## How to find current and voltage drop without knowing a resistor value and total current?

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I need to find the current and voltage drop across the light bulb, but I wasn’t given the resistor value for the light bulb so I do not really know how to start. I do know that this involves Kirchoff’s rules.
I believe I am not suppose to approximate any values as this is a physics problem not a lab.

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### 2 Responses to “How to find current and voltage drop without knowing a resistor value and total current?”

1. Sourabh Patidar said:

We first have any awear ness about kind of that object that appx. What amount of electricity has been conducted by it and then also having awearness that in which approx. voltage it can be opereted then we can also conclude about its resistors, voltage,and current of it.

2. aaroonn said:

You cant get a voltage and current value with this, you will need to know something else like the power, voltage drop or current in any resistor, however, you can get the equation in function of a variable
English is not my First language so excuse me if its bad. When I say “Given something” I mean that that is the variable for the equations.

The Kirchhoff voltage rule: The total voltage in a closed loop = 0
The Kirchhoff current rule: The total currents in and out a node = 0

start from P clockwise: Vp-12+Vb=0

Voltage in bulb: Vb=12-Vp

The teacher must give you the power of an element (R1, R2, Bulb or Source), the current of an element (Total or in R1:10ohm), the resistance of the bulb (even if it wasn’t given to you you could write the equation in function of that) or the voltage in P (the easiest way).

Constants an variables:

Voltage in bulb: VbBulb current: IbCurrent in R1:I1
voltage in P: VpCurrent in P: I

Rb:Bulb resistance
R1:10ohm
R2:5ohm
Re:R1 in parallel with Rb
Re=R1*Rb/(R1+Rb)
Rt:total resistance

-Given Vp:

I=vp/5
I1=Vb/10

Vb=12-Vp
Ib=I-I1

-Given the resistance of the bulb:

Re=10Rb/(Rb+10)
Rt=Re+5

I=12/Rt
I1=vb/10

Vb=I*Re
Ib=I-I1

-Given the power

with the power you can get V or I

Power of the source: I=Ps/12

I in a resistor: Ix=(Px/Rx)^.5
V in a resistor: Vx=(Px*Rx)^.5

Get Vp or Ip and use the equations above

-Given Current

Get the Get Vp and/or vb and use the equations above

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