Phy Book Notes
Unit 1
Chapter 21: Electricity
• Electric forces hold things together
• Gravity is not an electric force
• Elektron = amber
• Two types of electric charge: repel and attract or + and -
o + charge is on glass
o - on plastic/amber
• Equal and opposite charges
o Conservation of energy
• When creating static electricity, the rubbing transfers electrons from the cloth/towel to
the object
o Towel has + charge
o Object has - charge
• When rubbed, charge returns to neutral state quickly
o Because of polarity of water
o Charge is not distributed uniformly
o “Leak off” into the air
o Static electricity is greater on a dry day because fewer water molecules in the air
• Charge is transferred by conductors
o Especially metals
o Electrons are bound loosely and can move freely within material
§ Free/conduction electrons
o Free electrons attracted to a positively charged object
§ Repelled from negatively charged
o Nonconductors/insulators are wood and rubber
§ Even insulators conduct a small amount of electricity
§ Electrons are tightly bound to nucleus
o Semiconductors are between conductors and insulators
§ Silicon and germanium
§ Fewer free electrons than conductors
• Induced charge
o Free electrons pass over to the positively charged object if objects touch
§ The original object that was neutral now has a positive charge because it
loses electrons
§ Charging by conduction or charging by contact
• Two objects have same charge
o If objects do not touch
§ Electrons do not leave but still move towards the positive charge
§ Creates a positive charge on one end of the object
§ Charge has been induced at both ends of the object
, § Charges have been separated; net charge is still zero
§ If separate object into two pieces, there would be one positive and one
negative
o Net charge
§ Can be induced by a conducting wire or pipe into the ground
§ Object is grounded
§ Earth easily exchanges electrons
§ How lightning rods work
o Electroscope detects charge
§ Same principle used to make electrometers
§ Electrometers more sensitive
• Coulomb’s Law
o Electrostatic force
o Electric charge exerts attraction or repulsion
o Small spheres with magnitudes of charge where ratio of charge is known, but not
actual charge
!" "
o 𝐹 = #!" "
§ k is proportionality constant
§ k=8.99*109 Nm2/C2
§ 1 C is the amount of charge that will result in a force of k
o Magnitude of force that one charge exerts on the other
o Direction is always along the line joining the charges
o If same sign, repel. If opposite sign, attract.
o 𝑒 = 1.602 ∗ 10$%& C
o Similar to law of universal gravitation, except gravity is only attractive
o k in terms of ∈'
§ Permittivity of free space
%
§ 𝑘 = ()∈
#
%
§ ∈' = ()! = 8.85 ∗ 10$%+
§ Precise for point charges
• Spatial size negligible compared to other distances
o Principle of Superposition
§ Net force on any charge will be the vector sum of the forces due to each
other
§ Vector Form
!" "
• 𝐹⃗%+ = # ! "" 𝑟̂+%
"!
• 𝐹⃗%+ = vector force charge on Q1 due to Q2
• 𝑟̂+% = unit vector pointing from Q2 towards Q1
• If Q1 and Q2 have the same sign, force on Q1 points away from Q2
o Repulsive
• The Electric Field
o Electric field extends outward from every charge to permeate all space
, § If a second charge is near the first, it feels a force exerted by the field
o Investigate field surrounding charge(s) by measuring the force on a small positive
test charge at rest
§ Test charge(q): a charge so small that the force it exerts does not
significantly affect charges creating an electric field
,⃗
o Electric field: 𝐸2⃗ = .
