1. Journal 4/19/17 Objective Tonight’s Homework
Have you ever seen one of these boxes? They’re usually found near neighborhoods and have something to do with electricity. They’re usually right next to the high power lines. What do you think these boxes are for?
Objective Tonight’s Homework
To learn how we change electricity to be usable
p 433: WDYL 1-3

2. Transformers
Last class we talked about transformers. How magnetism can go from one loop to another and change the voltage.
Now we want to measure this.

3. Transformers E V S N1 / N2 = V1 / V2
N1 = number of loops on the first transformer
N2 = number of loops on the second transformer
V1 = voltage of the first transformer
V2 = voltage of the second transformer
This equation tells us how much voltage we’re changing in a transformer.

4. Transformers
Example: The voltage in a high-powered cable carries electricity at 700,000 volts. Your house needs electricity at 110 volts. If the transformer box at the base of the pole has 20 loops on the low voltage side, how many loops should be on the high voltage side?

5. Transformers
Example: The voltage in a high-powered cable carries electricity at 700,000 volts. Your house needs electricity at 110 volts. If the transformer box at the base of the pole has 20 loops on the low voltage side, how many loops should be on the high voltage side?
N1 / N2 = V1 / V2
N1 / 20 = 700,000 / 110
N1 = (700,000 • 20) / 110
N1 = 127,272.7
N1 ≈ 127,273

6. Transformers
Our previous example just showed us that we would need a transformer with over 100,000 loops on one side! This is totally not practical.
So instead of jumping down in voltage all in one go, most power lines do it in steps.
Most power goes from the high- powered lines into an electrical substation, where the voltage drops several steps.

7. Transformers
Any transformer that causes a drop in voltage is called a step-down transformer. Similarly, any transformer that causes a rise in voltage is called a step-up transformer.
In general, the process of getting power to your house looks like this:

8. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?

9. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?
Let’s start with 1 transformer and see what it can do. If we want it to step down, we need the 50 loops on the high-voltage side. We’ll put just one loop on the opposite side to get as big a drop as possible.

10. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?
N1 / N2 = V1 / V2
50 / 1 = 500,000 / V2
(with a setup like this, we can flip both fractions)
1 / 50 = V2 / 500,000
V2 = (1 / 50) • 500,000
V2 = 10,000

11. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?
Ok. So we know that after 1 transformer, we can drop it from 500,000 to 10,000. Let’s do it again.

12. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?
N1 / N2 = V1 / V2
50 / 1 = 10,000 / V2
(with a setup like this, we can flip both fractions)
1 / 50 = V2 / 10,000
V2 = (1 / 50) • 10,000
V2 = 200

13. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?
Now we’re getting close! Now we just need to drop it from 200 volts to 120 volts. Let’s see how many loops our third transformer needs.

14. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?
N1 / N2 = V1 / V2
50 / N2 = 200 / 120
(with a setup like this, we can flip both fractions)
N2 / 50 = 120 / 200
N2 = (120 / 200) • 50
N2 = 30

15. Transformers
Example problem: A high-power line carries 500,000 volts. It needs to get changed to 110 volts before heading to a person’s house. If none of the transformers can have more than 50 loops, how many transformers do we need between the high power line and a home?
So there’s our answer. We’ll need 3 transformers, with the first 2 stepping things down at a ratio of 50:1, and the last one stepping things down at a ratio of 50:30.

16. Practice Problems
1) Calculate the number of loops needed in the secondary winding of a transformer if we want to transform a primary voltage of 300 volts down to a secondary voltage of 180 volts. The primary winding has loops.
2) A doorbell requires 6V to work.   It is connected to a transformer whose primary contains 2000 loops and is connected to 110-V household outlet.  How many loops should there be in the secondary? 
3) A transformer starts with a voltage of 300 V and ends with 800 volts. If it starts with 35 loops, how many loops need to be on the other side?

17. Exit Question
A transformer has 200 loops on the starting side, and 100 loops on the ending side. Is a transformer like this going to increase or decrease the voltage?
Increase
Decrease
Neither
Not enough information