11.3 Physical Science: Matter, Forces, Energy, Heat, Light, Electricity & Magnetism

Physical Science

Physical is one of the four Stanford 10 Science content clusters (11 of 40 items on a real Intermediate 2 report). It covers matter and its changes, forces and motion, energy, heat, light, sound, electricity and magnetism. Many physical science questions involve measurement and what stays the same or changes, which is the focus of the Constancy process cluster.

---

Physical Science: Matter, Atoms, and Chemical Changes

Matter is anything that possesses mass and occupies physical space. It exists in four primary states: solid (definite shape and volume, tightly packed vibrating particles), liquid (definite volume, indefinite shape, sliding particles), gas (indefinite shape and volume, high-velocity particles), and plasma (ionized gas with free electrons).

Physical vs. Chemical Changes

• Physical Change: Alters the physical form, state, or appearance of a substance without changing its chemical composition or molecular bonds (e.g., melting ice into water, dissolving salt in water, chopping timber, boiling ethanol).
• Chemical Change: Breaks existing molecular bonds and forms entirely new chemical substances with distinct properties (e.g., iron rusting, wood combusting, milk curdling, acid reacting with metal). Hallmarks of chemical reactions include gas bubble evolution, color change, precipitate formation, and temperature shifts.

Atomic Structure

Every atom contains three subatomic particles: positively charged protons ($+1$) and neutral neutrons ($0$) packed tightly in the central nucleus, surrounded by negatively charged electrons ($-1$) orbiting in electron shells. The atomic number equals the number of protons and uniquely defines the element.

---

Classical Mechanics: Newton's Laws and Simple Machines

Isaac Newton's three laws of motion govern macroscopic physical interactions:

1. First Law (Law of Inertia): An object at rest remains at rest, and an object in uniform motion continues moving at constant velocity in a straight line, unless acted upon by an unbalanced external net force.
2. Second Law ($F = ma$): The acceleration of an object is directly proportional to the net force acting upon it and inversely proportional to its mass:

$\text{Force (Newtons, N)} = \text{Mass (kg)} \times \text{Acceleration (m/s}^2)$

1. Third Law (Action-Reaction): When one body exerts a force on a second body, the second body simultaneously exerts an equal and opposite force on the first body.

Work and Simple Machines

Mechanical work is done when an applied force moves an object across a distance in the direction of the force:

$\text{Work (Joules, J)} = \text{Force (N)} \times \text{Distance (m)}$

Simple machines do not reduce the total work required; they reduce the necessary input force by increasing the distance over which the force is exerted. The six classical simple machines are the lever, pulley, wheel and axle, inclined plane, wedge, and screw.

---

Forms of Energy and Conservation Principles

Energy is the capacity to perform work. It divides into two broad states:

• Potential Energy ($PE$): Stored energy of position or configuration (e.g., gravitational potential energy $PE = mgh$, chemical energy in petroleum bonds, elastic tension in a stretched bow).
• Kinetic Energy ($KE$): Energy of active motion ($KE = \frac{1}{2}mv^2$).

The Law of Conservation of Energy (First Law of Thermodynamics) states that energy cannot be created or destroyed in an isolated system. When a roller coaster descends a steep hill, gravitational potential energy is converted into kinetic energy and thermal frictional losses, while the total energy of the closed system remains constant.

---

Forces and Motion in Everyday Terms

• Speed is distance divided by time: a bicycle that travels 30 meters in 6 seconds moves at $30 \div 6 = 5$ meters per second.
• Balanced forces (equal and opposite) leave motion unchanged; unbalanced forces change speed or direction.
• Friction opposes motion between surfaces and turns some motion energy into heat; rough surfaces and heavier objects mean more friction.
• Gravity pulls objects toward Earth; without air resistance, a heavy ball and a light ball dropped together land at the same time.

Heat, Light and Sound

Electricity and Magnetism

• A circuit needs a complete loop: an energy source, wires and a device. Opening a switch breaks the loop.
• In a series circuit, one path connects every bulb, so one burned-out bulb stops them all. In a parallel circuit, each bulb has its own path.
• Conductors such as copper and aluminum let current flow easily; insulators such as rubber, plastic and glass do not.
• Magnets have north and south poles: like poles repel and opposite poles attract. Magnets attract iron, nickel and cobalt, not aluminum or copper.