U=q+w or U=q-w

[Holland Smith 3C]

I have found conflicting information about this equation and have been getting confused. while looking at solutions and walk throughs online (and looking up the first law of thermodynamics) has the equation U=q-w. BUT the equation sheet for the exam is U=q+w. If anyone can clear this up that would be amazing!

[Chem_Mod]

The equation is U= q + w. The negative appears because you need to evaluate if the work value is positive or negative. For example, is the system in question doing work or is work being done in it? Answering this will give you the sign of the work value and that can be plugged into the equation.

[AlexandriaHunt2J]

Hi! To further elaborate on the sign of work, if work is being done by the system, then its sign will be negative (representing a decrease in energy). If work is being done on the system, then its sign will be positive (representing an increase in energy). However, I'd say it's easiest to use the equation U = q+w. Alternatively, because w = -(p)(deltaV), you can also use the equation U = q - p(deltaV).

[405669838]

The equation is U=q+w, but sometimes work is negative, which is when you may get the negative sign.

[Kaira Shibata 1E]

The equation is U=q+w. However the subtraction can occur if the system is doing work (ie work is negative).

[Lesley Kim 1K]

The standard equation is U=q+w, but w might be negative if the change in volume is negative making the equation U=q+w. Another way to think of it is to determine whether work is being done ON the system or BY the system. If work is being done BY the system, w should be negative making the equation U=q+w. However, if work is done ON the system, work is positive and the equation should remain U=q+w.

[Martha Avila 1I]

Hello. So the correct formula is U = q + w. Like my peers have stated the only way that your equation will be U = q - w is if your work is a negative number. Essentially you just need to check the sign of your work and if it is positive you will use U = q + w, if it is negative you will use U = q - w. Hope this helps!

[Aanjaneyaa]

The main equation is U=q+w. The w changes when it is positive or negative.

[105743571]

I like to look at it conceptually. If your system is doing work on the surroundings then w is negative. If your system is getting work done on it, then w is positive. If your system is losing heat, then q is negative. If your system is absorbing heat, then q is positive.

[Andrew Nguyen 1E]

The equation ΔU = q - w assumes that the system does work or expands if its a gas, which isn't always true.
ΔU = q + w is the general form for the change in internal energy.

[Sidharth Paparaju 3B]

It depends on how you define w. Since we define w as -pdeltav, it means that when work is done on a system, w is positive, so we add w or U=q+w.

[Madison Nguyen 3L]

The positive or negative sign of w is entirely contingent on how work is being done: either on the system or by the system. If work is being done on the system, then the system is gaining energy, so therefore work is positive. If the system is doing work, it is "exerting" energy, so by the same token, it's losing energy in the form of kJ or J and is negative.

[Anna Furton]

Hi, the equation is ∆U=q+w! However, the confusion might come from the fact that work, or w, is often negative. w is negative when work is done by the system, and w is positive when work is done on the system.

Also, be careful using U instead of ∆U. This equation is useful for calculating the change in internal energy of the system (how much energy is added or taken away in the form of heat or work), not the total energy overall.

[Amanda Dankberg 1B]

The main equation is deltaU=q+w but both q and w can be negative depending on wether heat is absorbed or released or wether work is being done on or by the system

[Erin Chin 1L]

The equation for the change in internal energy is deltaU = q+w but you have to determine whether work is being done on the system or by the system. If work is being done by the system (losing energy), w will be negative, so in that case, it would be q-w.

[305589333]

U=q+w is the correct formula; however, work can be negative in which your answer will be negative.

[Eszter Kovacs 1A]

U=w+q, if the system does work w will be negative.

[Emily Quill 2H]

The equation is U=q+w, but w can be negative or positive depending on whether work is done on or by the system.

[Madelyn_Rios_2c]

The equation is U = q + w, but w will be negative when the system is doing work.

[Isabelle Rieke-Wey 2G]

Hi!
the equation is W=q+w, but when the system is doing work, w will be negative.

[Shawn Suen]

It depends on the type of work. If the work is adding energy to the system, then the w will be positive, but if the system is doing work, which takes energy out of the system, then the w will be negative.

[Madison Rhynhart 3H]

The formal equation would be U = q+w however, the negative sign in front of the w may have something to do with if the work is either negative or positive. However, if we want to denote the use of negative we can merely write, U = q + (-w) which would be the same as U = q- w. Therefore these are related equations just changing the sign of w depending on if the system or an external factor is doing the work (losing or gaining energy).

