AC Current

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

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.

Worksheet

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

0 2 4 6 8 10

Vo

ltag

e (

V)

Time (s)

AC Voltage

-10.0

-5.0

0.0

5.0

10.0

0 2 4 6 8 10

Cu

rre

nt

(mA

)

Time (s)

AC Current

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

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.

0

10

20

30

40

50

60

70

80

0 1 2 3 4 5 6 7 8 9 10

Po

we

r (m

W)

Time (s)

AC Power

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

Ave P

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.

Multimeters and most AC voltmeters and ammeters give a reading of the RMS value for voltage or current.

2

maxVVRMS

2

maxIIRMS

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;

Now because average AC power output (½ peak power) = DC power outputand so;

the same is true for I

AI

mA

mWI

R

PI

24

2

36

.

VV

V

PRV

58

236

.

2

2

22

2

21

max

max

max

;

VV

so

VV

R

V

R

V

RMS

RMS

RMS

2

maxIIRMS

Rectification

• Rectification is the process of converting AC into DC.

• Diodes set up in a bridge are one method of rectifying AC.

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

Exercises

ESA Pg 265

Activity 16A

ABA

Pg 176-179