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Battery Anatomy Understanding EMF

Batteries and Electromotive Force (EMF)

Introduction: As we have already discussed batteries serve as the powerhouse of a circuit, much like the heart in our circulatory system. They provide the necessary push or energy to set electrons in motion. We have already discussed them generally in our overview so now let’s take a deeper dive and look at them in more detail.


Batteries are described in terms of their EMF, often represented by the symbol \( \varepsilon \), is not actually a force. It’s a measure of the potential energy supplied per coulomb of charge that passes through the source, such as a battery.

To grasp the concept of EMF, imagine the heart at rest, yet poised to pump blood. The maximum potential pressure it can exert on the blood is akin to the EMF of a battery. Just as the heart’s potential pressure is not always fully utilized (due to factors like resistance from blood vessels), a battery’s EMF isn’t always fully applied because of internal resistance within the battery.

Internal Resistance: Every real battery has some degree of internal resistance due to the materials inside it. This resistance can affect the actual output voltage of the battery when supplying current.


With these factors in play, the actual pressure that drives the electric current through a circuit is not just the EMF. It’s the EMF minus the internal resistance, expressed in the equation V=EMF−IR. This voltage is akin to the blood pressure that’s felt in the arteries, the effective force that pushes the blood through the circulatory system, ensuring life to tissues and organs.

For a battery with an internal resistance \( r \) delivering a current \( I \), the actual terminal voltage \( V \) is: \[ V = \varepsilon – IR \]

Analogy in Action: A Weakening Heart and a Fading Battery

Imagine a battery that’s not at its best — it’s like a heart struggling to pump efficiently. This reduced EMF is similar to a heart condition that weakens its force. And when internal resistance goes up, it’s like our arteries stiffening or narrowing, a condition known as aortic stenosis. Both scenarios demand more work from the heart and battery alike, impacting their performance and the system they support.

How EMF Is Tested

EMF is a lesser tested concept and you generally won’t be asked to calculate a specific value using the EMF equation instead the MCAT is likely to ask about the proportionalities between the various variables. Here’s a simplified breakdown:

  • When Internal Resistance (IR) Changes:
    • If the internal resistance increases while EMF remains constant, the overall voltage (V) decreases.
    • Conversely, if internal resistance decreases, the voltage (V) increases, assuming EMF is unchanged.
    • This means IR and V are inversely related to one another 
  • When EMF Changes:
    • If the electromotive force (EMF) increases and the internal resistance (IR) remains constant, the voltage (V) across the external circuit increases.
    • If EMF decreases, while IR stays the same, the voltage (V) decreases.
    • This means EMF and V are directly related to one another