# Conservation of energy – energy is never destroyed, it is always changed into other types of energy. The water behind the dam has gravitational potential.

## Presentation on theme: "Conservation of energy – energy is never destroyed, it is always changed into other types of energy. The water behind the dam has gravitational potential."— Presentation transcript:

Conservation of energy – energy is never destroyed, it is always changed into other types of energy. The water behind the dam has gravitational potential energy, this changes to kinetic energy as it flows down the pipe. The generators change this into electrical energy. Some will also be changed into heat and sound energy. Eg Hydroelectricity For calculations we often assume no energy is changed to heat and sound so At top At bottom mgh = ½ mv 2

Reminders F w = mg E = Pt E k = ½mv 2 E p = mgh E H = cmΔT E H = ml Friction changes kinetic energy into heat and sound. This results in work done. E w = Fd At top At bottom mgh = ½ mv 2 + Fd

Immersion heater water Electrical energy in = Heat energy out IVt = cmΔT P = IV and E = Pt This assumes that no heat energy is lost to the surroundings. To improve this experiment insulating material should be placed around it and a lid put on it. Also all of the immersion heater should be covered in water.

1. While repairing a school roof, workmen lift a pallet of tiles from the ground to the top of the scaffolding. This job is carried out using a motorised pulley system. The pallet and tiles have a total mass of 230 kg. (a) Calculate the weight of the pallet and tiles. (b) The pallet and tiles are lifted to a height of 12 m. Calculate the gravitational potential energy gained by the pallet and tiles. (c) When the tiles are being unloaded onto the scaffolding, at a height of 12 m, one tile falls. The tile has a mass of 2·5 kg. (i) Calculate the final speed of the tile just before it hits the ground. Assume the tile falls from rest. (ii) Explain why the actual speed is less than the speed calculated in (i).

2. A child of mass 42 kg is playing on a water slide at a water park. (a) The child climbs 7·5 m to the top of the slide. Calculate the gain in potential energy of the child. (b) When sliding down, an average frictional force of 15 N acts on the child. This causes 1050 J of heat energy to be produced. Calculate the length of the slide. (c) Calculate the speed of the child at the end of the slide.

Some cars are fitted with a system that stores the energy normally lost as heat in the brakes. Using your knowledge of physics, estimate the kinetic energy of a moving car. List any you data you would need and how you would handle this data to determine kinetic energy. Qu 3 Open ended question

4. An experimental geothermal power plant uses heat energy from deep underground to produce electrical energy. A pump forces water at high pressure down a pipe. The water is heated and returns to the surface. At this high pressure the boiling point of water is 180 °C. The plant is designed to pump 82 kg of heated water, to the surface, each second. The specific heat capacity of this water is 4320 J/kg °C. (a) The water enters the ground at 20 °C and emerges at 145 °C. Calculate the heat energy absorbed by the water each second.

5. A manufacturer has developed an iron with an aluminium sole plate. A technician has been asked to test the iron. The technician obtains the following data for one setting of the iron. Starting temperature of sole plate: 24 °C Operating temperature of the sole plate: 200 °C Time for iron to reach the operating temperature: 35 s Power rating of the iron: 1·5 kW Operating voltage: 230 V Specific Heat Capacity of Aluminium: 902 J/kg °C (a) Calculate how much electrical energy is supplied to the iron in this time. (b) Calculate the mass of the aluminium sole plate. (c) The actual mass of the aluminium sole plate is less than the value calculated in part (b) using the technician’s data. Give one reason for this difference.

Qu 6

Pressure p = F ÷ A

“GO ANYWHERE” TYRES Our new wide tyres will take you across any terrain, any country and in any weather! BUY NOW!! Look at this advert : Using your knowledge of physics, comment on the claims made by the “Go Anywhere” tyre company in the above advert. Qu 7 Open ended question

Kinetic theory explanation of the gas laws for a fixed mass of gas Increase T means higher average speed so particles hit the container walls more frequently and harder so pressure increases Constant V Decreasing V means the particles hit the walls more frequently so pressure increases Constant T Increase T means higher average speed so the volume must increase as the pressure stays constant, Constant p

Combined Gas Equation p 1 V 1 p 2 V 2 T 1 T 2 Constant mass Remember temperature must be in Kelvin. A gas filled syringe has a volume of 20 cm 3 when the temperature is 20 o C. It is then heated to 100 o C, what will its new volume be?

A student carries out an experiment to investigate the relationship between the pressure and temperature of a fixed mass of gas. The apparatus used is shown. 8. A student carries out an experiment to investigate the relationship between the pressure and temperature of a fixed mass of gas. The apparatus used is shown.

9. The pressure and temperature of the gas are recorded using sensors connected to a computer. The gas is heated slowly in the water bath and a series of readings is taken. The volume of the gas remains constant during the experiment. The results are shown. (a) Using all the relevant data, establish the relationship between the pressure and the temperature of the gas. (b) Use the kinetic model to explain the change in pressure as the temperature of the gas increases. (c) Explain why the level of water in the water bath should be above the bottom of the stopper. Pressure/kPa100105110116121 Temperature/°C15.030.045.060.075.0 Temperature/K288303318333348

10. Ice at a temperature of –10 ºC is heated until it becomes water at 80 ºC. The temperature change in kelvin is A 70K B 90K C 343K D 363K E 636 K. 11. One Pascal is equivalent to A 1 Nm B 1 Nm 2 C 1 Nm 3 D 1 Nm –2 E 1 Nm –3

12. A cylinder of compressed oxygen gas is in a laboratory. (a) The oxygen inside the cylinder is at a pressure of 2.82×10 6 Pa and a temperature of 19.0 °C. The cylinder is now moved to a storage room where the temperature is 5.0 °C. Calculate the pressure of the oxygen inside the cylinder when its temperature is 5.0 °C. (b) The valve on the cylinder is opened slightly so that oxygen is gradually released. The temperature of the oxygen inside the cylinder remains constant. Explain, in terms of particles, why the pressure of the gas inside the cylinder decreases. (c) After a period of time, the pressure of the oxygen inside the cylinder reaches a constant value of 1.01 × 10 5 Pa. The valve remains open. Explain why the pressure does not decrease below this value.

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