if you get a 100w @ 120v light bulb and installed it in to a lamp which had a voltage of only 115v what are the amps the light bulb use?

Question

if you get a 100w @ 120v light bulb and installed it in to a lamp which had a voltage of only 115v what are the amps the light bulb use?

Answer 1

DEPENDS ON THE DESIGN OF THE LIGHT BULB!!!



incandescent bulbs will take slightly less power with a reduction in voltage.

an LED bulb will take the same, less, or more current (depending on design) as some are designed to output the same lumens even with voltage changes.



Notes:

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110, 115 and 120vac are all considered the same voltage in America.



110V is a legacy term left over from ages gone by

115V comes from the design side, equipment is normally designed to run on 115V +/- 10%

120V comes from the supply side, under standard conditions electrical utilities deleiver electricity at 120V +/- 5%



In the end for us the numbers mean the same thing, just as 220/230/240 mean the same thing BUT 208V does NOT.

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But in real world if you lower the voltage a little, incandescent light bulbs will last longer, and LED bulbs might burn out sooner if designed poorly.

Answer 2

That 120 volt value is nominal, it is not an exact number.

Answer 3

you are only provided with operating information at 120 volts. at 120 volts the current draw is 100/120 or 0.8333 amp. However at 115 volts, the operating temperature of the filament changes. The filament resistance is not a constant so Ohm's "Law" does not apply, the resistance is proportional to the absolute temperature (measured as Kelvin or Rankine). In fact if you were to measure the resistance of the bulb at room temperature it would be about 0.08 ohm because room temperature is about 1/10th the operating temperature at 120 volts.(300K vs 3000K) it is not constant. This fact was utilized in the early days of radio (1920's - 1930's) to regulate the power line voltage fed to the radio set. A light bulb (called a "ballast tube") was fed by a series resistor (the line cord) to provide constant voltage which is similar to a modern circuit for a zener diode voltage regulator. This is the reason the instructions for a radio said to NEVER run the power cord under a rug, because the power cord was designed to generate heat as part of the regulating function. It also meant you could only replace a power cord on a tube radio with an original factory part.

Answer 4

I assume you have an incandescent bulb in mind,

Imprimis, it will light up and look quite normal,

Secundus, there will be little reduction in current, because Tungsten metal has a very high positive thermal coefficient of resistance, so the slight cooling because of a 5 volt drop in applied voltage will reduce the resistance of the filament slightly which helps maintain the current more or less constant.

Answer 5

R = 120^2 / 100



CURRENT WITH 115V POWER SOURCE = 115 / R = 115 / 120^2 / 100

Answer 6

Assuming a constant bulb resistance.

Watts at 115V =~ [(115^2)/(120^2)]*(100W) = 91.84027778 Watts

Amps bulb will use at 115 Volts =~ 91.8402778W/115V =~ .798611111 Amps =~ 798.61 mA

Answer 7

We need to determine the resistance of the wire filament in the light bulb.



P = V * I

100 = 120 * I

I = 5/6 amps



Let’s determine the resistance.

V = I * R

120 = 5/6 * R

R = 144 Ω



Let’s use the same equation to determine the current at 115 volt.



115 = I * 144

I = 115 ÷ 144



This is approximately 0.8 amps. Your answer is correct.

Answer 8

If you use ohm's law to work it out, there will be a small error because the tungsten filament changes its resistance with temperature. When the voltage goes down, so does the filament's temperature and it's resistance goes up. The best way is empirical. Actually measure the current and voltage and multiply them. The truth is, lightbulbs don't behave exactly like resistors.

Answer 9

With info given, can't be more accurate, but less than 100/120. Remember a filament does not obey ohms law because the resistance changes with temperature.

Answer 10

I get 0.7986 amps. But how will you know if I am correct?

Answer 11

You can do the math assuming the bulb resistance is constant, but it's not. You can assume the bulb is exactly 100W but it will actually be somewhere between 80 and maybe 110W at 120V. It's just pointless. Light bulbs are not precision devices, no where need the 4% change in voltage you are asking about. Think around 20% accurate. Products made for sale in North America must operate correctly with power between 105 to 135Volts, and are called the nominal value 120V.

Answer 12

The first thing you do is work out the lamp resistance using Ohms Law, R = V/I

Now you have this, use the new voltage and Ohm's Law again to work out the new current.

I=V/R

You do the maths.

Answer 13

The calculation if the lamp was a constant resistance could be calculated to be 0.7986 amps. Tungsten filament resistance will decrease with heat, increasing current, slightly. The current change is not linear, most of the current curve is experienced from 0 to 20v, but there would be a slight increase, just a few milliamps, not enough to increase the watts.

Answer 14

100 X 115/120