Circuits on the MCAT

Table of Contents

Though it may seem like one of those things that seems irrelevant when going into medicine, we can apply some of the basic concepts of circuits when modeling the body’s physiology, most specifically our circulatory system!

While this complete guide has some overlap with our “Electrostatics on the MCAT!” chapter overview, you’ll see that some of the concepts covered there have just been applied in a more practical sense.

After going through this chapter overview, you’ll have the basic ideas of circuits on the MCAT before diving into our more in depth articles. Let’s go!

Circuits on the MCAT: What You Need to Know

Topics on circuits will be tested on the Chem/Phys section of the MCAT and can appear both as passage based and fundamental discrete questions. 

This is definitely one of the more high yield physics questions so maybe expect around 8-9 questions covering circuits on the MCAT!

Introductory physics accounts for 25% of the content covered in the Chemical and Physical Foundations of Biological Systems.

Important Sub-Topics: Circuits

As mentioned before, there is some overlap between this chapter overview and our “Electrostatics on the MCAT!” chapter overview! We suggest going over that one first so that you can get a better foundation before reviewing this chapter overview!

In addition, these physics topics might show up on the MCAT more as calculation based questions. Of course, this is not to discredit the theory behind the circuits, but it may give you a better idea of what to focus on when reviewing!

1. Current and Conductance 

Circuits generally have 3 main components: 1) a conductive pathway usually in the form of wires, 2) a power source with voltage such as a battery, and 3) a load which uses electrical energy such as a light bulb. 

The battery voltage source has a negative, anode terminal and a positive, cathode terminal. This drives how a circuit generates electrical energy as oxidation-reduction reactions take place where the anode loses electrons and the cathode gains the electrons lost by the anode. 

Current simply refers to the flow of electrical charges within the circuit, just like the current of water that flows in a river! Current (I) is measured in the SI units amps, which is the amount of charge (Q) flowing per unit time (Δt).  
Current and Conductance - A

We should note an important convention in regard to current direction. The negatively charged electrons move toward the positive, cathode terminal as depicted above. 

However, the proper convention of current direction is actually based on the direction a positive charge would go. Thus, the direction of the current (by proper convention) is opposite to the one shown above. 

An important property that wires should have for a circuit to function is conductance, which basically measures how well a substance can support an electric current. Another way to view conductance is through an equation! 

As shown below, the conductance of a material is actually the reciprocal of its resistance which will be discussed later.

Current and Conductance - B

(Coming Soon!) Full Study Notes : Current and Conductivity

For more in-depth content review on current and conductivity, check out these detailed lesson notes created by top MCAT scorers. 

2. Voltage and Ohm’s Law

Voltage is usually described as the electric potential difference, but let’s try to get a better understanding of what this means! A better way of understanding voltage is as the ratio of how much work (in joules) a charge can accomplish traveling within the circuit from one terminal to another. 

For example, a battery that has a voltage of 1.5 V means that 1 joule of work can be done when 1 coulomb of charge moves from one end of the battery to another. What determines a battery’s voltage is the chemical oxidation-reduction reactions within the battery which yields the “pressure” which allows the electrons to move. 

As mentioned above, another way to think about voltage is the “pressure” exerted by the battery cell on the electrons to move them from one terminal to the other terminal. 
Voltage and Ohm’s Law - A

From this, an important law can be generated which is Ohm's law, which states that voltage is equal to the product of the current and resistance within a circuit. From here, we can make some important relationships between the variables. We’ll cover resistance shortly in the next section!

Most notably, we can see that the current is directly proportional to the voltage supplied. When we use our “pressure” analogy, we can say that an increase in voltage (“pressure”) increases the current!

Voltage and Ohm’s Law - B

(Coming Soon!) Full Study Notes : Voltage and Ohm’s Law

For more in-depth content review on voltage and how it applies to Ohm's law, check out these detailed lesson notes created by top MCAT scorers. 

3. Resistance and Resistors

When applied to circuits, resistance is exactly what it sounds like! It refers to any opposition to the flow of charges within the circuit. Just as with other properties in circuit physics, we can model resistance (using ohms units -- 𝝮) as shown below!

Resistance and Resistors - A

Usually, this equation will be used in the context of the wires within the circuit as even the wires themselves can elicit some form of resistance! Note also the difference between resistivity and resistance: resistivity is an intrinsic property of the material while resistance accounts for all variables of resistivity, length, and cross-sectional area. 

Likewise, resistors are simply circuit components that offer resistance to the circuit current. Because a current experiences resistance when it passes through a resistor, there is a voltage drop associated with the resistor due to Ohm’s law. 

