Ch 4 EnergyCh 4 Energy

Sec 1 The Nature of Energy

Energy (E) is the ability to do work to cause change

Many forms: radiant, electrical, chemical, thermal, nuclear

Basic unit of E is the joule (J) named for British scientist Prescott Joule

Usually when you think of E, you think of ACTION!!

The E that involves motion is Kinetic E (KE)

Amount depends on the mass and the velocity of the object

Semi truck going 100 km/hr has more KE than a motorcycle also going 100 km/hr

Identical vehicles—the faster has more KE

KE KE Equation KE = ½ m x v2

m= mass in kg v= velocity in m/s Let’s try one! A soccer ball with a mass

of .5 kg is moving at a velocity of 20.0 m/s. What is the soccer ball’s KE?

1. KE = ½ m x v2

2. m= .5 kg v= 20.0 m/s 3. KE= ½(.5)(20)2

4. KE= 100 J

Whiteboard Whiteboard Practice!!Practice!!

A jogger has a mass of 75.0 kg and is jogging at a speed of 2.0 m/s. What is the jogger’s KE?

A race car is speeding around a track at 48.0 m/s. The mass of the car and driver is 560.0 kg. What is the car’s KE?

Potential Energy (PE)

E that does not involve motion is stored energy

Amount depends on the position of the object above Earth-this type is called gravitational potential energy or GPE

Also the condition of the object—relaxed spring has less PE than a tightly coiled spring

This type called elastic potential energy (EPE)

PE can also be stored in foods, fuels and dry cell batteries—chemical potential energy (CPE)

GPE= mghm= mass (kg) g= 9.8m/s2 h= height (m)

1. Your text book has a mass of 1.5 kg and is sitting on the top of your locker 1.75 m above the floor. What is its GPE?

2. An 65.0 kg cliff diver is ready to jump. The cliff is 15.0 m above the water. What is the diver’s GPE?

Sec 2 Law of Conservation of E

E cannot be created or destroyedE cannot be created or destroyed E can change form E can change form from one type of E to another (KE one type of E to another (KE

to PE) but it is never destroyedto PE) but it is never destroyed Total amount of E in the universe stays constantTotal amount of E in the universe stays constant

E conversions

An incandescent lightbulb transforms electrical E into light and thermal E

Green plants convert light E from Sun into chemical E

Engines change the chemical PE in gasoline into KE of moving vehicles

Mechanical energy (ME) is KE + PE

There is no change in ME when PE changes into KE or KE into PE

Nuclear fission and fusion Fission is when the

nuclei of an atom is broken apart

Nuclear power plants generate electrical E this way

Fusion is when nuclei of hydrogen isotopes come together to make a helium atom—happens in our Sun and other stars