chapter 20 static electricity
DESCRIPTION
-. +. Chapter 20 Static Electricity. Charge by Conduction. + + +. -. +. + + +. + + +. Chapter 20 Static Electricity. Charge by Induction. Chapter 21 Electric Fields. Electric Field A property of space around a charged object that causes forces on other charged objects. - PowerPoint PPT PresentationTRANSCRIPT
Chapter 20Static Electricity
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Charge by Conduction
Chapter 20Static Electricity
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Charge by Induction
Chapter 21Electric Fields Electric Field A property of space
around a charged object that causes forces on other charged objects. Vector quantity It has both direction and
magnitude The direction of the force is away from the
positive and towards the negative. The electric field is the strongest when the
lines are close together Field lines do not exist-only a pictorial
guide.
Chapter 21Electric Fields
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Chapter 21Electric Fields
Chapter 21Electric Fields
Chapter 21Electric Fields
Van de Graaf Generator
In the Van de Graaf generator, charge is transferred onto a movingbelt A, and then onto the metaldome B, An electric motor does thework needed to increase the electric potential energy.
Chapter 21Electric Fields
Van de Graaf Generator
Chapter 21Electric Fields
Chapter 21Electric Fields
Chapter 21Electric Fields
Electric Field Intensity
E Electric field intensity (N/C)F Force (Newtons)q’ Test Charge (Coulombs)
E F
q'Similar to Gravitational Field Intensity
g F
m
g Gravitational field intensity (N/kg)F Force (Newtons)m mass (kg)
Chapter 21Electric Fields
gIncrease in gravitationalpotential energy
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Increase in electricpotential energy
E
Chapter 21Electric Fields
The electric potential difference (V) is the work done in moving a test charge in an electric field divided by the magnitude of the testcharge.
V Won q'
q'Electric potential difference is measured in joules per coulomb.One joule per coulomb is a volt.
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HIGH V
LOW V
Chapter 21Electric Fields
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HIGH V
LOW V
Chapter 21Electric Fields
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Chapter 21Electric Fields
V EdElectric potential difference in a uniform field
And E = F/q so
V
Fq
d or qVFd
V
NC
m volt
Chapter 21Electric Fields
A force of .032 N is required to move a charge of 4.2 x 10-5 Cin an electric field between two points that are 25 cm apart. What potential difference exists between the points?
EdV q
Wd
q
FV
C4.2x10
5m)(.032N)(.2V 5-
V = 190 volts
Chapter 21Electric Fields
An electron is accelerated by a machine that subjects it to a potential difference of 50 Megavolts. What energy has the electron acquired?
VqWorq
WV
W = (50 x 106V)(1.6 x 10-19C) = 8 x 10-12J
Chapter 21Electric Fields
•Electric current The flow of electrons•Electric current can be maintained only if the electrons are returned to areas of high electron concentration
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pump
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Chapter 21Electric Fields
Millikan’s oil drop experiment Early 1900s – Determined
electric charge
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F1
F2
•When the forces are balanced, F1 = F2
•Eq = mg so q = mg/E•Found that the charge is quantified•Multiples of 1.6 x 10-19C
Chapter 21Electric Fields
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F1
F2
An oil drop has a mass of 1.9 x 10-16 kg and is suspended in an electric field with intensity of 6000 N/C. Find the charge on the drop and the number of excess electrons.
F1= F2 so Eq = mg
6000N/C
)kg)(9.8m/s(1.9x10
E
mgq
216
= 3.1 x 10-19C
C3.1x10
x
C1.6x10
1e1919
one extra electron
All systems are in equilibrium when the energy of the system is a minimum.
The ball comes to rest when the potential energy is the least. It is the greatest at A and the least at B.
A
B
Chapter 21Electric Fields
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- - - - - -The potential of A decreases and thepotential of B increases and both areat the same potential
Acharged sphere
Bneutral sphere
A is the charged sphere withhigh potential energy. B is neutralwith zero potential energy
Chapter 21Electric Fields
What happens with a large sphere and a small sphere?
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Low V High V same q
High q Low q same V
Chapter 21Electric Fields
•Capacitor A device that stores a charge•As charge is added, the potential of the body increases.•For a given charge, the ratio of the charge to the potential q/V is a constant.•Capacitance is the ability to store a charge.
V
qC
C Capacitance (farads)q Charge (Coulombs)V Potential (Volts)
Chapter 21Electric Fields
A 3 x 102 pF capacitor has a potential difference of 30 volts acrossit. What is the charge on the capacitor?
V
qC
C = 3 x 102 pF = 3 x 10-10 Fq =V = 30 volts
q = CV = (3 x 10-10F)(30V) = 9 x 10-9C
Chapter 21Electric Fields