Energy, Work, Power
Unit 6
Videos • Brainiac Pendulum Demo: h>p://www.youtube.com/watch?v=ZbCLAZSPLNk
• Bill Nye Part 1: • h>p://www.youtube.com/watch?v=zTXW9aRO23Y
• Bill Nye Part 2: • h>p://www.youtube.com/watch?v=Vfcsq6ylRzg • Bill Nye Part 3: h>p://www.youtube.com/watch?v=XkDG02nzfSk
Topics
• 9 Different forms of Energy • Law of Conserva[on 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 • Defini[on: 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 Poten&al Energy (Chemical, Gravita[onal, Elas[c)
• Kine&c Energy (i.e. “movement energy”) • Light, Thermal, Electrical, Nuclear, Sound Energy
Poten[al Energy (PE) • Poten[al Energy is some[mes 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 ba>ery 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 elas[c PE • An object liied up contains gravita[onal PE
Principle of Conserva[on 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 equa[ons of KE and GPE: • Formula for GPE: – GPE = mgh
• Formula for KE – KE = ½ mv2
• 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 aier it has fallen 5 m? (assume energy loss to fric[on)
• GPE → KE • mgh = ½ mv2
• m(10)(5) = ½ mv2
• v2 = 100 • v = 10 ms-‐1
Prac[ce Task
• Task 1: GLM Pg 126 Qn 1 • Task 2: GLM Pg 128 Qn 9 • Task 3: GLM Pg 128 Sec[on 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 direc[on 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 direc&on 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 direc[on of mo[on is NOT in the direc[on of the force, no work is done.
• E.g. I lii 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
• Some[mes, it is difficult to calculate work done directly, but you can calculate it indirectly using conserva[on 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
Prac[ce 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: Wa>s (W) • There are two (similar) equa[ons for power • Power = Work Done / Time • Power = Energy Converted / Time • Note: unit of [me must be in seconds
Prac[ce Task
• Task 1: GLM Pg 125 Qn 1(b) • Task 2: GLM Pg 127 Qn 4 • Task 3: GLM Pg 128 Qn 8
Efficiency
• Equa[on for Efficiency: • (Useful Energy/Total Energy) x 100% • Note: Efficiency is a percentage, so the answer is expected to be between 0 to 100%
Prac[ce 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?
Prac[ce Task 2
• A crane liis 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 Conserva[on of Energy • Equa[ons 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
Exit Card