Joule (Joule for short, symbol J) isenergyandWorkInternational units of.1 Joule energy is equal to the action point of 1 Newton force moving 1 meter in the direction of forceWork[1]。
The symbol J is in memory of British physicistsJames Prescott Joule And named.1 J=1 N · m, also equal to the work done by 1 W power in 1 second, 1 J=1 W · s.[2]
Joule's main contribution is that he studied the equivalent relationship between heat and mechanical work.Joule's initial research direction was electromagnetic engine. He wanted to replace the steam engine used in his father's brewery with electromagnetic engine to improve work efficiency[3]。
In 1837, Joule made a battery driven electromagnetic machine, but because the current supporting the operation of the electromagnetic machine came fromZinc batteryThe price of zinc is expensive, and the cost of using an electromagnetic engine is higher than that of using a steam engine.Although Joule did not achieve his original goal, he found the phenomenon that electricity can do work from experiments[3]。
Joule law experiment[9]
In order to further explore the law of thermal effect of current, Joule put the ring coil into a test tube filled with water and measured the water temperature at different current intensities and resistances.Through this experiment, he found that the heat released by the conductor in a certain time is proportional to the square product of the resistance of the conductor and the current intensity.Shortly thereafter, Russian physicistslenz A large number of his experimental results were published, which further verified the correctness of Joule's conclusion on the thermal effect of current.Therefore, this law is called Joule Lenz law[3]。
After completing the research on the thermal effect of current, Joule carried out the conversion experiment of work and heat.Joule believes that the energy of nature is indestructible. When mechanical energy is consumed, corresponding heat energy can always be obtained.Therefore, there must be a certain quantitative relationship between work done and heat transferred, that is, thermal work equivalent[3]。
In 1843, Joule designed a new experiment to find this relationship.He wound a small coil on the iron core, measured the induced current with a galvanometer, placed the coil in a container containing water, measured the water temperature to calculate the heat.In this way, when there is no external power supply, the rise of water temperature is only the result of the conversion of mechanical energy into electrical energy and electrical energy into heat[3]。
This experiment made Joule think of the relationship between mechanical work and heat. After repeated experiments and measurements, Joule measured the thermal work equivalent, that is, the heat of 1 kcal is equivalent to the work of 460kg/m.However, this result is not accurate, and Joule conducted more accurate experiments[3]。
Joule thermal work equivalent experimental apparatus[8]
In 1847, Joule designed a more ingenious experiment. He filled the calorimeter with water, installed a rotating shaft with blades in the middle, and then let the falling weight drive the blades to rotate. Due to the friction between the blades and water, the water and the calorimeter became hot.According to the falling height of the weight, the converted mechanical work can be calculated;According to the rising temperature of water in the calorimeter, the rising value of internal energy of water can be calculated.The accurate value of thermal work equivalent can be obtained by comparing the two numbers[3]。
laboratory appliance [7]
Later, Joule also used whale oil or mercury instead of water to do experiments. He carried out more than 400 experiments with various methods. After more accurate measurementsThermal equivalentThe value is 1 card=4.15 joules, very close to the current 1 card=4.184 joules.Under the conditions at that time, it was not easy to make such an accurate experiment.Joule accurately measured the thermal work equivalent, further proving that the laws of energy conversion and conservation are objective truth.The determination of this law declares the manufacturing“Perpetual motion machine”The illusion of[3]。
Joule's law
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Joule's law refers to the transformation relationship between electric energy and heat energy, which was discovered by British physicist Joule in 1841.The specific content of Joule's law is that the heat generated by the current passing through the conductor is proportional to the square of the current, the resistance of the conductor, and the energizing time[5]。
The mathematical formula of Joule's law is Q=ItwoRt, where Q is the heat, in joules;I is the current in amperes;R is the resistance in ohms;T is the time in seconds.This formula applies to allCurrent heating effectCalculation of[5]。
When Joule used the resistance wire to heat water, he found that the heat generated by the resistance wire was different and the temperature of water was different with different parameters.He decided to carry out a quantitative study.Through a lot of experiments, Joule finally discovered Joule's law.Joule's law provides a basis for circuit lighting design, electric heating equipment design and calculation of electric equipment heating[5]。
stayPure resistance circuitBased on the formula of Joule's law, other formulas for calculating circuit heat can also be derived.However, it should be noted that the formula of Joule's law is applicable to all circuits, while the formula derived is only applicable to pure resistance circuits[5]。
International System of UnitsA unit of work or energy expressed in joules [ears] (J).1 Joule is equal to the work done by moving 1 meter in the direction of 1 N force;In electricity, it is equal to 1W ・ s, that is, the energy released within 1s when 1A current flows through a resistance of 1 Ω[6]。
Joule heat
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withcapillary electrophoresisFor example, capillary electrophoresis requires an electric field to do work. If an electric field does work, it will generate heat. This isJoule heat。Depending on its degree, this joule heat can form different temperature gradients, or even cause solution convection and bubbles.Bubbles will interrupt electrophoresis, while temperature gradient and convection will greatly reduce separation efficiency[4]。
In traditionelectrophoresisIn order to avoid convection, various substances that are difficult to flow or do not flow are used as electrophoresis support media, such as cellulose and gel, which is actually a "blocking" method[4]。
On the contrary, in capillary electrophoresis, the strategy of eliminating "source" is adopted, that is, reducing the inner diameter of the capillary to speed up the heat dissipation, so as to overcome the Joule heat effect.It can be predicted that the heat dissipation capacity of different capillaries must be different, and their separation effects must also be different. Therefore, it would be very useful to predict the heat dissipation performance or temperature distribution of capillaries in the electrophoresis process in advance[4]。
