You are Here: Home >< Physics

# Root mean squared values??? Watch

1. Hi, i dont get root mean squared values at all so i was wondering if anyone could help me understand root mean squared values of a sinusoidal wave.

Specifically where resistors are involved.

E.g. i have a question here where i dont know how to even start!

A sinusoidal alternating voltage which has an rms value of 230V is connected to a 60(ohms) heating element designed to operate at an rms value of 230V. Calculate

the peak voltage

The voltage change from positive peak to negative peak

The rms current

The peak current

The peak power

The mean power

2. (Original post by chemicalX)
Hi, i dont get root mean squared values at all so i was wondering if anyone could help me understand root mean squared values of a sinusoidal wave.

Specifically where resistors are involved.

E.g. i have a question here where i dont know how to even start!

A sinusoidal alternating voltage which has an rms value of 230V is connected to a 60(ohms) heating element designed to operate at an rms value of 230V. Calculate

the peak voltage

The voltage change from positive peak to negative peak

The rms current

The peak current

The peak power

The mean power

The peak value of current or voltage is the rms value x √2
This is a relationship you should know.

AC voltages and currents are usually quoted as rms values. For example, your mains domestic voltage supply, given as 240V, will be 240 V rms.
The peak value will be 240 √2 (about 339V)

You just use V=IR in this question to find unknown currents or voltages.
You can use peak or rms in the equation so long as you use the same for both current and voltage.
3. (Original post by chemicalX)
Hi, i dont get root mean squared values at all so i was wondering if anyone could help me understand root mean squared values of a sinusoidal wave.

Specifically where resistors are involved.

E.g. i have a question here where i dont know how to even start!

A sinusoidal alternating voltage which has an rms value of 230V is connected to a 60(ohms) heating element designed to operate at an rms value of 230V. Calculate

the peak voltage

The voltage change from positive peak to negative peak

The rms current

The peak current

The peak power

The mean power

Peak voltage swing = 2 x root2 x r.m.s.

+ve pk to -ve pk = root2 x r.m.s

I = VR

so for rms current use rms voltage and resistance

for peak current use pk voltage and resistance

P = V2/R or I2R

So for peak power use the peak voltage or current

Mean Power is the average continuous power. So use either rms voltage or current.
4. (Original post by uberteknik)
Peak voltage swing = 2 x root2 x r.m.s.

+ve pk to -ve pk = root2 x r.m.s

I = VR

so for rms current use rms voltage and resistance

for peak current use pk voltage and resistance

P = V2/R or I2R

So for peak power use the peak voltage or current

Mean Power is the average continuous power. So use either rms voltage or current.
(Original post by Stonebridge)
The peak value of current or voltage is the rms value x √2
This is a relationship you should know.

AC voltages and currents are usually quoted as rms values. For example, your mains domestic voltage supply, given as 240V, will be 240 V rms.
The peak value will be 240 √2 (about 339V)

You just use V=IR in this question to find unknown currents or voltages.
You can use peak or rms in the equation so long as you use the same for both current and voltage.

Hi sorry i know this is off topic but what is meant when a fully charged capacitor is discharged through a resistor with a time constant of 0.2ms?

is it thaat the charge will fall to 37% of its original value in 0.2ms? if it is why is it correct? to me it seems right but i dont know why!?

Thank you

PS your other explanation helped a lot!
5. (Original post by chemicalX)
Hi sorry i know this is off topic but what is meant when a fully charged capacitor is discharged through a resistor with a time constant of 0.2ms?

is it thaat the charge will fall to 37% of its original value in 0.2ms? if it is why is it correct? to me it seems right but i dont know why!?

Thank you

PS your other explanation helped a lot!
It's a result of the fact that when a capacitor C discharges through a resistor R the amount of charge on that capacitor falls exponentially (like radioactive decay) according to the formula

Q=Qo e (-t/CR)

This means that if t = CR then
Q=Qoe-1
or
Q=Qo/e
or Q=0.37Qo as 1/e = 0.37

The value of t that equals CR is called the time constant of the circuit.

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: May 12, 2013
Today on TSR

### Falling in love with him

But we haven't even met!

### Top study tips for over the Christmas holidays

Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• Poll
Discussions on TSR

• Latest
• ## See more of what you like on The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

• 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

Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.