electric charge page: unit: electricity & magnetism...

Add Important Electric Charge Page: 424

Notes/Cues Here Unit: Electricity & Magnetism

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AP Physics 1: Algebra-Based Mr. Bigler

Electric Charge Unit: Electricity & Magnetism

NGSS Standards: N/A

MA Curriculum Frameworks (2006): 5.1, 5.4

AP Physics 1 Learning Objectives: 1.B.1.1, 1.B.1.2, 1.B.2.1, 1.B.3.1

Knowledge/Understanding Goals:

electric charge

properties of electric charges

conductors vs. insulators

Language Objectives:

Understand and correctly use the terms “electricity,” “charge,” “current,” “conductor,” “insulator,” and “induction.”

Accurately describe and apply the concepts described in this section using appropriate academic language.

Labs, Activities & Demonstrations:

Charged balloon making hairs repel, attracting water molecules.

Water sprayed on balloon neutralizes the charge.

Wimshurst machine.

Van de Graaff generator.

Notes:

electric charge: a physical property of matter which causes it to experience a force when near other electrically charged matter. Electric charge is measured in coulombs (C).

positive charge: the type of charge carried by protons. Originally defined as the charge left on a piece of glass when rubbed with silk. The glass becomes positively charged because the silk pulls electrons off the glass.

negative charge: the type of charge carried by electrons. Originally defined as the charge left on a piece of amber (or rubber) when rubbed with fur (or wool). The amber becomes negatively charged because the amber pulls the electrons off the fur.





elementary charge: the magnitude (amount) of charge on one proton or one

electron. One elementary charge equals C101.60 19 . Because ordinary

matter is made of protons and electrons, the amount of charge carried by any object must be a multiple of the elementary charge.

Note however, that the quarks that protons and neutrons are made of carry fractional charges; up-type quarks carry a charge of +⅔ of an elementary charge, and down-type quarks carry a charge of −⅓ of an elementary charge. A proton is made of two up quarks and one down quark and carries a charge of +1 elementary charge. A neutron is made of one up quark and two down quarks and carries no charge.

static electricity: stationary electric charge, such as the charge left on silk or amber in the above definitions.

electric current (sometimes called electricity): the movement of electrons through a medium (substance) from one location to another. Note, however, that electric current is defined as the direction a positively charged particle would move. Thus electric current “flows” in the opposite direction from the actual electrons.





xkcd.com. Used with permission.

Devices that Produce, Use or Store Charge

capacitor: a device that stores electric charge.

battery: a device that uses chemical reactions to produce an electric current.

generator: a device that converts mechanical energy (motion) into an electric current.

motor: a device that converts an electric current into mechanical energy.





Conductors vs. Insulators conductor: a material that allows charges to move freely through it. Examples of

conductors include metals and liquids with positive and negative ions dissolved in them (such as salt water). When charges are transferred to a conductor, the charges distribute themselves evenly throughout the substance.

insulator: a material that does not allow charges to move freely through it. Examples of insulators include nonmetals and most pure chemical compounds (such as glass or plastic). When charges are transferred to an insulator, they cannot move, and remain where they are placed.

Behavior of Charged Particles Like charges repel. A pair of the same type of charge (two positive charges or

two negative charges) exert a force that pushes the charges away from each other.

Opposite charges attract. A pair of opposite types of charge (a positive charge and a negative charge) exert a force that pulls the charges toward each other.

Charge is conserved. Electric charges cannot be created or destroyed, but can be transferred from one location or medium to another. (This is analogous to the laws of conservation of mass and energy.)





Charging by Induction

induction: when an electrical charge on one object causes a charge in a second object.

When a charged rod is brought near a neutral object, the charge on the rod attracts opposite charges and repels like charges that are near it. The diagram below shows a negatively-charged rod repelling negative charges.

If the negatively-charged rod above were touched to the sphere, some of the charges from the rod would be transferred to the sphere at the point of contact, and the sphere would acquire an overall negative charge.





A process for inducing charges in a pair of metal spheres is shown below:

(a) Metal spheres A and B are brought into contact.

(b) A positively charged object is placed near (but not in contact with) sphere A. This induces a negative charge in sphere A, which in turn induces a positive charge in sphere B.

(c) Sphere B (which is now positively charged) is moved away.

