Unit 7, Chapter 21

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Unit 7, Chapter 21
CPO Science Foundations of Physics
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Unit 7 Electricity and Magnetism
Chapter 21 Electric Charges and Forces

• 21.1 Electric Charge
• 21.2 Coulombs Law
• 21.3 Capacitors

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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 charge.
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.

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Chapter 21 Vocabulary Terms

• charge
• electrically neutral
• static electricity
• positive charge
• negative charge
• electric forces
• charge by friction
• electroscope
• protons
• neutrons

• electrons
• gravitational field
• charged
• induction
• Coulombs law
• capacitor
• parallel plate capacitor
• microfarad
• coulomb
• electric field

• capacitance
• charge
• polarization
• shielding test
• charge
• farad
• field inverse
• square law
• discharged field
• lines

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21.1 Electric Charge

• Key Question
• How do electric charges interact?

Students read Section 21.1 AFTER Investigation
21.1
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21.1 Electric Charge

• All ordinary matter contains both positive and
  negative charge.
• 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 charge.

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21.1 Electric Charge

• Objects can lose or gain electric charges.
• The net charge is also sometimes called excess
  charge because a charged object has an excess of
  either positive or negative charges.
• A tiny imbalance in either positive or negative
  charge on an object is the cause of static
  electricity.

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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.

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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.

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21.1 Electric forces

• The forces between the two kinds of charge can be
  observed with an electroscope.

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21.1 Electric forces

• Charge can be transferred by conduction.

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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 current.
• In solid metal conductors, only the electrons can
  move, so current is carried by the flow of
  negative electrons.

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21.1 Electric current

• Current is the movement of electric charge
  through a substance.

Charge that flows (coulombs)
Current (amps)
Time (sec)
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21.1 Calculate current

• Two coulombs of charge pass through a wire in
  five seconds.
• Calculate the current in the wire.

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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
  movethey are tightly bound inside atoms.

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21.1 Conductors and insulators

• A semiconductor has a few free electrons and
  atoms with bound electrons that act as insulators.

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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 friction.
• Objects charged by this method will attract each
  other.

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21.2 Coulomb's Law

• Coulombs law relates the force between two
  single charges separated by a distance.

Constant 9 x109 N.m2/C2
Force (N)
Charges (C)
Distance (m)
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21.2 Coulomb's Law

• The force between two charges gets stronger as
  the charges move closer together.
• The force also gets stronger if the amount of
  charge becomes larger.

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21.2 Coulomb's Law

• The force between two charges is directed along
  the line connecting their centers.
• Electric forces always occur in pairs according
  to Newtons third law, like all forces.

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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.

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21.1 Coulomb's Law

• The force between charges is inversely
  proportional to the square of the distance
  between them.
• 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.

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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.

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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.

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21.2 Fields and forces

• Mass creates a gravitational field that exerts
  forces on other masses.

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21.2 Fields and forces

• Gravitational forces are far weaker than electric
  forces.

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21.2 Drawing the electric field
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21.2 Electric fields and electric force

• On the Earths 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.

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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.

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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 them.
• Electron beams are used in x-ray machines,
  televisions, computer displays, and many other
  technologies.

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21.2 Electric shielding

• Electric fields are created all around us by
  electric appliances, lightning, and even static
  electricity.
• These stray electric fields can interfere with
  the operation of computers and other sensitive
  electronics.
• Many electrical devices and wires that connect
  them are enclosed in conducting metal shells to
  take advantage of the shielding effect.

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21.2 Coulombs Law

• Key Question
• How strong are electrical forces?

Students read Section 21.2 BEFORE Investigation
21.2
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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.

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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.

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21.3 Capacitors

• The current flowing into or out of a particular
  capacitor depends on four things
• The amount of charge already in the capacitor.
• The voltage applied to the capacitor by the
  circuit.
• Any circuit resistance that limits the current
  flowing in the circuit.
• The capacitance of the capacitor.

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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 together.
• Wires conduct charges coming in and out of the
  capacitor.

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21.3 How a capacitor works inside

• The amount of charge a capacitor can store
  depends on several factors
• The voltage applied to the capacitor.
• The insulating ability of the material between
  the positive and negative plates.
• The area of the two plates (larger areas can hold
  more charge).
• The separation distance between the plates.

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21.3 Capacitance

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

Capacitance (coulombs/volt)
Charge (C)
q C V
Voltage (volts)
Cameras use capacitors to supply quick bursts of
energy to flash bulbs.
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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.

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21.3 Calculate capacitance

• A capacitor holds 0.02 coulombs of charge when
  fully charged by a 12-volt battery.
• Calculate its capacitance and the voltage that
  would be required for it to hold one coulomb of
  charge.

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21.3 Capacitors

• Key Question
• How does a capacitor work?

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