Chapter 22 - Electric Fields Flashcards by Ollie Cooke

What is an electric (or electrostatic) field

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

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

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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do electrons and protons have electric fields

yes, they are charged objects

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define electric field strength

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

E = F/Q

units of E are Nc^-1

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Is electric field strength a vector or scalar

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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how do we use electric field lines to map electric fields

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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what is the value of E0

8.85x10^-12

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what is the equation linked to coulombs law and any proportionalities

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

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where can coulombs law be applied

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

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describe the method to investigate coulombs law

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

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

where Q is the charge on the sphere

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

Gravity - mass, and always attractive

Electric Fields - like charges repel, opposite charges attract

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

F = EQ

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what is the equation linking work done and voltage

W = VQ

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what is the equation linking electric field strength and voltage

E = V/d

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

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

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

area, Capacitance = kA

- distance, capacitance = k/d

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

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

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

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

How can you analyse the results of the Eo practical

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

C = E0ErA/d

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

it experiences a constant force due to F = EQ

- this means it experiences a constant acceleration

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

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

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

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

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

``` W = VQ VQ = 1/2mV^2 eV = 1/2(me)V^2 ```

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

- 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

what is the main equation for electrical potential energy

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

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

- it is the shape of a Y = 1/X^2 graph

how can we derive the equation for energy

- 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

define electric potential energy

"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"

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

it means there's an attractive force

Define electric potential

"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"

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

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

what are the units for electric potential

Joules per coulomb or Volts | JC^-1 = V

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

- Capacitors store charge - isolated spheres can also store charge - therefore isolated spheres are capacitors

what is the equation for capacitance including Eo

C= (Eo)(Er)A/d where E = EoEr and Er is the relative permittivity of the dielectric

what to remember when defining electric field strengths etc.

per unit POSITIVE charge

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

Q = 4(pi)(Eo)(r)V charge stored is directly proportional to radius and voltage

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

positive is on the left | negative is on the right

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

- 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

what is relative permittivity

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

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

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

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Chapter 22 - Electric Fields Flashcards

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