Energy, Work, Power Unit 6

Videos Brainiac Pendulum Demo: Nk Nk Bill Nye Part 1: 23Y 23Y Bill Nye Part 2: Bill Nye Part 3: k k

Topics 9 Different forms of Energy Law of Conservation of Energy KE and GPE Work Done and Power Efficiency

What is Energy? In biology, we say “the sun is the source of energy for all life on Earth” What does this statement mean? Where does the Energy in the sun come from??

Energy The concept of energy is confusing for two reasons: (i) the way we use the English word “energy” may not be the same way to use the word in Physics (ii) even when we use the word in Physics, energy can take many different forms

Energy Definition: Energy is the capacity to do work Qn: so….what is work? Ans: we will get to that second half of this chapter Unit: Joules (J)

Forms of Energy There are 9 forms of energy which are in your syllabus: 3 types of Potential Energy (Chemical, Gravitational, Elastic) Kinetic Energy (i.e. “movement energy”) Light, Thermal, Electrical, Nuclear, Sound Energy

Potential Energy (PE) Potential Energy is sometimes referred to as “stored” energy. (note: cannot use “stored energy” in O levels) Energy which is doing nothing right now, but is able to do work later A battery contains chemical PE (you will learn more in A level chemistry), so do fossil fuels and food (e.g. bio SPA) A stretched rubber band contains elastic PE An object lifted up contains gravitational PE

Principle of Conservation of Energy (CoE) Energy cannot be created or destroyed, but can be converted from one form to another. The total energy in an isolated system is constant.

ENERGY CONVERSION WORKSHEET

KE and GPE You are required to be familiar with these two equations of KE and GPE: Formula for GPE: – GPE = mgh Formula for KE – KE = ½ mv 2 Note: units of KE and GPE is Joules ONLY IF, all other units are S.I. Units.

Example A ball is dropped from rest. What is its velocity after it has fallen 5 m? (assume energy loss to friction) GPE → KE mgh = ½ mv 2 m(10)(5) = ½ mv 2 v 2 = 100 v = 10 ms -1

Practice Task Task 1: GLM Pg 126 Qn 1 Task 2: GLM Pg 128 Qn 9 Task 3: GLM Pg 128 Section B Qn 1(a)-(b)

Assignment 6A TYS Topic 7 Paper 1 Qn 1, 3, 5, 10, 11, 14, 17 Paper 2 Qn 4

Work Done When a force pushes an object and the object moves in the direction of the force, it is said that work is done by that force. Work done has units of Joules (J), same as energy Formula: – Work = F x Distance (same direction as force)

Worked Example If a force of 3 N pushes an object for a distance of 5 m what is the work done by the 3 N force? Ans: Work = Fd = (3)(5) = 15.0 J (3 s.f.) 3 N

Work Done IMPORTANT: When the direction of motion is NOT in the direction of the force, no work is done. E.g. I lift carry 20 N of books and walk 10 m forward. How about if I carry 20 N of books and I climb up the stairs 4m? How about if I climb down the stairs?

Work Done Sometimes, it is difficult to calculate work done directly, but you can calculate it indirectly using conservation of energy.

Worked Example If the mass of the object is 2 kg, what is the work done in pushing it up the smooth slope? 5 m

Practice Task Task 1: GLM Pg 122 Qn 1(b) Task 2: GLM Pg 127 Qn 7 Task 3: GLM Pg 129 Qn 2(a)

Power Units for Power: Watts (W) There are two (similar) equations for power Power = Work Done / Time Power = Energy Converted / Time Note: unit of time must be in seconds

Practice Task Task 1: GLM Pg 125 Qn 1(b) Task 2: GLM Pg 127 Qn 4 Task 3: GLM Pg 128 Qn 8

Efficiency Equation for Efficiency: (Useful Energy/Total Energy) x 100% Note: Efficiency is a percentage, so the answer is expected to be between 0 to 100%

Practice Task 1 A machine uses 50 J of energy per second but only produces 35 J of useful energy per second. What is the machine’s efficiency?

Practice Task 2 A crane lifts up a mass of 700 kg up a distance of 20 m. If the energy used by the crane is 200 kJ, determine the efficiency of the crane.

Summary 9 types of Energy Law of Conservation of Energy Equations for GPE and KE Work Done Power Efficiency

Quiz

Assignment 6B TYS Topic 7 Paper 1 Qn 1, 2, 4, 6, 7, 9, 13, 18 Paper 2 Qn 3, 7

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