What is Empty: Definition and 271 Discussions

In mathematics, the empty set is the unique set having no elements; its size or cardinality (count of elements in a set) is zero. Some axiomatic set theories ensure that the empty set exists by including an axiom of empty set, while in other theories, its existence can be deduced. Many possible properties of sets are vacuously true for the empty set.
In some textbooks and popularizations, the empty set is referred to as the "null set". However, null set is a distinct notion within the context of measure theory, in which it describes a set of measure zero (which is not necessarily empty). The empty set may also be called the void set.

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  1. S

    Empty Tank: Work Calculation w/ Origin at Apex of Cone

    I take the origin to be at the apex of the cone. Using the similarity of the triangle, where ##r## is radius of water and ##y## is height of water from the apex of cone: $$\frac{r}{y}=\frac{4}{10}$$ $$r=\frac{2}{5}y$$ The mass of water = ##\rho .V## = ##\rho . \pi r^2~\Delta y## = ##\rho . \pi...
  2. A

    I Does electron beam in empty space generate magnetic fields?

    Does electron beam in empty space generate magnetic fields around them just as with current in conductor. If yes, then is it experimentally proven that two parallel electron beam would attract each other.
  3. C

    Does a full fridge cost less money to run than an empty fridge?

    With the cost of living crisis. my fridge has a bottle of milk in it by itself most of the time. Would it cost less in energy if i fill the fridge with bottles of water to fill the gaps? hope this makes sense.
  4. CalvinandHobbes

    Engineering Electrical energy to empty reservoir with a siphon water pipe

    Calculation of the potential electrical energy generation of a siphon pipe, emptying an enclosed reservoir and end of siphon pipe connected to a pelton turbine with efficiency of 75% Known inputs: Height difference of siphon(Hd) = 4,5m Height lift of Siphon (Hu) = 6m Total siphon pipe length...
  5. Manshah

    Time to Empty a Tank with Water Flowing at 2h kg/s

    a tank is filled with water up to its brim a hole was made at the bottom of tank find time taken to empty tank if water flows at rate of 2h kg/s where h is height of liquid column and is equal to 20m radius is equal to h/2 [Moderator's note: moved from a technical forum.]
  6. Haorong Wu

    I Is the Vacuum Really Empty? Exploring Vacuum Fluctuations and Quantum Mechanics

    Hello, there. From another post, I read the insights Learn the Physics of Virtual Particles in Quantum Mechanics and Explore The Vacuum Fluctuation Myth in Quantum Theory. Also, I sometimes see a statement that the vacuum is not empty from some media. Most often, it says the vacuum can create...
  7. C

    Inserting pulling metal wires in existing empty conduit

    Inserting wires in existing empty conduit There is an empty PVC conduit size 1.5748" (40mm) in the main entrance panel. It was built around 8 years ago. There is metal wire (see picture above) inside the conduit to pull the main electrical wires from the inside or outside. Is there any...
  8. kyphysics

    Safe to Empty a "Pee Bottle" After 13 Days?

    So, another poster triggered this question in my mind when saying old urine can smell like ammonia. If this is TMI (talking about urine), please feel free to tune out of this thread. :-p For those sticking around, I use a pee bottle at times on the road (this was even pre-COVID, before...
  9. U

    I Explaining technicalities involving the empty set

    For instance, I attached two problems in the the thumbnail below. I’m curious why A cannot be the empty set in 18b, but A is not excluded from being the empty set in 17a. In 17a, if A is empty, then all the hypothesis can be satisfied (the composition will be empty too, obviously), but g need...
  10. funmi

    Voltage between two points on an empty wire and also across a resistor

    In a closed circuit the battery sets up a potential difference which causes charge to flow. I understand potential difference as the measure of electric potential energy per unit charge, the amount of energy required to move a charge from one point to another per charge. for example when a...
  11. S

    A Does the empty set have a complement?

    In an elementary school version of set theory, we can take the complement of the empty set to obtain ##\emptyset ^ C = \mathbb{U}## However, in a sophisticated version of set theory, the concept of a "universal set" ##\mathbb{U}## is problematical. ( So says the current Wikipedia article on...
  12. I

    Running a microwave oven empty

    So I know it's generally not a good idea, but I wondered what actually is the limiting factor. Does the magnetron heat up much more quickly? - and if so what's the physics going on that's causing it to heat up - is it increased current in the resonant cavities ? Also is it bad for the cathode...
  13. Flying_Dutchman

    Electrodynamics: divergence of E in empty space

    What is the physical significance of fundamental law del.E=0 in free space ?
  14. H

    I Can empty space be considered ‘elastic’ and expandable?

    The intention of this question is to get to the heart of the geometrical properties of space-time according to GR, and to focus entirely on what the theory allows, and not so much on what we actually see. I would like to consider a perfect vacuum, in a euclidian infinite void, completely devoid...
  15. Arman777

    I Why does an Empty Universe have to obey Negative Curvature?

