Electric Field

 Electric Field

The aim in this lab was to show how electric field works and how it can be applied to human being daily live.

In this picture, students were asked to present some concepts about the electric field by using the ideas or the knowledge they have about gravity. Here are some taking from the picture:

1. the electric field is is caused by a charge of some object, like proton or electron. 
2. the magnitude of the electric field depends on the magnitude of the charge that produces the field. 

Likewise, gravity also affect any object, because every object has mass and is affected by gravity that pulls things and avoid them to 'travel' throughout the planet or 'galaxy'

In this picture the students analyzed the forces produced by one charge into another. And the final thoughts, resuming, was that when q1 is near the q2 the field becomes bigger, and father it is, the electric field creases exponentially.  In fact, by keeping the force constant and make the charge varying, the electric field will increase when the the charge is small, and decrease when the charge is bigger. This can be viewed by using the mathematical formula E=F/Q. E (electric field), F (force), and Q (charge). 

 The first picture represents the relationship between the electric field and the force. Either one E=F/Q or E=kq/(r^2) will give the same answer. Nonetheless, all depend what information is convey on a problem.
The second picture is the continuation of the second picture. The conclusions are that the force experienced by a charge remains the same, but the charge itself change sign. The formula F=qE is connected with this result.

In this picture the intention is to see what happen when two objects with the same charge get near each other. For example, when a circular or spherical object has a negative charge and we pass a charged rod with also a negative charge, the charge on the rounded object will spread equally throughout it, making equal angles between them. Numerically speaking, the electric field depends also on the distance between the fixed point and the object that is charged passes near the fixed point. For example, calculating the electric field from a fixed point to a rod with 10 cm in length, we will find that when we get near the center, the electric field is smaller than the one at the beginning of the rod. As stated previously, and like the force, the electric field also depends on the distance between the two charged objects; bigger the distance smaller the electric field.

This worksheet describes the experience made by students in where by increasing the radius between the charges either one the force and the electric field will vary accordingly with the distance. 

Here is a demonstration in how the electric field can be calculated when the charges are not directly pointing to the fixed charge. To a more accurate calculations, the electric field is calculating using the x-axis and y-axis so the direction and the magnitude of it can be determined easily, and are given in terms of vector notation.  

    

Electrical Forces

Electrical Forces

This lab had the intention to show to the students how the electrical forces can work by using material that we are in contact with on daily basis.

Our physics professor Dr. Mason started the lesson rubbing cat hair and a balloon to see what happens when the glass and the balloon are in contact (first picture). By rubbing this two materials, electrons will be transferred from one to another, causing the balloon to increase its electrical charges. When the balloon was placed near the glass they sticked together as they were glued. This happens because each one presents different chargers when in contact. The balloon may have negative and the glass positive charge, or the glass may have negative and the balloon positive charge. In fact, if they both would have the same charge, positive or negative, they would not be sticked because same charge always repels each other. 

In the second photo, a silk was used to see if the balloon and the glass would behave in the same way. Although the rubbing object used was different the effect was the same. This two experiences had shown that no matter how a person may rub a balloon, or other mass the can gain electrical charge, it will always generate an electrical charge in such away that when placed near a mass with an opposite charge they will stick together. 

The third photo explains what happens in both experiences, on the left side of the last photo is a free body diagram which represents the forces acting on the balloon and the contact force. On Y-axis: Force due to gravity (+Fg, taking the positive down) and Friccion (-Fr, because it goes in the opposite direction of the Fg), and on X-axis: Force that the balloon does on the glass (Fb) and the force that the glass does on the balloon (-Fg, or normal force). The forces in the X-axis are equal in magnitude but opposite in direction, so when added the net force is zero. And that is the reason why the balloon and the glass in both cases can stick together. 

Aclaration: Even Though the red balloon is sticked to the wall, initially it was with the glass. The intention was to show that two methods were used, and they had the same effect.

After doing the practical learning, student went to the theoretical learning which consist in try to find a definition for electrical charges. And at the end, the simple definition that was found is: Charge is how much matter interacts with other matter electrically.

