You are Here: Home >< Physics

# Why cant I just use V=IR for the output voltage in this circuit? watch

1. Why cant I just use V=IR for the output voltage in this circuit? Is it because it wants the output voltage, not the voltage through the resistor? Is it that simple?

Thanks

2. Its a voltage divider network.
3. Same thing as on the other thread

(Original post by Joinedup)
Absolutely!

you'll do I=V/(Rfixed+RLDR) each time (resistances are added because they're in series)

then

VLDR=I x RLDR

then on to the next value of LDR resistance and calculate the current again.
except the stages are combined into one formula
so in this case

I=Vin / (R1+(R2) [eq1]

Vout=I R2 [eq2]

substitute eq1 for the I in eq2

Vout=(R2 Vin ) / (R1+(R2)

which is the same as what's shown in the book

Vout=Vin x R2/(R1+R2)

and all the rules are obeyed...
same current in both series resistors
sum of voltages across each individual series resistor adds up to the input voltage
voltage across each individual resistor equals the current through that resistor multiplied by it's resistance

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 27, 2017
Today on TSR

### Summer Bucket List is Back!

Start yours and you could win £150!

### GCSE Physics Equations sheet!

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE