27 January Electric Charges, Forces & Fields All matter ...
27 January
Electric Charges, Forces & Fields All matter is made up of charged particles. There are two types of charge, + and -. Opposite charges attract, like charges repel. We usually describe the electric force in terms of the electric field. For now, this is a convenient tool, but we’ll see that the electric field is real. Electric fields cannot penetrate a conductor. All excess charge stays on the surface and the field is zero inside.
Charge Diagrams Rub a plastic rod with a piece of wool. Negative charges are immobile on the rod’s surface. The other end is still neutral. The positive charge on the wool is equal to the negative charge on the rod. Charge is transferred to a piece of metal upon contact. The charges repel each other. Charge spreads over the surface of the metal very fast.
Atomic View Charging by contact An electrically neutral molecule + friction → positive molecule ion + negative molecular ion The bonds were broken by friction. The positive half of the molecule lost an electron as the bond broke. The negative half of the molecule gained an extra electron as the bond broke. Fixed positive, mobile negative The nucleus contains positive protons. The electron cloud is negatively charged. Dipoles Forces from an external charges cause the negative charge and the positive charge to be slightly offset.
Hydrogen bonding The slightly negative oxygen is attracted to the slightly positive hydrogen of a neighbor. Water molecules form hydrogen bonds with four neighbors.
Electrostatic forces make DNA do its thing. The base pairs of the two strands can only join in certain combinations. Three hydrogen bonds join guanine and cytosine. Two hydrogen bonds join thymine and adenine. The bases are comprised of hydrogen, nitrogen, carbon, and oxygen.
Units & Equations The unit of charge is the coulomb (C). The symbol for charge is q, or Q. A typical charge on a rubbed object is about 1 nC. Particle
Mass (kg)
Charge (C)
Proton
1.67 x 10-27
+e = 1.60 x 10-19
Electron
9.11 x 10-31
-e = -1.60 x 10-19
F=
K ∣q1∣∨q2 ∨ ¿2 r ¿ Same on both charges Attractive for opposite, repulsive for like We’ll use K (electrostatic constant) = 9.0 x 109
Nm 2 C
2
Two spheres are touching each other. A charged rod is brought near. The spheres are then separated, and the rod is taken away. In the first case, the spheres are aligned with the rod, in the second case, they are perpendicular. After the charged rod is removed, which of the spheres is positive? Negative? Neutral? When aligned, the ball nearest the rod is negatively charged; the ball farther from the rod is positively charged.
When perpendicular, both balls are neutral.
If a charged plastic rod is brought near an uncharged metal rod on an insulating stand, an uncharged metal ball near the other end of the metal rod is attracted to this end of the rod. Explain the motions of charges that give rise to this force. Negatively charged plastic rod attracts positive charges in uncharged metal rod Uncharged metal rod now has positive charges on side near rod, negative charges on side near ball Charges in ball align so positive charges are near negatively charged end of rod, attracting the ball to the rod
Two 0.10 g honeybees each acquire a charge of +23 pC as they fly back to their hive. As they approach the hive entrance, they are 1.0 cm apart. What is the magnitude of the repulsive force between the two bees? How does this force compare with their weight? Prepare +23 x 10-12 C
+23 x 10-12 C .01 m
F=
K ∣q1∣∨q2 ∨ ¿2 r ¿
Solve F = 4.8 x 10-8 N Assess W = mg = (.1 x 10-3 kg) (-9.8
m s2 )
= 9.8 x 10-4 N
What is the magnitude and direction of the force on the positive charge? Prepare -2.0 x 10-9 C F1
.02 m
-3.0 x 10-9 C
F2 .03 m
+1.0 x 10-9 C F
θ F2 Solve F1 = 4.5 x 10-5 N F2 = 3.0 x 10-5 N F=
√F +F 2 1
θ = sin-1 (
2 2
= 5.4 x 10-5 N
F1 F ) = 56°
F1
Electric Charges, Forces & Fields All matter is made up of charged particles. There are two types of charge, + and -. Opposite charges attract, like charges repel. We usually describe the electric force in terms of the electric field. For now, this is a convenient tool, but we’ll see that the electric field is real. Electric fields cannot penetrate a conductor. All excess charge stays on the surface and the field is zero inside.
Charge Diagrams Rub a plastic rod with a piece of wool. Negative charges are immobile on the rod’s surface. The other end is still neutral. The positive charge on the wool is equal to the negative charge on the rod. Charge is transferred to a piece of metal upon contact. The charges repel each other. Charge spreads over the surface of the metal very fast.
Atomic View Charging by contact An electrically neutral molecule + friction → positive molecule ion + negative molecular ion The bonds were broken by friction. The positive half of the molecule lost an electron as the bond broke. The negative half of the molecule gained an extra electron as the bond broke. Fixed positive, mobile negative The nucleus contains positive protons. The electron cloud is negatively charged. Dipoles Forces from an external charges cause the negative charge and the positive charge to be slightly offset.
Hydrogen bonding The slightly negative oxygen is attracted to the slightly positive hydrogen of a neighbor. Water molecules form hydrogen bonds with four neighbors.
Electrostatic forces make DNA do its thing. The base pairs of the two strands can only join in certain combinations. Three hydrogen bonds join guanine and cytosine. Two hydrogen bonds join thymine and adenine. The bases are comprised of hydrogen, nitrogen, carbon, and oxygen.
Units & Equations The unit of charge is the coulomb (C). The symbol for charge is q, or Q. A typical charge on a rubbed object is about 1 nC. Particle
Mass (kg)
Charge (C)
Proton
1.67 x 10-27
+e = 1.60 x 10-19
Electron
9.11 x 10-31
-e = -1.60 x 10-19
F=
K ∣q1∣∨q2 ∨ ¿2 r ¿ Same on both charges Attractive for opposite, repulsive for like We’ll use K (electrostatic constant) = 9.0 x 109
Nm 2 C
2
Two spheres are touching each other. A charged rod is brought near. The spheres are then separated, and the rod is taken away. In the first case, the spheres are aligned with the rod, in the second case, they are perpendicular. After the charged rod is removed, which of the spheres is positive? Negative? Neutral? When aligned, the ball nearest the rod is negatively charged; the ball farther from the rod is positively charged.
When perpendicular, both balls are neutral.
If a charged plastic rod is brought near an uncharged metal rod on an insulating stand, an uncharged metal ball near the other end of the metal rod is attracted to this end of the rod. Explain the motions of charges that give rise to this force. Negatively charged plastic rod attracts positive charges in uncharged metal rod Uncharged metal rod now has positive charges on side near rod, negative charges on side near ball Charges in ball align so positive charges are near negatively charged end of rod, attracting the ball to the rod
Two 0.10 g honeybees each acquire a charge of +23 pC as they fly back to their hive. As they approach the hive entrance, they are 1.0 cm apart. What is the magnitude of the repulsive force between the two bees? How does this force compare with their weight? Prepare +23 x 10-12 C
+23 x 10-12 C .01 m
F=
K ∣q1∣∨q2 ∨ ¿2 r ¿
Solve F = 4.8 x 10-8 N Assess W = mg = (.1 x 10-3 kg) (-9.8
m s2 )
= 9.8 x 10-4 N
What is the magnitude and direction of the force on the positive charge? Prepare -2.0 x 10-9 C F1
.02 m
-3.0 x 10-9 C
F2 .03 m
+1.0 x 10-9 C F
θ F2 Solve F1 = 4.5 x 10-5 N F2 = 3.0 x 10-5 N F=
√F +F 2 1
θ = sin-1 (
2 2
= 5.4 x 10-5 N
F1 F ) = 56°
F1