After the definition, students had to perform a experience with tape. Four strips of tape were used, and each one was labeled. Two were written of top and the other two bottom. They had to induce some charge on each of it by sticked them on the table and pulled. After that, they had to three different combinations, such as, top and top, bottom and bottom, and top and bottom. The conclusions were: when the two tops were placed together, they would repeal each other because the have the same electrical charges. The same happened when the bottom were placed together. When top and bottom were placed together, they sticked since they had different electrical charges.

The first photo represents the derivation of the force used to calculate the electrical forces when one mass is approaching another one with the same electrical charge, and the second one starts to repel the first one. 

The second and the third pictures are simple calculations and the answers for the questions on the lab. 

Those formulas are also known as the Coulomb's Law, which mathematically says that the force of the electrical interaction between too chargers depends how apart they are from each other. Bigger the distance, smaller the force. And the most important of all, the distance cannot be zero, otherwise the force would be infinity. 

   

  

Entropy and Enthalpy

Entropy and Enthalpy

Entropy is defined as one way process. It measure the disorder of a gas or a system can have. It is one way process because the amount of energy needed to cause a disorder on a system is much less than the amount of energy needed reverse the process. Indeed, it is almost or impossible to reserve a process. For example, the process in which chemical energy is transformed in mechanical is considered an irreversible process because when the gasoline is ignited to make the pieces inside the engine rotate to do mechanical work, it is impossible to reverse the process or working backwards.
Enthalpy is the amount of heat used or release from a system at constant pressure. Takes the same units as Q(heat) used to calculate the amount of heat needed to melt or fuse water into ice.

The first picture shows the two definitions. Entropy can be found using (heat (Q))/(temperature (T)), or (heat (Q))/(thermal energy (KT)). A few examples of Entropy are boiling water, burning staff, and the diffusion of a gas.

The next is the enthalpy that is represented with the letter H and is calculating by adding the internal energy of the system (U) plus pressure times delta V(volume) or work done on the system.

The two pictures that can be seen are simples calculations to help everyone to see what each of us is buying special when it comes to Air Conditioners.

Video

This video has the intention to show how by heating water, and use its vapor and balancing with ice, mechanical work can be done. The process is: water is heated until 100 degree Celsius, after that the recipient is placed under the small engine cylinder. Ice is place over the it to balance the temperature differences. Once the small engine gain enough vapor to start moving, it will transform the vapor into mechanical work. Yet, the reverse process does not occur because mechanical movement cannot be transformed into vapor. It is an example of an entropy process as one way process.     

This lab had the intention to make students aware that any phase transition of any system have only one way and they are not reversible. Otherwise the cost to make it would not compensate the transformation of any article in an useful tool. For example, it is possible to cut a tree and produce wood furniture. But it is impossible to reverse the process, pass the wood furniture in a tree.  

Applications of the First Law of Thermodynamics (Continuation)

Applications of the First Law of Thermodynamics (Continuation)

The picture shown here were presented to see how adding some heat (positive, high temperature, or negative, low temperature) the device (the upper photo) can do work. Since the heat will go from one side to another (hottest to coolest), the rotational part will do work accordingly with how the heat flows. However, it also does work when it is applied negative heat, and the rotational part will rotate in the opposite direction.

The second works almost the way as the first picture, but just when positive heat is added. Once heat is added, the seringy will start going up because work is been done by the gas to its surroundings, which will expand and increasing the volume of it. To compress the gas, the heat source is pulled over and and the surroundings will do some work on the gas. This can be used to lift a reasonable mass, like an apple.

This picture shows the full cycle of a heat engine. The process the this engine passes through are Isothermal (constant temperature) and Isobaric (constant pressure). Here students were asked to find the network for the interi cycle, which would be the area of the figure. 

This two pictures represent the process of a heat engine and its calculations for internal energy, heat in and out (Qh, Qc) and its efficiency. In this case the engine were considered to be ideal. 

The conclusion about the applications of the first law of thermodynamics gave to students a better understanding about an engine cycle. Although the ones used by students were considered ideals, the are made to make studies in how an engine will behave in the real world, where all the heat that goes to the engine will be divided in work and some of them will be lost and never recovered. It is also a way to see how, in an ideal conditions, an engine can be highly efficient. Nonetheless, this efficiency is never applied to the real world because the heat that goes to the engine, will never be totally recovered. Some will do work and some will be lost, but just a portion of it is again reused to repeat the cycle again.  

