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Simple Electrical Question

  1. Aug 20, 2008 #1
    Hey guys I have a few simple questions:

    So on a light bulb I have connected to my lamp it says "120 V" and 25 Watts. From the outlet we receive 120 V. If that is true is the light bulb using the maximum E.M.F. the outlet can provide (120V)? and therefore it is using a very low current (p=vi = 25=120i i=25/120 A)

    Is that right?

    if that is right, do all things made in US for example use 120 V? Or if we plug something in the max they can use is 120 V but the outlet provides anything below that as well.

    What is the maximum CURRENT our home outlets can give?... (Is there a maximum)?


    There is always a ground or neutral wire, i know it is there to prevent consequences from short circuits but when does the current flow through this wire? What causes the current to start going into the neutral wire? At what point does the current "know" to start flowing through the neutral wire?

    If im not being clear please tell me.
    Last edited: Aug 20, 2008
  2. jcsd
  3. Aug 20, 2008 #2
    Home convenience outlets in the USA are normally rated for use with devices designed for operation at 115 or 120 volts. The rating on the outlet device may be listed as 125 volts. The voltage applied to the terminals will be 120 volts nominal.

    Things designed to be plugged into these outlets usually have a voltage rating of 120 volts. You will find some lamps rated at 130 volts. (Put these in a 120 volt circuit and they will last damn near forever.)

    Whatever is plugged into the outlet will "pull" current according to its resistance.

    A 100 watt lamp will pull .8333 amps because its resistance is about 144 ohms.

    A 200 watt lamp will pull 1.66 amps because its resistance is about 72 ohms.

    A 15 watt lamp will pull .125 amps because its resistance is about 960 ohms.

    The lower the consumption rating of the driven device, the higher its resistance to current flow is. The higher the resistance is, the lower the current is. Thus a low wattage lamp has a higher resistance to current flow than a high wattage lamp. They are all driven by the same 120 volt supply voltage.

    This is applicable to 120v outlets only. We also have incandescent lamps designed for 240 volts...and 277 volts.

    You should NOT plug things into the outlet unless they are rated for 115/120 volts, especially if the rating is lower than 115 volts. Applying more voltage than the equipment is designed for will cause overcurrent which leads to overheating which leads to fire which leads to destruction of materials and possibly death.
    Last edited: Aug 20, 2008
  4. Aug 20, 2008 #3


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    It's limited by the fuse/circuit breaker to prevent too much current being drawn which would overheat the wiring.
    Usually the US limit is 15Amps, but since this is at the fuse box it is shared by all the appliances on the circuit. Large appliances like ovens, washers, dryers will be on a separate 30A circuit which is often also 240V.
    In the UK it is typically 32A at 220V for the circuit with a separate 13A fuse in the plug for each appliance. This is why electric kettles and electric heaters are more common in europe.

    You are confusing ground and neutral.
    The neutral wire completes the circuit so the light bulb (or whatever) is connected between the incoming live wire and the outgoing neutral - exactly the same current flows in both.
    The earth wire is a separate wire which is usually connected to the neutral at the fuse box. It never normally carries any current but is connected to any external metal parts of the appliance. The idea is that if the live wire should touch the external metal (due to some fault) a large current will flow down the earth wire which will blow the fuse and so disconnect the faulty appliance.
    A more modern solution is an RCD which measures the current flowing in through the live and back through the neutral - if there is any difference it turns off the circuit. This stops any current flowing to earth through a different path such as your body!
  5. Aug 20, 2008 #4
    so the wattage on the bulbs are as a result of calculations made on VOLTAGE(120V) and the RESISTANCE.
  6. Aug 20, 2008 #5
    ok so the ground wires are connected to each load ?
    Last edited: Aug 20, 2008
  7. Aug 20, 2008 #6


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    Yes - it's a little tricky because the resistance of the filament changes as it heats up and as it ages - the wattage is only an approximation.

    The ground wires are all connected together at the fuse box and then usually connected to a copper rod into the ground or a cold water pipe.
  8. Aug 20, 2008 #7
    I made a quick diagram if u can help me,


    can u give me an example of how a ground would be connected

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  9. Aug 20, 2008 #8


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    The ground would be a separate wire from the -ve terminal of the supply to the metal case of the equipement.
    At the negative terminal it would also be connected to the Earth (in the real world through a buried metal rod).
    Last edited: Jan 13, 2009
  10. Aug 20, 2008 #9


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    The load might be enclosed in a metal case, and the ground wire connected to that case. Somewhere else in the house, the ground wire and neutral are connected together (perhaps near the "source" in your diagram) and also (as mgb_phys mentioned in post #6) "usually connected to a copper rod into the ground or a cold water pipe."

    Normally, the ground wire and metal case are not part of the circuit. But if the wiring inside the box becomes loose and touches the metal walls, that completes the circuit. Lots of current flows, trips the circuit breaker or fuse, resulting in no voltage being applied to the device.

    It's a safety feature. Without that ground circuit, if the wiring in the box gets loose and touches the metal case, there is no complete circuit ... until somebody comes along and touches the box (which is in contact with the live wire) and electrocutes themself.

