Q4.
Draw a circuit diagram with three cells connected in parallel
with an open switch.
Q5.
How does a fuse prevent the damage of an electric
Answers
Answer:
Fuses and circuit breakers disconnect an overloaded circuit, protecting the house against fire and often saving appliances before serious damage occurs. In a fuse, a piece of wire safely melts and breaks the circuit; in a circuit breaker, a spring opens up a switch.
A fuse or a circuit breaker automatically disconnects a circuit from the electric power the moment more current is drawn than its design allows. Electrical codes require one or the other to be installed at the point where power first enters a building. The electric supply to a home is usually divided into several circuits, each with its own fuse or breaker, of a rating appropriate for the thickness of the circuit's wires. That way, if the power is interrupted in part of the home, the rest of the home stays connected.
A fuse is an insert plugged into the circuit, containing a piece of a special wire, able to carry a regular current for a long time without wearing out, but melting quickly if the current, say, doubles in intensity. The fuses one buys either screw into sockets like those of light bulbs, or are (like car fuses) short insulated tubes with metal ends, fitting into clips that hold them.
The old home where I grew up had fuses of a simpler kind, rectangular ceramic plugs with brass contacts at their ends (drawing). Usually a plug was held by a pair of clips (shielded from the user) inside a ceramic box: one clip was connected to the input power and held the contact at one end of the plug (drawing). The other clip was connected to the home's circuit and held the brass contact at the other end. The ceramic block had a narrow hole, and the brass contacts had screws attached, between which a piece of "fuse wire" (spools of which were sold in stores) was threaded through the hole. If the circuit got overloaded, the wire would melt somewhere inside the hole (where it could not cause any fire!) and the fuse would be "blown", stopping the current.
To replace the fuse one pulled out the plug, unwound the ends of the broken wire from the screws and discarded them. One now threaded a fresh piece of "fuse wire" through the hole, wound its ends around the two screws and tightened the screws to hold the wire firmly. Then the plug would be replaced. Before replacing it, of course, one needed to find why the fuse blew--otherwise, it was likely to "blow" again, often, at once
Nowadays fuses have been largely replaced by circuit breakers, automatic switches with compressed springs that try to open them up and thus break the connection. A wire holds them closed, but it is part of the circuit: if the current in the circuit is too large, the wire heats up and expands, its grip on the switch loosens and the spring pushes the switch open. Some circuit breakers are screwed into sockets like fuses and have "reset" buttons (allowing the homeowner to replace fuses with them), but most modern ones are plastic and rectangular, and plug into fitting electrical contacts in the "entrance box" where electric power enters the home.
Before doing any work or repair on a circuit in the home, find its connections in the entrance box, the metal box where power enters the house and where all circuit breakers are located. Then disconnect that part of the circuit by resetting its switch from "ON" to "OFF", or unscrew the circuit breaker from its socket. Then check and make sure that the circuit you interrupted was indeed the one with the problem and is now "dead." For this you can use an electricians small "neon light tester" described further below, or an electric meter.
When a fuse or circuit breaker "blows," that is a sign its circuit was drawing a current larger than it was designed to handle. That could be caused by a short circuit somewhere--but not necessarily: an overload will also occur if too many appliances are plugged into the same circuit. If a kitchen circuit is disrupted when a toaster, coffee maker and microwave oven operate simultaneously, it may be a smart idea to delay using at least one of them: overloaded circuits have also been known to start fires. If an extension cord feels unusually warm, that too is a warning sign. In any case, wait some minutes before resetting a tripped circuit breaker: because it was released by some wire heating up and expanding, it will not allow itself to be reset until that wire cools down again.
Explanation:
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