Flashlight Transformer
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Can anyone help me with my physics presentation?
There are a few things that I'm stuck on.. can anyone help?
1.) In a house, circuit breakers for a line delivering power to small appliances are usually designed to open when the current exceeds 15 amperes. If the line voltage is 115 volts, what is the least resistance you could connect to a line and not have the circuit breaker open? How many 100-watt bulbs could you connect to a line and not have the circuit breaker operate?
2.) If a flashlight batter is connected to the primary of a step-up transformer, a friend claims a voltage appears across the secondary as long as the battery is connected. Is he right or wrong? Why?
3.) Household door bells and chimes use a transformer to reduce the 115-volt house voltage to six volts. Does the primary winding contain more or fewer turns that the secondary in these transformers? If the secondary winding contains 1,000 turns, how many turns are there on a primary?
1. WARNING: wattage ratings on lightbulbs are only valid when used at the specified voltage. Typically, household lightbulbs are connected in parallel, thus 115 volts (rms) are across each one. 115 volts (rms) is assumed the standard voltage across the bulbs.
IF you don't know what rms means, it stands for root-mean-square. What it means is an equivalent DC voltage which will deliver the same average power output.
Anyway, since we can agree that the wattage ratings are valid, we can simply add them up. Call the power rating P.
Thus, net power consumed is:
Pnet = n*P
Use the current-voltage relation:
Inet*V = Pnet
Thus:
Inet*V = n*P
n = Inet*V/P
Data:
Inet:=15 amperes; V:=115 V; P:=100 W;
Result:
n = 17.25 bulbs
WE MUST ROUND DOWN and conclude that 17 lightbulbs can be used, and the circuit breaker will not be "tripped".
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2. Your friend is wrong. Transformers are only designed to work with alternating current in their intended function.
When you initially install the battery, you have an increasing current through the primary loop. Eventually, this current steadies out to a steady state current.
The secondary loop only has an induced voltage when the magnetic field within it is changing. The magnetic field is proportional to the current in the primary loop.
Conclusion, only a voltage will appear across the secondary loop at the sudden addition of the battery. After a while, the primary current will steady out and the secondary loop will not sense any changing magnetic field. Thus no voltage will appear across the secondary loop.
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3. You are discussing a step-down transformer. In a step-down transformer, the secondary winding contains fewer coils than the primary winding.
We intend to reduce the voltage by 6/115. Thus 1000 turns on the secondary winding means about 19167 turns on the primary winding.
Transformer Flashlight Machine Gun!
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