+ Post New Thread
Results 1 to 16 of 16

21st February 2011, 08:03 #1
 Join Date
 Feb 2011
 Location
 India
 Posts
 65
 Helped
 1 / 1
 Points
 1,311
 Level
 8
Why does less resistance dissipate more power
I have two resistances which I will call them as loads... and they are purely resistive...
First one is a heavy load( A geyser which has 1000Ω resistance coils to heat up the flowing water)
Second is a very light load( A hand held electric fan of 10Ω )
Intuition says the geyser should be consuming more power as it is a heavy load..
but calculation shows it otherwise...
Help me understand

21st February 2011, 08:41 #2
 Join Date
 Aug 2010
 Location
 San Jose area
 Posts
 701
 Helped
 197 / 197
 Points
 4,234
 Level
 15
Re: Why does less resistance dissipate more power
Resistance is just a measure indicating how easy/hard the current can flow and it does not directly dictate how much power it would consume. Power, in pure physics terms, is how much work the system can do in a unit time, and amount of current and potential would directly determine the power by definition. Of course, you can relate the power to the resistance since these three properties are linked by Ohm's law and it can increase/decrease the amount of current, but by definition, current and potential should determine the power.
A simple analogy. Two idential water towers, and one with wider pipe(low resistivity) down to the ground, and another with narrower pipe(high resistivity) down to the ground and run the water through those pipes.. Which one causes more damages on the ground in a given time ? That's the interpretation of power in physics.Last edited by lostinxlation; 21st February 2011 at 09:20.

21st February 2011, 08:42 #3
 Join Date
 Apr 2005
 Posts
 881
 Helped
 128 / 128
 Points
 7,315
 Level
 20
Re: Why does less resistance dissipate more power
The power dissipated in a resistive load is:
P = V² / R
So, in case of the 1000Ω load, the power dissipated is 220²/1000 = 48.4W
while in case of the 10Ω load, the power dissipated is 220²/10 = 4840W
Power dissipation : BASIC CONCEPTS AND TEST EQUIPMENT
Hope this helps.

21st February 2011, 08:42

21st February 2011, 08:58 #4
 Join Date
 Mar 2008
 Location
 Greece
 Posts
 11,889
 Helped
 2084 / 2084
 Points
 37,838
 Level
 47
 Blog Entries
 6
Re: Why does less resistance dissipate more power
Heavier load is the one with the lower resistance and the resistance in that case is lower for the fan,
your calculations have to do only with the given data not with the image of the devices.
It is true that in reality the geyser would consume more power but for the data given for calculation your results are correct.
Alex

21st February 2011, 12:34 #5
 Join Date
 Feb 2011
 Location
 India
 Posts
 65
 Helped
 1 / 1
 Points
 1,311
 Level
 8
Re: Why does less resistance dissipate more power
Heavier load is the one with the lower resistance
But it doesn't???

21st February 2011, 13:03 #6
 Join Date
 Mar 2008
 Location
 Greece
 Posts
 11,889
 Helped
 2084 / 2084
 Points
 37,838
 Level
 47
 Blog Entries
 6
Re: Why does less resistance dissipate more power
Load is considered the total resistance seen by the source,
you have 2 circuits with the same source but different loads, in that case the heavier load is the one with the lower total resistance because it forces the source to provide more current.
When you have 2 loads in series then the current provided from the source is the voltage divided with the total resistance and the current is common for both loads,
the voltage on the other hand will be more on the load with the higher resistance (because of the voltage divider formed) and the power dissipation will be grater on that load that has more resistance.
The wire is a heavier load if you connect it as a load not in series with a second load, for example calculate the curent for a wire with resistance of 0.1 ohm.
Alex
1 members found this post helpful.

21st February 2011, 13:03

21st February 2011, 14:36 #7
 Join Date
 Feb 2011
 Location
 India
 Posts
 65
 Helped
 1 / 1
 Points
 1,311
 Level
 8
Re: Why does less resistance dissipate more power
Yes alexan..,
I guess I am getting it now... I have read tat if two terminals are shorted by a wire... the whole line will get heated up and tats why we go for fuse....
So a 100Watt bulb should have lower resistance that a 60Watt bulb.... Is it right??

