Brainscape's Knowledge GenomeTM

Browse over 1 million classes created by top students, professors, publishers, and experts.


See full index

Physics 2 > Electricity and Magnetism > Flashcards

Electricity and Magnetism Flashcards

You may prefer our related Brainscape-certified flashcards:
Physics 101 flashcards AP® Physics flashcards
1
Q

how did Faraday discover EM induction

A

a voltage was produced when magnetic field through circuit changed

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

modern version of Faraday’s experiment

A
  1. magnet at rest = no current in coil
  2. magnet moving relative to coil = current induced in coil
  3. second current-carrying coil moving relative to stationary coil = current induced in stationary coil
  4. second current-carrying coil at rest relative to outer coil = current induced in outer coil only when current in inner coil changes
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

magnetic flux

A

similar to electric flux

divide any surface into elements of area dA

magnetic flux through surface element is B.dA

total magnetic flux is sum of these contibutions i.e integral of B.dA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

units of magnetic flux

A

Tm^2 = webber Wb

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Faraday’s law

A

induced emf in a coil is proportional to the negative of the rate of change of magnetic flux through the coil

for coil with N turns, emf is N times bigger

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

emf

A

voltage generated by battery or by magnetic force

energy per unit charge

available by chemical energy for battery, magnetic for induction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

B.A=BAcostheta where theta is

A

the angle between the direction of the area vector (normal to the plane of the coil) and the direction of the magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

what can give rise to an induced emf

A

changing any of B, A or theta

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

right hand rule

A
  1. point thumb of right hand in +ve direction of area vector
  2. induced emf is +ve in direction of curled fingers
  3. positive charges in direction of emf
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

if emf is induced in a closed loop

A

it will cause a current

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

direction of induced conventional current

A

in same direction as induced emf

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

Lenz’s law

A

electrons flowing along wire represented as a current flow (+ to -) in opposite direction

current flow creates a magnetic field around wire

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

how to work out field direction

A

RH grip rule

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

RH grip rule

A

grip wire with right hand
point thumb in direction of (conventional) current flow
fingers wrap in direction of magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

current induced by

A

a change in magnetic flux

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

current induced by a change in magnetic flux creates additional…

A

magnetic field, B induced

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

the induced current produces a magnetic field that tends to…

A

oppose the change in magnetic flux that gave rise to the induction

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

eddy current created when

A

a moving conductor experiences changes in a magnetic field generated by a stationary object

or when a stationary conductor encounters a varying magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

relative motion causes circulating eddies of current within the conductor which in turn…

A

create magnetic fields that oppose the effect of the applied magnetic field

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

eddy currents create losses due to

A

resistive heating (ohmic heating) as the heating power generated in an electrical conductor of resistance E through which a current I is flowing if P prop to I^2R

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

eddy current use

A

brakes on trains

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

how to reduce eddy currents

A

thin laminations (insulated layers) that minimise current flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

induced emf - area of coil is A and magnetic field strength is B so magnetic flux through coil is

A

ΦB = B.A = BAcos theta

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

induced emf - as the loop is rotating uniformly Φ(t)=

A

Φ0 + wt

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

induced emf - according to faraday’s law ε=

A
  • dΦB/dt = BAwsinΦ

this is the basis of AC generators

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

AC generator

A

mechanical energy input to a generator turns the coil in the magnetic field

voltage proportional to the rate of change of the area facing the mag field is generate in the coil

sinusoidal voltage output

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

commercial AC generators

A

single power station sized generator

B field provided by electromagnet to get large B

reverse stationary and rotating bit so rotating B field and stationary coils

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

slidewire generator - flux through the loop is changing because

A

area of the loop in the uniform field is changing as the rod is slid to the right

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

induced emf in slidewire generator

A

A and B point into page so ΦB=BA

area and width increasing uniformly

A(t) = Lw(t) = A0 + Lvt

faraday’s law: ε= - dΦB/dt = -d/dt[B(A0+LVT)] = -BLv

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

magnitude of induced emf in slidewire generator

A

-BvL

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

the -BvL emf was caused only by

A

moving conductor

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

isolated conducting rod emf

A

same emf developed between its ends

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

force on a charge Q in magnetic field B

A

F=Qv x B

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

emf between ends of rod

A

ε= integral of E.dL where path integral is along the rod

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

motional emf - using F=QE

A

sub in E=F/Q to integral of E.dL

and then sub in F= Qv x B and then cancel Qs

so ε=vBL

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

changing flux causes an induced emf and hence current, how?

