MCAT Physics and Math Review

Chapter 5: Electrostatics and Magnetism

Introduction

Electrostatics is the study of stationary charges and the forces that are created by and which act upon these charges. Without electrical charge, we would not be able to do many of the activities that we enjoy or consider essential to basic living. But living with electrical charge can also be dangerous and even deadly: magnify the small shock you receive from a doorknob after walking across a carpet, and you have the equivalent of a lightning bolt strong enough to stop a heart. This same concept can be used in life-saving therapy as well: cardioversion and defibrillation create a strong electrical current through the heart’s conduction system that attempts to resynchronize a pulse.

In this chapter, we will review the basic concepts essential to understanding charges and electrostatic forces including conductors and insulators. We will review Coulomb’s law, which describes the attractions and repulsions between charged objects. Next, we will describe the electric fields that all charges create, which allow them to exert forces on other charges. After we’ve discussed how charges set up these fields, we’ll observe the behavior of charges that are placed into these fields. In particular, we will note the motional behavior of these test charges inside a field in relation to the electrical potential difference, or voltage, between two points in space. We can then determine the change in electrical potential energy as the charge moves from a position of some electrical potential to another. Next, we will describe the electric dipole and solve a problem involving one of the molecular dipoles most important to life on this planet: the water molecule. Finally, we will explore the topic of magnetic fields and forces.

5.1 Charges

Charged subatomic particles come in two varieties. One, the proton, has a positive charge; the other, the electron, has a negative charge. While opposite charges exert attractive forces, like charges—those that have the same sign—exert repulsive forces. Unlike the force of gravity, which is always an attractive force, the electrostatic force may be repulsive or attractive depending on the signs of the charges that are interacting.

MCAT EXPERTISE

While many of the particles we discuss in electrostatics are very, very tiny, do not forget that they still do have mass. We can use equations such as the kinetic energy equation when solving problems with charged particles, and the MCAT will sometimes require us to do just that.

Most matter is electrically neutral, as a balance of positive and negative charges ensures a relative degree of stability. When charges are out of balance, the system can become electrically unstable. Even materials that are normally electrically neutral can acquire a net charge as result of friction. When you shuffle your feet across the carpet, negatively charged particles are transferred from the carpet to your feet, and these charges spread out over the total surface of your body. The shock that occurs when your hand gets close enough to a metal doorknob allows that excess charge to jump from your fingers to the knob, which acts as a ground—a means of returning charge to the earth. Static charge buildup or static electricity is more significant in drier air because lower humidity makes it easier for charge to become and remain separated.

The SI unit of charge is the coulomb, and the fundamental unit of charge is

e = 1.60 × 10−19 C

A proton and an electron each have this amount of charge, although the proton is positively charged (q = +e) while the electron is negatively charged (q = −e). Even though the proton and the electron share the same magnitude of charge, they do not share the same mass; the proton has a much greater mass than the electron.

KEY CONCEPT

The fundamental unit of charge is e = 1.60 × 10−19 C. A proton and an electron each have this amount of charge; the proton is positively charged (q = +e), while the electron is negatively charged (q = −e).

INSULATORS AND CONDUCTORS

Insulators and conductors vary in their ability to both hold and transfer charges. An insulator will not easily distribute a charge over its surface and will not transfer that charge to another neutral object very well—especially not to another insulator. On a molecular level, the electrons of insulators tend to be closely linked with their respective nuclei. By extension, most nonmetals are insulators. Experimentally, insulators serve as dielectric materials in capacitors as well as in isolating electrostatic experiments from the environment to prevent grounding.

In contrast, when a conductor is given a charge, the charges will distribute approximately evenly upon the surface of the conductor. Conductors are able to transfer and transport charges and are often used in circuits or electrochemical cells. Conductors are often conceptualized as nuclei surrounded by a sea of free electrons that are able to move rapidly throughout the material and are only loosely associated with the positive charges. Conductors are generally metals, although ionic (electrolyte) solutions are also effective conductors. Figure 5.1 demonstrates the behaviors of an insulator and a conductor when a negative charge is placed on them.

Figure 5.1. A Negatively Charged Insulator and Conductor Insulators will not distribute charge over their surface; conductors will.

MCAT Concept Check 5.1:

Before you move on, assess your understanding of the material with these questions.

1.    When placed one meter apart from each other, which will experience a greater acceleration: one coulomb of electrons or one coulomb of protons?

2.    Categorize the following materials as either conductors or insulators: blood, hair, copper, glass, iron, sulfuric acid, distilled water

·        Conductors:

·        Insulators:

3.    What is the net charge of an object with one coulomb of electrons and 3 moles of neutrons?