Take Home Quiz

Applications of the First Law of Thermodynamics

Applications of the First Law of Thermodynamics

The first law of thermodynamics  describe how heat and work can change the internal energy of a system for an adial gas. In this lab, students were able to prove how it works and also how the change in the total mechanical energy of the system will affect the work and the amount of heat, when it existe, that is transferred to or from the system.

In this picture students were asked to predict on three different situations, what would happen when the candle was placed inside a jar, a small one and big one. The prediction, the upper left side, was that the candle's flame would dime faster and then go out. The reason is because no air was circulating inside the jar, and since oxygen, one of the most important factors to keep the flame 'vivid', called also fuel, was cut, the flame went out.
In the second prediction, the down left side, a tube had been added. This time the flame kept the flame more brightly since it had air that was circulating between the jar and the tube. The closer the tube was to the candle, more dimed the flame was. And when the tube was far from the candle, more agitated the flame was. This happens because the space between the candle and the tube had a bigger column of air.
In the third prediction, the candle was placed inside of a big bottle of glass and drooped from a height of 6 inches. The result was for instance the candle's flame was burned brightly since it, presumedly, the column of air was compressed and was burn faster as the big bottle was dropped; and at the end started to dime because the oxygen had almost been consumed by the flame.

In the first picture, the students were trying to see if the what happens when a mass in placed on the top of  cylinder. In this case, the mass will do work on the gas ( negative work, -W) to compress it. As the gas is compressed, the temperature increases since the molecules will not have the same space to move around and because they will be hitting each other with more frequency; the volume decreases, causing what was explained before, the decreasing in space between the molecules and the walls of the cylinder.
The second picture shows a mechanism that can be used to actually see how pressure and temperature can increase, while the volume of the gas is decreased. Starting as an ideal mechanism, means that no heat is lost, and the internal energy is conserved, the gas is slowly compressed, the gage will start reading the increasing pressure inside the cylinder as well as the digital thermometer.

This photo represents the various types of processors that can be found. the first one is A (Isobaric, iso - unique, baric - pressure), means that the phase changes that a gas suffer will be at constant pressure, while the volume will keep increasing. The second one is B (Isochoric, choric - volume), means that the changes that a gas suffer, from one phase to another, will be at constant volume, while the pressure will keep increasing. The third and fourth graph are very similar, but one is faster than the other; In C (Adiabatic) the phase change, for example from point 'a' to 'b', is so fast that the only thing that is kept constant is the pressure and the volume will decrease at same rate that the pressure increases. On D (Isothermal, thermal - temperature) since is a slow process, the gas will 'have time' to increase its temperature as the pressure as well. Likewise, the volume here will decrease because the space between the molecules and the walls of the recipient will get smaller.  

The first picture explains how a rubber band when heated will contract and lift an object, move it from one place to another, either a higher one or one that is besides of the first one, drooped when is cooled, since it will extend, and then repeat the same process all over again. However, this process is to slow and the rubber band would not last longer. This happens because until a certain point the rubber ban will keep its elastic properties, each are the ones that even been heated and cooled, it will 'never' change its initial properties. After a long using of this process, the rubber band will pass to a plastic phase, where the initial conditions are no longer present. It means that every time that it is heated, to return to the nearest original form will take more time, and will never be the same. 

P.S.: Although in the elastic process its assume that returns to its original, this happens to the ideals processes. The true is that once the rubber band, and to any material, is heated, some of its original properties are lost, and it does not return completely to its original position. Yet, because it is so small the lost, it is considered that the mass retain its properties when it gains and losses heat. 

Video

This picture above, is an illustration of how a Carnot Cycle works. Heat is added to the engine through the hot reservoir, and at the same time the gas starts to do work because it gain the temperature that the hot reservoir hat. At this point no heat is lost. In this case the process in which the increasing of volume is done isobarically (the gas does work, +W), at constant pressure, from 1 to 2. Next, from 2 to 3, the gas does work on its surroundings, which means that heat is been delivery by the system to its surroundings. To keep the process reversible, the same amount of heat the gas losses must be the same that the surroundings will gain. And this process is done isochorically, at constant volume. From 3 to 4, the gas suffers again an isobaric process, but this time the gas is been compressed. It means that the cylinder is doing work on the gas (-W), reducing the volume of it by having the same pressure. Then, from 4 to 1, the gas is heated again, and work is done on it isobarically.  

