Physics II For Dummies
von: Steven Holzner
For Dummies, 2010
ISBN: 9780470640678
Sprache: Englisch
384 Seiten, Download: 4796 KB
Format: EPUB
Chapter 1
Understanding Your World: Physics II, the Sequel
In This Chapter
Looking at electricity and magnetism
Studying sound and light waves
Exploring relativity, radioactivity, and other modern physics
Physics is not really some esoteric study presided over by guardians who make you take exams for no apparent reason other than cruelty, although it may seem like it at times. Physics is the human study of your world. So don’t think of physics as something just in books and the heads of professors, locking everybody else out.
Physics is just the result of a questioning mind facing nature. And that’s something everyone can share. These questions — what is light? Why do magnets attract iron? Is the speed of light the fastest anything can go? — concern everybody equally. So don’t let physics scare you. Step up and claim your ownership of the topic. If you don’t understand something, demand that it be explained to you better — don’t assume the fault is with you. This is the human study of the natural world, and you own a piece of that.
Physics II takes up where Physics I leaves off. This book is meant to cover — and unravel — the topics normally covered in a second-semester intro physics class. You get the goods on topics such as electricity and magnetism, light waves, relativity (the special kind), radioactivity, matter waves, and more. This chapter gives you a sneak preview.
Getting Acquainted with Electricity and Magnetism
Electricity and magnetism are intertwined. Electric charges in motion (not static, nonmoving charges) give rise to magnetism. Even in bar magnets, the tiny charges inside the atoms of the metal cause the magnetism. That’s why you always see these two topics connected in Physics II discussions. In this section, I introduce electricity, magnetism, and AC circuits.
Looking at static charges and electric field
Electricity is a very big part of your world — and not just in lightning and light bulbs. The configuration of the electric charges in every atom is the foundation of chemistry. As I note in Chapter 14, the arrangement of electrons gives rise to the chemical properties of matter, giving you everything from metals that shine to plastics that bend. That electron setup even gives you the very color that materials reflect when you shine light on them.
Electricity studies usually start with electric charges, particularly the force between two charges. The fact that charges can attract or repel each other is central to the workings of electricity and to the structure of the atoms that make up the matter around you. In Chapter 3, you see how to predict the exact force involved and how that force varies with the distance separating the two charges.
Electric charges also fill the space around them with electric field — a fact familiar to you if you’ve ever felt the hairs on your arm stir when you’ve unloaded clothes from a dryer. Physicists measure electric field as the force per unit charge, and I show you how to calculate the electric field from arrangements of charges.
Next up is the idea of electric potential, which you know as voltage. Voltage is the work done per unit charge, taking that charge between two points. And yes, this is exactly the kind of voltage you see stamped on batteries.
With those three quantities — force, electric field, and voltage, you nail down static electric charges.
Moving on to magnetism
What happens when electric charges start to move? You get magnetism, that’s what. Magnetism is an effect of electric charge that’s related to but distinct from the electric field; it exists only when charges are in motion. Give an electron a push, send it sailing, and presto! You’ve got magnetic field. The idea that moving electric charges cause magnetic field was big news in physics — that fact’s not obvious when you simply work with magnets.
Electric charges in motion form a current, and various arrangements of electric current create different magnetic fields. That is, the magnetic field you see from a single current-bearing wire is different from what you see from a loop of current — let alone a whole bunch of loops of current, an arrangement known as a solenoid. I show you how to predict magnetic field in Chapter 4.
Not only do moving electric charges give rise to magnetic fields, but magnetic fields also affect moving electric charges. When an electric charge moves through a magnetic field, that charge feels a force on it at right angles to the magnetic field and the direction of motion. The upshot is that left to themselves, moving charges in uniform magnetic fields travel in circles (an idea chemists appreciate, because that’s what allows a mass spectrometer to sort out the chemical makeup of a sample). How big is the circle? How does the radius of the circle correlate with the speed of the charge? Or with the magnitude of the charge? Or with the strength of the magnetic field? Stay tuned. The answers to all these questions are coming up in Chapter 4.
AC circuits: Regenerating current with electric and magnetic fields
Students often meet electrical circuits in Physics I (you can read about simple direct current [DC] circuits in Physics For Dummies). In Chapter 5, you get the Physics II version: You take a look at what happens when the voltage and current in a circuit fluctuate in time in a periodic way, giving you alternating voltage and currents. You also encounter some new circuit elements, the inductor and capacitor, and see how they behave in AC circuits. Many of the electrical devices that people use every day depend on such elements in alternating currents.
In reading about the inductor, you also encounter one of the fundamental laws that relates electric and magnetic fields: Faraday’s law, which explains how a changing magnetic field induces a voltage that generates its own magnetic field. This law doesn’t just apply to inductors; it applies to all electric and magnetic fields, wherever they occur in the universe!
Riding the Waves
Waves are a huge topic in Physics II. A wave is a traveling disturbance that carries energy. If the disturbance is periodic, the amount of disturbance repeats in space and time over a distance called the wavelength and a time called the period. Chapter 6 delves into the workings of waves so you can see the relationships among the wave’s speed, wavelength, and frequency (the rate at which cycles pass a particular point). In the rest of the chapters in Part III of this book, you explore particular types of waves, including electromagnetic waves (such as light and radio waves) and sound.
Getting along with sound waves
Sound is just a wave in air, and the various interactions of sound waves are just a result of the behaviors shared by all waves. For instance, sound waves can reflect off a surface — just let sound waves collide with walls and listen for the echo. Sound waves also interfere with other waves, and you can hear the effects — or silence, as the case may be. These two kinds of interaction form the basis for understanding the harmonic tones in music.
The qualities of a sound, such as pitch and loudness, depend on the properties of the wave. As you may have noticed by hearing the change of pitch of a siren on a police car as it passes by, pitch changes when the source or the listener moves. This is called the Doppler effect. You can take this to the extreme by examining the shock wave that happens when objects move very quickly through the air, breaking the sound barrier. This is the origin of the sonic boom. I cover all this and more in Chapter 7.
Figuring out what light is
You focus on light a good deal in Physics II. How light works is now well-known, but that wasn’t always the case. Imagine the excitement James Clerk Maxwell must’ve felt when the speed of light suddenly jumped out of his equations and he realized that by combining electricity and magnetism, he’d come up with light waves. Before that, light waves were a mystery — what made them up? How could they carry energy?
After Maxwell, all that changed, because physicists now knew that light was made up of electrical and magnetic oscillations. In Chapter 8, you follow in Maxwell’s footsteps to come up with his amazing result. There, you see how to calculate the speed of light using two entirely different constants having to do with how well electric and magnetic fields can penetrate empty space.
As a wave, light carries energy as it travels, and physicists know how to calculate just how much energy it can carry. That amount of energy is tied to the magnitude of the wave’s electric and magnetic components. You get a handle on how much power that light of a certain intensity can carry in Chapter...