EMF, which stands for electromotive force, is the largest possible potential difference between two electrodes of an electrochemical cell when there isn’t a current flowing. It’s measured using the net voltage between the oxidation and reduction half-reactions.
Measuring the EMF of an electrochemical cell can help you to determine whether it’s galvanic, meaning whether or not it has an electric current. It’s also very useful when designing batteries as it allows you to calculate how many cells are needed to achieve a certain voltage.
Read on to learn more about electrochemical cells and how to calculate and measure electromotive force.
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What is an electrochemical cell?
An electrochemical cell is an instrument you can use to either generate electricity from a chemical reaction, or generate a chemical reaction.
A battery consists of one or more electrochemical cells. These cells produce an electrical potential or voltage that becomes an electrical current when a load is connected to it. The strength of the electrical potential and current depends on the chemical reactions in the cells and the materials that are used.
Chemical reactions in electrochemical cells involve the transfer of electrons between electrodes. Put simply, one electrode donates electrons while the other receives electrons. The electrode that spontaneously donates electrons is known as the anode and it’s here where the oxidation reaction occurs. The anode acts as a negative electrode in a galvanic cell and a positive electrode in an electrolytic cell.
Meanwhile, the electrode that spontaneously attracts electrons is called the cathode. This is where the reduction reaction occurs. The cathode serves as the positive electrode in a galvanic cell and a negative electrode in an electrolytic cell, as shown in the diagram below. 
The voltage strength depends on the way the cells are arranged and whether they’re connected in parallel or in a series. While size has no bearing on the net voltage, the electrical storage capacity is directly proportional to the size of the cells.
You can compute the electromotive force of a cell based on the type of material that’s used as an electrode. However, it’s worth noting that the computed ideal value usually differs from the actual measured value due to variables such as temperature and the concentration of the electrolyte.
What are the components of an electrochemical cell?
An electrochemical cell is made up of two electrodes, an electrolyte solution, a bridge, and a container. You can create a simple electrochemical cell by using a lemon and two electrodes of different metal types. The most common metals used as electrodes in laboratory experiments are copper and zinc.
What is the electrochemical potential of a cell?
The electrochemical potential of a cell is the potential difference between two half cells. It’s based on the voltage potential of the electrodes, which is directly proportional to their tendency to donate or receive electrons.
What is the EMF of an electrical cell?
The EMF (electromotive force) of an electrical cell is the measure of its electric potential or voltage. Essentially, it’s the difference between the half-cell reaction potentials of two different electrodes. 
You can think of EMF as the potential energy of an electrical cell. There must be a higher level from which the electrical current can flow into the lower level. As one electrode requires a higher oxidation number than the other, you can’t have two electrodes made of the same material.
One electrode must serve as the cathode from which electrons will be spontaneously removed (known as an oxidation reaction), while the other acts as the anode to which the electrons must be added (a reduction reaction).
Which way do electrons flow in an electrochemical cell?
When there’s a significant difference in the oxidation number of two different materials, current can flow from the electrode with a higher oxidation number to the one with a lower number. In both galvanic and electrolytic cells, electrons always flow away from the anode towards the cathode (although the respective charges of the electrodes are reversed in an electrolytic cell). 
In galvanic cells, the electrodes consist of different metals with different oxidation numbers that react with the electrolyte. While oxidation reactions in galvanic cells are spontaneous, oxidation reactions in electrolytic cells are not. Instead, they require electrical input from an external power source. A galvanic cell or battery is itself a power source.
How to calculate electromotive force (EMF)
Calculating the electromotive force or electric potential of a cell is relatively straightforward. You simply need to subtract the lower oxidation potential from the higher oxidation potential.
The oxidation potential can either be positive or negative. Whichever has the lower oxidation potential also has the higher reduction potential. That means that even if an electrode has a positive oxidation number, it can also serve as a reduction electrode or cathode if it has a lower positive value.
To calculate electromotive force, you can either refer to an oxidation/reduction potential table or write the half reactions based on the periodic table of elements. You can then use the above formula to determine the EMF of a cell given the materials used as electrodes.
Summary
The electromotive force (EMF) of a cell is a measure of its electric potential, which is usually expressed as voltage. Different types of metals and other materials have corresponding electric potential, which is based on the oxidation number. You can calculate the EMF of an electrochemical cell using the table of standard reduction potentials or the periodic table of elements.