The link below the picture can illustrate the Carnot Cycle. Here, heat is added to a gas inside a cylinder, and it starts to move from one side to another. It is assumed that no heat is lost, so the total internal energy is zero, and the total heat that is been added to the cylinder is converted into work.  

 

In this three pictures, the students were asked to analyze the Carnot Engine. French engineer Sadi Carnot developed an idealized engine that was able to explain the first law of the thermodynamics, where the idea was to reach the maximum efficiency that could be reached. This means that the engine would have a reversible process, where every heat added to the engine, could be recovered to be reused. This method is now known as the Carnot Cycle. Students were able to calculate the quantities asked, such as heat hot and cold, it means the heat added (hot) and losted (cold), work, and work net, that is the difference between the heat hod and heat cold. They also calculated the efficiency of it as shown. From this, it can be seen that the internal energy does not conserve because the engine was not 100 percent efficient. Some heat was lost during each process, and converted in to heat that could not be recovered. 

First Law of Thermodynamics

First Law of Thermodynamics

In this lab, students were able to connect Work, Kinetic Energy, and Potential Energy, which is known as Internal Energy for gases, with the Ideal Gas Law. It was also found that the velocity can be calculated using Kinetic Energy and Ideal Gas Law equations. However, the velocity has resumed to be three: The average velocity that is the mean speed of a particle; the most probable velocity, which is the speed where a majority of the molecules move; and the root mean squared speed, which is simple the square root of the average velocity squared of the molecules in the gas. The students were able also to learn how the change in pressure and volume, and where the temperature will increase also can make a matter ignited.

This picture represents the calculation of the final temperature where a piece of paper would ignited. The experiment consists in a simple fire syringe. By taking the necessary measurements, such as initial length, final length after the total compression, both of the air column, the inner radius, calculating the initial and final volume, and the initial temperature, students were requested to find at what temperature the piece of paper would ignited. The initial temperature was 298 K, and the final one was 663 K. Although the calculated temperature had shown an increasing in temperature, the small piece of paper did not. One reason could be that the compression of the air inside the syringe was not fast enough; other one could be that the actually temperature to ignited the paper was higher than the one calculated.

Ideal Gases Lab

Ideal Gases Lab

This laboratory had the the chance to teach physics' students the behavior of the gases within certain parameters, such as, at constant temperature, pressure, volume, and number of moles that the gas has. However, students were also able to see what can happen when one, two, or more parameters are changed.

Video

The first picture shows what happens when a soda can with water inside is heated. The outcome was that the rising in the temperature made some of the water become steam, increasing also the pressure inside the can and the temperature of the can itself. Once the can was put in contact with cold water, the pressure inside dropped dramatically, making the steam condensed and creating a 'vacuum'. Since the outside pressure is greater than the inside because of this momentary vacuum, the soda can will rapidly imploded.

This photo represents a relationship between temperature (T) vs pressure (p). Here students were able to see the that T and p are both related. If T increases the p will also increase, creating also a decreasing in the volume of the gas as the second picture shows. Although the graph is a straight line because the apparatus had a leak, the real graph would be a inverse relationship between p and volume.

The picture shown here, simple relates what happens when the volume (v) and pressure (p) are changed. The picture below, was a prediction made by the students to see what would happen when the p is taken from the vacuum machine above. Although it talks about to mass that were studied, the result was the same as it is written below.

1. When the p is taken from the vacuum machine, the 'molecules' of the study mass will release some of its molecules; in other words, they would have more space to occupy since the force caused by the pressure (downwards) will decrease, and the force that the study mass does (upwards) will increase.

2. When pressure is introduced to the vacuum machine again, there is an increasing in pressure, thus increasing the downwards force causing the study mass to become smaller than its original size. This happens because some of the molecules of the study mass leak when the pressure was taken. The downward force will continue to grow until the upwards force, made be the molecules from the study mass, reach the equilibrium with downwards.  

