Gavin Howe
The concept of negative pressure in gases is a subject of debate among physicists. While some argue that pressure cannot be negative, others suggest that real gases, as opposed to ideal gases, may sustain negative pressure due to attractive intermolecular forces. This idea stems from the understanding that liquids can sustain negative pressures because of these intermolecular forces. However, gases have larger distances between molecules, making the attractive forces weaker, and any gas that could potentially sustain negative pressure would immediately condense into a liquid. The term negative pressure is often used in engineering to describe a situation where an enclosed volume has a lower pressure than its surroundings, causing substances to flow inward. This understanding of negative pressure is related to gauge pressure, which is commonly used to indicate the effective force exerted by a gas.
| Characteristics | Values |
|---|---|
| Liquids can sustain negative pressure | Due to intermolecular attractive forces |
| Gases condense to liquids | When the attractive forces become strong enough to make negative pressures possible |
| Gases cannot sustain negative pressure | Any gas that could immediately condenses to a liquid |
| Gauge pressure | Indicates the resultant force of a gas |
| Absolute pressure | 1 bar in a tyre would have no real effect |
| Gay-Lussac's Law | Pressure of a given amount of gas held at constant volume is directly proportional to the Kelvin temperature |
| Charles' Law | Volume of a gas is directly proportional to its Kelvin temperature |
| Ideal Gas Law | Used to calculate any property of a gas |
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What You'll Learn
- Liquids can sustain negative pressure due to intermolecular attractive forces
- Gases cannot sustain negative pressure as they condense to liquids
- Negative pressure is caused by ambient air molecules
- Pressure is related to the work done in changing the volume
- Gay-Lussac's Law states pressure is directly proportional to Kelvin temperature
Liquids can sustain negative pressure due to intermolecular attractive forces
The concept of negative pressure in gases is a subject of debate in thermodynamics. Some sources state that pressure cannot be negative, while others argue that negative pressure states have existed since the beginning of the universe. While the existence of negative pressure in gases is questionable, it is generally accepted that liquids can sustain negative pressure due to their intermolecular attractive forces.
Liquids, such as water, possess intermolecular attractive forces that allow them to sustain negative pressures. These forces arise from the interaction between the positive and negative parts of molecules. The molecules in liquids are held together by intermolecular forces, which are generally weaker than the covalent bonds within individual molecules. The strength of a substance's intermolecular forces is directly related to its boiling point; stronger intermolecular forces result in a higher boiling point.
There are several types of intermolecular forces at play in liquids. One type is the dipole-dipole interaction, which occurs between polar molecules with permanent dipoles due to differences in electronegativities. Another type is the London dispersion force, which acts between nonpolar substances due to temporary fluctuations in electron distributions, resulting in short-lived dipole moments. Additionally, hydrogen bonding is a unique type of intermolecular attraction that involves a covalent bond between a hydrogen atom and an electronegative atom like oxygen, nitrogen, or fluorine.
The average distance between molecules in a liquid also plays a role in sustaining negative pressure. Liquids have smaller distances between molecules compared to vapors, resulting in stronger intermolecular interactions. When a liquid is stretched, it enters a metastable region, which is less stable than the initial stable liquid state. This metastability contributes to the ability of liquids to sustain negative pressure.
While liquids can sustain negative pressure, they cannot maintain a pressure lower than their equilibrium vapor pressure without forming a second phase. Even if the liquid pressure is less than atmospheric pressure, it still experiences positive absolute pressure. This distinction between gauge pressure and absolute pressure is crucial when discussing negative pressure in liquids.
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Gases cannot sustain negative pressure as they condense to liquids
Gases are fluids that do not have a definite volume. They assume the volume of the container they are confined to. Gases can be compressed and are affected by temperature and pressure. The volume of gas in an enclosed container is the volume of the container.
Gases cannot sustain negative pressure because any gas that could would immediately condense to a liquid. Liquids can sustain negative pressure due to their intermolecular attractive forces. The average distance between molecules in a vapor is larger than in a liquid, and the attractive tail of the intermolecular interactions is significantly weaker in gases than in liquids.
If the attractive intermolecular forces of a liquid are what allow it to sustain negative pressure, then real gases should also be able to sustain negative pressure because the molecules in a real gas can be either attractive or repulsive. When the internal pressure of a gas is positive, the intermolecular forces are attractive, and in theory, this should allow gases to sustain negative pressure.
However, gases cannot sustain negative pressure because they would immediately condense to liquids. This is because the attractive forces become strong enough to make negative pressures possible, and the gas condenses to a liquid.
Liquids cannot sustain a pressure less than their equilibrium vapor pressure without forming a second phase. Even though the liquid pressure may be less than atmospheric, it is still under positive absolute pressure. Therefore, although the gauge pressure may be negative, the absolute pressure is positive.
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Negative pressure is caused by ambient air molecules
The concept of negative pressure is often confusing because, despite the name, pressure cannot be negative—it is always zero or positive. The term "negative pressure" is used to refer to pressures below atmospheric pressure, but above absolute zero pressure. In other words, negative pressure is caused when an enclosed volume has lower pressure than its surroundings. This causes substances to flow inwards.
