How to Determine EMF for Desired Current Through a Resistor?

  • Thread starter Thread starter Snazzy
  • Start date Start date
  • Tags Tags
    Parallel
Click For Summary

Discussion Overview

The discussion revolves around determining the electromotive force (EMF) required to achieve a specific current through a resistor in a circuit involving two different EMFs. Participants explore the application of Kirchhoff's laws and the implications of connecting voltage sources in parallel, as well as the mathematical approach to solving the circuit equations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant questions how to determine the EMF needed for a 1.78 A current through a 7.00 Ohm resistor, indicating uncertainty about where to start.
  • Another participant suggests using Kirchhoff's laws and notes that the voltage across the resistor can be calculated as 12.46 V.
  • A participant states that the voltages in other branches must also equal 12.46 V and formulates three equations based on this assumption.
  • Another participant challenges the count of equations and unknowns, asserting that there are three equations but only two unknowns.
  • Concerns are raised about the practicality of connecting two voltage sources in parallel, with one participant noting potential risks such as explosions or damage to batteries.
  • Discussion includes the idea that real voltage sources have internal resistances, which complicates the parallel connection of ideal sources.
  • A participant describes real-life scenarios where voltage sources are connected in parallel, such as jump-starting a car battery.
  • One participant expresses confusion while solving the equations and receives advice to clarify the direction of currents in their equations.
  • A later reply indicates that the confusion was resolved, and the equations now work out correctly.

Areas of Agreement / Disagreement

Participants express differing views on the implications of connecting voltage sources in parallel, with some arguing it is problematic while others provide examples of practical applications. The discussion remains unresolved regarding the best approach to solving the circuit equations, as participants have varying interpretations of the equations and their relationships.

Contextual Notes

Participants mention the importance of correctly identifying current directions in circuit equations and the potential for confusion when applying Kirchhoff's laws. There is also a discussion about the limitations of ideal voltage sources versus real-world applications.

Snazzy
Messages
470
Reaction score
0
Good day, mates.

I was wondering if anyone could me out with this circuit question involving two different EMFs. I can't seem to figure out how to do it and I don't quite know where to begin.

1zcg7e9.jpg


What must the emf be in order for the current through the 7.00 Ohm resistor to be 1.78 A? Each emf source has negligible internal resistance.

I've exhausted all but one try for this question and I was wondering if anyone can shed some light on it.

Cheers.
 
Physics news on Phys.org
Snazzy said:
Good day, mates.

I was wondering if anyone could me out with this circuit question involving two different EMFs. I can't seem to figure out how to do it and I don't quite know where to begin.

1zcg7e9.jpg


What must the emf be in order for the current through the 7.00 Ohm resistor to be 1.78 A? Each emf source has negligible internal resistance.

I've exhausted all but one try for this question and I was wondering if anyone can shed some light on it.

Cheers.


Kirchoff's Laws are pretty good for this sort of problem...

Alternatively, since you know the value for the current through the 7R resistor, you can work out the voltage across it.

The solution follows from that...
 
Well yeah, the voltage across it is 12.46 V but I'm not sure where to go from there.
 
the voltage in the other two branches also have to be 12.46. So,

24 + 3*i1 = 12.46

E + 2*i2 = 12.46

i1+i2 = 1.78

That's 3 equations and 3 unknowns
 
No.

That's 3 equations and 2 unknowns...

Like I said, it's Kirchoff's laws...
 
Just curious . . I know how to solve this problem mathematically, but isn't putting two voltage sources in parallel a bad thing in real life?

Also, it appears the EMFs are not really in parallel according to the figure you provided.
 
It can be... though the example above is somewhat similar to connecting a battery charger to a car.

It's not a real life problem as such, but if you did that with real batteries, one would probably explode... or be otherwise damaged...
 
TheAnalogKid83 said:
Just curious . . I know how to solve this problem mathematically, but isn't putting two voltage sources in parallel a bad thing in real life?

Also, it appears the EMFs are not really in parallel according to the figure you provided.

Putting two voltage sources in parallel goes against Kirchoff's voltage law. In your circuit the 24V and the E source are not in parallel, since there are the 3 ohm and 2 ohm resistors in the circuit.
If both sources are providing current to the load, there is no problem. If one of the sources provides current both to the load and to the other source, there could be a problem if the source receiving the current is not rechargeable.
 
