An alternating current is a type of current which periodically reverses direction. The root mean square values are a type of average used for sinusoidal waveforms.
Equations
description
equation
R.m.s. voltage
Vrms=2V0
R.m.s. current
Irms=2I0
Variable definitions
quantity name
symbol
derived units
si base units
voltage
V
V
kgm2s−3A−1
current
I
A
A
Alternating current
An alternating current (AC) is a type of electric current which periodically reverses direction. It is used in generators, motors and power distributors due to being more efficient than direct current (DC), which is current flowing in only one direction.
An alternating current follows a waveform such as the one shown below:
1.
Current
2.
Time
The frequency of the current is the number of times the generators spin per second. The UK mains electricity has a frequency of 50Hz.
Root mean square voltage and current
Since alternating current follows a waveform it has peak values. These are the maximum values, regardless of sign, of sinusoidal waves.
The peak voltage V0 is the maximum voltage value, regardless of sign, of an alternating current.
The peak current I0 is the maximum value, regardless of sign, of an alternating current.
To determine the average voltage or current of an alternating current, a simple mean would not work as it will be 0 due to it being a wave. Scientists therefore use the root mean square (r.m.s.) values which are a different type of averaging technique.
The r.m.s. voltage Vrms can be calculated with the formula:
Vrms=2V0
The r.m.s. current Irms can be calculated with the formula:
Irms=2I0
Example
The UK mains electricity has a peak voltage of ±325V, calculate its r.m.s. value.
First, write down what you know:
V0=325V
Next, write down the formula for the r.m.s. voltage:
Vrms=2V0
Substitute the values and calculate the r.m.s. voltage:
Vrms=2325=230V
The r.m.s. voltage of the UK mains electricity is 230V.
Oscilloscopes
An oscilloscope is a type of voltmeter where you can manipulate how readings are displayed. You can do this by changing the time-base (x−axis) and the amplitude (y−axis). An oscilloscope can be used to obtain information about an alternating current, such as peak values for current and voltage.
1.
Display
2.
Time-base dial
3.
Amplitude dial
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FAQs - Frequently Asked Questions
What is an alternating current?
An alternating current is a type of current which periodically reverses direction.
What is a root mean square value?
A root mean square value is a type of average used for sinusoidal waveforms.
What is the peak value of a sinusoidal waveform?
The peak values of a sinusoidal waveform are the maximum values, regardless of sign, of the wave.