Physics · Class 9-10 · Chapter 1 · Physical quantities and measurement
Physics, its scope and its purpose.
No formulas in this section, no arithmetic. Just an answer to one question: what is physics, and how far does it reach. Once you have the answer, the rest of the book gets a good deal easier to read.
1. The definition, in the book's words
Physics is the branch of science that deals with matter, energy, and the interaction between the two. It is a short definition with three words in it, and all three earn their place: matter, energy, and the interaction between them. Drop the last one and the definition collapses, because physics is not a list of things. It is an account of what things do to each other.
The word physics comes from the Greek phusikos, meaning nature. The name tells you the subject. And what is science itself? Organised knowledge that comes from observation and experiment. Notice that both words are there: looking is not enough on its own, you have to test what you saw.
Science has three main branches: physics, chemistry and biology. Physics is called the foundation of the other two, and the reason is plain enough. Without the structure of the atom there is no chemistry, and without the behaviour of molecules there is no biology. You will see that claim drawn out in a moment.
check yourselfpick one answer
What is inside the definition
Which of these is not one of the three words in the definition of physics?
Right. The definition reads: the branch of science that deals with matter, energy and the interaction between the two. The living cell belongs to biology, even though the laws of physics hold inside a cell as well.
Matter is the first word of the definition. Anything with mass that takes up space is matter.
Energy is the second word. It is the capacity to do work, and most of physics is about it changing form.
Interaction is the most important word of the three. What matter and energy do to each other is the actual subject.
2. Scope: from how small to how large
The boldest claim physics makes is about its reach. It runs from something like 10⁻¹⁵ m inside a nucleus out to something like 10²⁶ m across the universe. The gap between those two ends is too large to picture, and yet the same handful of laws explains both of them. The gravity that pulls an apple down is the gravity that holds a galaxy together.
But you do not need distant stars to make the point. The room you are sitting in has enough examples. Tap the six points on the drawing below and see which branch of physics is at work behind each one.
look aroundtap a dot on the drawing
A whole physics syllabus in one room
The ceiling fan. Inside the motor, electrical energy is turning into the kinetic energy of a spin: that is current electricity. The blades then push air across your skin, which is mechanics and heat.
The tube light. A current runs through the gas inside and that energy comes back out as light. Electricity and optics are doing one job in a single tube.
Light from the window. Light travels in straight lines, which is why the shape of the window lands on the floor. Reflection, refraction and shadow are all optics.
The phone. The chip inside it is made of semiconductors, which is solid state physics. It talks to the rest of the world with electromagnetic waves.
The wall clock. Something inside it comes back to the same state at equal intervals. That repeating motion is the basis of measuring time, and time is one of the base quantities in this chapter.
The rolling ball. It rolls, and then it slows and stops. Why it stops is a question about friction, and the whole business is mechanics.
All six. Notice that not one of them is laboratory equipment. They are just the things in a room.
The drawing is a model, not a scale drawing of a real room.
Six things in one room, and at least six different branches. Which tells you the branches are not separate subjects so much as different angles on the same world. Still, it is worth knowing the divisions, because the rest of the book is arranged along exactly these lines.
3. Two halves, then the branches
Physics is split in two, and the split is by date. The branches built before the twentieth century are classical physics; the ones that arrived in the twentieth century on the back of quantum mechanics and relativity are modern physics. Open the branches below and read the names.
branchesopen a branch
The branches of physics
Physics
Classical physics
Mechanics motion and force
Heat and thermodynamics heat, temperature, energy changing form
Sound waves and vibration
Light reflection, refraction, lenses
Static and current electricity charge and flow
Magnetism and electromagnetism magnetic fields, induction
Modern physics
Quantum mechanics the rules inside the atom
Relativity high speed and gravity
Atomic and nuclear physics the atom and its nucleus
Solid state physics semiconductors, chips
Particle physics the fundamental particles
Astrophysics and cosmology stars, galaxies, the universe
Classical does not mean wrong, and it does not mean obsolete. Building a bridge, driving a car or installing a pump still runs on classical physics. The modern branches are needed when a thing is very small, or very fast, or very heavy. The phone in your hand, as it happens, is a product of both.
check yourselfpick one answer
Which half, and the base of what
Which half does solid state physics belong to, and which technology rests on it?
Right. Solid state physics arrived in the twentieth century on the back of quantum mechanics, so it is modern. Its largest product is the semiconductor: transistors, diodes, chips, which is to say all of electronics.
The steam engine is a product of classical physics, but of thermodynamics, not solid state. And solid state is a modern branch.
Lenses and microscopes come from optics, and optics is a classical branch. Neither half of this answer is about solid state physics.
The half is right: modern. But the tides are a matter of gravity, which is classical mechanics, and nothing to do with solid state.
