# How do you find the p.d across a resistor?

Asked by Timebomb (240) December 21st, 2009

Lets say I have a series circuit, 10v flowing through, two resistors one being a 2 ohm another a 3ohms.

What’s the p.d. across the 2ohm resistor?

I’m more interested in knowing how I work this out than the answers but what ever way helps in explaining it to me would help.

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Whats p.d

DrBill (16036)

@DrBill
Potential difference in other words voltage.

Timebomb (240)

You might want to send a comment to Ivan.

He is our physics nerd.

jrpowell (40429)

I think by P.D. You mean Power Dissipation? This is measured in watts.
This link will take you to a chart of Ohms law that will help.
I won’t do it for you but I will give you some help.
You need to calculate current for the entire circuit first then use this to calculate watts for each resistor.
http://www.aoce.com/images/EV_EFUNm01s034e_ohmsLawFormulas2.jpg

csimme01 (550)

@csimme01
No I said it means potential difference.

Timebomb (240)

@timebomb Then you mean the voltage.
Total Voltage = 10 Volts
Total Resistance = 5 Ohms (R1+R2)
V/R = I
Total Current = 2 Amps
I*R=V
I*R1=V1
I*R2=V2
As a double check..
V1+V2 should equal Total V.

csimme01 (550)

According to Ohm’s Law V = IR.

ETpro (34378)

@csimme01
Cheers I thought it would be like that.

Timebomb (240)

First of all you need to tell if the resistors are connected in parallel or in series.If they are connected in parallel,the PD across them is equal to the total voltage drop = 10V.
If they are connected in series, you have to calculate the current flowing through the circuit
which would be 2 amperes in this case(10/(2+3))
.
Next step is multiply the current with the resistance value of each resistor to get the voltage drop.In the above case it will be 4v and 6v respectively according to Kirchoff’s law.

@engineeristerminatorisWOLV “Lets say I have a series circuit,.,.” That means that the second case is correct.

jerv (31025)

Both @csimme01 and @engineeristerminatorisWOLV are correct in that you need to know the total amperage before you can solve for the individual voltage drops.

However, I have to say that @csimme01 did a better job of showing how to calculates it and saved me a bit of typing. (Sorry @engineeristerminatorisWOLV )

jerv (31025)

@jerv : Thanks for the approval.I’ve to agree with you that @csimme01 did it in a more systematic and detailed way.

Awww.. Geee whiz blush Thanks guys.
I use Mr. Ohm quite a bit at work.
@Timebomb I have never heard it called D.P.
Where are you from?

csimme01 (550)

V = IR, where R is the sum of all series resistances.

Ivan (13419)

Personally, I learned Ohm’s Law with voltage represented by the letter E, but c’est la vie.

Simple algebra dictates that if E=IR then I=E/R and R=E/I

jerv (31025)

@jerv
In school E=IR was the rule.
Personally I think V=IR is easier to teach and understand for the novice.

csimme01 (550)

@csimme01 Yeah. I never really understood why they used E anyways. I always thought E meant Energy, not voltage.

jerv (31025)

@jerv :We also know Voltage as EMF or electromotive force(Though, it’s not a force in physical sense) and hence E in short just like V for voltage..

@engineeristerminatorisWOLV That makes sense then. Thanx.

jerv (31025)

Yeah, in certain circumstances, the Greek letter Epsilon is used to represent potential difference, which is often referred to as “E”.

Ivan (13419)

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