A

changing magnetic flux causes an induced electric field in the conductor, must be non-conservative since it does net work on a charge as it is driven around loop

i.e. integral E.dL = - dΦB/dt

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

changing current will give rise to

A

changing magnetic field produced by the first circuit and hence a changing magnetic flux through the second

Faraday’s law then tells us there will be an induced current in the second circuit

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

induction between two adjacent coils

A

current 1 in coil 1 creates a magnetic field in region of coil 2

magnetic flux through coil 2 is N2ΦB2 (N is no of turns in coil) which is prop. to B and therefore I1

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

mutual inductance

A

proportionality constant M such that

N2ΦB2=MI1

unit Henry (H)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

what does value of mutual inductance depend on

A

properties (geometric etc) of the two coils

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

reciprocity theorem

A

other way round same thing ie

N1ΦB1=MI2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

solenoid

A

used interchangeably with coil

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

induction happens when

A

current in one of the coupled coils changes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

mutual inductance and emf

A

start with N2ΦB2=MI1

take time derivative of both sides

use faraday’s law to get emf induced in coil 2

(same applies if started with other way round)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

transformers - if current flowing through coil 1 is I1=I0coswt, emf induced in coil 2 is

A

ε2= -MdI1/dt = MI0wsinwt

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

induction applications

A

transformers used: powering low-voltage devices from mains, generation of high voltage eg ignition sparks in car engines

also used: metal detectors, wireless chargers and security tags

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

self inductance L

A

Proportionality factor depends on number of turns, geometry and material inside coil, unit is henry

NΦB=LI

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
48
Q

flux through a single turn

A

ΦB=BA

=μ0NI/l piR^2

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
49
Q

if inductor has a material in its core, we need to

A

replace vacuum magnetic permeability μ0 with μrμ0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
50
Q

if the current in the inductor varies, then so does

A

the flux through its N turns

NΦB=LI

(take time derivs and use faraday’s law)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
51
Q

potential change or voltage across an inductor

A

ε=-N dΦB/dt = -L dI/dt

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
52
Q

electric field lines

A

originate at positive charges and direction is direction of the net field at that point

never cross each other

density of field lines at a location indicates magnitude of field there

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
53
Q

outward electric flux

A

positive charge inside the box

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
54
Q

electric flux

A

analogous to magnetic flux

ΦE= integral E.dA

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
55
Q

Gauss’ law

A

electric flux over the surface of a volume V is prop. to electric charge contained in V

ΦE= integral over surface of V E.dA = Qenclosed/ε0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
56
Q

an inductor opposes…

A

(by generating the induced emf) any change in the
current flowing through it

So work must be done by an external source to change the current

57
Q

resistors vs inductors

A

resistor with current I: energy is dissipated

inductor with current I: energy is stored

58
Q
  • Going round a loop, if an inductor is traversed in the same direction as
    the direction of flow of positive (conventional) current
A

the “potential
change” (the potential at the output node of the inductor minus the
potential at the input node) is

-L dI/dt

59
Q
  • Gauss’s law provides an easy way of finding
A

the electric field
for charge distributions that exhibit a high degree of
symmetry

60
Q

symmetry in some problems allows an intelligent choice of

A

integration (Gaussian) surface

61
Q

General recipe for using Gauss’s law to calculate E

A

1.From the symmetries of the problem, deduce the direction of the
electric field you want to calculate

