P3. Energy, Work & Power
P3.1 Energy
The concepts of work and energy are closely tied to the concept of force because an applied force can do work on an object and cause a change in energy. Energy is defined as the ability to do work.
The energy of an object is measured in Joules (J). 1000 Joules = 1 kilojoule (kJ)
The power of an object is measured in Watts (W). 1000 Watts=1 kilo Watt (kW)
All objects have what we call, internal energy in its molecules. Internal energy is made out of Kinetic Energy (due to the motion of the molecules), and Potential Energy (due to the position of the object).
You can’t create or destroy energy, energy is conserved. However, energy can be transferred and stored. For example, when you jump of a cliff (god forbid), the Potential Energy will slowly be converted to Kinetic Energy. The NET Energy remains the same, but you see a transfer of energy from one form to another.
The energy of an object is measured in Joules (J). 1000 Joules = 1 kilojoule (kJ)
The power of an object is measured in Watts (W). 1000 Watts=1 kilo Watt (kW)
All objects have what we call, internal energy in its molecules. Internal energy is made out of Kinetic Energy (due to the motion of the molecules), and Potential Energy (due to the position of the object).
You can’t create or destroy energy, energy is conserved. However, energy can be transferred and stored. For example, when you jump of a cliff (god forbid), the Potential Energy will slowly be converted to Kinetic Energy. The NET Energy remains the same, but you see a transfer of energy from one form to another.
Energy Conversion
Conservation of energy principle: energy cannot be created or destroyed, when work is done, energy is changed from one form to another. The most everyday example of this is when we move, our cells turn chemical energy (in glucose bonds) into thermal and kinetic energy.
Conservation of energy- Trolley and falling mass
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Energy Transfer
Sankey diagrams summarise all the energy transfers taking place in a process. The thicker the line or arrow, the greater the amount of energy involved. This Sankey diagram for the lamp shows that it transfers most of the energy by heating, rather than by light:
Efficiency
The efficiency of a device such as the lamp above can be calculated using the efficiency equation:
efficiency = (useful energy transferred ÷ energy supplied ) × 100
The efficiency of the filament lamp is (10 ÷ 100) × 100 = 10%.
This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy).
efficiency = (useful energy transferred ÷ energy supplied ) × 100
The efficiency of the filament lamp is (10 ÷ 100) × 100 = 10%.
This means that 10% of the electrical energy supplied is transferred as light energy (90% is transferred as heat energy).
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3.2 Energy Resources
Renewable sources of energy, just like it sounds, are resources that theoretically can be used for ever if used at a rate which is consistent with the rate in which it is replenished. Examples include solar, wind and water energy. Every day, the sun comes out and we can indefinitely use its energy—for the time being at least.
Non-Renewable sources of energy are resources that get exhausted when used, and basically when you use all of them, that’s it, they’re gone. Examples include fossil fuels, oil and coal.
*The Sun is the source of ALL energy for all our energy resources except geothermal and nuclear
Non-Renewable sources of energy are resources that get exhausted when used, and basically when you use all of them, that’s it, they’re gone. Examples include fossil fuels, oil and coal.
*The Sun is the source of ALL energy for all our energy resources except geothermal and nuclear
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Chemical Energy stored in fuel (Fossil fuels, coal, natural gas)
- Burn the chemical fuel to produce heat.
- The heat is then used to heat a liquid such as water which is then turned into steam.
- The pressure from the steam is used to spin a turbine, and it is this spinning that produces electricity
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Tidal & Wave
- Underwater turbines spin like windmills
- The turbines are mounted on a gearbox shaft, which generated electricity.
- Underwater cables then help carry the electricity to the shore.
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Geothermal
- Possible to use the natural heat of the earth to generate electricity.
- Cold water is first pumped underground.
- The cold water then comes out as steam.
- This steam can be used for heating or powering turbines to create electricity.
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Nuclear Fission
- Electricity can be generated through nuclear fission. During nuclear fission, energy is released, and electricity can be produced in the Nuclear Reactors.
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Heat and light from the Sun (solar cells and panels)
- Heat from the sun is trapped in solar panels and subsequently converted into electricity.
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P3.3 Work
How much work is done when a man pushes a box with 10N of force across the floor 2m away?
Work done = force x distance = 200N x 2m = 400J
Work done = force x distance = 200N x 2m = 400J
P3.4 Power
Power is the rate at which work is done. It is the work/time ratio.The standard metric unit of power is the Watt (W). As is implied by the equation for power, a unit of power is equivalent to a unit of work divided by a unit of time. Thus, a Watt is equivalent to a Joule/second (J/s).
What is the power output for a 60.0-kg woman who runs up a 3.00 m high flight of stairs in 3.50 s, starting from rest but having a final speed of 2.00 m/s? (See Fig. 1)