Electric Charges and Forces CPO Science

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					CPO Science
Foundations of Physics

  Unit 7, Chapter 21
Unit 7: Electricity and Magnetism
Chapter 21 Electric Charges and Forces

       21.1 Electric Charge

       21.2 Coulomb’s Law

       21.3 Capacitors
               Chapter 21 Objectives
1. Describe and calculate the forces between like and
   unlike electric charges.
2. Identify the parts of the atom that carry electric
3. Apply the concept of an electric field to describe how
   charges exert force on other charges.
4. Sketch the electric field around a positive or negative
   point charge.
5. Describe how a conductor shields electric fields from
   its interior.
6. Describe the voltage and current in a circuit with a
   battery, switch, resistor, and capacitor.
7. Calculate the charge stored in a capacitor.
           Chapter 21 Vocabulary Terms
   charge                  electrons                capacitance
   electrically neutral    gravitational field      charge
   static electricity      charged                  polarization
   positive charge         induction                shielding test
   negative charge         Coulomb’s law            charge
   electric forces         capacitor                farad
   charge by friction      parallel plate           field inverse
   electroscope             capacitor                square law
   protons                 microfarad               discharged field
   neutrons                coulomb                  lines
                            electric field
            21.1 Electric Charge

Key Question:
How do electric charges

*Students read Section 21.1
AFTER Investigation 21.1
              21.1 Electric Charge
 All ordinary matter contains
  both positive and negative
 You do not usually notice
  the charge because most
  matter contains the exact
  same number of positive
  and negative charges.
 An object is electrically
  neutral when it has equal
  amounts of both types of
               21.1 Electric Charge
 Objects can lose or gain electric
 The net charge is also sometimes
  called excess charge because a
  charged object has an excess of
  either positive or negative
 A tiny imbalance in either positive
  or negative charge on an object is
  the cause of static electricity.
               21.1 Electric Charge
 Electric charge is a
  property of tiny particles
  in atoms.

 The unit of electric charge
  is the coulomb (C).

 A quantity of charge
  should always be
  identified with a positive
  or a negative sign.
               21.1 Electric forces
 Electric forces are created between all electric charges.

 Because there are two kinds of charge (positive and
  negative) the electrical force between charges can
  attract or repel.
             21.1 Electric forces
 The forces between the two kinds of charge can
  be observed with an electroscope.
           21.1 Electric forces
 Charge can be transferred by conduction.
                21.1 Electric current
 The direction of current was historically defined as the
  direction that positive charges move.
 Both positive and negative charges can carry current.
 In conductive liquids (salt
  water) both positive and
  negative charges carry
 In solid metal conductors,
  only the electrons can
  move, so current is carried
  by the flow of negative
            21.1 Electric current
 Current is the movement of electric charge through
  a substance.
                                  Charge that flows
     Current                        (coulombs)
     (amps)          I=q
                                   Time (sec)
          21.1 Calculate current

 Two coulombs of charge pass through a wire
  in five seconds.

 Calculate the current in the wire.
     21.1 Conductors and insulators

 All materials contain electrons.

 The electrons are what carry
  the current in a conductor.

 The electrons in insulators are
  not free to move—they are
  tightly bound inside atoms.
  21.1 Conductors and insulators
A semiconductor has a few free electrons and atoms
with bound electrons that act as insulators.
    21.1 Conductors and insulators
 When two neutral objects are
  rubbed together, charge is
  transferred from one to the
  other and the objects become
  oppositely charged.
 This is called charging by
 Objects charged by this
  method will attract each
           21.2 Coulomb's Law
 Coulomb’s law relates the force between two
  single charges separated by a distance.

                           9 x109 N.m2/C2

    (N)        F = K q1 q 2          Charges (C)

                                 Distance (m)
              21.2 Coulomb's Law

 The force between two
  charges gets stronger as
  the charges move closer

 The force also gets
  stronger if the amount of
  charge becomes larger.
              21.2 Coulomb's Law

 The force between two
  charges is directed along
  the line connecting their

 Electric forces always
  occur in pairs according to
  Newton’s third law, like all
              21.1 Coulomb's Law
 The force between charges
  is directly proportional to the
  magnitude, or amount, of
  each charge.

 Doubling one charge
  doubles the force.

