Mechanical and Electrical Energy Converted to Heat
When macroscopic mechanical or electrical energy is irreversibly converted to thermal energy, conservation gives Q_received = eta Delta E_available, followed by the appropriate sensible-heat and phase-change relations.
Why this shows up in the exam
Bullet and hammer heating · Waterfall temperature rise · Inelastic-collision and oscillator dissipation
Learn the idea
Dissipated mechanical or electrical energy becomes internal energy according to an explicit efficiency or partition. A falling stream, stopping bullet, collision, or heater supplies an energy budget; only the stated fraction that remains in the chosen body may be used for its temperature or phase change.
🧠 Memory hook: Track where the lost mechanical energy actually goes.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- Q_received = eta Delta E_mechanical — stated fraction eta of lost mechanical energy heating the selected system
- Delta T = eta Delta E/(m c) — temperature rise when no phase change occurs
- E_electric = P t — electrical energy delivered by constant power P over time t
How to approach it
- 1Choose the thermodynamic system
- 2Compute energy lost and apply the stated fraction
- 3Follow the body's thermal path and test whether melting begins
Common slip-ups that cost marks
- •Assigning all energy to one body without permission
- •Using initial kinetic energy when final kinetic energy remains
- •Ignoring latent heat when melting occurs
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
A wire 1 m long and cross-sectional area 2 mm^2 extends by 1 mm under a 200 N load. Find Young modulus.
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