Which of the following terms refers
to the potential difference provided by a cell in a circuit? (A) Electromagnetic force, (B)
electrostatic force, (C) electromotive force, (D) electrochemical force.
The question asks us to choose the
term that refers to the potential difference provided by a cell in a circuit. Firstly, we can reject option (A),
electromagnetic force, as there is no magnet present in a cell or in a conducting
wire. So, any forces that act are
unrelated to magnets. A cell stores chemical energy. When a cell is connected to a
circuit, the chemical energy of the cell is released as electric energy. This suggests that the potential
difference provided by a cell might be called electrochemical force. Is this correct though?
Firstly, we know that a potential
difference exists when we separate opposite charges, and we know that these opposite
charges attract each other and will move towards each other if they are not fixed in
place. The force that acts on the charges
is called electrostatic force. To separate opposite charges, work
must be done on the charges. For a particular separation
distance, the greater the charge that is separated, the more work required. The potential difference produced
when opposite charges are separated is the work done on the charges to separate them
per coulomb of charge that is separated.
It is important to understand that
we have now described two different quantities: the force on each of a pair of
opposite charges called electrostatic force and the work done per coulomb of
separated charge called potential difference. This shows that potential
difference is not the same quantity as electrostatic force. We reject option (B), electrostatic
force. It is important to realize that
work done per coulomb of separated charge is not actually any kind of force at
all. Recall that work equals force
multiplied by distance. This means that potential
difference equals force times distance divided by charge, or equivalently, that
potential difference equals force times the quantity distance divided by charge.
We see then that potential
difference is not equal to force, but equal to force multiplied by distance divided
by charge. It seems then that none of the
answers are correct. In fact, the correct answer is
option (C), electromotive force. But why?
The name “electromotive force” was
invented a long time ago, when scientists studying electricity did not clearly
understand what electric charge, electric current, electric potential difference, or
electric energy were. Scientists at that time did not
know that electrons existed; they only knew that a conducting wire connected to a
cell seemed to have something flowing through it. This “something” was named
“electricity” by the scientists, just so that they could call it by a name to
distinguish it from other things.
The term “electromotive force” just
means that which makes electricity move; it is not a force in the scientific meaning
of force. Today, we know that a potential
difference is not actually a force, but we still use the old term “electromotive
force” as it was used so often by scientists in the past that since then scientists
everywhere have become accustomed to using it.