1 Škola: Gymnázium, Brno, Slovanské náměstí 7 Šablona: III/2 – Inovace a zkvalitnění výuky prostřednictvím ICT Název projektu: Inovace výuky na GSN prostřednictvím ICT Číslo projektu: CZ.1.07/1.5.00/ Autor: RNDr. Bc. Miroslav Hruška Tematická oblast: Dynamics of particles (dynamika hmotných bodů) Název DUMu: Power and efficiency Kód: VY_32_INOVACE_FY.3.18 Datum: Cílová skupina: Žáci středních škol s CLIL výukou fyziky v angličtině Klíčová slova:Power, watt, average power, instantaneous power, input power, output power, efficiency, work, force Anotace: Prezentace slouží k zavedení veličiny výkon a účinnost.

Dynamics of particles 18. Power and efficiency

3 Exercise (first fill in the gaps)  A 500-gram ball was freely dropped from a ………… of 2 metres. It bounced back up to a height of 80 cm. Calculate how much …………… energy was transformed into …………… energy. Solution J 6 joules were transformed.

4  The law of ……………… of energy says that the energy of an ………… system is ……………  Energy cannot be ………… nor destroyed. It can only be ………………… from one form to another  Friction and fluid ……………… cause mechanical ………… to change to ………… energy Fill in the gaps

5  The law of conservation of energy says that the energy of an isolated system is constant  Energy cannot be created nor destroyed. It can only be transformed (convered) from one form to another  Friction and fluid resistance cause mechanical energy to change to internal energy Fill in the gaps – solution

6 Find the only correct answer Binding energy is a kind of  A) nuclear energy  B) mechanical energy  C) electromagnetic energy  D) internal energy

7 Average power (průměrný výkon)  Average power equals work divided by time:  In SI, watt, W, is a unit of power, it is a derived unit: W = J/s (joule per second) W = J/s = N·m/s = kg·m/s 2 ·m/s = kg·m 2 /s 3 Ws = J ⇒ 1 Wh = 3,600 J ⇒ 1 kWh = 3,600,000 J  Physicist: James Watt (1736–1819)

8 Instantaneous power P  Instantaneous power is defined as the average power in a very short time interval (in an instant)  If a work is being done “uniformly”, the instantaneous power = the average power

9 Input power, output power  For machines, we define input power (příkon) as the energy given to ( put in ) a machine divided by time:  Output power (výkon ! ) = the power of a machine = the work done by the machine divided by time:

10 Efficiency (účinnost) of a machine  Efficiency of a machine = output power divided by input power  Efficiency is a dimensionless quantity, it is often expressed as a percentage, e.g = 72 %  Note: dimensionless quantity is also called a quantity of dimension one; we can write [η] = 1 (unit of efficiency is 1)

11 1. Power can be negative 2. Potential energy can be negative 3. Kinetic energy can be negative 4. James Watt was English 5. Efficiency can be more than 100 % 6. If efficiency is less than 100 %, it means that some energy was destroyed True or false?

That is all for now Thank you for your attention

13  Materiál je určen pro bezplatné používání pro potřeby výuky a vzdělávání na všech typech škol a školských zařízeních. Jakékoliv další využití podléhá autorskému zákonu. Zdroje:  Bednařík, M., Široká, M. Fyzika pro gymnázia, Mechanika. Dotisk 3. vyd. Praha: Prometheus, s. ISBN  Hornby, A. S. Oxford Advanced Learner’s Dictionary of Current English. 5. vyd. Oxford: Oxford University Press, s. ISBN