[erud]

The correct equation is U= q + w.

[Emily Wan 1l]

The standard form of the equation is U = q + w. We should determine if the sign of w is positive or negative based on if the system is doing the work or if work is being done to the system.

[Arden Napoli 1E]

Hi,

The equation is U=q+w, however w can be negative based on if the system is doing work

[Alice Weber 3I]

Hi. This depends on whether work is negative or positive. To figure that out, you must find out if work is being done on or by the system.

[Misha_Khan_2H]

The original equation is U=q+w. Sometimes when work is being done by the system, then work becomes negative, so the equation become U=q-w.

[Rishab_Haldar_3B]

The equation is ∆U=q+w. w is negative when work is done by the system and is positive when work is done on the system.

[sophiavmr]

The equation is U=q+w, in which w is negative when the system is doing work, and w is positive when work is being done to the system

[Trisha Badjatia 2L]

Hi! I think this depends on how the equation frames work. Typically, we use ∆U = q + w because if the system does work, that work is negative, and is seen as energy leaving the system. I assume the equation ∆U = q - w is including the negative sign to account for work the system is doing. I would just use the first equation though. Hope that helps!!

[Allen Mayo 1E]

To clear it up, w is work and work can either be positive or negative so there is no set sign for the value of work. Therefore, it all depends on the value of work.

[Irene Kim 3E]

You can also think of it this way - q is the energy already within the system, w is the energy leaving/entering the system, and U is the total energy taking into account both q and w. Therefore, if energy is entering the system, it is a positive change and we add w, whereas when energy is leaving the system, it is a negative change and we subtract w.

[Mya_Chiarappa_2C]

The equation is U = q+w, but sometimes w is negative when work is done by the system.

[ThomasL1L]

If work is being done by the system ie. the system is expanding then the value is negative but when the system is being compressed then the work volume should be positive.

[Samantha Toscano 2C]

The equation stands as U=q+w, but we need to determine if the work is + or -. Then if the work is either positive or negative, you input that value into the equation. For example, W= - 109 then you would write it out like this (U=q+ (-109)).

[Melody Khoshneviszadeh 3C]

U is change in internal energy and it is summed up by work done by the system and heat but if work is done on the system then w is positive and if work is done by the system then w is negative.

[Nicole Weinstein 3E]

The equation is U=q+w, but can change if the system does work on its surroundings, making it -w instead. Releasing heat can also lead to a -q. To know which is right, it is usually signified in the wording of the question!

[Joelle Tran 1K]

It should be U = q + w. The + or - in front of the w is determined by if the problem says work is done to (+) or by (-) the system, just like when it says that heat (q) is released (-) or added (+) to the system.

[Hannah Carsey 1B]

The general change in the internal energy equation is ∆U = q + w, though the signs of q and w are dependent on the details of the problem you are solving. If heat is going into the system then q is positive (+q), if heat is going out of the system then q is negative (-q). If the system is losing energy in the form of work of expansion (work is being done by the system) then w is negative (-w), if the system is gaining energy in the form of work of compression (work is being done on the system) then w is positive (+w). Then solve for ∆U according to the sign and values of q and w.

[SuryaDham 3E]

The equation is U = q+w, this can be confusing since w = -PdeltaV.

[Rebekah Jung 1C]

The sign for w depends on the context of the problem. Positive, +w, would indicate that work is being done to the system. Negative w would indicate work is being done by the system.

[Aaron Kim 1J]

The equation is U = q + w. However, when a system does work on the surroundings, w is a negative value, creating -w.

[Chris Van 2J]

The true equation is U = q+w, however, this can change when work is negative.

[Hannah Thornton 1F]

The correct formula is U = q + w !

[Natasha]

The correct formula would be U = q +w but it can fluctuate depending on whether or not work is being done on the system or the surroundings.

[Joanne Sarsam 2F]

It depends upon the sign of work. In the most general form we can write the equation as U=q+w. If work is negative (the system is doing work on the surroundings), we can modify the equation to be U=q-w. If the system gains energy tehn work is being done ON the system BY the surroundings so work is positive and U=q+w. So, it's all situational!

