**Start revising A-level & GCSE with 7 million other students**

# The Basics and RMS Values

## You are here

***Please note: you may not see animations, interactions or images that are potentially on this page because you have not allowed Flash to run on S-cool. To do this, click here.***

## The Basics and RMS Values

Cells produce currents that travel in the same direction all of the time, **direct currents.** However, this is not always useful - for instance, transformers will only work if the current is constantly changing.

**An alternating current is constantly changing direction.** It is normally sinusoidal.

The **frequency** of an alternating current supply, f, is the number of cycles completed per second.Measured in Hertz (Hz).

The period, T, of an alternating supply is the time taken to complete one cycle.

The **peak values** of **current, I _{o},** and

**voltage,V**are the maximum values at the crest or trough. They are equivalent to the amplitude of a wave. Sometimes we quote the peak-to-peak value, which is of course, double the peak value.

_{o},Wouldn't it be great if you could just use the electricity equations you are already comfortable with for d.c. circuits in your a.c. calculations? The problem is - what value do you put in, because the a.c. values are always changing.

Well, the **rms values** of current and voltage are the answer to your problem.

**RMS values are the d.c. equivalent of an a.c. value.** In other words, if you had two circuits, one d.c. and one a.c., and you wanted them to use exactly the same amount of power (energy each second) then you would choose the d.c. values of current and voltage to be the same as the rms values of current and voltage in the a.c. circuit:

The power being used in each circuit is the same, so you can use all your old electrical equations.

*For example,* V = IR, P = IV, P= I^{2}R.

For a.c. circuits, as long as you use the **rms values** of current and voltage, a.c. is no longer difficult!

**So how do you lay your hands on this magical value?**

**And**

**Where V _{0} and I_{0} are the peak values.**

** Note:** rms values are less than the peak values of voltage and current.

So, when given an a.c. question simply find the rms values (using the peak value and above equation) and then use your normal electrical equations.