It's possible to do the math correctly on an MCAT physics question and still end up with the wrong answer.
For example, maybe you've calculated that a device transfers 200 joules of energy, but the question asks for power. Your calculation isn't necessarily wrong. You just haven't calculated the quantity the question is asking for.
This is where understanding the difference between joules and watts becomes useful. The two units are related, but they measure different things, and confusing them can make an otherwise straightforward problem unnecessarily difficult.
What does a joule measure?
A joule (J) is the SI unit of energy. It's also used to measure work and energy transferred as heat.
In mechanics, one joule is the work done when a force of one newton moves an object one meter in the direction of that force.
You'll see joules in several areas of MCAT physics, including kinetic energy, potential energy, electricity, and thermodynamics.
The important distinction is that joules describe an amount of energy, not how quickly that energy is transferred.
For example, knowing that a heater transferred 200 joules of energy tells you how much energy was transferred. It doesn't tell you whether that happened over two seconds, ten seconds, or an entire minute.
To describe how quickly the energy transfer occurred, you need a different quantity.
What does a watt measure?
A watt (W) is the SI unit of power, which measures the rate at which energy is transferred or work is performed.
One watt is equal to one joule per second.
1 W = 1 J/s
This means that a device transferring energy at a constant rate of 20 joules every second has a power output of 20 watts.
Two devices can transfer the same amount of energy but have different power outputs if one takes longer than the other.
That's why it's important to distinguish between the amount of energy involved and the rate at which it's transferred.
A simple example
Suppose a laboratory heating element transfers 200 joules of energy to a solution over 10 seconds.
The question asks for the average power transferred to the solution.
Since power measures energy transferred per unit time, we divide the energy by the time interval.
Power = 200 J / 10 s = 20 J/s = 20 W
The answer is 20 watts.
If the question had asked how much energy was transferred, the answer would have been 200 joules. Since it asks for average power, we need to account for how long the transfer took.
This is a simple example, but it illustrates why identifying the quantity you're calculating matters just as much as getting the math right.
Why the letter W can be confusing
There's another detail worth knowing, especially if you've been memorizing physics equations.
The letter W is commonly used as the variable for work, even though W is also the symbol for the unit watt.
For example, you may have learned the equation:
W = Fd
In this equation, W represents work, measured in joules, assuming the force is constant and acts in the direction of the displacement.
But when you see a measurement written as 20 W, the W means watts, which measure power.
The same letter appears in both situations, but its meaning depends on whether it's being used as a variable in an equation or as a unit.
It's a small distinction, but understanding the actual physical quantities makes the notation much less confusing.
How this helps with MCAT physics questions
When you're working through a calculation-heavy passage, it's easy to focus on remembering the appropriate equation and getting through the math.
But even if the math works out, you still need to make sure your result represents what the question asks for.
If you're supposed to calculate power and your result is in joules, that's a sign to revisit your setup. Power should be expressed in watts or another equivalent unit of energy per time.
Of course, units can't tell you everything. An equation can have the correct units and still be scientifically incorrect. You still need to understand the underlying physics and whether the relationship you're using applies to the situation.
But paying attention to what units mean is a useful way to catch mistakes before committing to an answer.
One related question: Is a kilowatt-hour energy or power?
Despite having the word watt in its name, a kilowatt-hour (kWh) is a unit of energy.
It represents the energy transferred when one kilowatt of power is sustained for one hour.
Since one kilowatt is 1,000 joules per second and one hour contains 3,600 seconds, one kilowatt-hour equals 3.6 million joules.
This is another example of why understanding the relationship between power, energy, and time is more useful than relying on the names of units alone.
Final thoughts
The main difference to remember is that joules measure energy, while watts measure the rate of energy transfer or work.
When a physics problem involves both energy and time, understanding that distinction can help you recognize whether you've actually answered the question.
It's also a good example of how paying closer attention to units can make MCAT calculations easier to organize.
If you struggle with knowing how to approach calculation-heavy Chem/Phys questions, I've created a free introduction to the approach I teach in Unit Hack for the MCAT® exam.
You can explore the Unit Hack Free Preview here.