Linear Thermal Expansion Lab

Linear Thermal Expansion Lab:

The first picture the students were asked t predict what could happen when a metal hole is heated to see if it expands or contracts. Since the iron hole is made with the same material, it will expand allowing that a small iron ball can pass through. This indicates that when heat is applied to a metal made by the same material, it will expand at the same ratio and keeping always the distance between two points on the metal. This is called the Linear Thermal Expansion.

The second picture the material was made with different metals, such as, invar and brass. Here students were asked to predict the behavior of the metal bar.  All depends on the coefficient of linear expansion. Bigger it is, faster it will expand because it will allow more heat to excite its molecules, and by doing so, it will create a vibration and an expansion between them. And when cooled, happens the reverse of its expansion. The material will contract. In these experiment, the Brass was the one the expanded when heated, and contracted when cooled. Its Coefficient of Linear Thermal Expansion is 2.0x10^(-5)/K, and the invar is 0.09x10^(-5)/K. This means that the brass will allow more heat to cause vibration on its molecules, and causing the molecules to expand and increase its size. The same will happen when cooled, but this time the brass will bend and its molecules are been contracted, degreasing its vibration.

   
This picture represents the calculation of the Linear Thermal Expansion of a steel rod. Here the rod was heated with steam. Both the steam and the rod are at different temperature. Steam will be blow until the steam and the rod reach the same temperature. After that, students will have to calculate the angular displacement and linear displacement that the rod had; after a 300 second blowing steam through the rod, they finally reached the same temperature. The angular displacement of the rod was 0.28 rad, and the linear displacement was 12.2x10^(-5)/K. The rod is made by steel, since the calculated value is the same as the tabled linear thermal expansion's value.

 Heating Water

This picture represents a few measures that can be taken to made the mix Ice/Water going from 0 degrees Kelvin to 100 Kelvin. It also presents the formulas that can be used to perform the necessary calculations that can be asked, and the graph that describes of what happened.

The photo represents the how from the conservation of energy theorem (Heatgain=Heatlost) students were able to find the mass of water after all ice was melted. To begin, the mass of Ice was 255 g @ -5 degrees Celsius. And using the conservation of energy theorem showing on the picture, the mass of the water after melt the ice was just 14.5 g. That means that for each gram of water there will be 17.6 grams of ice.

This graph represents all the process from where the water and ice are at 0.30 degrees Celsius to 99.0 degrees Celsius, which was the boiling point of the water. The two abrupt increases in temperature showing on the graph was due to the contact between the immersion and the thermal sensor.

The picture shows the mass of the ice and the water + beaker. The starting temperature is also displayed as to be 0.30 degrees Celsius, and it took 130 s, or 2.17 m at 14 degrees Celsius to melt all the ice. The total amount of energy added until the ice melted was 37628.344 J. Students had to consider that there was a phase change, and the heat of fusion (Lf) had to be added to the formula Q=mici(Tf-Ti)+miLf. 

In this picture, the students calculated the latent heat of fusion (Lf) and the the specific heat (cw) of water. The result shown for the latent heat of fusion was not the experimental one, which is 336 J/g. And the specific heat was found to be 4.944 J/g-K. The water boiled for 86 s, and the mass of water that was turned into steam during that time was 17.5 g at 34.969 J. The Latent Heat of Vaporization (Lv) found was 1420.2 J/g has shown on the picture.

The values for the latent heat of fusion were very close to the real one, with just an .67% diference between them. Nevertheless, the specific heat and latent heat of vaporization were higher than the accepted one, where the cw was of by 18.2% and Lv by 37.05%. One possible source that can cause such discrepancy can be the simple fact the students had not considered the amount of heat that was lost, or the readings that the equipment does. Another source of error can be the room where the experiment is been held. Since the AC was on, he temperature inside the room was not at the 26 degrees Celsius, which affects the results. 

  

The last two pictures are the calculations of the uncertainty. For the Lf the uncertainty was +/- 0.085 J/g. For the Lv the uncertainty was +/- 0.503 J/g. Both Lf and Lv are calculated on the first picture. For the cw the uncertainty was +/- 0.1 J/g-k (which I believe that there was a mistake during the calculations we did. I just notice know).