    Its stated that empty universe should have a hyperbolic geometry (Milne Universe) but I don't understand how its possible. $$H^2=\frac {8\pi G\epsilon} {3c^2}-\frac {\kappa c^2} {R^2a^2(t)}$$For an empty universe when we set ##\epsilon=0## we get $$H^2=\frac {-\kappa c^2} {a^2(t)}$$...
  16. Arman777

    I Pair Production in Empty Space

    I am not sure this question has been asked here before but I am curious about it. From the Modern Physics Course, I learned that we need a nucleus to create an electron and positron pair (with a photon). And the reason is stated as to conserve linear momentum. If this is the case then how the...
  17. opus

    Learning Go: Why is the Empty Spot Neutral?

    Trying to learn the game Go. I have a question in regards to the uploaded pic. Why is the empty spot neutral? Couldnt white just place a stone there and capture all of the black stones since all of blacks liberties would be taken?
  18. stoky

    Fill an empty bottle with R-134a

    Homework Statement We want to fill a bottle with R-134a. The fluid comes from a fluid line. The bottle is initially empty, the bottle volume is 12 litres, the bottle is insulated (adiabatic filling). The fluid line contains R-134a at 20 bar, saturated steam. Homework Equations Using the...
  19. W

    I Empty Lattice approximation/Nearly-free electron model

    Hi all, I am having trouble understanding the some ideas presented in some notes I've been reading, help is greatly appreciated! I've uploaded screenshots of the material I'm referring to below, the last two images are what I'm mainly referencing, and the first few are to provide context...
  20. I

    How long will it take for a gas cylinder to empty?

    The main aim of my question is to find time taken by gas filled tank while emptying (from full to zero) it into atmosphere? I require a set of equations which might be used in the calculation of such a situation. Known parameters are: 1. Pressure inside the gas tank. 2. Dimension and shape of...
  21. Islam Hassan

    I Black Holes: Draining Empty Spacetime?

    If a black hole that is feeding is actually ingesting the spacetime around it (and hence whatever may lie in that spacetime including light) what does it do when it is not ingesting but nonetheless has very strong gravitational attraction? Does it sit there dragging empty spacetime into it? If...
  22. M

    When will the barrel become half empty?

    Homework Statement [/B] I'm not a native english speaker, so I apologize if my explanations are a bit awkward. I do not have a solution to this problem and I'd be grateful if someone could check this/help. I also have absolutely no idea how to even begin. Asked my colleagues, none of them know...
  23. A

    I How Does a Pressurized Tank Empty Over Time Through a Hole?

    I having difficulty wrapping my head around a concept that I wish to solve. I have a 10L tank of compressed air at 100psi. When an outlet hole of cross-sectional area of 0.115 in^2 is opened, I wish to develop a curve of pressure vs. time from t=0 until pressure drops to atmospheric. Many things...
  24. Mr Davis 97

    Showing that the empty set is subset of every set

    Homework Statement Let ##J = \emptyset## and ##A## be any set. Then ##J \subseteq A##. Homework EquationsThe Attempt at a Solution Proof: Suppose that it is false that ##J \subseteq A##. Then ##\exists x (x \in J ~~\text{and}~~ x \not \in A)## is true. But this is a contradiction, since...
  25. N

    I Can matter come into existence in empty space?

    A question arose over a simplification I wrote on another subject. My i information say's that matters antimatter pairs are generated in what is known to be very empty space such as the voids within the cosmic web. When I read about this it was considered anomalous but definitely verified. Now...
  26. R

    When is the tank empty? [differential equation]

    Homework Statement problem 23: A large tank is filled to capacity with 500 gallons of pure water. Brine containing 2 lbs of salt per gallon is pumped into the tank at a rate of 5 gal/min. The well mixed solution is pumped out at the same rate. Find the number A(t) of lbs of salt in the tank at...
  27. Jeirn

    How to Calculate Gas Cylinder Drain Time from Pressure to Atmospheric?

    Do you know where I can find a formula to calculate how long it would take to drain a cylinder under pressure to atmospheric pressure? if I know the volume of the cylinder and the size of the opening.
  28. T

    I Object suspended over an empty bucket

    I was recently tutoring a first year student, and a question of her assignment was as follows: Suppose that that you have a bucket of water over a scale. If you then partially submerge an iron rod in the water, while holding the rod so that it does not touches the bucket, will the reading on...
  29. D

    B Empty domains and the vacuous truth

    So, here's my question. I read somewhere that all universal truths on empty domains are vacuously true, whereas all existential are false. However, if all statements of the form (∀x∈A)(P(x)) , where A is an empty set, are vacuously true, then the statement (∃x∈A)(P(x)) should also be true...
  30. M

    MHB Intersection of all subspace of V is the empty set

    Hey! :o Let $V$ be a $\mathbb{R}$-subspace with basis $B=\{v_1 ,v_2, \ldots , v_n\}$ and $\overline{v}\in V$, $\overline{v}\neq 0$. I have shown that if we exchange $\overline{v}$ with an element $v_i\in B$ we get again a basis. How can we show, using this fact, that the intersection of all...
  31. Mr Davis 97

    I Relation of the empty set to vacuous truth?

