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Electricity & Magnetism 2 Flashcards

1
Q

Current

A

Rate of change of charge

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2
Q

Electric circuit

A

Charges follow conductive path that forms a closed loop
Electric potential energy transferred from a source to a device in which that energy is stored or converted

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3
Q

Electrons do not escape from metal as

A

They are attracted to its positive ions

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4
Q

With steady electric field E in the conductor, electric force F=qE imposes

A

A small drift in the direction of the force

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5
Q

Microscopic view of Ohm’s law

A

Electrons speed up nd colllide with stationary ions which vibrate around equilibrium position
Much electrical energy wasted as heat instead of moving electrons
Each collision randomly changes direction of motion
Net effect: in addition to radndom motion, there is a very slow net motion, drift velocity CONSTANT

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6
Q

Drift velocity

A

F=ma and F=qe gives a=qe/m
V=u+at but with mean free time

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7
Q

Drift velocity for E=0

A

Typical electron has V0 at random direction so average velocity over many electrons is zero

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8
Q

Drift velocity for E not =0

A

Electric field causes acceleration a=qE/m of every electron

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9
Q

Why is switching on a light almost instantaneous

A

Electric field in metal travels close to c so electrons everywhere start moving with drift velocity

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10
Q

Electron concentration

A

Conductor with n atoms per unit volume
n=Nap/A

=avogadro. Density/ atomic mass

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11
Q

Direction of drift

A

Constant velocity in direction opposite to E

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12
Q

Total charge through cross section A of conductor in time dt

A

dQ=nqAvddt

no= free charge per volume
Avddt= volume

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13
Q

Current is

A

I=dQ/dt=nqAvd =JA where J is current density

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14
Q

Current density

A

J=I/A

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15
Q

Conductivity (sigma)

A

Scalar depending on conductor material

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16
Q

Resistivity

A

1/conductivity ( unit ohm m)

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17
Q

Ohm’s law

A

J=signage=E/p

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18
Q

Does J change

A

Vector
Can vary around a circuit as A changes

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19
Q

Conductivity and thermal conductivity

A

Good electrical conductors are also good thermal conductors

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20
Q

Perfect conductor

A

Zero resistivity

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21
Q

Perfect insulator

A

Infinite resistivity

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22
Q

Greater resistivity,

A

Greater field needed to cause given current density

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23
Q

Non ohmic materials

A

Resistance has non linear response

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24
Q

Resistivity

A

Material property that is temperature dependent , increases with T for metal conductors