[Joanne Sarsam 2F]

It depends upon the sign of work. In the most general form we can write the equation as U=q+w. If work is negative (the system is doing work on the surroundings), we can modify the equation to be U=q-w. If the system gains energy tehn work is being done ON the system BY the surroundings so work is positive and U=q+w. So, it's all situational!

[Madison Kiggins 1E]

Hi, the equation is always U=q+w, but since work is sometimes negative (when the system is doing work), it will be -w. In this case, the equation will end up being U=q+-w, which could be written in a simpler manner of U=q-w.

[elenadworak]

The equation sheet says U = q + w, its up to you to say if work done is positive or negative. Following the equation sheet will make things simpler and prepare you better for tests too if you are familiar with those forms of equations

[505734174]

It depends on the whether the sign for work is positive or negative. In all honesty, this can be really confusing so my best advice is to just look at the problem carefully and figure out what is doing the work, the system or the surroundings.

[isabella garcia - 1G]

The equation is U = q + w, but in certain instances where w is negative, the + sign is omitted and replaced with a - for simplicity.

[Daniel Tabibian 3K]

The equation is U=q+w. The only time it would be q-w is if the work is negative.

[Jonathan Shyu 3L]

The official equation is U=q+w. However, the situation that w is subtracted or negative comes from whether work is being done by the system or on the system. It is better to think of U=q+w, that way if work is being done on the system and w is positive, it is just added. If work is being done by the system on the surroundings, w is negative, thus it is being subtracted or to reduce confusion, add a negative value.

[Kayla Ziebell 1H]

If the system is doing work then the value of w is negative. This gives us U=q-w. On the other hand, if the system is getting work done on it then you would use the equation U=q+w, where w is positive.

[Polo Morales 3C]

The equation is U=q+w. Sometimes, however, the value of w is negative. However, this would just be adding a negative value. Therefore, the equation holds true. Hope this helps!

[Esther Kim]

U=q+W

[kaylasaens2A]

Great question! The correct equation is U = q + w. You will only utilize the U = q - w is if your work is a negative number. Ultimately, use U = q + w if your work is positive and U = q - w if your work is negative.

[Evan L 2B]

Either equation is correct, but you need to evaluate whether the work is done on or by the system. If work is being done on the system then energy is being added and you should add the value of the work done. If work is done by the system then energy is lost and you should subtract the work done.

[Tristan Friet 3G]

U=q+w is correct. When we're looking at a system that is at constant pressure or when expansion work is being done, that where the negative comes in as we get the equation ΔU=ΔH - PΔV. So it's more dependent on if the work is compression (positive) or expansion (negative)

[Sophia Dominguez 3I]

U=q+w is the correct equation. The positive and negative before w changes depends on the work being done. For instance, if work is being done on the system, w will be will be positive and the equation will be U=q+w. If work is being done by the system, w will be negative and the equation will be U=q-w.

[Aeson Salcedo 3L]

The equation is U=q+w. However we will sometimes have a negative number for work which is then why it is seems as if its -w.

[radhika singh 1A]

It is important to keep in mind that it is delta U and not just internal energy

[Kendall Brown]

It is U = q+w

[Gabby Burgess 2I]

The equation is the U=q+w one, but you may get negative values for w which can make it negative, but I would not memorize the equation that way.

[azurexrose]

The overall equation is U = q + w, but you need to look at whether the work done is positive or negative.

[Allie Kinsey 3H]

For the majority of the time are we going to need to solve to q and then solve for delta U afterwards, or are we given q for some problems?

[Anabelle Pham 2C]

The sign will be negative if the system is doing work. The sign will be positive if you are doing work on the system.

[Emily Ang 2B]

In some cases we might have to solve for q, so to be on the safe side, I think we should just be aware of how to solve it. One thing I would think we would have to look out for is that when having to solve q is that it also refers to a change in enthalpy. So the equation could also look like : U = (delta)H + W

[Emma Fernandez 1C]

The equation is U= q + w, but sometimes w will be negative so it will change the equation to be U = q - w. This is when work is being done by the system.

[Matthew Do 2G]

The equation itself is U= q + w.

However, I like to avoid being strict with these positive/negative signs in the equation by taking note of the system's changes. For example, if there is heat applied to the system, q would be positive. And if the equation is putting work out of the system (for example, volume getting larger), then w would be negative. To sum it all up, if there is something going in, it is positive, if there is something going out, it is negative.

I hope this helps!