Unit 4.2 Electrostatic Fields (No escape velocity & distance approach) Flashcards
What does electrostatically charging an object involve?
(2,way)
Redistribution, addition or removal of electrons
What are the 2 ways to charge and object?
- Charging by friction
- Charging by induction
What is charging by friction?
(4-way)
- Process of friction generates energy
- … to remove some electrons from the surface
- … of the material and place them
- … onto the duster
What is charging by induction?
(2-way + 2-way)
- Electrons redistribute themselves on the leaves
- … of the electroscope
- Positive charge induced on the
- … top plate of the electroscope
Examples of everyday charging?
(Just know that one)
- Lightning clouds
- Hair n stuff
- Christmas jumper
- Comb??
What is the 2 principles of electrostatics?
- Like charges repel
- Unlike charges attract
Show me like charges repel?
(2 light things)
- Positive to a positive repel
- Negative to a negative repel
Show me unlike charges attract?
Positive to a negative attract each other
Define induction in general?
When it’s not touching
Define conduction in general?
When it’s touching
Explain gold-leaf electroscope
(Check up waaat thaaa)
Later
Explain electric field lines for a positive point charge?
Arrows directed away from center
Explain electric field lines for a negative point charge?
Arrows directed towards the center
What would u see for electric field lines if there’s a positive point charge near a negative point charge?
(2-way + 3-way)
(Or just draw it)
- Arrows from positive point charge
- directed towards negative point charge
- The direction of the arrows some are straight
- Others are curved
- Based on closeness
How would u increase electric field lines curve strength?
(2 things)
- Decrease distance
- Increase quantity of point charges
Diagram of field between 2 parallel plates?
Whiteboard if u need it
Explain electric field between 2 oppositely charged parallel plates?
(1 + 2-way + 1 thing)
- Field lines = straight & parallel to one another
- ∴ field’s strength uniform throughout
- … except edges
- At edges = field strength slightly less
What happens to the field if u increase the quantity?
(2 comparisons)
- Magnitude doubles if u add 1 (+2Q)
- Magnitude triples if u add 2 (+3Q)
What would happen to the field for ‘unlike point charge’
(Field between two point charges)
(2 things)
- Attraction
- Basically looks like magnet directions/arrows
(u should probably draw it)
What would happen to the field for ‘like point charge’
(Field between two point charges)
(2 things + 2-way)
- Repel
- Similarities for less curves
- Likewise to negative version
- …. but arrows towards negative
What’s the jig with field for ‘like point charge’?
(1 thing + 3-way)
- It has a point where the field is zero
- Due to vector lines at these points…
- are equal and in opposite direction
- ∴ they cancel out
Diagram for the field ‘like point charge’
(Field between two point charges)
Whiteboard
What does the density of field lines indicate?
Its relative strength of the field
What does the direction of the field line indicate?
(3-way)
- Direction of the force
- experienced by a small positive charge
- placed in that position
How do u calculate the strength of a charges’ (Q) field (E)?
(2-way + 1 way?)
- Place a positive test charge q
- in the field
- Measure force acting on it
Electric field strength equation?
(Analogous to capacitance version O_o)
(NOT IN DATA BOOKLET)
E = f/q
Define E
(Electric field strength equation)
… The electric field strength
(NC-1)
Define f
(Electric field strength equation)
Force
(N ig?)
Define q
(Electric field strength equation)
Charge
(C)
Electric field strength equation expressed in words?
Electric field strength = Force/Charge
Definition of the electrical field strength?
Force per unit charge
The 2 different units for E (electrical field strength)?
- NC-1
- Vm-1 (E = v/d, capacitance version)
The 2 things to note for electrical field strength equation + positive test charge?
(Lil check up)
- Magnitude of the field strength at any point in the field is independent of the test charge
- The direction of the field depends upon the charge of Q which generates the field; however, a positive test charge must always be used to define the direction
What equation for E in a radial field?
(In data booklet)
E = (1/4πε0) x Q/r2
Define 1/4πε0
(E in a radial field equation)
= 9 x 109 Fm-1
A constant…. kind of.
What’s the jig with 1/4πε0?
k = 1/4πε0 = 9 x 109
Define Q
(E in a radial field equation)
Charge
(C)
Define r
(E in a radial field equation)
Radial distance from point charge
(m)
What’s the ε0 known as?
Permittivity of free space
For E in a radial field, what is the relationship between charge and E?
They’re directly proportional
(E ∝ Q)
For E in a radial field, what is the relation between distances and E?
They’re inversely proportional
(E ∝ 1/r2)
What’s the relationship between E and radial distance another example of?
The inverse square law
If r doubles, what happens to E?
(Relationship between E and r)
E becomes 1/4 of its previous strength
If r is halved, what happened to E?
(Relationship between E and r)
E becomes 4 times greater
Combining electric fields using vector addition
(lil check up)
I’ll come back to it.
It’s actually a bit similar to basic physics vectors.
Tho obviously, “analogous”
Tell me what u know so far for the combining electric fields
(Future self)
- Once again, like resolving vectors
- Tho apparently mostly u wouldn’t be needed sin + cos
- (Horizontal & vertical components)
- Typical equation is E = k x Q/r2
- k obviously the constant
- Take in consideration of what happens between + against -
- or - against - or + against +
- Don’t forget to square
- In addition, finding magnitude and direction
- I’ll stick with this
What’s it called for if we consider force experienced by a second charge (q), place in a specific position in some rando’s field
Electrical field force (Coulomb’s Law)
The typical equation for finding force in an electric field?