    Edit added: just saw mgb_phys's figure. A picture is worth a thousand words!
  11. Aug 20, 2008 #10
    so when exactly then will current go into this ground wire?
    like if there is too much current? but if there is too much current flowing into the load how does it know to start flowing in the ground wire? Wouldnt the current try to pick the least resistive path and as a result continue to travel through the load?

    Thanks to everyone by the u guys are helping a lot
  12. Aug 20, 2008 #11
    The wattage will determine the current draw at a specific voltage. For purely resistive devices (no phase shift associated with the current), the wattage will be the product of the voltage and the current.

    W = E x I

    We already know that E = I x R

    so, in the W = E x I formula, replace E with I x R and you get

    W = I x R x I = I^2 R

    It is not so simple for inductive/capacitive devices because the power factor must be considered. When dealing with incandescent lamps, it is negligible.

    The green ground wire is usually connected to the metal casing of the electric device, or to the mounting yoke on a receptacle. The connection to the casing of the device plugged in is through the rounded hole on the receptacle. The white neutral is connected to the lamp through the screw-shell and usually goes through the shorter of the two vertical slots on the receptacle. The hot wire is connected to the other side of the lamp filament, usually through the button on the bottom of the lamp and is wired through the longer of the two vertical slots on the receptacle.

    The green ground wire is not part of the current carrying circuit as are the white neutral and the colored hot wire. The green ground wire and the white neutral are connected together at the main panelboard only.
  13. Aug 20, 2008 #12


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    If the wiring becomes loose inside the case. It's not supposed to happen, but it can and does. Screws holding the wires in place can become loose. Soldered joints can crack. Most electric devices warm up when in use, then cool down when turned off. With heating and cooling comes thermal expansion and contraction. This places stress on wiring joints. After many times of the stress pulling on the joint or screw one way (while heating up) and then the opposite way (while cooling down), the joint can fail eventually. When that happens, you have a loose, live electrical wire inside the box.

    If the loose, live electrical wire makes contact with the walls of the box, it's a safety hazard. A person who touches the box while standing on the ground (or floor) completes a circuit, and becomes electrocuted.

    By making a path from the box to ground (via the ground wire), the circuit breaker is tripped and power is shut off. No more safety hazard.
  14. Aug 20, 2008 #13


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    Only if there is a fault in the equiepment and the live wire touches the metal case.
    A light bulb isn't a good example since there isn't ussually anything metal to connect the ground wire to. Modern light fittings often have a screw terminal in the plastic base to connect the ground wire to. This is completely pointless but is needed to meet wiring codes - this might be what is confusing you!
    If you look inside a metal desk lamp or a toaster you will see a green/yellow (at least in europe) wire connected to the case.

    No, the fuse is there to blow if there is to much current. The earth wire is needed to blow the fuse.
    As Redbelly said, suppose there was no earth and the live wire came loose and touched the case. There would be no path for the current and so the fuse wouldn't blow - but if you come along and touch the metal case the current can flow through you (bad!). The earth wire is there so that if the live does touch the case it will flow down the earth wire - this short circuit will pull a lot of current and blow the fuse.

    there is less resitance directly down the earth wire than through the load. The load must have some resistance to do work. The earth wire is designed to have much lower resistance.

    edit - don't know why the picture doesn't show up properly.
  15. Aug 20, 2008 #14
    The circuit through the device (lamp, motor, lamp ballast, solenoid, whatever) is normally carried through insulated wires. They are insulated from contact with the supporting case either by insulation material or by physical separation. When the insulation breaks down due to age and excess heat, the circuit can conduct current through the metal case. It is at this stage (failure of containment of the current to its intended path) that the green ground wire becomes helpful. It carries the current to the grounding electrode outside the house and keeps it from going through people that may be touching the metal case. When the current through the multi-breaker (or fuse) becomes too high, the breaker will open (or the fuse will melt) causes the current flow to stop completely.
  16. Aug 20, 2008 #15
    OOOkkkk i never knew that. THANKS! So basically this ground we are talking about protects from US completing the circuit due to some reason as stated in ur postss .... I was always confused about thattttttt thats coooooool...

    BUT in an outlet the "third prong," what is that?
  17. Aug 20, 2008 #16


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    That is for the ground (or "Earth") wire which is connected to the metal case.
  18. Aug 20, 2008 #17


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    That's the ground, on the equipment (plug) side it just goes to the metal case, on the supply (socket) side it is the earth wire going back to the fuse box and then into the Ground.

    In most countries the earht prong is sligghtly longer than the other two, to guarrantee that it is connected first - so if the plug is only partly in your are sure the case is earthed BEFORE any power can be supplied.
  19. Aug 20, 2008 #18

    I hope this is right to say... CHECK PIC :)??



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  20. Aug 20, 2008 #19


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    Yes, that's it. You also got the narrow & wide slots correct!
  21. Aug 20, 2008 #20

    thanks to everyone who helped me, i was very confused about that for a while now.

    THANKS A LOT!!!!!!!
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