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Voltage, Current and Resistance across a series circuit

I have to do a question which I don't understand, there are two graphs:
Fig 1 (Resistor): 2, 0.1; 4, 0.2; 6,0.3 where x,y x=Voltage (V) y = Current (A)
Fig 2 (Lamp): 2, 0.33; 4, 0.5; 6, 0.6 ditto

If we placed these in a series circuit of 14V supply, what would the current be?
Original post by Annual Learner
I have to do a question which I don't understand, there are two graphs:
Fig 1 (Resistor): 2, 0.1; 4, 0.2; 6,0.3 where x,y x=Voltage (V) y = Current (A)
Fig 2 (Lamp): 2, 0.33; 4, 0.5; 6, 0.6 ditto

If we placed these in a series circuit of 14V supply, what would the current be?


Draw a quick circuit. We know that the supply voltage will be 14V and this is placed across the lamp in series with the resistor:

VS = 14 Volts

Rtotal = Rresistor + Rlamp

The question asks for the current flowing in the series circuit.

from ohms law:

I = VS / Rtotal

So you need to find Rtotal by calculating the two resistances (lamp and resistor) using the voltage/current graphs.

The resistor is easiest since the gradient is a straight line and again from ohms law:

Rresistor = V / I

The lamp is more tricky since the points on the plot are not on a straight line. You can choose to draw a line that best fits the points and then using the same method for the resistor above, find Rlamp.

Alternately, find the average resistance of the lamp by working out the resistance value for each pair of points on the voltage/current graph. Then find the average of those resistances:

Rlamp = (R2V + R4V + R6V) / 3

Finally, use the two resistance values above (lamp and resistor), together with the supply voltage (14V) plugged back into the equation at the beginning and bingo. :smile:

Hint:

Spoiler

(edited 6 years ago)
Thank you. That was very helpful.
(edited 6 years ago)

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