4. Naming the branch from the question
More useful than memorising the names is being able to look at a question and say which branch it belongs to, because that is what gets asked. Tap each of the ten questions below and see whose territory it falls in.
whose territorytap a question
Which branch does this question belong to?
Optics. Air scatters blue light more than the rest, so blue is what reaches your eye from every direction.
Thermodynamics. Heat is being moved from inside to outside, which is why the box is cold and the coil at the back is warm.
Solid state physics. A semiconductor such as silicon, whose ability to conduct can be switched at will.
Static electricity. Charge builds up in a cloud, suddenly finds a path, and that discharge is the lightning.
Nuclear physics. Light nuclei are fusing into heavier ones deep inside the sun, and that reaction is where the energy comes from.
Acoustics. The skin vibrates, the vibration spreads through the air, and when that wave reaches your ear you hear it.
Mechanics and gravity. A satellite is falling. It is just moving sideways fast enough that the earth keeps curving away beneath it.
Magnetism. The field around the magnet lines up the tiny magnets inside the iron, and then pulls on them.
Cosmology. Distant galaxies are moving away, and the further they are the faster they go. That is the evidence that the universe is expanding.
Atomic physics. X-rays are electromagnetic waves from inside the atom, and bone and flesh stop them by different amounts, which is what makes the picture.
All ten. Notice that none of the questions is hard to ask, and yet answering them takes ten different branches.
5. Why it is called the foundation
Saying “physics is the foundation of the other sciences” in a paragraph is just a claim. Draw three circles side by side and it becomes something you can look at. Press the buttons and see what lands in each region.
overlappress a button
Where the three sciences meet
Physics reaches into all of it, because the rules of atoms, energy and force are the ground the other two stand on. This picture is what the word foundation means.
Physical chemistry. The structure of the atom, the energy in a bond, thermochemistry.
Biophysics. The lens of the eye, pressure in the bloodstream, the electrical signal along a nerve.
Biochemistry. Enzymes, metabolism, the chemical structure of DNA.
All three at once. Take MRI: the magnetic field is physics, the hydrogen nucleus in a water molecule is chemistry, and the tissue being imaged is biology.
Notice that the overlaps have names of their own, and that they are whole subjects. Physical chemistry and biophysics are not casual blends; universities have departments for them. The divisions of science are lines we drew for convenience. Nature does not have them.
6. Three purposes
One, uncovering what nature is doing. Why the sky is blue, how lightning happens, where the sun gets its energy. Asking the question, then staying with it until there is an answer. The purpose here is only to know, not yet to use.
Two, finding the laws. Getting from a pile of separate observations to one general rule. Newton's discovery that the law which brings an apple down is the same law that keeps the moon going round is still the best example of what a law buys you.
Three, turning the laws into technology. Electromagnetic induction gives you a generator; semiconductors give you the transistor and then the phone; nuclear reactions give you a power station; X-rays and MRI give you a way to look inside a patient. The law has to come first. The machine follows.
cut from the notetap to turn over
Three purposes, six examples
ExampleThe generator
Which purposePurpose 3, technology. Faraday's law of induction, put to work by spinning a magnet to make electricity.
ExampleWhy the sky is blue
Which purposePurpose 1, uncovering. Air scatters blue light more than the other colours.
ExampleThe law of gravitation
Which purposePurpose 2, finding the law. The falling apple and the orbiting moon obey the same rule.
ExampleMRI
Which purposePurpose 3, technology. Nuclear magnetic resonance turned into a picture of the inside of a body.
ExampleLightning
Which purposePurpose 1, uncovering. The sudden discharge of charge built up in a cloud.
ExampleThe chip in a phone
Which purposePurpose 3, technology. The quantum behaviour of semiconductors turned into a transistor.
This is where the difference between science and technology gets clear. Science is for knowing; technology is for using. Technology is a product of science, not the same thing as it. When Faraday found the law of induction in 1831 he had no plan for a generator. The machine came long after the law, and that order never runs backwards.
check yourselfpick one answer
Which comes first
Which pair shows the order “science, then technology” the right way round?
Right. Faraday found the law of induction in 1831 with no generator in mind. The generator came after the law was known.
Back to front. The generator is a result of the law, not its cause. Without the law nobody could have built the machine.
Back to front. Understanding semiconductors came first, then the transistor, and only then was a phone possible.
Back to front. Nuclear magnetic resonance is a natural effect that was discovered first; the MRI machine is a device built on top of it.
From here the chapter goes straight into measurement, and the reason is hiding in this section. Physics means measuring. How long, how heavy, how long it took, how fast. So the whole of chapter 1 is really the answer to two questions: how do we measure it, and how far can we trust the number.