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Physics Component 2 > Electricity, Resistance, DC Circuits, Capacitors, Materials > Flashcards

Electricity, Resistance, DC Circuits, Capacitors, Materials Flashcards

1
Q

[def] Electric Current

A

Rate of flow of charge
UNIT: A or Cs-1

A= ΔQ/ Δt

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2
Q

Charge through conductors?

A

Conduction in metals from drift of free e-

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3
Q

I=nAve define

A 
I = Current (A)
n = number for free electrons PER UNIT VOLUME (m-3)
A= Cross sectional area (m2)
v= drift velocity (ms-1_
e= charge of a single charge carrier (eg electron) C
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4
Q

Derive I=nAve (x6 lines)

A 
Length =vΔt
Volume =Area* length =AvΔt
e- in wire = nAvΔt
Charge in wire Q=nAvΔte 
                                              I=Q/t
I=(nAvΔte)/Δt
I=nAve
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5
Q

vt graph for e- in wire explained

x3

A

Constant grad as pd remains the same
Distance= area underneath
v rapidly down when e- collide with wire

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6
Q

Why light switches on immediately?

x3

A

electrons all in wire
battery switched on = electric field in wire
electrons begin to move immediately

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7
Q

[def] Potential Difference (pd), V

A

The pd between twho points is the ENERGY CONVERTED from electrical potential energy to some other form PER COLOUMB of charge flowing from one point to the other
UNIT: V = JC-1
[Energy transferred per unit charge]

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8
Q

[def] Ohm’s law

A

The current in a metal wire AT CONSTANT TEMPERATURE is proportional to the pd across it

V=IR

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9
Q

[def] Electrical Resistance, R

A

Resistane of a conductor is the pd placed across it divided by the resulting current through it
R= V/I
UNIT: Ω = VA-1

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10
Q

Filament lamp IV graph

explained

A 
increased current
=increased vibrations from collisions
=increased temp
=collisions more likely
=increased resistance

= shallower grad (when I =y axis)

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11
Q

Metal wire at constant temp graph

Described

A

Linear, through origin

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12
Q

LED graph
Described
Explained

A

Rapid increase in I when moves past bias voltage (1.8V for Red LED)
–> or prevents certain direction of current flow

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13
Q

Resistance Factors (x3, one explained)

A

Thick wire= more free e- to conduct = less R
R ∝ 1/CSA
R ∝ ρ

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14
Q

Why resistance?

A

Collisions between free electrons and large +ve ions

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15
Q

Conductors and resistance?

A

Conductors = more free e- = less resistance

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16
Q

Resistance and temp?

A

Resistance INCREASES with temp (more energy, more vibrations, more collisions)

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17
Q

Resistance variation trend in metals

A

Resistance of metals varies almost linearly with temp over a wide range

18
Q

LDR

A

+Light = free e- travel
so resistance DECREASES with LIGHT

symbol: resistor rectangle, in circle, two arrows towards

19
Q

Thermistor

A

resistance DECREASES with HEAT

symbol: resistor rectangle, line through on diagonal with flat base

20
Q

Variable resistor symbol

A

resistor rectangle, with arrow through

21
Q

Superconductors
What?
How?
EG?

A

Loose all electrical resistance when BELOW a certain temp
Current can continue flowing for a very long time
EG Tin

22
Q

[def] Superconducting Transition Temperature, Tc

A

The temperature at which a material, when cooled, looses all its electrical resistance, and becomes superconducting.
Some materials eg COPPER NEVER become superconducting, no matter how low the temperature becomes

23
Q

Typical Superconducting Transition Temp?

A

Most metals show superconductivity, with transition temps a few degrees above absolute zero (-273 degC)

24
Q

~special~ super conducting transition temps?

A

HIGH TEMPERATURE SUPERCONDUCTORS have transition temps above the boiling point of nitrogen (-196 degC)
therefore use liquid nitrogen to make cheaper

25
Q

Kirchoff’s 1st Law

A

Sum of currents = 0 at a junction

26
Q

Kirchoff’s 2nd Law

A

Sum of EMF = sum of IR in a closed loop

[voltage supplied = voltage used in a closed loop]

27
Q

Kirchoff’s laws and x2 things to calculate circuits

A

Batteries (-ve to +ve): END ON +VE = +ve voltage

Resistor: Through resistor in same direction as current = -ve voltage

28
Q

Current in a circuit: general,
series/ parallel

What is conserved?

A

Current splits and each branch depending on resistance

Series: same
Parallel: splits

CONSERVATION OF CHARGE

29
Q

[def] Law of conservation of charge

A

Electric charge cannot be created or destroyed
(though positive and negative charges can neutralize each other)
charges cannot pile up at a point in a circuit

30
Q

Voltage in a circuit:
series/ parallel

What is conserved?

A

Series: Shared amongst components (sum= pd across supply)

Parallel: equal to pd across supply, same across all components

CONSERVATION OF ENERGY

31
Q

Resistance in a circuit:

series/ parallel

A

Series: Rtotal = R1 +R2 +R3

Parallel: 1/Rtotal = 1/R1 + 1/R2 + 1/R3
Rtotal = product/sum (for only 2 resistors in parallel

32
Q

[def] EMF

A

ENERGY CONVERTED from some other form (eg chemical) to ELECTRICAL POTENTIAL ENERGY,
per coulomb of charge flowing through the source

UNIT: V

33
Q

Potential Divider

A

A combination of resistors in series connected across a voltage source to produce a required pd

34
Q

Potential divider usag (x2)

A

to calibrate sensors

  • -> Light (LDR) turns on in darkness
  • -> Temperature sensor (from thermistor)

Use these items as one of the resistors

35
Q

Internal Resistance equation

A

EMF (ε)

ε= IR +Ir

36
Q

EMF cells:

series / parallel

A

Cells in series: EMF added together
Cells in parallel: EMF same

[LIKE VOLTAGE]

37
Q

Electrons? Conventional current?

A

E- drift in opposite direction to conventional current

–> remember when drawing I=nAve diagram

38
Q

Drift Velocity and temperature changes?

A

Drift velocity HIGHER at COLDER temps

  • ->Ions vibrate less @lower temps
  • -> so less freq collisions between e- and ions
39
Q

Current and temperature changes?

A

Current DECREASES with HOTTER temps

  • -> e- flow obstructed as e- collide with ions
  • -> higher chance of collisions with more temperature because larger ion vibrations
40
Q

Adding a component in parallel
V
R
I

A

Voltage across one will fall

  • -> lower R in parallel bit
  • -> Lower total R
  • -> therefore increased current
41
Q

Resistance across a voltmeter

A

assumed to be INFINITE

Physics Component 2 (2 decks)

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