Welcome to AS Level Physics!
Welcome to your first step in mastering Physics! Before we dive into how the universe works, we need to learn the "language" of science. Just like you can't build a house without measuring the wood, you can't do Physics without Physical Quantities and SI Units. This chapter is the foundation for everything else you will learn, so let's make it crystal clear!
1.1 What is a Physical Quantity?
In Physics, a physical quantity is anything that can be measured. To describe one, you always need two parts: a numerical magnitude (the number) and a unit.
Think of it like ordering pizza. If you tell the shop "I want 5," they won't know if you want 5 pizzas or 5 slices. The "5" is the magnitude, and "pizzas" is the unit!
Example: A piece of string is 2.5 meters long.
2.5 = Numerical Magnitude
meters (m) = Unit
Making Reasonable Estimates
Part of being a good physicist is having a "feel" for numbers. You should be able to guess the size of things roughly. Don't worry if this seems tricky at first; it just takes practice!
Here are some common estimates to remember:
- Mass of an apple: 100 g (0.1 kg)
- Height of a classroom door: 2 m
- Mass of an adult: 70 kg
- Current in a hairdryer: 5 A to 10 A
Key Takeaway:
Every measurement must have a number and a unit. Always ask yourself: "Does this number make sense in the real world?"
1.2 SI Base Units: The Building Blocks
There are millions of things to measure, but in the SI system (International System of Units), we only use a few "Base Units" to build all the others. According to your syllabus, you must recall these five base quantities:
1. Mass: measured in kilograms (kg)
2. Length: measured in meters (m)
3. Time: measured in seconds (s)
4. Electric Current: measured in ampere (A)
5. Temperature: measured in kelvin (K)
Quick Review Tip: Remember that mass is kg, not grams! In Physics, the "kilo" is part of the base unit.
Memory Aid: You can remember these as "My Large Tiger Always Kicks" (Mass, Length, Time, Ampere, Kelvin).
1.3 Derived Units: Putting the Bricks Together
Most other units are derived units. This means they are made by multiplying or dividing base units together. Think of base units like LEGO bricks—you can combine them to build more complex shapes.
How to find a Derived Unit step-by-step:
Let's find the SI base units for Force.
1. Start with the formula: \( F = ma \)
2. Identify the units of each part:
- Mass (\( m \)) is kg
- Acceleration (\( a \)) is m s\(^{-2}\)
3. Multiply them together: \( \text{kg} \times \text{m s}^{-2} \)
4. Result: The unit of Force (the Newton) is actually kg m s\(^{-2}\) in base units.
Common Mistakes to Avoid: Don't confuse the symbol for the quantity with the symbol for the unit. For example, the symbol for the quantity time is \( t \), but the symbol for the unit is \( s \).
Key Takeaway:
You can break down any unit (like Joules or Watts) into the 5 base units by using their defining formulas.
1.4 Homogeneity: The "Apple and Orange" Rule
In Physics, an equation is homogeneous if the units on the left side are exactly the same as the units on the right side. You cannot add 5 meters to 10 seconds—that would be like adding apples and oranges!
How to check for Homogeneity:
Imagine the equation: \( \text{speed} = \frac{\text{distance}}{\text{time}} \)
- Units on the Left: m s\(^{-1}\)
- Units on the Right: \( \frac{\text{m}}{\text{s}} = \) m s\(^{-1}\)
The units match, so the equation is homogeneous!
Did you know? Checking units is the best way to see if you've remembered a formula correctly in an exam. If the units don't match, the formula is wrong!
1.5 SI Prefixes: Making Numbers Manageable
Physics deals with the very big (stars) and the very small (atoms). Instead of writing lots of zeros, we use prefixes.
Large Prefixes (Multiples):
- Tera (T): \( 10^{12} \)
- Giga (G): \( 10^{9} \)
- Mega (M): \( 10^{6} \)
- Kilo (k): \( 10^{3} \)
- Deci (d): \( 10^{-1} \)
- Centi (c): \( 10^{-2} \)
Small Prefixes (Sub-multiples):
- Milli (m): \( 10^{-3} \)
- Micro (\( \mu \)): \( 10^{-6} \)
- Nano (n): \( 10^{-9} \)
- Pico (p): \( 10^{-12} \)
Example Conversion:
If you have 5 nm (nanometers), you can write it as \( 5 \times 10^{-9} \) meters.
Simple Trick: When moving from a prefix to a power of 10, just replace the symbol with the number.
Example: Replace 'k' with \( 10^{3} \). So, \( 7 \text{ km} = 7 \times 10^{3} \text{ m} \).
Key Takeaway:
Prefixes are just shorthand for powers of 10. Learn the symbols and their powers by heart—they are used in almost every calculation!
Quick Review Summary
1. Physical Quantities = Magnitude + Unit.
2. The 5 Base Units: kg, m, s, A, K.
3. Derived Units: Created by combining base units (e.g., \( \text{m s}^{-1} \)).
4. Homogeneity: Units must be the same on both sides of an equals sign.
5. Prefixes: Used to handle very large or very small numbers (from Pico to Tera).