CEL said:
Putting two voltage sources in parallel goes against Kirchoff's voltage law. In your circuit the 24V and the E source are not in parallel, since there are the 3 ohm and 2 ohm resistors in the circuit.
If both sources are providing current to the load, there is no problem. If one of the sources provides current both to the load and to the other source, there could be a problem if the source receiving the current is not rechargeable.

i know this but what about real life when there is no such thing as an ideal voltage source. since there is impedance between all components of a voltage source, can they really be considered in parallel? And there are limits on current and power on the components as well .. so I've never actually put two in parallel. Do they explode or do they overheat or melt? What if the voltage difference is almost 0 (say a 5V and a 5.01V supply are put in parallel) to where KVL is almost satisfied, will a power supply still funciton then??
 
  • #10
TheAnalogKid83 said:
i know this but what about real life when there is no such thing as an ideal voltage source. since there is impedance between all components of a voltage source, can they really be considered in parallel? And there are limits on current and power on the components as well .. so I've never actually put two in parallel. Do they explode or do they overheat or melt? What if the voltage difference is almost 0 (say a 5V and a 5.01V supply are put in parallel) to where KVL is almost satisfied, will a power supply still funciton then??

Real voltage sources have internal resistances. In the circuit of the example you can see the two ideal voltage sources in series with the 2 and 3 ohm resistors as models of real voltage sources with the open circuit voltage represented by the ideal sources and the internal resistances as the resistors in series.
We do put real voltage sources in parallel often. If your car battery is discharged, you can put a good battery in parallel using jumpers and you are able to start the car.
The charging of the car battery is another example. The alternator plus rectifier is a voltage source that is in parallel with the battery and charges it.
 
  • #11
Okay, I said that

[tex]24-E -2I_2-3I_1=0[/tex]

[tex]24-7I_3-3I_1=0[/tex]

[tex]E-7I_3-2I_2=0[/tex]

[tex]I_1 + I_2 = 1.78[/tex]

And I proceed to solve for I1 using the 2nd equation, but when I plug that into equation 4 to see what I2 equals, and then use equation 3 to find E, I get a wrong answer.
 
Last edited:
  • #12
Snazzy said:
Okay, I said that

[tex]24-E -2I_2-3I_1=0[/tex]

[tex]24-7I_3-3I_1=0[/tex]

[tex]E-7I_3-2I_2=0[/tex]

[tex]I_1 + I_2 = 1.78[/tex]

And I proceed to solve for I1 using the 2nd equation, but when I plug that into equation 4 to see what I2 equals, and then use equation 3 to find E, I get a wrong answer.

You are confusing the directions of the currents in your equations. Subtracting equation 3 from equation 2 you get:
[tex]24-E +2I_2-3I_1=0[/tex]
where the term [tex]2I_2[/tex] has a different sign from that in equation 1.
Redraw your circuit with the directions of the currents clearly specified and use the Kirchoff's laws.
 
  • #13
Oh, thanks. It works out now.
 

Similar threads

Analyze this circuit with 2 sources and 4 resistors
  • · Replies 37 ·
2
Replies
37
Views
7K
Solving the Missing Resistor Problem in a Parallel Circuit
  • · Replies 6 ·
Replies
6
Views
3K
DC current through a resistor circuit
  • · Replies 6 ·
Replies
6
Views
3K
How do I find total current when there are two emfs?
  • · Replies 4 ·
Replies
4
Views
2K
Calculating Current in a Series RC Circuit
  • · Replies 4 ·
Replies
4
Views
3K
Engineering Calculating the voltage at a certain Resistor
  • · Replies 3 ·
Replies
3
Views
2K
Engineering Unknown resistor in a series-parallel circuit
  • · Replies 17 ·
Replies
17
Views
13K
Is the source in parallel or series with the resistor?
  • · Replies 1 ·
Replies
1
Views
2K
Coil and resistor in parallel -- How to calculate the phase angle?
  • · Replies 4 ·
Replies
4
Views
3K
3 resistors in parallel: how to get there
  • · Replies 5 ·
Replies
5
Views
3K