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Physics Year 13 > Chapter 22 - Electric Fields > Flashcards

Chapter 22 - Electric Fields Flashcards

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1
Q

What is an electric (or electrostatic) field

A

It’s a field formed by a charged objects, other charged object experience a force in this field

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2
Q

what is the easiest/most efficient way to test for an electric field

A
  • have an insulator with a charged piece of gold
  • Charge the piece of gold by briefly touching it against a charged sphere
  • it will deflect away from the sphere when placed nearby
  • it will deflect less with distance
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3
Q

do electrons and protons have electric fields

A
  • yes, they are charged objects
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4
Q

define electric field strength

A

“the electric field strength, E, of a field is defined as the force experienced per unit positive charge at that point”

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5
Q

what is the equation for E (electric field strength) involving F (Force), what are the units

A

E = F/Q

units of E are Nc^-1

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6
Q

Is electric field strength a vector or scalar

A

it is a vector quantity

the positive direction is the direction in which a positive charge will move when placed in the electric field

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7
Q

how do we use electric field lines to map electric fields

A
  • the arrow of the line represents the direction of the field (N to S)
  • they are always perpendicular to the surface of the conductor
  • equally spaced lines imply a uniform electric field
  • closer lines imply a stronger electric field
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8
Q

what is the value of E0

A

8.85x10^-12

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9
Q

what is the equation linked to coulombs law and any proportionalities

A

F = Qq/4(pi)(E0)r^2
F is inversely prop to r^2
F is directly prop to Qq

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10
Q

where can coulombs law be applied

A
  • to point charges
  • to spheres where the charge in uniformly distributed
  • to objects where r» size of the object
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11
Q

describe the method to investigate coulombs law

A
  • charge two spheres
  • place one on an insulating rod and the other on an insulating rod attached to a mass balance
  • move them closer and observe how the mass balance reading changes, use this to calculate F
  • measure R to centre of spheres
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12
Q

what is the equation for the strength of the electric field at a distance from a charged sphere

A

E = Q/4(pi)(E0) R^2

where Q is the charge on the sphere

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13
Q

what is the difference between gravity and electric fields in terms of what produces the attraction and the nature of the attraction

A

Gravity - mass, and always attractive

Electric Fields - like charges repel, opposite charges attract

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14
Q

what is the formula for the force experienced by a charge in an electric field

A

F = EQ

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15
Q

what is the equation linking work done and voltage

A

W = VQ

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16
Q

what is the equation linking electric field strength and voltage

A

E = V/d

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17
Q

what is the derivation of the electric field strength and voltage equation and what is the condition on this working

A 
W = Fd, W = VQ, F = EQ
so 
VQ = Fd
VQ = EQd
V = Ed
E = V/d
this only works for Parallel plates
18
Q

what are the factors on the capacitance of a parallel plate capacitor

A
  • area, Capacitance = kA

- distance, capacitance = k/d

19
Q

what are the equations for capacitors for the different types of dielectric

A

capacitance is directly proportional to area over plate seperation so

C = (Eo)A/d
if vacuum or air is used
or 
C = (Eo)(Er)A/d
where EoEr = permittivity of the dielectric used
20
Q

what is a practical that you can do to work out (Eo)

A
  • set up a circuit with a cell and voltmeter, capacitor, and coulombmeter in parallel with a flying lead from the capacitor branch
  • charge the capacitor using the cell and the flying lead
  • record the voltage across the cell and therefore capacitor
  • move the flying lead to the coulombmeter to determine the charge Q
  • measure the seperation and area of the plates
  • repeat for different values of V
  • plot a graph of Q against V
  • grad = c = EoA/d
21
Q

How can you analyse the results of the Eo practical

A
  • plot a V-Q to a graph
  • measure the gradient, this is capacitance (C)

C = E0ErA/d

22
Q

what occurs to a charged particle in an electric field in terms of accleration

A
  • it experiences a constant force due to F = EQ

- this means it experiences a constant acceleration

23
Q

what is the equation for the force on a charged particle when it is between parallel charged plates and derive it

A

E = V/d
F = EQ
F = VQ/d
where V is the voltage across the two plates
this can be combined with F = ma to find the constant acceleration

24
Q

what are the useful equations for a charged particle travelling at right angles to the electric field

A
  • for a ‘tube’ of length L and width D, electric field strength E and a particle of charge Q

t = L/Vh

a = EQ/m

this can also be used alongside the suvat equations to calculate the other quantities

25
Q

what is another useful equation you can use when you want voltage and/or velocity

A 
W = VQ
VQ = 1/2mV^2
eV = 1/2(me)V^2
26
Q

what is a useful bit of evidence to show that electric potential energy occurs

A
  • work must be done against a resistive force to push two like charges together
  • this means energy is being put into the electric potential energy store
27
Q

what is the main equation for electrical potential energy

A

Energy = Qq/4(Eo)(pi)r

28
Q

what does a force-distance graph look like for two like charges as they are pushed together

A
  • it is the shape of a Y = 1/X^2 graph
29
Q

how can we derive the equation for energy

A
  • we want to find the area under the graph between r and infinity
  • so you integrate the graph equation (equation for force)
  • the area under a force distance graph is equivalent to energy
30
Q

define electric potential energy

A

“E is the electrical potential energy for 2 point charges Q and q at a distance r, it is the energy required to completely separate the particles to infinity”

31
Q

what is the real terms meaning of a negative sign when calculating electrical potential energy

A

it means there’s an attractive force

32
Q

Define electric potential

A

“the electric potential, V, at a point is defined as the work done per unit charge in bringing a positive charge q, from infinity to a point r away from a charge Q”

33
Q

what is the equation for electric potential and how to derive it

A 
V = Energy/q = (Qq/4(Pi)(E0)r) x 1/q
V = Q/4(pi)(E0)r
34
Q

what are the units for electric potential

A

Joules per coulomb or Volts

JC^-1 = V

35
Q

Why can an isolated sphere often be thought of as a capacitor

A
  • Capacitors store charge
  • isolated spheres can also store charge
  • therefore isolated spheres are capacitors
36
Q

what is the equation for capacitance including Eo

A

C= (Eo)(Er)A/d

where E = EoEr and Er is the relative permittivity of the dielectric

37
Q

what to remember when defining electric field strengths etc.

A

per unit POSITIVE charge

38
Q

state the equation for charge stored on an isolated sphere and what are the factors on charge stored

A

Q = 4(pi)(Eo)(r)V

charge stored is directly proportional to radius and voltage

39
Q

if a field is said to be going from left to right, which polarity is on the left side and which is on the right

A

positive is on the left

negative is on the right

40
Q

what is always the effect of adding a different dielectric to a vacuum in between plates in a capacitor and why

A
  • it increases the capacitors permittivity, E, because the relative permittivity Er of a dielectric is always greater than 1 and given E = EoEr this must increase E
  • Hence as Q = VC and C = EA/d, the charge stored can increase
41
Q

what is relative permittivity

A

E is the permittivity in a given capacitor
Eo is the permittivity of free space (a vaccuum)
Er is a relative permittivity, this is a ratio of the permittivity of a different dielectric relative to Eo
so a vacuum is 1 by definition
hence
E = EoEr

42
Q

for an isolated sphere, what is the equation for its capacitance, derive it

A

C = Q/V
V = Q/4(pi)(Eo)(r) (electric potential)
so Q = 4(pi)(Eo)(r)(V)

C = 4(pi)(Eo)(r)

Physics Year 13 (15 decks)

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