    I need a clear-cut explanation of vacuous truth, as I can't seem to wrap my head around it. I guess this more precisely comes down to why we organize the truth table for the conditional statement the way that we do. Also, in connection to this, I'm wondering why the empty set is a subset of all...
  32. P

    I Why can't the interior of a black hole be empty?

    Can someone explain to me why there must be a real/meaningful space inside of a black hole? I have been autodidactically working on understanding the mathematical concepts that general relativity is based on, so I've never had anyone to ask questions to (until it occurred to me to find a forum...
  33. Quantum Velocity

    I How stuff in empty space cools down?

    If a body have heat and floating in space do it loss heat? If it does loss heat and the how? Thanks!
  34. durant35

    I How will the universe empty out?

    I have a problem understanding where does the matter/radiation go to because of the expansion of our universe. Eventually all normal matter and radiation will leave our causal patch, but the word leave is pretty unconvincing in this case. They will stay in their own causal horizon until each...
  35. H

    Error: no plot because it would be empty, using mcnpx

    Hello everyone! I would like to ask you if anyone can help me solving my Problem with mcnpx. I created a file with a watertank, a nozzle and a slit blend on the surface. I also inserted a protonbeam and would like to plot the dose and fluence for examplte with Moritz. But when I execute the...
  36. Arman777

    Electromagnetic Field in Empty Space

    Our Professor said there will be still an electric field in empty space even without charges.Hows that possible.Space-time has a some quantum property that acts like this ?
  37. Ed Lenarduzzi

    B Is the center of the universe empty?

    If the big bang through matter in every direction away from itself (the center) shouldn't there be a huge void there?
  38. T

    Voltage between two points on an empty wire

    I'm a computer science student. I'm currently learning electronics so that I can program embedded devices effectively. I'm reading from the free book at allaboutcircuits.com. Why is the voltage between two points on a wire in a closed circuit equal to zero? Note that between these two points...
  39. wolram

    B Weird quantum property of empty space?

    https://www.sciencedaily.com/releases/2016/11/161130082804.htm Is there any other detection's to back this up?By studying the light emitted from an extraordinarily dense and strongly magnetized neutron star, astronomers may have found the first observational indications of a strange quantum...
  40. V

    Flushing Data in Loop? -- Keep Entering Empty Space

    Hello, I am in my first class for C++, so I apologize in advance if this question is amateurish. To be clear, this project is already finished, and what I am trying to do already counted points off against me; I am not interested in cheating, I just want to know what the answer to my issue is...
  41. conquest

    I Theoretical problem with an empty seat going faster than light

    Hi everyone, Recently I randomly thought of a thought experiment of something going faster than light, along the lines of a shadow or some other "non-information carrying object". In this case it would be an empty seat. The idea is to take 800 million and one chairs of half a meter breadth...
  42. L

    I Empty Set Confusion: Why is it Not Counted in Cardinality?

    The empty set has a cardinality of 0, yet it is found in all sets and as we know any sets within a set are counted as a single element. But it never counted as an element in the cardinality of a set? For example: The set {1,2,3} contains 8 distinct subsets including the empty set, as taken by...
  43. L

    I Why is the empty set a proper subset of every set?

    I know what a proper subset is, but I never understood why every set has the empty set as its subset? I mean, is the reasoning something primitive like this: if I have x objects, the number of unordered sets of elements I can make are 2^x, including the case where I throw out x objects and...
  44. T

    Centre of Mass of an empty and filled goblet

    1. A goblet consists of a uniform thin hemispherical cup of radius r, a circular base of the same material thickness and radius as the cup, and an intervening stem of length r and whose weight one quarter of that of the cup. (a) Show that the height of the centre of gravity above the base is...
  45. T

    Bel-Robinson Tensor in empty spacetime

    Homework Statement This is Exercise 15.2 in MTW - See attachment Homework Equations See attachment The Attempt at a Solution [/B] My attempt at a solution is also in the attachment. Are my initial assumptions OK? If not can someone nudge me in the right direction. If my initial...
  46. C

    B What takes up the empty space of an atom?

    Please post scientific analysys like measurements. I want to see what they measured with measuring devices. I tried searching Google. Couldn't find any good paper on the subject. Only answers that it's empty and that it's not empty. Always contradictions when searching on Google!
  47. Dr. Strange

    What is the energy in a cubic km of empty space?

    If I have a 1 km cube of empty space at a constant temperature of 2.7 K, how much energy do I have? That is, if I know the total volume of space in the universe and I know what the average temperature of that space is, can I computer to total energy of the universe?
  48. G

    Ptolemy's Theory of the Empty Focus of an Elliptical Orbit

    An ellipse has two foci. For a planet in such an orbit, the star is at one of the foci. The other is empty. According to Ptolemy, if we draw a line connecting the planet and the empty focus, we will find that the line moves at a constant angular velocity. Is this true, or is it a crude...
  49. T

    If you were to empty a full tank 16 gallon tank of fuel, how

    I am kind of wondering how much for example CO2 you would put out or other greenhouse gases would be emitted everytime i use a full tank of gas. It would help me if you could explain it in volume, like how much it would take to contain the emissions on its own, and how much it would take to...