Two different EMFs in parallel

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In summary, the conversation involved a group of individuals discussing a circuit question involving two different EMFs and how to solve it using Kirchoff's Laws. They also discussed the potential dangers of putting voltage sources in parallel in real life and how to solve the problem mathematically. One individual was initially struggling with finding the solution but was able to solve it with guidance from others.
  • #1
Snazzy
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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.
 
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  • #2
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...
 
  • #3
Well yeah, the voltage across it is 12.46 V but I'm not sure where to go from there.
 
  • #4
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
 
  • #5
No.

That's 3 equations and 2 unknowns...

Like I said, it's Kirchoff's laws...
 
  • #6
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.
 
  • #7
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...
 
  • #8
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.
 
  • #9
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.
 

Related to Two different EMFs in parallel

1. What is the concept of "Two different EMFs in parallel"?

The concept of "Two different EMFs in parallel" refers to the situation where two different sources of electromagnetic force (EMF) are connected in parallel to a circuit. This means that the two EMFs are applied to the same circuit at the same time, resulting in a combined effect on the circuit.

2. How does having two different EMFs in parallel affect the circuit?

Having two different EMFs in parallel will result in a total EMF that is equal to the sum of the individual EMFs. This means that the circuit will experience a stronger overall force, potentially resulting in higher currents and increased power consumption.

3. What types of devices use two different EMFs in parallel?

Many electronic devices use two different EMFs in parallel, such as batteries, generators, and power supplies. These devices often have multiple sources of EMF to provide a steady flow of electricity to the circuit.

4. Can having two different EMFs in parallel cause any problems?

In some cases, having two different EMFs in parallel can cause problems with the circuit. If the EMFs have different frequencies or are out of phase with each other, they can interfere with each other and result in unstable or unpredictable electrical currents.

5. How can the effects of two different EMFs in parallel be controlled?

To control the effects of two different EMFs in parallel, it is important to make sure that the EMFs have compatible frequencies and are in phase with each other. This can be achieved through proper circuit design and using components that are specifically designed for parallel EMF applications.

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