Phys 102 – Lecture 2 Coulomb’s Law & Electric Dipoles 1.

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Phys 102 – Lecture 2 Coulomb’s Law & Electric Dipoles 1

Today we will... Get practice using Coulomb’s law & vector addition Learn about electric dipoles Apply these concepts! Molecular interactions Polar vs. nonpolar molecules Hydrophilic vs. hydrophobic Permanent vs. induced dipole Chemistry! Phys. 102, Lecture 2, Slide 2

Recall: Coulomb’s Law Force between charges q 1 and q 2 separated a distance r: Magnitude “Coulomb constant” Direction Opposite charges attract, like charges repel “Permittivity of free space” Phys. 102, Lecture 2, Slide 3 “Force on q 1 due to q 2 ”

Superposition principle Total force on charge due to other charges = sum of individual forces q2q2 q3q3 q4q4 q1q1 Ex: what is the force on q 1 due to q 2, q 3, and q 4 ? Phys. 102, Lecture 2, Slide 4 Order does not matter!

Calculation: four charges Calculate the total force on charge q 1 = +2 μC due to charges q 2 = +7 μC, q 3 = –3.5 μC Fundamental concept: Superposition Approach: Draw forces Calculate magnitudes of forces Add vectors Decompose into x-, y-components Add like components 3 m 4 m 3 m Phys. 102, Lecture 2, Slide 5 May need geometry, trigonometry q3q3 q2q2 q1q1

ACT: four charges Which vector best represents the total force on charge q 1 = +2 μC due to charges q 2 = +7 μC and q 3 = –3.5 μC? A. B. C. D. E. Phys. 102, Lecture 2, Slide 6 q3q3 q2q2 q1q1

Calculation: four charges Calculate the total force on charge q 1 = +2 μC due to charges q 2 = +7 μC and q 3 = –3.5 μC Calculate magnitudes of forces 3 m Phys. 102, Lecture 2, Slide 7 4 m q3q3 q2q2 q1q1

ACT: components What is the x-component of, F 12,x ? 3 m Phys. 102, Lecture 2, Slide 8 x y A. 3/4 F 12 B. 3/5 F 12 C. –4/5 F 12 4 m Decompose vectors into components q3q3 q2q2 q1q1

ACT: components What is the y-component of, F 13,y ? 3 m Phys. 102, Lecture 2, Slide 9 x y A. 3/4 F 13 B. 3/5 F 13 C. –4/5 F 13 4 m Decompose vectors into components q3q3 q2q2 q1q1

Calculation: four charges Add like components 3 m 4 m 3 m Calculate the total force on charge q 1 = +2 μC due to charges q 2 = +7 μC and q 3 = –3.5 μC q4q4 q2q2 q1q1 Phys. 102, Lecture 2, Slide 10

Calculation: four charges 3 m 4 m 3 m Calculate the total force on charge q 1 = +2 μC due to charges q 2 = +7 μC and q 3 = –3.5 μC Magnitude of total force Direction of total force q3q3 q2q2 q1q1 Phys. 102, Lecture 2, Slide 11

ACT: CheckPoint 1.1 Q 2q2q q q Phys. 102, Lecture 2, Slide Consider three charges on a circular ring, q 1 = +2q, q 2 = q 3 = +q. A charge +Q is placed at the center of the circle. A. F x > 0 B. F x = 0 C. F x < 0 What is the x-component of the total force on Q? x y

ACT: CheckPoint 1.2 Q 2q2q q q Phys. 102, Lecture 2, Slide Consider three charges on a circular ring, q 1 = +2q, q 2 = q 3 = +q. A charge +Q is placed at the center of the circle. A. F y > 0 B. F y = 0 C. F y < 0 What is the y-component of the total force on Q? x y

Electric dipole & dipole moment A positive and negative charge of equal magnitude q separated by a (usually small) distance d = +q+q –q Phys. 102, Lecture 2, Slide 14 +q+q –q Dipole moment is measure of separated + and – charges Note: opposite from Lewis notation (Chemistry) = Direction From – to + charge (by convention) Magnitude d What are examples of electric dipoles? definition

Molecular dipole Phys. 102, Lecture 2, Slide 15 Ex: H 2 0 (water) δ+ δ+δ+ 2δ-2δ- Ex: CO 2 (carbon dioxide) 2δ+2δ+ δ- Electrons are not shared equally between chemically bonded atoms Charge imbalance creates a bond dipole Ex: HF (hydrofluoric acid) δ+δ+δ-δ- Slightly positive Slightly negative Polar p tot > 0 Nonpolar p tot = 0

ACT: CheckPoint 2.1 An electric dipole is placed near a large positive charge +Q. In what direction is the net force on the dipole? A. LeftB. ZeroC. Right Phys. 102, Lecture 2, Slide 16 +Q+Q +q+q –q

ACT: Dipole & 2 charges Phys. 102, Lecture 2, Slide 17 +Q+Q +q+q –q –Q–Q Consider an electric dipole placed an equal distance from a +Q and a –Q charge. Does the dipole move? A. YesB. No

Ion-dipole interactions Phys. 102, Lecture 2, Slide 18 Ionic compounds (ex: salts) dissolve in water “Hydration shell” Dipole moment aligns away from + charge, toward – charge Ex: ions in water & solubility Polar molecules are attracted to ions

Dipole-dipole interactions Phys. 102, Lecture 2, Slide 19 Polar molecules interact together Dipole moments align end-to-end + to – Like magnets! Ex: hydrogen bond is a dipole-dipole interaction between water molecules Hydrogen bondStructure of iceSnowflake

Hydrophilic vs. hydrophobic Polar molecules interact with charged & polar molecules Hydrophobic “repel water” Hydrophilic “attract water” Ex: charged & polar molecules attract water, nonpolar molecules do not Nonpolar Polar Oil and water Cell membranes Hydrophobic Hydrophilic Protein structure Nonpolar inside Polar outside “coiled coil” Phys. 102, Lecture 2, Slide 20

ACT: Charge & conductor An uncharged conducting sphere is placed next to a fixed + charge. What happens when the uncharged sphere is released? + A. Nothing B. Attracted to + sphere C. Repelled from + sphere Phys. 102, Lecture 2, Slide 21 DEMO

Molecular interactions Phys. 102, Lecture 2, Slide 22 Interactions between molecules are understood in terms of charges and electric dipoles interacting by Coulomb’s law Ion-dipole Yes! Two nonpolar molecules can induce dipoles in each other and interact! London dispersion or van der Waals force Dipole-dipole Ion-induced dipole Dipole-induced dipole Induced dipole- induced dipole?

Summary of today’s lecture Coulomb’s law Superposition principle Electric dipole & dipole moment Permanent vs. induced dipole Phys. 102, Lecture 2, Slide 23