1. Alternating and Direct Current
St. Augustine Preparatory School
May 9, 2016

2. Two Types of Current
As you already know, there are two main types of electrical current:
Alternating Current (AC)
Direct Current (DC)

3. Users/Uses of Direct Current
Batteries
Fuel cells (electricity produced from chemical reactions)
Solar cells
Cell phones, flashlights; almost all electronics

4. What is Direct Current?
With direct current, electricity flows in one direction at a constant voltage and current.
This container of water can be
used as an analogy. Water only flows
in one direction. Once the tank is
empty, water will quit flowing.
Batteries are the same, but with
electrons, not water.

5. The following contains an animation of DC and AC current:

6. Alternating Current
Alternating current reverses the direction that it flows many times per second.
The frequency of AC in Canada, Mexico, Nicaragua, and the USA is 60 Hz.
The voltage of AC can be easily stepped up (increased) and stepped down (decreased).
Youtube Video on AC:

7. Electricity Transmission
Electricity is transmitted at high voltages (~115kV or 115,000V, but as high as 760kV depending on the country and area)
However, it is stepped down to around V before it enters your house.
High voltage and low current results in the least amount of lost energy during transmission.
The voltage is changed by using a transformer.

8. Transformers
Transformers allow us to change the voltage of AC easily by using electromagnetic induction.

9. Components of Transformers
Primary and secondary coils (input and output coils)
Soft iron core (easily magnetized)

10. Formula for Transformers
To complete calculations for transformers, we can use the following equation:
Voltage across primary = voltage across secondary
number of primary turns number of secondary turns

11. Example Problem
A transformer with a primary coil of 1200 turns and a secondary coil of 120 turns has 240 V connected to its primary. What is the output voltage? Is this a step up or step down transformer?

12. Solution 240V / 1200 Turns = V / 120 turns V = 24 V
Voltage across primary = voltage across secondary
number of primary turns number of secondary turns
240V / 1200 Turns = V / 120 turns
V = 24 V
This is a step down transformer.

13. Example Problem
What is the ratio of the primary turns to the secondary turns for a transformer that has an input of V and an output of V?

14. Solution
Voltage across primary = voltage across secondary
number of primary turns number of secondary turns
The ratio of the voltage will be the same as the ratio of the turns so:
np:ns = 20,000:400,000 = 1:20

15. Stepping Up/Down Voltage Affects Current
If voltage is stepped down by a factor of 20, then current will be stepped up by a factor of 20.
Bigger currents require thicker wire, so step down transformers will have primary coils of thin wire and secondary coils of thick wire.
Step up transformers will have primary coils of thick wire and secondary coils of thin wire.