Physical processes

This course is a part of MCAT, a 14-course Topic series from Khan Academy.

Foundational Concept 4: Complex living organisms transport materials, sense their environment, process signals, and respond to changes using processes that can be understood in terms of physical principles.

This course contains 30 segments:

Vectors and scalars

4A: It’s gym day. Today you are going to run, swim, and lift weights. In physics terms, we can describe these actions in terms of vectors and scalars. For instance, if you throw a discus across a field with a 20-pound force westward, that is an example of a vector: an entity with both magnitude and direction. However, when you show off about how many laps you can swim in an hour, you are referring to speed, a scalar, which does not specify direction. An understanding of vectors and scalar will provide a foundation to many of the other basic concepts in physics.

Speed and velocity

4A: With every beat, your heart pumps blood throughout the vessels of your body. Let’s look at the radial artery, one of the vessels in your arm. How fast is the blood going at a particular point in time, and in what direction? The speed is an absolute number without direction - 1.8 kilometers per hour in this case. Velocity expands on the speed, which is an absolute value, by adding direction - for instance, 1.8 kilometers per hour downward. It’s that easy! you already grasping the concept of speed and velocity, terms used to describe the rate of change in our distance over time.

Acceleration

4A: Let’s imagine blood flowing through the aorta, one of the great vessels in our body. At peak velocity, blood may flow at 92 cm/sec (during contraction) or perhaps 60 cm/sec during relaxation. Going from 60 cm/sec to 92 cm/sec in 1 second is a lot different from going from 60 cm/sec to 92 cm/sec in 10 seconds. That difference is acceleration, that is, how quickly the car changes its velocity over time. (When you hear automobile enthusiasts compare the “0 to 60” capabilities of the latest sportscar, they are referring to the acceleration).

Newton's laws and equilibrium

4A: Apples would not fall without a force to propel them downward. These three laws described by the famous Englishman centuries ago still inform our understanding of the physical world today. When you push a patient’s bed down the halls of the hospital, you are providing a force in a direction. If we know the magnitude of the force we provide as well as the mass of the patient and the hospital bed, we can calculate the acceleration of the patient and his bed using Newton’s Second law (F=ma).

Normal forces

4A: How do forces work inside an elevator or when you slide a box across the floor? Normal forces are forces which act perpendicularly to surfaces. When you see a patient in a hospital bed, the reason he does not fall through the bed is because the ground provides the patient with a normal force that directly opposes the force of the Earth’s gravity on your the body (this is why you aren’t sinking to the Earth’s core right now). We will walk through real-world physical examples like these to shed light on normal forces.

Force of tension

4A: Let’s picture an adventurous rock-climber hanging at the edge of a cliff. Gravity is providing a downward force. The only reason he does not fall down to the valley below is due to the force of tension upward, provided by the muscles of his arm. You will work through some interesting real-world problems involving tensile forces in these tutorials.

Forces on inclined planes

4A: If you've ever moved from one town to another, you are likely familiar with inclined planes. When you push up that 50 pound box into the long flatbed truck , there are several forces at play: the weight of the box, the frictional force between the box and the ramp, and the force of your push. We will explain how these forces interact so next time the move won't be so back-breaking.

Work and energy

4A: Work doesn’t always have to break your back. In physics, it has a different meaning. When a force is applied to an object and results in displacement, work has been done. When an apple falls from a tree, gravity has done work on the object as it descends earthward. And as work is done by gravity on the object, its gravitational potential energy is converted into kinetic energy (which is also why it hurts as it lands on your head, hopefully resulting in a brilliant Newtonian idea). Let’s get moving!

Fluids at rest

4B: Fluids can be fun even when they’re not moving. A still pond has more going on with it than meets the eye - why exactly do you even float when you decide to jump in on a scalding summer day? It’s because you displace a volume of water that provides a buoyant force upward, counteracting the downward pull of gravity. You will find out why the great Archimedes ran through the streets of Syracuse shouting "eureka" millennia ago in Ancient Greece as we also delve into the concepts of pressure, specific gravity, buoyancy, and Bernoulli's principle.

Fluids in motion

4B: In these videos you'll learn more about fluids. Bernoulli's equation will describe the behavior of fluids in motion. The Venturi effect, turbulence, surface tension and viscosity will be discussed as well as the difference between arteries and veins.

Gas phase

4B: Imagine taking a balloon and heating it up by putting it in a sauna. What would happen? Gases, though you may not be able to see them, are perpetually in motion. You will encounter them if you ever frequent an operating room, as the anesthesiologist holds in his arsenal several gases with doze-inducing properties. We will walk through the history and application of the ideal gas law to real-world problems as you also come to appreciate the meaning of partial pressures and STP in these tutorials.

Kinetic molecular theory of gases

4A: Though we may not be able to always visualize them, gases are comprised of atoms and molecules. We’ll explore the molecular behavior that determines the pressure and temperature of a gas. In addition, we will discuss the first law of thermodynamics and heat capacity.

Electrostatics

4C: Ouch! have you ever heard of people being struck by lightning? Have you ever seen a defibrillator used to shock a patient back from the jaws of death? These amazing phenomena are possible due to the action of electric charges. Like masses, electric charges can have an associated force and a potential energy within a field. In these tutorials, you will discover how electric charges interact with one another and will arrive at an understanding of the concepts of electric force, fields, and potential.

Current and resistance

Always follow the path of least resistance. In these videos you will learn how to interpret circuit diagrams and the rules that govern the flow of electrons - or current - through wires and resistors.