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25
For a conductor to have steady state current
Must be part of path that forms a closed loop/complete circuit
26
Four components of electric circuit
Source of electrical power Conductors (wire with low R) A load A means of control (etc)
27
In an electric circuit, charge moves
From higher to lower electric potential (similar to water in fountain due to gravity) Source of emf causes charges to move against electrostatic force
28
Ideal source of emf
Maintains constant potential difference between conductors
29
Real sources of emf
Have different potential difference than E Ring due to charge moving through material of the emf source
30
Voltaic cell
Voltage generated by connecting different metals in circuit due to different electronegativities Daniel cell uses copper and zinc electrodes in electrolytes
31
Idealised voltmeter
Infinite,y large resistance so no current flow
32
Idealised ammeter
Zero resistance so no voltage drop associated with it
33
Closed circuit condition
R —>0 I—> E/Rint
34
Short circuit condition
R=0 Vab =0 I —> infinity for small Rint
35
Open circuit condition
Vab=E I—>0
36
Power
dUdt
37
What does E.I represent
It is rate of work done by forces in source Represents rate of conservation of non electrical energy to electrical energy within the source
38
I^2.Rint
Rate of dissipation of energy
39
Junction
Point where three or more conductors meet
40
Loop
Any closed conducting path
41
Junction rule
Algebraic sun of currents into any junction is zero In +, out -
42
Loop rule
Algebraic sum of potential differences in any loop must equal zero
43
Sign conventions for junction rule
In + Out -
44
Loop rule - sign conventions for emfs
+E travel direction from - to + -E travel direction from + to -
45
Loop rule - sign conventions for resistors
+IR travel opposite to current direction -IR travel in current direction
46
Easier way to set up junction rule
Flows in = flows out
47
Where to start when using Kirchoff’s rules
Highest potential in the loop
48
Thevenin’s theorem
Any combination of batteries and resistors with two terminals can be replaced by a single voltage source e and a single series resistor r
49
Thenevin voltage e
The open circuit voltage between terminals A and B
50
Thenevin resistance r
resistance seen between terminals A and B when voltage sources are replaced with short circuits *take load out and calculate Vab*
51
Thenevin steps
Short all voltage sources Find the equivalent resistance
52
Charge conservation
In junction rule
53
Energy conservation
In loop rule
54
Open loop systems
No way of checking if results have been achieved Eg central heating without a thermostat
55
Closed loop systems
A KE to correct themselves in order to meet target results
56
Feedback
Sensor used to look at the output and adjust the process accordingly
57
Op amp main purpose
To amplify signals
58
How we treat op amp
Like a black box Analysing only it’s external behaviour
59
Three fundamental properties of op amps
1. Gain A (multiplier between differential input and output represented by diamond shape) 2. Input impedance (Rin) resistance at the input of the circuit, very high Output impedance (Rout) resistance seen from output of the circuit, very low
60
Closed loop system response
Distinguish between negative and positive feedback depending on the phase relationship
61
Negative feedback
Portion of the output is combined out of phase with the input Promotes stability and setting to equilibrium
62
How does negative feedback loop reduce perturbations
Fluctuation in input cause flux. in out which fed back to in compensating original fluc so minimising out fluc
63
Examples of negative feedback loop
Thermo regulation (sweating)predator and prey population (more mice = more food for cats, more cats = more mice eaten, less mice = less cats
64
Closed loop positive feedback
Portion of output is combined in phase with the input Minus sign in equation
65
What does positive feedback promote
Instability and chaos through exponential growth
66
Example of positive feedback
Panic buying toilet roll Motivation and success
67
Condition A0B=1 results in
Gain becoming infinite
68
Non inverting op amp
Produces bigger version of original graph
69
Inverting op amp
Bigger and flipped version of original
70
Graph of Vin against Vout
Horizontal- saturation region Slope = gain A0 through linear region Horizontal + saturation region
71
Ideal op amp
Slope is infinite (vertical line) A0= infinity Linear region only occurs at Vin=0 (virtual short circuit)
72
2 golden rules for circuit analysis with op amps
1. Both input terminals are at the same voltage 2. No current flows in or out of either input
73
Transfer function
Independent of gain A Non inverting
74
Non inverting op amp
Vout/Vin=R2+R1/R1
75
Inverting op amp
Vout/Vin = =Rf/Rin
76
Summing op amp
If Rf=R1=R2 then Vout=-[v1+v2]
77
Subtracting op amp
Vout=-[V1-V2]
78
Rules for adding binary numbers
As expected except 1+1=0 carry 1
79
Logic gate
Idealised or physical device implementing a Boolean function Made of transistors or diodes acting as electronic switches
80
AND gate
Read as A,B,AandB 0,0,0 0,1,0 1,0,0 1,1,1 A AND B need to be 1 for A and B to be 1
81
OR gate
If A or B is high then output high
82
XOR gate
Same as or except if both inputs are 1, output 0
83
NAND gate
0,0,1 0,1,1 1,0,1 1,1,0
84
NOR gate
Inverse So 0,0,1 0,1,0 1,0,0 1,1,0
85
Half adder
Performs binary addition on two input numbers
86
Flip flop
Basic data storage Stores a single bit of data
87
SR latch
Made from two cross coupled NAND gates

Physics 1 (10 decks)

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