 Doubling both charges
  quadruples the force.
                21.1 Coulomb's Law
 The force between charges is
  inversely proportional to the
  square of the distance between
 Doubling the distance reduces
  the force by a factor of 22 = (4),
  decreasing the force to one-
  fourth its original value (1/4).
 This relationship is called an
  inverse square law because
  force and distance follow an
  inverse square relationship.
             21.2 Calculating force

 Two balls are each given a static electric charge of
  one ten-thousandth (0.0001) of a coulomb.
 Calculate the force between the charges when they
  are separated by one-tenth (0.1) of a meter.
 Compare the force with the weight of an average 70
  kg person.
            21.2 Fields and forces
 The concept of a field is used to describe any quantity
  that has a value for all points in space.
 You can think of the field as the way forces are
  transmitted between objects.
 Charge creates an electric field that creates forces on
  other charges.
            21.2 Fields and forces
 Mass creates a gravitational field that exerts
  forces on other masses.
           21.2 Fields and forces
 Gravitational forces are far weaker than
  electric forces.
21.2 Drawing the electric field
  21.2 Electric fields and electric force
 On the Earth’s surface, the gravitational field creates
  9.8 N of force on each kilogram of mass.

 With gravity, the strength of the field is in newtons per
  kilogram (N/kg) because the field describes the
  amount of force per kilogram of mass.
  21.2 Electric fields and electric force
 With the electric field, the strength is in newtons per
  coulomb (N/C).

 The electric field describes the amount of force per
  coulomb of charge.
                21.2 Accelerators
 An electric field can be
  produced by maintaining a
  voltage difference across any
  insulating space, such as air
  or a vacuum.
 Electric fields are used to
  create beams of high-speed
  electrons by accelerating
 Electron beams are used in x-
  ray machines, televisions,
  computer displays, and many
  other technologies.
             21.2 Electric shielding
 Electric fields are created all
  around us by electric appliances,
  lightning, and even static
 These stray electric fields can
  interfere with the operation of
  computers and other sensitive
 Many electrical devices and wires
  that connect them are enclosed in
  conducting metal shells to take
  advantage of the shielding effect.
             21.2 Coulomb’s Law
  Key Question:
  How strong are electrical forces?

*Students read Section 21.2 BEFORE Investigation 21.2
                21.3 Capacitors

 A capacitor is a storage device for electric charge.

 Capacitors can be connected in series or parallel
  in circuits, just like resistors.
                 21.3 Capacitors
 A capacitor can be charged by connecting it to a
  battery or any other source of current.
 A capacitor can be discharged by connecting it to any
  closed circuit that allows current to flow.
                 21.3 Capacitors
The current flowing into or out of a
   particular capacitor depends
   on four things:
 1. The amount of charge already in
    the capacitor.
 2. The voltage applied to the
    capacitor by the circuit.
 3. Any circuit resistance that limits
    the current flowing in the circuit.
 4. The capacitance of the
     21.3 How a capacitor works inside
 The simplest type of
  capacitor is called a parallel
  plate capacitor.
 It is made of two conductive
  metal plates that are close
  together, with an insulating
  plate in between to keep the
  charges from coming
 Wires conduct charges
  coming in and out of the
  21.3 How a capacitor works inside
The amount of charge a capacitor can store
  depends on several factors:
 1. The voltage applied to the capacitor.
 2. The insulating ability of the material between
    the positive and negative plates.
 3. The area of the two plates (larger areas can
    hold more charge).
 4. The separation distance between the plates.
                21.3 Capacitance

The ability of a capacitor to store charge is called
 capacitance (C).

      (C)         q = CV              Voltage (volts)

 Cameras use capacitors to supply quick bursts of
  energy to flash bulbs.
                 21.3 Capacitance

 Capacitance is measured in farads (F).

 A one-farad capacitor can store one coulomb of charge
  when the voltage across its plates is one volt.

 One farad is a large amount of
  capacitance, so the microfarad
  (μF) is frequently used in place
  of the farad.
        21.3 Calculate capacitance

 A capacitor holds 0.02
  coulombs of charge when
  fully charged by a 12-volt

 Calculate its capacitance
  and the voltage that would
  be required for it to hold
  one coulomb of charge.
                  21.3 Capacitors

   Key Question:
   How does a capacitor work?

*Students read Section 21.3 BEFORE Investigation 21.3
Application: How a Television Works

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