(Capacitance type shi)
F = E x q
Therefore, the equation for the electric field force?
(In data booklet except in k terms)
F = k x (Q1Q2)/r2
Define F
(Electric field force eqn)
Force between charges
(N)
Define Q
(Electric field force eqn)
Charge
(C)
Define k
(Electric field force eqn)
- Constant of proportionality
- Coulomb’s constant
(Nm2C-2 aka 1/4πε0)
Define r
(Electric field force eqn)
Separation of charges
(m)
Define ε0
(Electric field force eqn)
Permittivity of free space
(Constant)
Definition for Coulomb’s law?
(4-way)
- Force between charges is directly proportional…
- to the product of charges
- and inversely proportional to
- … the distance between charges squared
What to take in consideration for electric field force eqn?
(2-(way) + 2 things)
- For finding the charge, be aware whether it’s + or -
- (Rules of magnets n stuff)
- If attracting, Force = negative
- If repelling, Force = positive
Glad i did that hw asap
But now how much can I recall?
+ i forgot what i was gonna say
Once again, now u better be sorry
…
Explanation for the equation for electrical Potential Energy in terms of force/distance graph?
(1, 3-way)
- In a Uniform Field,
- area under force/distance graph
- = work done when moving
- charge through a distance (x)
Hence, equation for “electrical Potential Energy” [in distance]?
△W = F △x
Definition of potential energy?
(In terms of charge ig)
(3-way)
- The energy required to move
- the charge from infinity to
- a point in a field
However, what about “electrical Potential Energy” in a radial field graph?
(1, 3-way)
- Same principle
- Area enclosed under…
- force/radius represents work done
- when charge moved from r1 to r2 (positions)
What does the force/radius graph look like for “electrical potential energy” in a radial field?
A curve
In addition, how would u gain the area in the force/radius graph?
(“Electrical Potential Energy” in a radial field)
Count the squares…
Thus, the equation of our line for force/radius graph?
(Electrical Potential Energy P.E)
F = k x q1q2/r2
(ofc k = 1/4πε0)
BUT ULTIMATELY, the actual equation for the electrical Potential Energy in a radial field?
(In data booklet)
PE = Q1Q2/4πε0r
better way for this, k constant.
Explain what it means by infinity?
(Electrical Potential Energy in a radial field)
(4-way)
- The zero energy reference but as…
- Electric Fields,
- can be attracting/repelling
- either max. or min. PE
What would it mean if the field is attractive?
(Electrical Potential Energy in a radial field)
(4 things)
- Q1Q2 is -‘ve
- PE = negative
- Maximum PE at ∞
- Work to move charge in field is greatest towards ∞
What would it mean if the field is repulsive?
(Electrical Potential Energy in a radial field)
(4 things + 1 optional special)
- Q1Q2 is +’ve
- PE = positive
- Minimum PE at ∞
- Work to move charge in field is lowest to ∞
- Like a spring pushed down…
Explain what the electrical potential (VE) due to a point charge is?
(4-way)
(Electrical Potential (VE of a position))
- The work done in…
- bringing a unit positive charge from
- infinity to that point
- in the field
Equation for Electric Potential (volt)?
(JC-1)… in data booklet but the only for finding W
(Electrical Potential (VE) of a position)
V = W/Test q
Data booklet edition for equation for Electric Potential (volt)?
(Electrical Potential (VE) of a position)
W = q △VE
Derive expression for Electrical Potential V using equation for electrical PE in radial field?
(3 by 3 step)
(Electrical Potential (VE) of a position)
- VE = PE/q
- PE = k x Q1Q2/r
- Hence, VE = k x Q/r
[Useful] way to imagine potential (work done per unit charge)?
(2-way)
- Imagine a “potential surface”
- created by a point charge
Surface of zero potential would be?
(Potential wells and hills)
Flat surface
Explain the positive charge in terms of…
(Potential wells and hills)
(a thing + 3-way + a things)
- Creates potential “hill”
- +’ve work done must be done to move
- a +’ve test charge closer to it
- “as tho u’re moving it up a hill”
- +’ve work done when against electric force
Explain the negative charge in terms off…
(Potential wells and hills)
(a thing + 3-way + a things)
- Creates a potential “well”
- -‘ve work done to move…
- a +’ve test charge closer to it
- “as tho u’re moving it down a well”
- -‘ve work done when moving with electric force
What’s electrical potential (VE) of a position similar to?
(1 imp + 3-way)
- Like PE of a charge in a field
- Electrical Potential (VE) of a point in a field is:
- negative in an attractive field
- positive in a repulsive field
How to find resultant potential at point x if multiple charges is present?
(1 thing but optionals if u want)
i.e. Q1, -Q2 Q3
- Algebraic sum of the potentials
- Resultant V at X = V1 - V2 + V3
Sketch and label how potential varies with distance for both a positive and negative charge (graph)
Stage is yours
Gradient for [how potential varies with distance (graph)]?
(3-way)
- Gradient of graph = rise/run = △VE/△r
- VE = k x Q/r
- ∴ gradient = k x Q/r2 = E
- but ultimately, gradient = -E
How’s that the gradient for [how potential varies with distance (graph)]?
(4-way)
- Direction given by charge producing field
- Negative charge producing -E (inwards)
- Positive charge producing +E (outwards)
- ∴, gradient = -E
It’s like, I forgot I had physics
SO, what’s left?:
- Escape velocity in electrical fields
(as much as i’d start this first…. it’s already a major sell that I didn’t do the previous work ¬.¬)
I have a choice:
- Make notes for escape velocity in electrical fields
- Boycott, start the real revision
