Why cell is a source of constant EMF but not constant current?
So the question here is why there is a constant potential difference between the two terminals of a battery rather than a “ ready to flow” constant current source.Potential energy is by virtue of its position , this will be like someone standing on a top of a hill and if we assume the bottom of the hill is at zero potential then that guy possess some energy by virtue of being at the top.Kinetic energy is one which a body possesses by virtue of being in motion. Suppose unfortunately that person fell somehow, at every moment of fall there is an energy conversion from potential to kinetic resulting in increased acceleration ultimately increase of velocity. During the fall the potential energy decreases as it is proportional to height and the kinetic energy increases as it is proportional to velocity.We may observe that at every point of his fall the the potential energy is converted to kinetic,i.e the kinetic energy is being created by potential energy. Existence of kinetic energy is only due to potential. Potential is an independent variable and kinetic is dependent on it.In case of battery EMF is the potential energy existing between the two terminals of a battery and the current is analogous to kinetic energy. Even the ohms law states “The current flowing through a metallic conductor is proportional to the Electromotive force applied across its ends, provided the temperature and all other conditions remain constant” and not the other way round.So the EMF can exist and the current will only be a mere function of EMF.We can easily make that EMF source to give a constant current . Again there can’t be any existence of a current source without a EMF source feeding it .As said earlier the current will be variable depending on the load you connect. Everything around us has potential energy unless something made it to convert it to kinetic form.The battery say a Galvanic cell the potential energy is produced by reduction oxidation reaction, when the electrodes are connected by a wire the current flows, again the existence of current is due to difference in potential only, if you don’t remember Galvanic cell anymore here is a video.I have narrated the story according to my understanding this can be partially or completely incorrect, and sorry if my writing skills are below par.
Calculate the standard emf of a cell that uses the Mg/Mg2+ and Cu/Cu2+ half-cell reactions at 25°C.?
Wow That is deep.
What is constant mass?
The instructions for some experiments tell you to "...heat to constant mass". This means that you keep heating and reweighing the reaction until its mass no longer changes, i.e. "constant mass".
Why should a cell of an ohmmeter have a constant EMF?
The ohmmeter part of a cheap analogue multimeter applies cell voltage across a series resistance circuit of which the resistor under test is part. The current measured in the circuit is measured by the meter movement and displayed as resistance value. Note that lower measured resistance gives a greater meter deflection.With this arrangement, the cell voltage needs to be constant during measurement to be sure of a reasonably accurate reading. A depleting battery can be compensated for somewhat by shorting the test leads and adjusting the zero before measurement.
Is emf of a cell extensive or intensive property?
It is an Intensive Property.The reason can be that,emf of a cell = E0 -RT/nF log(Keqm).Now, keqm (equilibrium ) does not change with concentration.Hence, it is an Intensive Property.
Why is the measured cell potential different from the theoretical potential?
There are many things which cause the cell potential to be less.Anything at all which causes any internal resistance in the cell or the circuit being used to measure it will reduce the measured potential.As well, frequently, non standard conditions may be used in a lab.The standard cells are for 1 M solutions. They are measured at 25 degrees Celsius. Differences in these conditions will cause changes. If a reactant or product is a gas, it needs to be bubbled over a platinum (inert) electrode with a pressure of 1 atm.Other much more significant differences also impact. Unless the electrodes are super clean, with a fresh metal surface that has no oxide layer at all, then the oxide layer will interfere with the reaction and reduce the measure potential. The salt bridge can cause interference to the easy flow of ions, and so cause internal resistance and lower the measured potential. How much will depend on a lot of factors about how it is made etc.If the connections within your circuit are not perfect, the measured potential will be less as well.I am sure you are beginning to imagine now that there are many factors which are very difficult to get just right.