  1. Set up a surface (“Gaussian surface”), chosen such that the
    calculation of the electric flux through the surface is easy
  2. Compute the electric flux through the Gaussian surface, which will
    involve the unknown electric field E as a variable
  3. Equate the flux to (enclosed charge)/ε0
  4. Calculate E
62
Q

in equilibrium, the electric field inside a conductor is

A

zero, and any excess
charge on the conductor resides solely on its surface

63
Q

the work done by the E field when moving a charge from A to B is

A

path independent

64
Q

the component of the E field that is parallel
to the surface is

A

continuous, the general boundary condition
on the E field

65
Q

electrostatic force is a

A

conservative force

66
Q

work done by the force round a closed path is

A

0

67
Q

potential for a conservative force

A

potential energy per unit charge, volts

68
Q

work done by the electric force when a unit charge moves
from a to b

A

Vab= Va - Vb

69
Q

equipotentials

A

always normal to electric field lines

(No work to move charges
along equipotentials so no
electric field component
along equipotentials)

70
Q

capacitor

A

A capacitor is a device for storing electric charge
* It has two conductors which are electrically separated
* One is positively charged, the other negatively

71
Q

various geometric possibilities for a capacitor including:

A
  • Parallel plates
  • Coaxial cylinders
  • Concentric spheres
72
Q
  • If charges +Q and –Q are present on the two plates there will be a
A

potential difference V between the plates
* The plate with +ve charge is at higher potential

73
Q

The electric field between the plates of a capacitor is proportional to

A

the magnitude of Q

so potential difference between the plates is also proportional to Q

74
Q

capacitance is defined by the ratio

A

C = Q/V

75
Q

SI unit of capacitance

A

Farad

1F=1C/V

1 farad is a huge capacitance

76
Q

the potential difference between the plates is given by

A

V=Ed

77
Q

instead of vacuum, most practical capacitors have

A

an insulating material between the electrodes

such material is called a dielectric

78
Q

why use a dielectric

A
  1. makes it easy to get conductors close together (larger capacitance) without danger of electrical contact
  2. gives bigger capacitance because of what dielectrics do to the electric field inside the capacitor
  3. can withstand higher voltages than air allowing higher stored charge and energy
79
Q

what does a dielectric do?

A

adding a dielectric to a charged capacitor reduced the potential difference between plates

removing it restores potential difference

80
Q

if charge stays the same with a dielectric and voltage reduces then…

A

capacitance must be increased

without dielectric: C0=Q/V0
with: C=Q/V

81
Q

dielectric constant (or relative permittivity) of material

A

εr=C/C0

82
Q

for fixed charge, presence of a dielectric reduces the

A

potential difference by a factor of εr

V=V0/εr

83
Q

for fixed free charge on the plates, if the voltage across the plates decreases then so

A

must the electric field in the gap

E=E0/εr

84
Q

net surface charges

A

all +ve charges slightly shifted one way, -ve other way

surface charges not free to move ie bound charges

**dielectric is polarised **

85
Q

if a dielectric with dielectric constant εr is inserted between the plates, electric field becomes

A

E=E0/εr= Q/A / εrε0

ε=εrε0 is permittivity of the material

86
Q

capacitance for parallel capacitors

A

voltage for both capacitors is same so

Ceq=Q/V = Q1+Q2/V = Q1/V +Q2/V = C1 + C2

87
Q

capacitance for capacitors in series

A

Ceq= Q/V = Q/V1+V2

1/Ceq = V1+V2/Q = V1?Q +V2/Q = 1/C1 +1/C2

88
Q

combining capacitors

A

think of like combining resistors with rules:

parallel - Ceq=C1+C2
series - Ceq= 1/C1 + 1/C2

89
Q

rate of charge transfer to capacitor’s left plate

A

I=dQ/dt

90
Q

RC circuit charging - voltages (loop rule)

A

ε = VR+VC
sub in VR=IR
VC=Q/C

I=dQ/dt

end up with diff.eqn.

