# Electricity-Resistance Question

• Bullkank

## Homework Statement

A Student needs to construct a circuit in which there is a voltage drop of 5.0v across a resistance combination, and a total current of 2.0mA flowing through the combination. She has a 4000 Ohm ( 4.0kOhm) resistor which she wants to use and proposes to add another resistor in parallel with it.

## Homework Equations

a) What value should she use for the second resistor

b) Determine the effective resistance of the combination

## The Attempt at a Solution

I have no idea how to work this out. The answers are

a) 6.7 kOhms

b) 2.5 kOhms

I just need to know how to get to that answer

Voltage drop across the "equivalent" resistance, meaning the resistance shown by the 2 resistances in parallel, equals 5V.

V = I*R => R = V/I

R = 5V/2mA = 2,500 Ohms

Now this R is the equivalent resistance R_eq, which can be found by:

1/R_eq = 1/R1 + 1/R2

or after rearranging: R_eq = R1*R2/(R1+R2) , where you know one of the resistors (R1 = 4 kOhm).