AS91526, NCEA Level 3 Physics, Alternating Current, Electricity

1. AC Current

2. AC -Alternating Current
In alternating current (AC) the movement of electric
charge periodically reverses direction. In direct current
(DC), the flow of electric charge is only in one direction.
AC electricity is widely used because:
• It is easily produced by generators
• The maximum voltage can be changed easily using a
transformer
• It can be controlled by a wide range of components
• It has a regular frequency which is useful for timing

3. AC Voltage
• The oscilloscope is
used to measure AC
voltage, period and
frequency
• The oscilloscope display
represents a graph of
voltage against time,
this graph can also be
drawn using the phasor
diagram.

4. Worksheet

5. AC Current
The current flowing
through the circuit can be
calculated using Ohms’
Law (V=IR) and is usually
measured with an
oscilloscope.
Notice that voltage and
current through the
resistor are in phase.
15.0
5.0
-5.0
-15.0
AC Voltage
0 2 4 6 8 10
Voltage (V)
Time (s)
10.0
5.0
0.0
-5.0
-10.0
AC Current
0 2 4 6 8 10
Current (mA)
Time (s)

6. AC Voltage and Current –looking closely
The average values for V and I over one cycle are zero,
because they have equal sized positive and negative
values.
So how do we assign a value to represent the effect of V
or I in an AC circuit? It is not really zero.
What evidence do you have to say that it is not zero?
Time (s) 0 1 2 3 4 5 6 7 8 9 10 Ave
Voltage (V) 0.0 7.1 11.4 11.4 7.1 0.0 -7.1 -11.4 -11.4 -7.1 0.0 0.0
Current (mA) 0.0 3.5 5.7 5.7 3.5 0.0 -3.5 -5.7 -5.7 -3.5 0.0 0.0
Power (mW) 0 24.9 65.1 65.1 24.9 0 24.9 65.1 65.1 24.9 0 32.7

7. Power Output of an AC Circuit
Considering the power output of one AC cycle is more useful.
The power output has two peaks as the power output is the
same no matter which direction the current flows.
Notice that the average power output is exactly half the peak
power output.
80
70
60
50
40
30
20
10
0
AC Power
0 1 2 3 4 5 6 7 8 9 10
Power (mW)
Time (s)
Ave P
Time (s) 0 1 2 3 4 5 6 7 8 9 10 Ave
Power (mW) 0 24.9 65.1 65.1 24.9 0 24.9 65.1 65.1 24.9 0 36

8. The RMS Value of AC Voltage
• The ‘average’ effect of V and I in an AC circuit is given
by the Root mean square (RMS).
• The RMS value of an alternating voltage or current is
the DC value that would produce the same average
power output.
max V
2
VRMS 
max I
2
IRMS 
Multimeters and most AC voltmeters and ammeters
give a reading of the RMS value for voltage or
current.

9. RMS –the maths
• The RMS value of an alternating voltage or current is the DC value that
would produce the same average power output.
• For the circuit we have been considering with an average power output of
36mW
• The same power output for a DC circuit (using P=I2R and P=V2/R) would be;
mW
36
mA
I
I
P
R
2


I  4 .
2
A
V PR
V
 
36 2
 .
Now because average AC power output (½ peak power) = DC power output
and so;
the same is true for I
V 8 5
V

RMS
2
2
2
2
2
1
2
max
max
max
;
V
so
V
V
V
R
V
R
V
RMS
RMS



max I
2
IRMS 

10. Rectification
• Rectification is the process of converting AC
into DC.
• Diodes set up in a bridge are one method of
rectifying AC.

11. Exercises
1. An AC circuit has a peak voltage of 16.97V. Calculate
the RMS voltage of this circuit.
12V
2. Calculate the RMS current in an AC circuit with a peak
current of 3.5mA.
2.5mA
3. A voltmeter shows a RMS voltage of 230V. What is the
peak voltage?
325V
4. An ammeter in a AC circuit gives a RMS reading of
10A. What is the peak current in this circuit?
14A

13. Exercises
ESA Pg 265
Activity 16A
ABA
Pg 176-179