Transformer current

  • #1
tanaygupta2000
204
14
Homework Statement:
An electrical device operates on 9V and has a resistance of 21 ohm. It is connected to a power supply of 120 V output through a transformer. The current in the primary of the transformer is? (a) 0.032 A, (b) Cannot be determined from the information supplied, (c) 0.23 A, (d) 2.32 A
Relevant Equations:
Primary power = Secondary power
VpIp = VsIs
Okay, so according to the transformer equation, VpIp = VsIs
In this question, I know I have to calculate Ip.
I think given, Vp = 9V, Vs = 120 V
Now I don't know what to use in the value of Is.
Please help!
The answer is 2.32 A.
 

Answers and Replies

  • #2
berkeman
Mentor
64,195
15,446
Homework Statement:: An electrical device operates on 9V and has a resistance of 21 ohm. It is connected to a power supply of 120 V output through a transformer. The current in the primary of the transformer is? (a) 0.032 A, (b) Cannot be determined from the information supplied, (c) 0.23 A, (d) 2.32 A
Relevant Equations:: Primary power = Secondary power
VpIp = VsIs

Okay, so according to the transformer equation, VpIp = VsIs
In this question, I know I have to calculate Ip.
I think given, Vp = 9V, Vs = 120 V
Now I don't know what to use in the value of Is.
Please help!

The answer is 2.32 A.
The more traditional direction for such a power transformer is to call the 120Vrms input the Primary side, and the 9V output the secondary side.

You have the secondary/output power determined by the 9V output and 21 Ohm load resistance. Equating input and output powers (for a first approximation which does not include losses in the transformer) will let you calculate the input current at 120Vrms to generate that power.

The answer is 2.32 A.
That looks very wrong to me. Can you post an image of the problem to be sure that you have transcribed it correctly? Thanks.
 
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  • #3
tanaygupta2000
204
14
The more traditional direction for such a power transformer is to call the 120Vrms input the Primary side, and the 9V output the secondary side.

You have the secondary/output power determined by the 9V output and 21 Ohm load resistance. Equating input and output powers (for a first approximation which does not include losses in the transformer) will let you calculate the input current at 120Vrms to generate that power.


That looks very wrong to me. Can you post an image of the problem to be sure that you have transcribed it correctly? Thanks.
Sure sir
 

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  • #4
tanaygupta2000
204
14
Please note sir that these answers are not always correct, as I found many wrong.
 
  • #5
cnh1995
Homework Helper
Gold Member
3,477
1,163
You have the secondary/output power determined by the 9V output and 21 Ohm load resistance. Equating input and output powers (for a first approximation which does not include losses in the transformer) will let you calculate the input current at 120Vrms to generate that power.
@tanaygupta2000, I think this is what the problem is all about. Power balance.
If you follow this procedure, your answer will match one of the options (not d of course ;))
 
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  • #6
tanaygupta2000
204
14
Option - (a)
 
  • #8
Vince Kamboj
1
1
Homework Statement:: An electrical device operates on 9V and has a resistance of 21 ohm. It is connected to a power supply of 120 V output through a transformer. The current in the primary of the transformer is? (a) 0.032 A, (b) Cannot be determined from the information supplied, (c) 0.23 A, (d) 2.32 A
Relevant Equations:: Primary power = Secondary power
VpIp = VsIs

Okay, so according to the transformer equation, VpIp = VsIs
In this question, I know I have to calculate Ip.
I think given, Vp = 9V, Vs = 120 V
Now I don't know what to use in the value of Is.
Please help!
The answer is 2.32 A.
Secondary power = Vs^2/Rs = 9×9/21 = 3.86W
Primary power = Vp×Ip = 120I
VpIp = VsIs
I = 3.86/120 = 0.032A
 
  • #9
berkeman
Mentor
64,195
15,446
Secondary power = Vs^2/Rs = 9×9/21 = 3.86W
Primary power = Vp×Ip = 120I
VpIp = VsIs
I = 3.86/120 = 0.032A
Welcome to PF, Vince. Since the OP has solved the problem a number of months ago, it is okay to post your solution to the problem. In general, though, in the schoolwork forums it is not allowed to post solutions to schoolwork/homework problems until after the OP has solved the problem correctly. The student must do the bulk of the work here at PF.

Again, Welcome to PF. :smile:
 

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