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

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

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

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.

The following contains an animation of DC and AC current:

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: https://www.youtube.com/watch?v=YNkD9TZtdbY

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 110-120V 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.

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

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

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

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?

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.

Example Problem What is the ratio of the primary turns to the secondary turns for a transformer that has an input of 20 000 V and an output of 400 000 V?

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

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.