An interesting thing about resistors is how each of the resistors can be organized within a circuit, with there being 2 possible ways: those connected in series and those connected in parallel.

When resistors are connected in series, we can simply add the resistance of all the individual resistors together to get the total resistance. Because the electrons can only follow one path when resistors are in series, the current remains constant while the voltage drop associated with each resistor is dependent on the resistor’s resistance. 
Resistance and Resistors - B

In contrast,  when resistors are connected in parallel, we actually add the reciprocals of each resistor’s resistance together which is set equal to the reciprocal of the total resistance — this is a lot easier visualized as an equation as we’ll show! 

In this case, adding resistors in parallel will actually decrease the total resistance because you increase the amounts of paths the current can take. This is the same thought process behind opening up more lanes on a highway to reduce traffic. 
Resistance and Resistors - C

(Coming Soon!) Full Study Notes : Resistance and Resistors

For more in-depth content review on resistance and resistors, check out these detailed lesson notes created by top MCAT scorers. 

4. Capacitance and Capacitors

Sometimes, we may not want to use a voltage battery for powering a lightbulb; in some cases, we want to use charges released by the battery and store them for different uses. This is exactly what capacitors do! 

Capacitors are generally composed of 2 metal plates: when a capacitor is connected to a circuit with a voltage source, one plate will accumulate a positive charge (+Q) and the other plate will accumulate a negative charge (-Q). 

To find the capacitance (given in SI units farads, F), it’s simply the ratio between the b on plates and voltage, as given by the equation below: 
Capacitance and Capacitors - A

Similar to resistors, multiple capacitors can also be arranged in circuits both in series and in parallel. However, the biggest distinction that should be made is that the additive properties of capacitors are the opposite of resistors.

When capacitors are placed in series, it is the same as adding resistors in parallel: we add the reciprocal of each capacitor's capacitance to the reciprocal of the total capacitance. 
Capacitance and Capacitors - B

Conversely, when capacitors are placed in parallel, it’s the same as adding resistors in series: we simply add all the capacitor’s capacitances together to get the total capacitance. Notice again, how it’s the opposite of how you do it for resistors!

Capacitance and Capacitors - C

(Coming Soon!) Full Study Notes : Capacitance and Capacitors

For more in-depth content review on capacitance and capacitors, check out these detailed lesson notes created by top MCAT scorers. 

5. Types of Meters

When we want to measure a certain value within a circuit, such as the circuit’s current and voltage, we can attach different types of meters to the circuit to help determine these values!  

There are 3 main types of meters: ammeters, voltmeters, and ohmmeters. The table below gives a great summary of what value each type of meter measures in a circuit. 
Types of Meters - 3 Main Types

(Coming Soon!) Full Study Notes : Types of Meters

For more in-depth content review on types of meter, check out these detailed lesson notes created by top MCAT scorers. 

Important Definitions and Key Terms

Below are some high yield definitions and key terms to refer to when reviewing concepts and ideas about circuits!




The amount of charge flowing per unit time; given in the SI units amps


Term used to describe how well a material can support an electric current; Mathematically, it’s the reciprocal of resistance


Described as the electric potential difference between 2 points; Can also be described as how much work in joules a charge generates

Ohm's Law

States the voltage is equal to the product of current and resistance


Term used to describe how well an object opposes the flow of charges


Circuit elements that have the ability to hold and store charge


Device that can be used to measure the current flowing with in a circuit


Device used to measure the voltage (electric potential difference) between 2 points


Device that can be used to measure resistance

Additional FAQs - Circuits on the MCAT

Are Circuits on the MCAT?

Yes! Topics on circuits are considered one of the more high yield topics covered on the MCAT so try and devote a good amount of your physics prep focusing on the basics of circuits as well as important calculations.

What are Circuits in Physics? – MCAT?

Circuits in the context of physics are quite similar to circuits in the context of chemistry. They mainly differ on the design of the voltage power source: the physics, the voltage power source comes in the form of a battery. In chemistry, the voltage source comes from electrolyte solutions. 

However, in both cases, there are oxidation-reduction reactions that occur which result in the release of electrons which can flow through the circuit.

What is the Electromotive Force – MCAT?

The electromotive force (emf) is essentially the voltage (i.e. potential difference) between the 2 terminal ends of the battery when not connected in the circuit. Ideally, the emf is equal to the total voltage of the circuit and not having any significant internal resistance.

What is a Circuit in Chemistry? – MCAT?

As mentioned above, a circuit in chemistry is quite similar to the circuit construction in physics, with the main difference being the voltage power source. We cover a lot more about electrochemistry in the context of general chemistry in another chapter overview!

Additional Reading Links (Coming Soon!) – Study Notes for Circuits on the MCAT

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