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Power and Efficiency – Dec 2

Power and Efficiency – Dec 2. Agenda ( 90 min ): Research Project: Due TODAY Discussion: Power Discussion: Efficiency Finish Pendulum Lab Upcoming Pendulum Lab – Due end of class Email : gregory.nestor@peelsb.com. Power (P). P = Δ E / Δ t It takes time to transform or use energy.

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Power and Efficiency – Dec 2

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  1. Power and Efficiency – Dec 2 Agenda (90 min): • Research Project: Due TODAY • Discussion: Power • Discussion: Efficiency • Finish Pendulum Lab Upcoming • Pendulum Lab – Due end of class • Email: gregory.nestor@peelsb.com

  2. Power (P) P = ΔE / Δt • It takes time to transform or use energy. • (e.g. Using an elevator) • Power(P) is the rate that energy is transformed • Power is measured in Watts (W)1 W = 1 J/s • Powerful machines: • Transform large amounts of energy (e.g. Transport Truck) • Transform energy very quickly (e.g. race car)

  3. Sample Power Questions • What is the power of an elevator motor if it uses 2.9x105 J to lift an elevator car one story in 16 s? • P = ΔE / Δt • P = 2.9x105 J / 16 s = 1.8x104 J/s = 1.8x104 W • If a 60 W light bulb is left on for ten hours (3.6x104 s), how much energy is transformed? • P = ΔE / Δt or ΔE = P x Δt • ΔE = 60 W x 3.6x104 s = 2.2x106 J

  4. Friction and Efficiency • In ideal systems we ignore friction • In real systems • Friction is always present • Friction converts energy into thermal energy • Friction in a system reduces the amount of useable mechanical energy. Energy is not lost only transformed. • e.g. The pendulum in the lab quickly slows down

  5. No real system is 100% efficient • Some percentage of energy is always converted to an non-useful form • E.g. Noise, heat, friction, etc. • For example, A standard light bulb • 95% energy wasted as heat • Only 5% useful light • A compact fluorescent light bulb is better • 20 % useful light, 80% wasted heat • Led light bulbs are the future • 75 % useful light, 25% wasted heat

  6. Efficiency efficiency = (Eout / Ein) x 100% • Where: • Ein is the energy used by the device • Eout is the useful energy transformed by the device • No real system is 100% efficient • Eout is always less than Ein

  7. Efficiency Questions • Fireflies use chemical energy to create a glow in their abdomen. What is a firefly’s efficiency if it transforms 4.13 J of chemical energy into 3.63 J of radiant energy? • efficiency = 88 % • What is the efficiency of a pulley system if 1.93x103 J of mechanical energy is used to lift a 20 Kg mass to a height of 7.5 m? • efficiency = 76%

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