91
Q

energy stored in capacitor is energy stored in

A

the electric field

In a plate capacitor, this E-field fills a volume Ad resulting in an enery density

92
Q

energy density in magnetic field

A

uB = 1/2B^2/μ

μ is permeability

93
Q

energy density in electric field

A

uE=1/2 εE^2

ε is permittivity

94
Q

in AC circuits, voltage and current vary

A

sinusoidally

95
Q

AC circuit sinusoidal form

A

V(t)=V0cos(wt+phi0)

96
Q

AC allows more efficient

A

power transmission (because of transformers)

97
Q

phasors

A

graphical representations of AC quantities

vector rotating anticlockwise

instantaneous value is projection onto horizontal axis

98
Q

phasor - resistor across AC source

A

V0/R cos(wt+phi0)
voltage and current in phase
parallel on phasor diagram

99
Q

phasor - inductor across an AC source

A

I(t)=I0cos(wt+phi0)
V(t)=L dI/dt = -LI0w sin(wt+phi0)

voltage leads current by 90 degrees

100
Q

phasor - capacitor across an AC source

A

V(t)=Vocos(wt+phi0)
I(t)=dQ/dt = d/dt CV(t) = -CV0wsin(wt+phi0)

voltage lags current by 90 degrees

101
Q

Lenz’s law tells us that induced emf resists…

A

the buildup of current when a voltage is applied to an inductor

takes a finite time for current to rise to max

so voltage across inductor leads current

102
Q

for a capacitor, the current that flows to charge up capacitor, causes voltage

A

across the capacitor to rise

so voltage lags the current

103
Q

rms voltage

A

Vo/root 2

104
Q

rms values are important for calculating

A

dissipated power when AC signals are used

105
Q

impedance, Z

A

ratio of complex voltage and current

Z=V/I

106
Q

impedance for a resistor

A

Z=R
simply resistance and it is real

107
Q

impedance for inductors and capacitors

A

Z is complex describing both the relative magnitude and phase of voltage and current

108
Q

impedance of an inductor

A

start with sinusoidal current
I=I0exp(iwt)

voltage is V=L DI/dt = iwLI

Z=V/I = iwL

109
Q

impedance of a capacitor

A

start with sinusoidal voltage
V=V0exp(iwt)

Q(t)=CV(t)

I=dQ/dt = iwCV

Z=V/I = 1/iwC

110
Q

Z for resistor

A

Z=R

111
Q

Z for inductor

A

Z=iwL

112
Q

Z for a capacitor

A

Z= 1/iwC

113
Q

phasor - impedance for resistor

A

V and I parallel

114
Q

phasor - impedance for inductance

A

V leads I by 90 degrees

115
Q

phasor - impedance for capacitor

A

V lags I by 90 degrees

116
Q

impedances in series

A

exactly like resistors

117
Q

impedance in parallel

A

exactly like resistors

118
Q

series LCR circuit

A

loop rule
differentiate wrt t
divide by L
use I(t)=dQ(t)/dt

119
Q

series LCR circuit is same as

A

damp, driven harmonic oscillator

120
Q

LC circuit

energy oscillates between being

A

stored in capacitor (in E field) and inductor (B field)

121
Q

impedance analysis

A

impedance of all the components adds up (series)

Z=R+L+C

122
Q

greatest current

A

resasonace

I=V/Z

123
Q

maximum |I| when

A

|Z| is minimal

124
Q

what w when |Z|minimal?

A

w=w0=1/sqrt(LC)

125
Q

expensive way to have voltage source whose voltage is independent of current

A

build huge redundancy into voltage source

126
Q

alternative way to have voltage source whose voltage is independent of current

A

use feedback to keep voltage constant

standard feedback component: op-amp

127
Q

key parts of an op-amp

A

non-inverting input V+
inverting input V-

voltage source,G=open-loop gain

Vout

128
Q

output of opamp is

A

input multiplied by a factor of G

129
Q

ideal open loop gain

A

infinity

130
Q

ideal input resistance

A

infinity

131
Q

ideal output resistance

A

0

132
Q

negative feedback example

A

if bicycle is heading too far right, you steer left

133
Q

non-inverting amplifier

A

voltage source attached to non-inverting part

134
Q

infinite input impedance means

A

no current flowing through inputs

135
Q

infinite G means

A

voltage at inverting and non-inverting inputs are the same

136
Q

exact parameters of op-amp don’t matter as long as

A

G»1
Rin»1
Rout«1

137
Q

wide tolerances of op-amp mean

A

very cheap to manufacture

138
Q
A

Physics 2 (8 decks)

Brainscape helps you reach your goals faster, through stronger study habits.
© 2024 Bold Learning Solutions. Terms and Conditions