Transcript Electric charge

Electricity and Magnetism
Electric Charges and Forces
Electric Charge
Coulomb’s Law
Capacitors
Objectives
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7.
Describe and calculate the forces between like and unlike electric
charges.
Identify the parts of the atom that carry electric charge.
Apply the concept of an electric field to describe how charges exert
force on other charges.
Sketch the electric field around a positive or negative point charge.
Describe how a conductor shields electric fields from its interior.
Describe the voltage and current in a circuit with a battery, switch,
resistor, and capacitor.
Calculate the charge stored in a capacitor.
Vocabulary Terms: homework
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charge
electrically neutral
static electricity
positive charge
negative charge
electric forces
charge by friction
electroscope
protons
neutrons
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electrons
gravitational field
charged
induction
Coulomb’s law
capacitor
parallel plate
capacitor
 microfarad
 coulomb
 electric field
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capacitance
charge
polarization
shielding test
charge
farad
field inverse
square law
discharged field
lines
Electrostatics
In Physics
Electric Charge
Key Questions:
1. Define electrically neutral
2. Define net charge or excess
charge
3. What causes static electricity
 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.
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.
Electric Charge
 Answer the following:
 4. What is the unit of electrical charge?
 5. What is the mass of an electron, proton, and neutron?
 6. What are the charges in coulombs of a protron, neutron,
and electron?
 7. Describe the attracting and repelling characteristics of
protons and electrons
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.
Electric Charge
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.
 8. Draw a picture of an electroscope with a charge (positive
or negative)
 9. Describe how the electroscope becomes positively
charged through conduction
 10. Describe how the electroscope becomes negatively
charged through conduction
Electric forces
 The forces between the two kinds of charge can be observed
with an electroscope.
Electric forces
 Charge can be transferred by conduction.
 11. What moving particles create a current in metals?
 12. Define current
 13. Write the formula for current
 14. What is the unit for 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.
Electric current
 Current is the movement of electric charge through a substance.
Current
(amps)
I=q
t
Charge that flows
(coulombs)
Time (sec)
 15. Two coulombs of charge pass through a wire in five seconds.
 Calculate the current in the wire.
Calculate current
 16. Compare the movement of an electron in a conductor
and insulator
 17. Describe a semi-conductor
 18. Describe charging by friction
 19. What causes a negatively charged balloon to stick to a
wall?
 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.
Conductors and insulators
Conductors and insulators
A semiconductor has a few free electrons and atoms
with bound electrons that act as insulators.
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.
20. Define coulomb’s law and write the formula
21. What is coulomb’s constant?
22. Describe the force between two charges if the distance
between them is tripled
23. Describe the force between two charges if both charges are
increased by a factor of two.
 Coulomb’s law relates the force between two single charges
separated by a distance.
Constant
9 x109 N.m2/C2
Force
(N)
F = K q1 q2
r2
Charges (C)
Distance (m)
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.
Coulomb's Law
 The force between two charges
is directed along the line
connecting their centers.
 Electric forces always occur in
pairs according to Newton’s
third law, like all forces.
Coulomb's Law
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.
 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.
 Now answer questions #22, 23
Coulomb's Law
 24. 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.
Calculating force
 25. Describe an electric field
 26. What type of field does mass create?
 27. Compare the gravitational force and electrical force
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between two charges.
28. Draw an electrical field between:
a. A proton and electron
b. A proton and proton
c. An electron and electron
29. Draw the electric field coming from a single proton and a
single electron
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.
Fields and forces
 Mass creates a gravitational field that exerts forces on other
masses.
Fields and forces
 Gravitational forces are far weaker than electric forces.
Drawing the electric field
 30. Describe the units and equations for both a gravity field
and electric field
 31. Describe how electric fields accelerate particles causing
changes in voltage
 32. Why is shielding necessary around electrical wires inside
of sensitive electronic equipment?
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.
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.
 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.
Accelerators
 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.
Electric shielding
Practice problems:
33. A particle in a microwave has a charge of -9 x 10-15 C and is
deflected into popcorn with a force of 4.5 x 10-2 N. What
is the strength of the electric field. USE F = Eq
q = -9 x10-15 C
F = 4.5 x 10-2 N
E = F/q = 4.5 x 10-2 N/- 9 x10-15 C = 5 x 1012 N/C
34. A positive and negative particle, each with a 5 x 10-8 charge,
are separated by a distance of 6 mm. What is the Force acting
on them? Use coulomb’s law
F = k q1 q2/r2
F = (9x109) (-) (5 x 10-8 ) (5 x 10-8 ) / (6 x 10-3)2 = -6.25x10-1N
35. What would the force be in number 34 if the distance were
tripled
F = k q1 q2/r2
If r is tripled the r2 = 32 = 9 so the force would be 1/9 as
strong = -6.25x10-1N/9
36. Find the size of the electrical field at the location of one
charge due to the other charge? USE F = Eq, Use
information from question #35 E = F/q = -6.25x10-1N/
(5 x 10-8 )
37. Find the potential energy of the charged pair.
Use UE = ke q1q2/r
Use UE = ke q1q2/r
= (9x109) (-) (5 x 10-8 ) (5 x 10-8 ) / (6 x 10-3)
 38. How many excess electrons does the microwave particle
in #33 have?
 Q = Ne
where
Q = the total charge
N = the number of electrons
e = electronic charge 1.6 x 10-19 C
N = Q/e = 9 x10-15 C/ 1.6 x 10-19 C = 5.625 x 104
39. What is a capacitor?
40. What is the symbol for a capacitor in a circuit diagram?
41. How is a capacitor charged?
42. How is a capacitor discharged?
43. What determines flow in and out of a capacitor?
44. Describe a parallel plate capacitor
45. Describe how the parallel plate capacitor works
Capacitors
Capacitors
 A capacitor is a storage device for electric charge.
 Capacitors can be connected in series or parallel
in circuits, just like resistors.
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.
Capacitors
The current flowing into or out of a
particular capacitor depends on four
things:
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 capacitor.
1.
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.
 46. What determines how much charge a capacitor can hold?
 47. Define capacitance
 48. What is the formula for capacitance
 49. What is the unit for capacitance
 50. What is a microfarad?
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.
How a capacitor works inside
Capacitance
The ability of a capacitor to store charge is called capacitance (C).
Cameras use capacitors to supply quick bursts of
energy to flash bulbs.
Capacitance
(coulombs/volt)
Charge
(C)
q = CV
Voltage (volts)
 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.
Capacitance
Calculate capacitance
 51. 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.
 C = q/V
Capacitors: electric field = V/d
52. 2000 volts moves across a parallel plate capacitor with a
separation of 5 mm. What is the electric field across the
capacitor?
E = V/d = 2000V/5x10-3m = 4 x 10 N/C
Application: How a Television Works