Capacitors

4C: Capacitors are simply components which store electrostatic energy in a field. They are similar to batteries - however, capacitors only store new electricity rather than producing it through a chemical reaction like a battery does. You will walk through a mathematical description of how capacitors function and how they work within electrical circuits.

Magnetism

Magnets - how do they work? Guaranteed way to win a nobel prize: find a magnetic monopole. If you don’t know what that is, these videos would be a good place to start. Notable achievements of magnets - they put current in our homes, the protect us from dangerous cosmic rays, and they make particles at CERN go really really fast so we can learn more about science!

Electrochemistry

Recall from chemistry that electrons are transferred between atoms during redox reactions. When redox reactions occur on a conductive surface, the moving electrons can be collected as a current, with the voltage being dependent on the species used in reaction. Find out how batteries are made and the chemistry that makes them function by watching these videos.

Sound

4D:Some sounds are loud (high amplitude) like someone yelling, while others are soft (low amplitude) like a whisper. Some sounds are low pitched (low frequency) like a fog-horn, while others are high-pitched (high frequency) like a pager. You may have even noticed that the pitch of an ambulance rises as it rushes towards you and drops as it moves away. This is the Doppler effect in action. Here you will learn about the basics of sound properties such as wavelength, frequency, and amplitude.

Light and electromagnetic radiation

4D: Believe it or not, light has both wave-like and particle-like properties, as evidenced by the concepts of polarization, interference, and the photon model. In this modern age of medicine, we have seen a rise in the clinical use of the laser (which actually stands for “light amplification by stimulated emission of radiation”). As we discuss theories outlined by geniuses like Max Planck several decades ago, you’ll discover how light rays interfere in double slit, single slit, and diffraction gratings.

Infrared and Ultraviolet/Visible spectroscopy

They aren't the same as what you see in CSI:Miami, but molecules do have fingerprints. No two molecules have the same absorption spectra. We can use this fact to determine the composition of a material, its concentration, and measure it’s kinetics. Learn how to use this fantastic tool now!

Proton nuclear magnetic resonance

Let’s shift gears and take a peak at 1H-NMR. When you bring a magnet close to iron filings, the filings align with the applied field. The hydrogen atoms of molecules behave similarly under a strong magnetic field, but bounce back and radiate when the field is removed. in this section you will learn how to interpret the spectra created by proton NMR.

Thin lenses

4D: Without lenses, we would not be able to examine the layers of the skin or to observe the habits of the pink flamingo from a distance. If you have ever used a microscope, binoculars, or a magnifying glass, you have benefited from the workings of a thin lens, which refracts rays of light as it passes through the medium. In these tutorials, you’ll learn how to use ray tracings and the thin lens formula to predict the sizes and orientation of images created by thin lenses. We will apply these concepts to a discussion of the human eye.

Spherical mirrors

4D: Have you ever been to a house of mirrors at a carnival, or maybe seen one in the movies? Curved mirrors are fun, and so is a description of their physics. It’s actually possible to roughly predict what an image produced by a mirror will look like using just pen and paper. You will discover the difference between real and virtual images as we draw ray tracing diagrams to show how images are formed by spherical mirrors.

Reflection and refraction

4D: Reflection and refraction of light rays allow us to take in the visual world around us. Perhaps you have seen surgeons outfitted with magnifying glasses in order for them to grasp the tiniest vessels of the body. And if you’ve ever looked in a mirror to comb your hair in the morning, you’ve benefited from the power of a reflective mirror. You may have also noticed that things look different when seen underwater. We will explore how light rays bend as they penetrate surfaces like water or reflect as they meet surfaces like that of a mirror.

Atomic nucleus

4E: An introduction to the nucleus including isotopes, mass defects, and radioactive decay.

Electronic structure

1s, 2s, red s, blue s. Electrons spin around a the nucleus of an atom in various spherical and teardrop shaped orbitals. The following section will show how the periodic table is built around these orbitals, and how the electronic structure of atoms dictates how they interact with light and other atoms.

Periodic table

4E: A little more than a century ago, the chemist Dmitri Mendeleev published an early form of the periodic table, which organizes the known elements of our world by ionization energy and electron affinity. His method of classifying the elements was so useful that we still use it even today. We will learn to apply this elegant table to an understanding of atoms and molecules in this tutorial. Hydrogen, helium, lithium, beryllium, boron, carbon…

Stoichiometry

4E: A cake recipe calls for 2 eggs and 4 cups of flour, but you have 3 eggs, and 3 cups of flour. In this example, flour is our limiting reagent, that is, its small amount preventing you from completing the reaction of making the cake since we do not have enough (while we have more than enough eggs to make the cake). We’ll extend this simple example to chemical reactions with atoms and molecules. You'll learn to balance chemical reactions, a concept that will be applied to several other problems in chemistry.

Balancing chemical equations

We are now going to look at chemical reactions. But as we do, we need to make sure that atoms aren't magically appearing or disappearing. Put another way, we need to sure that we have the same number of each constituent atom in the product of the reaction as we do in the reactants (the molecules that react)!

Redox reactions

4E: You’re the resident on call in the wards of the hospital and have been receiving calls all day long. When you finally plug in your phone for charging, a redox reaction takes place to refuel its battery. At one terminal of the battery, a reaction is producing free electrons, while the other end absorbs these electrons. We will delve into the mechanics of this elegant process - part of the reason we are able to live in an increasingly wireless world - in this tutorial.