Power of Transformer: Primary & Secondary Light Bulb

  • Context: Undergrad 
  • Thread starter Thread starter steve oliver
  • Start date Start date
  • Tags Tags
    Power Transformer
Click For Summary

Discussion Overview

The discussion revolves around the behavior of light bulbs connected to the primary and secondary sides of a transformer, specifically a step-up transformer. Participants explore the relationship between voltage, current, and power dissipation in the context of transformer operation, raising questions about calculations and measurements.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant observes that the secondary light bulb is brighter than the primary one and questions the apparent contradiction between using \( V^2/R \) and \( I^2R \) for power calculations.
  • Another participant points out that the transformer increases voltage while decreasing current, which may lead to confusion when considering the two sides independently.
  • A participant suggests analyzing the circuit as if the transformer were not present, using an equivalent resistance approach to clarify the situation.
  • There is a discussion about calculating power dissipated in two identical light bulbs connected to a step-down transformer, with different methods yielding conflicting results.
  • One participant asserts that power in the primary should equal power in the secondary, while another clarifies that this is not the case when considering power dissipated in the bulbs.
  • Questions arise about the relationship between currents in the primary and secondary circuits, particularly when considering different configurations of the bulbs.
  • Participants discuss Kirchhoff's current law in the context of connecting bulbs in the same circuit, leading to a combined current calculation.

Areas of Agreement / Disagreement

Participants express differing views on the relationships between voltage, current, and power in transformers, with no consensus reached on the calculations or the implications of connecting bulbs in various configurations.

Contextual Notes

Participants reference different methods for calculating power and express uncertainty about the assumptions underlying their calculations. The discussion highlights the complexity of analyzing transformer circuits and the potential for confusion when comparing primary and secondary sides.

steve oliver
Messages
12
Reaction score
0
when i compare the brightness of the light bulb in the primary transformer and the secondary transformer (step up transformer) the secondary light bulb is brighter . teacher say because of v^2/r after step up the v increase and so the power. however if i use i^2r after step up i decrease power should be decreased also. whu would i get a different result in different calculating methods
 
Physics news on Phys.org
steve oliver said:
however if i use i^2r after step up i decrease power

How are you measuring or calculating i?
 
I don't blame you for being confused. Your transformer raises the voltage and decreases the current, relative to what? Relative to the primary side voltage and current. But what happens on the secondary side also affects the primary side. Considering the two sides independently causes the confusion.

Look up transformers on Wikipedia. https://en.m.wikipedia.org/wiki/Transformer. In the ideal tranfpsformer equations , you can see that you can replace the transformer and the load resistance R, by a single resistance Rp=a*a*Rs, where a is the number of primary side turns divided by the number of secondary side turns, Rs is the load resistance of the light bulb. Rp is the apparent resistance of the light seen from the primary side.

Now you can analyze your circuit using V=I*Rp as if the transformer was not there. If you do that, then the seeming cotraditictions and confusion should go away.

If that is not clear, I can try to draw the pictures for you, when I get access to a real computer.
 
The point is that the current through the load does not decrease in this situation. The transformer increases the voltage and the load determines the current according to I = V/R. V has been increased (relative to what it was when the load was connected to the primary) so I increases. Hence my question..

CWatters said:
How are you measuring or calculating i?
 
i get it now thank you
 
Then in the following question how should i calculate.
Two identical light bubls x and y are connected respectively to the primary and secondary coils of an ideal step downtransformer.The turn ratio of the transformer is 2:1.Calculate the power dissipated in x and y.
method 1: i^2r x:y=1:4
method 2: v^2/r x:y=4:1
method 3 :vi x:y=1:1
why would be 3 different result?also shouldn't the power in primary =power in secondary?
 
You made mistakes somewhere...

steve oliver said:
method 1: i^2r x:y=1:4

ix = V/R
iy = 0.5V/R

So using W = i2R you get

Wx = (V/R)2 R = V2/R and
Wy = (0.5V/R)2 R = 0.25V2/R
Ratio is 4:1

steve oliver said:
method 2: v^2/r x:y=4:1

Vx = V
Vy = V/2

Using V2/R ...

Wx = V2/R
Wy = (V/2)2/R = 0.25V2/R
Ratio is 4:1

steve oliver said:
method 3 :vi x:y=1:1

Vx = V
Vy = 0.5V

Ix = V/R
Iy = 0.5V/R

Using W = IV

Wx = V * V/R = V2/R
Wy = 0.5V * 0.5V = 0.25V2
Ratio = 4:1

So they are all the same ratio 4:1
 
steve oliver said:
also shouldn't the power in primary =power in secondary?

You are confusing two different things..

a) Power in vs Power out of the transformer - yes it's the same
with
b) Power dissipated in the bulb when connected to the primary vs Power dissipated in the bulb when connected to the secondary - no it's not the same because the bulb draws more power when connected to the primary.
 
But shouldn't iy have double current that of ix?
 
  • #10
x and y are not the same circuit so Ix and Iy are not related by the transformer equation...

Circuit "x" has the bulb connected to the primary. In that situation there is nothing connected to the secondary of the transformer so the secondary current is zero. Therefore the current in the primary is also zero. The current in the bulb is V/R.

Circuit "y" has the bulb connected to the secondary. The current in the secondary is 0.5V/R (It's 0.5V because it's a 2:1 step down transformer). The current in the primary is equal to half the secondary current or (0.5V/R) / 2.
 
  • #11
I will try and do a drawing later. Have to go out for a few hours.
 
  • #12
Bulb XnY.jpg
 
  • #13
then what if the light bulb in primary coil is connected with the light bulb in secondary coil. i mean they are in the same circuit.
 
  • #14
Then the current drawn from the source V is just the sum of the two (kirchhoff's current law).

Itotal = Ix + Iin = (V/R) + (0.25V/R) = 1.25V/R

Everything else is the same.
 

Similar threads

Graduate What's different in a transformer core when coupling high & low power?
  • · Replies 77 ·
3
Replies
77
Views
7K
Undergrad Does a Step-Up Transformer Really Increase Light Bulb Brightness?
  • · Replies 4 ·
Replies
4
Views
2K
Maximum power from a transformer
  • · Replies 8 ·
Replies
8
Views
3K
Undergrad How does KVL work in transformers?
  • · Replies 2 ·
Replies
2
Views
4K
"Ground" and "Center Tapped" Power Transformer Secondary in guitar amp
  • · Replies 6 ·
Replies
6
Views
4K
Undergrad Getting a reduced voltage from a fixed secondary voltage of a transformer
  • · Replies 8 ·
Replies
8
Views
2K
Undergrad Short Circuiting the Secondary of a Transformer/Self Impedence
  • · Replies 17 ·
Replies
17
Views
4K
Transformer; make primary voltage x10 of the secondary voltage
  • · Replies 4 ·
Replies
4
Views
2K
Undergrad DIY Electromagnetic Induction Experiment
  • · Replies 2 ·
Replies
2
Views
3K
High School Melting a nail with high currents
  • · Replies 3 ·
Replies
3
Views
2K