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physics 112N
electric charges, forces and fields
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physics 112N 2
static electricity
! like charges repel, unlike charges attract
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physics 112N 3
atomic origin of charge
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physics 112N 4
conductors and insulators
! materials which permit the movement of charge through them are called conductors
! materials which resist the movement of charge through them are called insulators
! e.g. most metals
! e.g. plastic, wood ...
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physics 112N 5
charge induction
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physics 112N 6
charge induction in conductors
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physics 112N 7
induction
! inducing charge separation with a van der Graaf generator
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physics 112N 8
induction
! inducing charge separation with a van der Graaf generator
++ + +
+++ +
+ ++
+ !
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physics 112N 9
polarization of insulators
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physics 112N 10
polarization of insulators
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physics 112N 11
conservation of charge
! the total charge in a closed system is conserved
! we can move charge around, but we can’t create or destroy it
! the unit of electrical charge is the Coulomb
! 1 Coulomb is a lot of charge
! the charge of a proton (or electron) seems to be the minimum allowed unit of charge in nature, e = 1.60217653(14) ! 10-19 C
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physics 112N 15
the force between charges - Coulomb’s law
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physics 112N 16
the force between charges - Coulomb’s law
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physics 112N 18
the force between charges - Coulomb’s law
Three charges are fixed in position on a line as shown.
!+ !
What is the magnitude and direction of the total electrostatic force on the middle charge ?
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physics 112N 19
the force between charges - Coulomb’s law
Three charges are fixed in position as shown.
!
+
!
What is the magnitude and direction of the total electrostatic force on the -1.0 nC charge ?
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physics 112N 24
the electric field
suppose we set up the two charges as shown
! +
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physics 112N 25
the electric field
suppose we set up the two charges as shown
! +
a third charge placed in between feels a force
+
how does it know the other charges are there ?
the charges produce an ‘electric field’
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physics 112N 26
the electric field
suppose we set up the two charges as shown
! +
a third charge placed in between feels a force
how does is know the other charges are there ?
the charges produce an ‘electric field’
at every point in space there is a vector
a charge q placed at that positionfeels a force
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physics 112N 27
the electric field
suppose we set up the two charges as shown
! +
the charges produce an ‘electric field’
at every point in space there is a vector
put a ‘test charge’ at each point, measure the force it feels and divide out its charge
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physics 112N 28
the electric field
+!
electric field line diagram
arrows show direction of E-field, density of lines shows magnitude of E-field
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physics 112N 29
the electric field
+!
electric field line diagram
arrows show direction of E-field, density of lines shows magnitude of E-field
lines only begin and end on charges
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physics 112N 30
the electric field from a point charge
+Q
!Q
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physics 112N 32
superposition of electric fields
the total electric field at any point is the vector sum of the electric fields from all the point charges around it
! +
e.g.
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physics 112N 33
superposition of electric fields
the total electric field at any point is the vector sum of the electric fields from all the point charges around it
! +
e.g.
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physics 112N 34
superposition of electric fields
the total electric field at any point is the vector sum of the electric fields from all the point charges around it
! +
e.g.
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physics 112N 35
the electric field
+!
electric field line diagram
arrows show direction of E-field, density of lines shows magnitude of E-field
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physics 112N 36
the electric field
++
two equal positive charges
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physics 112N 41
two chargesTwo charges are separated by a distance of 1.40 m. The electric field produced by the two charges is found to be zero a distance of 0.40 m from the left charge along the line joining the two charges. Find Q2 .
! ?
1.40 m
0.40 m
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physics 112N 42
electric field configurations - charged parallel plates
large charged parallel plates cause an almost uniform field in the gap between them
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physics 112N 46
electron in a capacitor
+
+
+
+
+
+
+
+
+
+
!!
!
!
!
!
!
!
!
!
!
An electron escapes from the negative plate of a capacitor producing a uniform electric field of magnitude 2.00!103 N/C.
Starting from rest, the electron takes 2.00!10-8 seconds to travel to the positive plate. How far apart are the capacitor plates ?
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physics 112N 47
electric field in a conductor
inside a conducting material, the electric field must be zero in an electrostatic situation
imagine it wasn’t - the electric field would cause forces on the ‘free’ charges and they’d accelerate, so the situation wouldn’t be static
the ‘free’ charges will always rapidly rearrange themselves to produce zero E-field inside the conductor
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physics 112N 48
electric dipole in a uniform electric field
a dipole is shown - it is easy to see that the total force on the dipole is zero
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physics 112N 49
electric dipole in a uniform electric field
a dipole is shown - it is easy to see that the total force on the dipole is zero
but there is a torque on the dipole that’ll cause it to rotate
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physics 112N 50
electric dipole in a uniform electric field
but there is a torque on the dipole that’ll cause it to rotate
two orientations feel zero torque :
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physics 112N 52
polarization of insulators
an insulating material made from ‘polar’ molecules which behave like electric dipoles can thus be polarized
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physics 112N 53
electric dipole in an electric field
! use induction to form a dipole! watch how it lines up in the electric field from the vdG sphere
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physics 112N 54
electric dipole in an electric field
! use induction to form a dipole! watch how it lines up in the electric field from the vdG sphere
+!+