(d) The positively charged object is removed.

(e) The charges distribute themselves throughout the metal spheres.

AP questions dealing with charging by induction are common, so you should be sure to understand how and why every step of this procedure works.





Grounding

For the purposes of our use of electric charges, the ground (Earth) is effectively an endless supply of both positive and negative charges. Under normal circumstances, if a charged object is touched to the ground, electrons will move to neutralize the charge, either by flowing from the object to the ground or from the ground to the object.

Grounding a charged object or circuit means neutralizing the electrical charge on an object or portion of the circuit. The charge of any object that is connected to ground is zero, by definition.

In buildings, the metal pipes that bring water into the building are often used to ground the electrical circuits. The metal pipe is a good conductor of electricity, and carries the unwanted charge out of the building and into the ground outside.

Add Important Coulomb’s Law Page: 431




Coulomb’s Law Unit: Electricity & Magnetism

NGSS Standards: HS-PS2-4, HS-PS3-5

MA Curriculum Frameworks (2006): 5.4

AP Physics 1 Learning Objectives: 3.C.2.1, 3.C.2,2, 5.A.2.1

Skills:

understand & solve problems using Coulomb’s Law

Language Objectives:

Accurately describe Coulomb’s Law using appropriate academic language.

Set up and solve word problems relating to Coulomb’s Law.

Labs, Activities & Demonstrations:

Van de Graaff generator with inertia balance pan.

Charged balloon or Styrofoam sticking to wall.

Charged balloon pushing meter stick.

Notes:

Electric charge is measured in Coulombs (abbreviation “C”). One Coulomb is the amount of electric charge transferred by a current of 1 ampere for a duration of 1 second.

1 C is the charge of 6.2415 × 1018 protons.

−1 C is the charge of 6.2415 × 1018 electrons.

One proton or electron (elementary charge) therefore has a charge of 1.6022 × 10−19 C.





Because charged particles exert a force on each other, that force can be measured and quantified. The force is directly proportional to the strengths of the charges and inversely proportional to the square of the distance. The formula is therefore:

221

r

qkqFe

where:

eF = electrostatic force of repulsion between electric charges. A positive value

of eF

denotes that the charges are repelling (pushing away from) each

other; a negative value of eF

denotes that the charges are attracting

(pulling towards) each other.

k = electrostatic constant = 2

2

2

2

C

mN9

C

mN9 109.0108.9876

q1 and q2 = charges 1 and 2 respectively

r = distance (radius, because it goes outward in every direction) between the centers of the two charges

This formula is Coulomb’s Law, named for its discoverer, the French physicist Charles-Augustin de Coulomb.





Sample problem:

Q: Find the force of electrostatic attraction between the proton and electron in a hydrogen atom if the radius of the atom is 37.1 pm

A: The charge of a single proton is 1.60 × 10−19 C, and the charge of a single electron is −1.60 × 10−19 C.

N1067.1

)1071.3(

)1060.1)(1060.1)(100.9(

m103.71pm37.1

7

211

19199

221

11

e

e

e

F

F

d

qkqF

The value of the force is negative, which signifies that the force is attractive.

Homework Problems

1. What is the magnitude of the electric force between two objects, each

with a charge of C102.00 6 , which are separated by a distance of

1.50 m? Is the force attractive or repulsive?

Answer: 0.016 N, repulsive





2. An object with a charge of C1005.1 2 is separated from a second

object with an unknown charge by a distance of 0.500 m. If the objects

attract each other with a force of N1035.1 6 , what is the charge on the

second object?

Answer: C1005.2 3

3. An object with a charge of 1q is separated from a second object with

an unknown charge by a distance d. If the objects attract each other with a force F, what is the charge on the second object?

Answer: 1

2

2kq

Fdq

4. The distance between an alpha particle and an electron is m1000.2 25 .

What is the force of electrostatic attraction between the charges?

Answer: N1015.1 22





5. Three elementary charges, particle q1 with a charge of C1000.6 9 ,

particle q2 with a charge of C1000.2 9 , and particle q3 with a charge

of C1000.5 9 , are arranged as shown in the diagram below.

What is the net force (magnitude and direction) on particle q3?

Answer: N1016.7 9 at an angle of 65.2° above the x-axis.

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