8.4 Energy, Waves, and Electricity
Key Takeaways
- Kinetic energy is KE = ½mv² and gravitational potential energy is PE = mgh, both measured in joules.
- Energy is conserved: it transforms between forms — chemical to heat, potential to kinetic — but is never created or destroyed.
- Wave speed follows v = fλ; sound is a longitudinal mechanical wave needing a medium, while light is a transverse electromagnetic wave that travels through vacuum.
- Ohm's law V = IR links voltage, current, and resistance; current is measured in amperes, voltage in volts, resistance in ohms.
- Household wiring is parallel so each appliance gets full voltage and works independently; series circuits share voltage and fail together when one part breaks.
8.4 Energy, Waves, and Electricity
Why This Matters for the PUPCET
These three mini-topics close out the physics portion of the Science area. The exam typically asks for one energy computation, one conceptual question about waves or the difference between sound and light, and one Ohm's law or series-parallel question. Each rewards knowing a single formula cold plus one clean example.
Kinetic and Potential Energy
Energy is the ability to do work, measured in joules (J). Its two mechanical forms:
- Kinetic energy (KE) — energy of motion: KE = ½mv². Doubling the mass doubles the KE, but doubling the speed quadruples it, because velocity is squared.
- Gravitational potential energy (PE) — stored energy of position: PE = mgh, where m is mass in kg, g ≈ 9.8 m/s², and h is height in meters.
Worked example 1. A 2-kg ball rolls at 4 m/s. KE = ½ × 2 × 4² = 1 × 16 = 16 J.
Worked example 2. A 2-kg book sits on a shelf 3 m above the floor. PE = 2 × 9.8 × 3 = 58.8 J.
Conservation of energy says energy is never created or destroyed, only transformed. A mango hanging on a branch holds PE; as it falls, PE converts to KE, and just before it hits the ground nearly all of it is kinetic. A pendulum (or a child on a swing) trades PE and KE back and forth forever, losing a little each pass to friction and air resistance as heat. A roller coaster, a hydroelectric dam (water's PE → turbines' KE → electrical energy), and a stretched rubber band all tell the same story.
Forms and Transformations of Energy
Know the common forms and be ready to trace conversions:
| Everyday device or event | Energy transformation |
|---|---|
| LPG stove cooking rice | Chemical → heat (thermal) |
| Solar panel on a rooftop | Light (radiant) → electrical |
| Electric fan | Electrical → mechanical (kinetic) |
| Eating breakfast before a run | Chemical (food) → mechanical + heat |
| Battery-powered flashlight | Chemical → electrical → light |
| Striking a match | Chemical → heat and light |
Wave Properties
A wave is a disturbance that transfers energy without transferring matter. Its measurable parts: the wavelength (λ) is the length of one full cycle, the frequency (f) is how many cycles pass per second in hertz (Hz), the amplitude is the height of the wave and carries its energy, and the period is the time for one cycle. They connect through:
v = fλ (wave speed = frequency × wavelength)
Worked example. Waves in a pond have a frequency of 2 Hz and a wavelength of 3 m. v = 2 × 3 = 6 m/s.
Sound vs Light — a Guaranteed Comparison Item
| Property | Sound | Light |
|---|---|---|
| Wave type | Longitudinal (compressions) | Transverse |
| Needs a medium? | Yes — cannot travel in vacuum | No — travels through empty space |
| Speed in air | about 340 m/s | about 300,000,000 m/s |
| Loudness / brightness | Amplitude | Amplitude |
| Pitch / color | Frequency | Frequency |
This explains why you see lightning before you hear thunder: light reaches you almost instantly while sound needs several seconds. It also explains echoes — sound bouncing off a wall or canyon — and why astronauts on the Moon needed radios to talk.
Simple Circuits and Ohm's Law
An electric circuit is a closed path around which charges flow. Three quantities describe it:
- Current (I) — the flow rate of charge, in amperes (A), measured with an ammeter.
- Voltage (V) — the push or electrical pressure from a source, in volts (V).
- Resistance (R) — the opposition to current, in ohms (Ω).
Ohm's law ties them together: V = IR, and its rearrangements I = V/R and R = V/I.
Worked example 1. A 12-V battery drives current through a 4-Ω bulb. I = V/R = 12 ÷ 4 = 3 A.
Worked example 2. An appliance on a 220-V Philippine outlet draws 11 A. R = V/I = 220 ÷ 11 = 20 Ω.
Worked example 3. A 0.5-A current flows through a 100-Ω resistor. V = IR = 0.5 × 100 = 50 V.
Series vs Parallel Circuits
| Feature | Series | Parallel |
|---|---|---|
| Path for current | One single path | Two or more branches |
| Current | Same everywhere | Splits among branches |
| Voltage | Shared among components | Same across each branch |
| If one bulb burns out | Whole circuit goes dead | Other branches keep working |
| Adding more bulbs | Bulbs get dimmer | Brightness stays the same |
| Everyday use | Christmas light strings (old style) | Household wiring |
This is why homes are wired in parallel: every outlet receives the full 220 V and your refrigerator keeps running when a lamp burns out.
Electrical Safety
- Water conducts electricity — never touch switches or appliances with wet hands.
- Fuses and circuit breakers cut the current when it exceeds safe levels, preventing overheating and fires; never bypass a fuse with wire or a coin.
- Do not overload one outlet with many adapters; too much current heats the wires.
- The human body conducts — a current of even a fraction of an ampere through the heart can be fatal, which is why electrical tape, insulation, and grounding matter.
Exam Tips
- For KE problems, square the velocity first, then multiply by ½m — order of operations is where points are lost.
- Any question comparing sound and light is really asking about medium and speed.
- Ohm's law items give two of V, I, R and ask for the third — write V = IR and rearrange before touching the numbers.
- Parallel = full voltage per branch and independent operation; series = shared voltage and all-or-nothing.
A 2-kg ball rolls along the floor at 4 m/s. What is its kinetic energy?
An electric fan plugged into a 220-V outlet draws a current of 11 A. What is the resistance of the fan's motor circuit?