is typically about 0.01 cm/s. (That's about 10 seconds to travel between the two smallest marks on a meter stick!)
(b) The 'drift speed" of electrons in an AC circuit is typically zero. The electrons that flow in a typical circuit element originate in the circuit element itself - not in the battery or wall outlet. Chapter 34 Think & Explain Answers: No (but I'd bet that it would cause some alarm anyway!). 107 km/h is a pretty typical speed of a typical electron orbiting a typical nucleus.