§ Force exerted on test charge divided by magnitude of test charge
!" "
§ 𝐸 = # " = ()∈ # "
#
o Superposition principle: 𝐸2⃗ = 𝐸2⃗% + 𝐸2⃗+ + ⋯
§ If electric field at a point in space is due to more than one charge
• Field Lines
o Electric field = vector field
o Field line: series of lines to indicate direction of electric field at points in space
§ Indicate direction of force due to the given field on a positive test charge
(q)
§ Lines are drawn so magnitude of field (E) is proportional to number of
lines crossing unit area perpendicular to lines
§ Electric field lines start on positive charges and end on negative charges
• Number starting or ending is proportional to magnitude of charge
(Q)
§ Field lines never cross
• Electric field cannot have two directions at same point
• Electric field cannot exert more than one force on a test charge
(q)
§ Aka lines of force
§ Nearer to the charge, electric field is greater and lines are closer together
%
• 𝐹 ∝ #"
• Closer together the lines are, stronger the electric field
o Electric dipole: electric field lines due to two equal and opposite charges
§ /
§ 𝐸 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = ∈
#
• Between two closely spaced, oppositely charged, parallel plates
o Gravitational Field
Unit 1
Chapter 21: Electricity
• Electric forces hold things together
• Gravity is not an electric force
• Elektron = amber
• Two types of electric charge: repel and attract or + and -
o + charge is on glass
o - on plastic/amber
• Equal and opposite charges
o Conservation of energy
• When creating static electricity, the rubbing transfers electrons from the cloth/towel to
the object
o Towel has + charge
o Object has - charge
• When rubbed, charge returns to neutral state quickly
o Because of polarity of water
o Charge is not distributed uniformly
o “Leak off” into the air
o Static electricity is greater on a dry day because fewer water molecules in the air
• Charge is transferred by conductors
o Especially metals
o Electrons are bound loosely and can move freely within material
§ Free/conduction electrons
o Free electrons attracted to a positively charged object
§ Repelled from negatively charged
o Nonconductors/insulators are wood and rubber
§ Even insulators conduct a small amount of electricity
§ Electrons are tightly bound to nucleus
o Semiconductors are between conductors and insulators
§ Silicon and germanium
§ Fewer free electrons than conductors
• Induced charge
o Free electrons pass over to the positively charged object if objects touch
§ The original object that was neutral now has a positive charge because it
loses electrons
§ Charging by conduction or charging by contact
• Two objects have same charge
o If objects do not touch
§ Electrons do not leave but still move towards the positive charge
§ Creates a positive charge on one end of the object
§ Charge has been induced at both ends of the object
, § Charges have been separated; net charge is still zero
§ If separate object into two pieces, there would be one positive and one
negative
o Net charge
§ Can be induced by a conducting wire or pipe into the ground
§ Object is grounded
§ Earth easily exchanges electrons
§ How lightning rods work
o Electroscope detects charge
§ Same principle used to make electrometers
§ Electrometers more sensitive
• Coulomb’s Law
o Electrostatic force
o Electric charge exerts attraction or repulsion
o Small spheres with magnitudes of charge where ratio of charge is known, but not
actual charge
!" "
o 𝐹 = #!" "
§ k is proportionality constant
§ k=8.99*109 Nm2/C2
§ 1 C is the amount of charge that will result in a force of k
o Magnitude of force that one charge exerts on the other
o Direction is always along the line joining the charges
o If same sign, repel. If opposite sign, attract.
o 𝑒 = 1.602 ∗ 10$%& C
o Similar to law of universal gravitation, except gravity is only attractive
o k in terms of ∈'
§ Permittivity of free space
%
§ 𝑘 = ()∈
#
%
§ ∈' = ()! = 8.85 ∗ 10$%+
§ Precise for point charges
• Spatial size negligible compared to other distances
o Principle of Superposition
§ Net force on any charge will be the vector sum of the forces due to each
other
§ Vector Form
!" "
• 𝐹⃗%+ = # ! "" 𝑟̂+%
"!
• 𝐹⃗%+ = vector force charge on Q1 due to Q2
• 𝑟̂+% = unit vector pointing from Q2 towards Q1
• If Q1 and Q2 have the same sign, force on Q1 points away from Q2
o Repulsive
• The Electric Field
o Electric field extends outward from every charge to permeate all space
, § If a second charge is near the first, it feels a force exerted by the field
o Investigate field surrounding charge(s) by measuring the force on a small positive
test charge at rest
§ Test charge(q): a charge so small that the force it exerts does not
significantly affect charges creating an electric field
,⃗
o Electric field: 𝐸2⃗ = .
§ Force exerted on test charge divided by magnitude of test charge
!" "
§ 𝐸 = # " = ()∈ # "
#
o Superposition principle: 𝐸2⃗ = 𝐸2⃗% + 𝐸2⃗+ + ⋯
§ If electric field at a point in space is due to more than one charge
• Field Lines
o Electric field = vector field
o Field line: series of lines to indicate direction of electric field at points in space
§ Indicate direction of force due to the given field on a positive test charge
(q)
§ Lines are drawn so magnitude of field (E) is proportional to number of
lines crossing unit area perpendicular to lines
§ Electric field lines start on positive charges and end on negative charges
• Number starting or ending is proportional to magnitude of charge
(Q)
§ Field lines never cross
• Electric field cannot have two directions at same point
• Electric field cannot exert more than one force on a test charge
(q)
§ Aka lines of force
§ Nearer to the charge, electric field is greater and lines are closer together
%
• 𝐹 ∝ #"
• Closer together the lines are, stronger the electric field
o Electric dipole: electric field lines due to two equal and opposite charges
§ /
§ 𝐸 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 = ∈
#
• Between two closely spaced, oppositely charged, parallel plates
o Gravitational Field