21st February 2011, 14:36

21st February 2011, 14:45 #8
 Join Date
 Apr 2005
 Posts
 881
 Helped
 128 / 128
 Points
 7,315
 Level
 20
Re: Why does less resistance dissipate more power
So a 100Watt bulb should have lower resistance that a 60Watt bulb.... Is it right??
http://www.edaboard.com/thread202301.html#post852362

21st February 2011, 15:00 #9
 Join Date
 Mar 2008
 Location
 Greece
 Posts
 11,889
 Helped
 2084 / 2084
 Points
 37,838
 Level
 47
 Blog Entries
 6
Re: Why does less resistance dissipate more power
Yes, assuming that both are specified for the same voltage.
You only need two equations to do these calculations,
ohm's law I=V/R (which can also be transformed to V=I*R )
and P=V*I,
this can be also transformed to P= (I*R) *I which is the same as P=I²*R
or you can transform it as P=V*(V/R) which is the same as P=V²/R
using these equations you can calculate the power over a resistor using either V&I or V&R or R&I
You can also calculate the resistance from the power and voltage, P=V²/R can be transformed to R=V²/P,
so in your question above if the bulbs are intended for 220v then
the 100W bulb is 220v²/100w=48400/100=484 ohm
and for the 60W bulb 220v²/60w=48400/60=806.66 ohm
Alex
1 members found this post helpful.

21st February 2011, 16:01 #10
 Join Date
 Feb 2008
 Posts
 16
 Helped
 1 / 1
 Points
 1,081
 Level
 7
Re: Why does less resistance dissipate more power
When you actually take the resistance of a 60W light bulb, the resistance measures 20 Ohm. I wonder why?

21st February 2011, 16:05 #11
 Join Date
 Mar 2008
 Location
 Greece
 Posts
 11,889
 Helped
 2084 / 2084
 Points
 37,838
 Level
 47
 Blog Entries
 6

21st February 2011, 16:07 #12
 Join Date
 Jul 2004
 Location
 Hungary
 Posts
 463
 Helped
 187 / 187
 Points
 5,332
 Level
 17
Re: Why does less resistance dissipate more power

21st February 2011, 16:10 #13
 Join Date
 Mar 2008
 Location
 Greece
 Posts
 11,889
 Helped
 2084 / 2084
 Points
 37,838
 Level
 47
 Blog Entries
 6
Re: Why does less resistance dissipate more power
Incandescent light bulb  Wikipedia, the free encyclopedia
"Electrical characteristics
Incandescent lamps are nearly pure resistive loads with a power factor of 1. This means the actual power consumed (in watts) and the apparent power (in voltamperes) are equal. The actual resistance of the filament is temperaturedependent. The cold resistance of tungstenfilament lamps is about 1/15 the hotfilament resistance when the lamp is operating. For example, a 100watt, 120volt lamp has a resistance of 144 ohms when lit, but the cold resistance is much lower (about 9.5 ohms).[33][64] Since incandescent lamps are resistive loads, simple triac dimmers can be used to control brightness. Electrical contacts may carry a "T" rating symbol indicating that they are designed to control circuits with the high inrush current characteristic of tungsten lamps. For a 100watt, 120volt generalservice lamp, the current stabilizes in about 0.10 seconds, and the lamp reaches 90% of its full brightness after about 0.13 seconds.[65]"
Alex
1 members found this post helpful.

21st February 2011, 16:10

21st February 2011, 16:15 #14
 Join Date
 Feb 2008
 Posts
 16
 Helped
 1 / 1
 Points
 1,081
 Level
 7
Re: Why does less resistance dissipate more power
Awesome, learning has occured. Thanks.

21st February 2011, 17:51 #15
 Join Date
 Dec 2010
 Location
 USA, midwest
 Posts
 960
 Helped
 308 / 308
 Points
 5,915
 Level
 18
Re: Why does less resistance dissipate more power

21st February 2011, 18:51 #16
 Join Date
 Feb 2011
 Location
 India
 Posts
 65
 Helped
 1 / 1
 Points
 1,311
 Level
 8
Re: Why does less resistance dissipate more power
:Enlightened...:idea
+ Post New Thread
Please login