I’ve ranted plenty of times about what constitutes “high yield” vs. “low yield” MCAT topics. I don’t like these classifications because they encourage us to skip things that aren’t considered “high yield” for the sake of efficiency. While usually, yes, that is the way to go if we’re in a time crunch, it sometimes comes back and hurts us more than helps.
Case in point: the MCAT right-hand rule in Physics.
Arguably, the right-hand rule is a “low-yield” topic. On a single exam, you’ll probably only see a handful of points that are directly related to the right-hand rule, if you even see any questions about it at all! So, why am I making such a big deal out of such a “low-yield” topic?
Because it’s fast, easy to memorize, and can directly result in points earned (or missed) on your exam. On the off chance that you DO see a few questions about it, you should be able to quickly snag some free points and pad that MCAT Chem/Phys score, rather than stress and waste time in an already anxiety-inducing section.
What is the Right-Hand Rule?
The right-hand rule is a mnemonic device used to determine the direction of a magnetic field, force, or current. It provides a way to visualize how these elements interact in electromagnetic systems, which is crucial for solving these types of problems on the MCAT.
It’s also hilarious to watch other people in your testing center using the right-hand rule in real-time, so that’s a plus.
Understanding Magnetism and Its Relationship With Electricity
At its core, magnetism is a force that arises due to the motion of charges. Electromagnetism, the interplay between electricity and magnetism, is one of the four fundamental forces of nature. When an electric current flows through a conductor, it generates a magnetic field around it. This principle is the foundation for technologies such as electric motors and generators.
Applying the Right-Hand Rule in MCAT Scenarios
First things first, the right-hand rule always uses your right hand. Never your left. I cannot stress this enough. It’s named after it and everything. It’s also important to note that there are two separate versions of the Right-Hand Rule. Both versions are correct, they’re just used in different situations. Below we’ll cover how, and when, to use each version.
Charged Particles Moving in Magnetic Fields
The first version of the right-hand rule is a valuable tool for determining the direction of the magnetic force on a charged particle moving through a magnetic field. This rule helps visualize the interaction between the particle’s velocity, the magnetic field, and the resulting force.
Hand Positioning
First, you’re going to want to extend all the digits on your right hand straight out. Your four fingers should be parallel to each other, and your thumb should be perpendicular to the rest of your fingers. Imagine you’re going to give someone a high-five, a hand-shake, or you’re about to tell someone to “talk to the hand”.
Congratulations! You’ve successfully mastered the first right-hand rule for the MCAT. This pose, with your palm flattened and your fingers extended, is all you need to get points on the exam (told you it was easy).
Once you’re holding your hand in the proper position, apply the following variables to your digits.
Magnetic Force Right-Hand Rule: Key Variables
- Thumb: Represents the velocity (v) of a positively charged (+) particle. Point your thumb in the direction the particle is moving. If the particle isn’t moving, there is no resulting magnetic force!
- Fingers: Point in the direction of the magnetic field (B). This is the direction in which the magnetic field lines are oriented.
- Palm of Your Hand: The direction the palm of your hand is facing indicates the direction of the magnetic force (F) acting on the particle, perpendicular to both the velocity and the magnetic field.
Example
Scenario: A proton (positive charge) is moving vertically “up” through a magnetic field directed horizontally, from right to left.
- Thumb: Point up (direction of proton’s velocity).
- Fingers: Point from right toward the left (direction of the magnetic field).
- Palm: Will naturally point towards your chest, showing the direction of the force acting on the proton. The MCAT will say this direction is coming “out of the page”, because you can imagine the force coming out of the computer screen towards you.
Common Mistakes to Avoid
- Using the Wrong Hand: Always use your right hand. Using the left hand will give you the opposite direction.
- Misaligning Fingers: Ensure your fingers are parallel to each other, while your thumb is perpendicular to accurately represent the directions.
- Forgetting Charge Type: Remember, the rule directly applies to positive charges (+). For negative charges, reverse the direction of the force indicated by the palm of your hand.
Sign up to get expert tips and exclusive invites to free MCAT classes and medical school admissions workshops!
Current-Carrying Wires in Fields
The second version of the MCAT right-hand rule is used when visualizing the direction of the magnetic field around a current-carrying wire. This simple technique helps you understand how the magnetic field lines are oriented relative to the flow of electric current.
Hand Positioning
For this version of the right-hand rule, all you have to do is give a “thumbs up”. That’s it!
Current Carrying Wire: Key Variables
- Thumb: Points in the direction of the conventional current flow (from positive to negative). This represents the direction in which the electric current is moving through the wire.
- Curled Fingers: Wrap your fingers around your thumb (representing the wire). The direction in which your fingers curl represents the magnetic field lines encircling the wire.
Example
Scenario: Consider a vertical wire with current flowing upwards.
- Thumb: Point upwards, representing the direction of the current.
- Curled Fingers: Curl your fingers around the wire. They will naturally wrap in a counterclockwise direction when viewed from above, indicating the direction of the magnetic field lines around the wire.
Common Mistakes to Avoid
- Using the Wrong Hand: Always use your right hand. Using the left hand will give you the opposite direction of the magnetic field lines.
- Incorrect Thumb Direction: Ensure your thumb is aligned with the direction of the current flow to accurately determine the magnetic field direction.
- Misunderstanding Current Direction: Remember, the rule applies to conventional current direction (positive to negative), not electron flow (negative to positive).
Final Thoughts
As you prepare for the MCAT, don’t dismiss the right-hand rule as just another low-yield topic destined for the dusty corners of your study notes. Sure, you may not be calculating magnetic forces or untangling current-carrying wires daily as a future doctor, but mastering this nifty trick can unlock the door to understanding trickier concepts in physics (not to mention net you some pretty quick and easy points in the MCAT Chem/Phys section).
Think of it as the secret handshake to the club of electromagnetism. Plus, isn’t it comforting to know that if you ever find yourself in a bizarre situation where you need to direct a magnetic field with just your bare hands, you’ll be totally prepared?
You got this, keep up the hard work, and remember to keep those right hands ready and your minds open!
If you need more help with MCAT physics or any other MCAT topics, our experts are here to help! Whether you need the flexibility of a Self-Paced Course, the instruction of a Live Course, or 1:1 assistance of a private MCAT tutor, Blueprint MCAT has the MCAT prep option that works for your learning style!
Get started with a free MCAT diagnostic, one free practice exam, and tons more MCAT prep resources.
