Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and...

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1/29/16 1 Physics 132 Theme Music: Blondie Atomic Cartoon: Bill Amend FoxTrot January 29, 2016 Physics 132 Prof. E. F. Redish

Transcript of Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and...

Page 1: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

1/29/16 1 Physics 132

■ Theme Music: Blondie Atomic

■ Cartoon: Bill Amend FoxTrot

January 29, 2016 Physics 132 Prof. E. F. Redish

Page 2: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Potential energy and force

Work energy theorem

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F = − dUdx

Δ 1

2 mv2( ) = !

F ⋅d!r∫

Page 3: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Foothold ideas: Conservation of Mechanical Energy

■ Mechanical energy – The mechanical energy of a system of objects

is conserved if resistive forces can be ignored.

■ Thermal energy – Resistive forces transform coherent energy

of motion (energy associated with a net momentum) into thermal energy (energy associated with internal chaotic motions and no net momentum)

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Δ KE + PE( ) = 0KEinitial + PEinitial = KEfinal + PEfinal

This is why we define the PE with a negative sign.

Page 4: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Foothold ideas: Energies between charge clusters ■ Atoms and molecules are made up of charges. ■ The potential energy between two charges is

■  The potential energy between many charges is

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U12elec = kCQ1Q2

r12

U12...Nelec =

kCQiQj

riji< j=1

N

Page 5: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Foothold ideas: Forces from PE

■  For conservative forces, PE can be defined by ■  If you know U, the force can be gotten from

it via

■  In more than 1D need to use the gradient

■ The force always points down the PE hill.

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F ⋅ Δr = −ΔU

Page 6: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

The extra potential energy from adding Q as a function of position r of charge Q

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-q

-q

r 2q Q

ΔU = kCQ

qi

rQ→qii=1

3

∑ = kCQq1

r1

+q2

r2

+q3

r3

⎝⎜⎞

⎠⎟

Page 7: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

1/29/16 14 Physics 132

Page 8: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Foothold ideas: Bound states

■ When two objects attract, they may form a bound state – that is, they may stick together.

■  If you have to do positive work to pull them apart in order to get to a separated state with KE = 0, then the original state was in a state with negative energy.

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Page 9: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

The Gauss Gun: A classical analog of an exothermic reaction

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Page 10: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Sketch 1.  What the potential energy curve

must look like between the magnet and a metal sphere as a function of distance. (Hint: think about the WEΘ)

2.  Energy bar graphs that show the initial and final energies of each of the 4 spheres (when the moving one is far from the magnet)

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Page 11: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Energy conservation with chemical reactions: 1

■ Consider the collision of two molecules in isolation A + B → A + B

■  If the initial and final states both have the two molecules far apart, UAB ~ 0.

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KA + KB +UAB = constant

KA + KB = constant

Page 12: Theme Music: Blondie Atomic - UMD PhysicsFoothold ideas: Energies between charge clusters Atoms and molecules are made up of charges. The potential energy between two charges is The

Energy conservation with chemical reactions: 2

■ Consider the reaction of two molecules in isolation A + B → C + D

■  If the initial and final states both have the

two molecules far apart, UAB ~ UCD ~ 0.

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KA + EA( ) + KB + EB( ) +UAB = KC + EC( ) + KD + ED( ) +UCD

KA + EA( ) + KB + EB( ) = KC + EC( ) + KD + ED( )Note: The “E”s here are molecular internal energies and are negative since the molecules are bound. The (positive) bond energies from chemistry are given by E = -E > 0.