Energy, Work, Power

Unit 6

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 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

after it has fallen 5 m? (assume energy loss to friction)

• GPE → KE• mgh = ½ mv2

• m(10)(5) = ½ mv2

• v2 = 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

Exit Card