In the context of gas laws, the discussion of negative pressure revolves around whether gases can sustain negative pressure. Some sources suggest that gases cannot sustain negative pressure because they would immediately condense into liquids. This is because the attractive forces between gas molecules would become strong enough to cause this phase change. However, others argue that real gases, as opposed to ideal gases, should be able to sustain negative pressure due to the presence of both attractive and repulsive forces between their molecules.
The concept of negative pressure is closely related to the idea of a vacuum, which is a space devoid of matter. In engineering, negative pressure is often described in relation to a vacuum, with values below the vacuum pressure being considered negative. For example, in the US, pressure readings often use units of psig (pounds per square inch gauge pressure), which exclude ambient air pressure. However, other gauges, such as psia (pounds per square inch absolute), are based on a perfect vacuum being the zero point.
Ambient air pressure, or atmospheric pressure, refers to the weight of the air above a specific unit area at a given location. It is influenced by factors such as altitude, weather, and temperature. The Earth's atmosphere, composed primarily of nitrogen, oxygen, argon, and carbon dioxide, exerts this pressure on the Earth's surface due to the weight of its gas molecules. This pressure can vary depending on location and weather conditions.
In summary, negative pressure is caused by ambient air molecules when an enclosed volume has lower pressure than the surrounding atmospheric pressure. This concept is important in engineering and is often discussed in relation to the idea of a vacuum. While the term "negative pressure" may be confusing, it simply refers to pressures below atmospheric pressure but above absolute zero.
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Pressure is related to the work done in changing the volume
The concept of negative pressure is a topic of debate among physicists. Many physicists believe that pressure cannot be negative. However, some argue that negative pressure states have existed since the beginning of the universe and have been studied for centuries. The term "negative pressure" is often used in engineering to describe an enclosed volume with a lower pressure than its surroundings. This causes substances to flow inwards.
The relationship between pressure and volume work is described by the pressure-volume work equation: PV = W. This equation relates the work done on a system to the change in volume and pressure of the system. The negative sign in the equation indicates that work is done by the system when volume increases and work is done on the system when volume decreases.
For example, consider a piston in a cylinder with a movable lid. If the lid is pushed down, the piston moves down, the volume decreases, and work is done on the gas. This is represented by W = -P*ΔV, where W is the work done, P is the pressure, and ΔV is the change in volume. On the other hand, if the lid is pulled up, the piston moves up, the volume increases, and work is done by the gas. This scenario is represented by W = P*ΔV.
The pressure-volume relationship is essential in understanding the behaviour of gases and liquids. It is also crucial in various fields, including physics, chemistry, and engineering. The work done on or by a system can be calculated using the pressure-volume work equation, providing valuable insights into the system's behaviour and energy transfer.
In the context of gases and liquids, the concept of negative pressure is intriguing. While some argue that gases cannot sustain negative pressure due to their molecular properties, others suggest that real gases with attractive intermolecular forces may exhibit negative pressure. However, it is generally accepted that liquids can sustain negative pressure due to their intermolecular attractive forces.
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Gay-Lussac's Law states pressure is directly proportional to Kelvin temperature
Gay-Lussac's Law, discovered by French chemist Joseph Gay-Lussac (1778-1850), states that the pressure of a gas is directly proportional to its temperature, provided that the volume remains constant. In other words, as the temperature of a gas increases, its pressure increases as well, and vice versa. This relationship can be expressed mathematically as:
> \[ \frac{P_1}{T_1} = \frac{P_2}{T_2} \]
Where:
- \(P_1\) and \(P_2\) represent the initial and final pressures of the gas
- \(T_1\) and \(T_2\) are the initial and final temperatures in Kelvin
This law is particularly useful in understanding the behaviour of gases in closed systems, such as propane tanks used for barbeque grills. For example, on a hot day, the pressure gauge on a propane tank will register a higher pressure than on a colder day. This is because the air inside the tank heats up, leading to an increase in pressure.
It's important to note that Gay-Lussac's Law assumes that the volume of the gas remains constant. If the volume changes, other gas laws, such as the Combined Gas Law, may be more applicable.
Now, regarding the possibility of negative pressure in gas law, it is important to clarify that the term "negative pressure" is often used to describe a situation where an enclosed volume has a lower pressure than its surroundings. In gases, pressure is understood to be a positive value. This is because gases, due to their lower density and larger distance between molecules, lack the same attractive intermolecular forces that allow liquids to sustain negative pressures. Therefore, while negative pressures may exist in liquids or solids, they do not occur in gases.
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Frequently asked questions
Negative pressure is when an enclosed volume has lower pressure than its surroundings. This causes substances to flow inwards.
Most sources agree that gases cannot have negative absolute pressure. However, some sources suggest that certain gases under certain conditions can have negative gauge pressure.
Liquids can have negative pressure due to their intermolecular attractive forces.
Absolute pressure is the pressure exerted by a gas or liquid. Gauge pressure is the pressure exerted by ambient air molecules.
Trees pull water from the soil to their leaves through negative pressure.
