Physics Help Forum What is the current based on the graph?

 Electricity and Magnetism Electricity and Magnetism Physics Help Forum

 Mar 2nd 2018, 10:49 AM #1 Junior Member   Join Date: Feb 2018 Posts: 10 What is the current based on the graph? The graph is current produced when the certain light bulb connected to voltage. When V = 12Volt, current = 3.5 A So the R for light bulb = 3.5 ohm Now, that certain light bulb connected to 5ohm resistor and voltage 12 V. I guess that Power consumed by the light bulb is P. P = I^2R = V^2/R = VI. I am not sure which formula to use. After light bulb connected to 5ohm and 12 V current = 12/8.5 = 1.4 A P = I^2R = 6.86 Wt P = VI = 16.8 Wt P = V^2/R = 41.14 Wt Could it be Volt used is not 12 V But only Volt in light bulb? V = IR = 1.4*3.5 = 4.9 V So, P = V^2/R = 6.86 Wt P = IV = 1.4*4.9 = 6.86 Wt But my answer is wrong.. Last edited by helly123; Mar 3rd 2018 at 07:33 AM.
 Mar 2nd 2018, 07:13 PM #2 Senior Member   Join Date: Apr 2017 Posts: 499 The current read from the graph when V=12 is about 3.42 , nearly 3.5 This graph cannot relate to the circuit since resistance is 5 ohms plus resistance of light , current must be less than 2.4A studiot likes this.
 Mar 3rd 2018, 07:34 AM #3 Junior Member   Join Date: Feb 2018 Posts: 10 Sorry, i forgot to mentioned before, that the graph is relation between the current in light bulb when connected to voltage. Then later the light bulb connected to 5ohm and voltage 12 V
 Mar 3rd 2018, 10:41 PM #4 Senior Member   Join Date: Apr 2017 Posts: 499 from the graph we see the resistance of the light varies when the voltage across it is changed ... you have found the resistance of the light (3.5 ohms) when the voltage across just it is 12V .... Now we have a 5 ohm resistor in series ... the voltage across both it and the light is 12 ... This means the voltage across the light is less than 12 V and your value for it's resistance is no longer true!! Now what ya gona do ??? studiot likes this.
 Mar 4th 2018, 06:16 PM #5 Junior Member   Join Date: Feb 2018 Posts: 10 Since R remains unknown, V and I remains unknown
 Mar 4th 2018, 06:37 PM #6 Senior Member   Join Date: Apr 2017 Posts: 499 Let's try guessing ... Perhaps the current is 2A .... From the chart we see the voltage across the lamp is 2V when current is 2A V=IR .... resistance of lamp at 2 A is 1 ohm In the the whole circuit we have R for the lamp 1 ohm plus 5 ohm = 6 ohm And 6 ohm is just what we need to get 2 A from 12 V ....magick!!! There should be a more professional way of getting the answer , butI can't think of it at the moment.
 Mar 6th 2018, 02:59 AM #7 Junior Member   Join Date: Feb 2018 Posts: 10 I see, thanks anyway
 Mar 6th 2018, 06:30 AM #8 Senior Member   Join Date: Oct 2017 Location: Glasgow Posts: 426 It looks like the diagrams come from a book. Is there any chance you could scan the whole page and link it here? It might help shed some light on the problem XD

 Tags based, circuit, current, graph, power consumed, resistor

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