Electricity, Resistance, DC Circuits, Capacitors, Materials Flashcards
[def] Electric Current
Rate of flow of charge
UNIT: A or Cs-1
A= ΔQ/ Δt
Charge through conductors?
Conduction in metals from drift of free e-
I=nAve define
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
Derive I=nAve (x6 lines)
Length =vΔt
Volume =Area* length =AvΔt
e- in wire = nAvΔt
Charge in wire Q=nAvΔte
I=Q/t
I=(nAvΔte)/Δt
I=nAvevt graph for e- in wire explained
x3
Constant grad as pd remains the same
Distance= area underneath
v rapidly down when e- collide with wire
Why light switches on immediately?
x3
electrons all in wire
battery switched on = electric field in wire
electrons begin to move immediately
[def] Potential Difference (pd), V
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]
[def] Ohm’s law
The current in a metal wire AT CONSTANT TEMPERATURE is proportional to the pd across it
V=IR
[def] Electrical Resistance, R
Resistane of a conductor is the pd placed across it divided by the resulting current through it
R= V/I
UNIT: Ω = VA-1
Filament lamp IV graph
explained
increased current =increased vibrations from collisions =increased temp =collisions more likely =increased resistance
= shallower grad (when I =y axis)
Metal wire at constant temp graph
Described
Linear, through origin
LED graph
Described
Explained
Rapid increase in I when moves past bias voltage (1.8V for Red LED)
–> or prevents certain direction of current flow
Resistance Factors (x3, one explained)
Thick wire= more free e- to conduct = less R
R ∝ 1/CSA
R ∝ ρ
Why resistance?
Collisions between free electrons and large +ve ions
Conductors and resistance?
Conductors = more free e- = less resistance
Resistance and temp?
Resistance INCREASES with temp (more energy, more vibrations, more collisions)
Resistance variation trend in metals
Resistance of metals varies almost linearly with temp over a wide range
LDR
+Light = free e- travel
so resistance DECREASES with LIGHT
symbol: resistor rectangle, in circle, two arrows towards
Thermistor
resistance DECREASES with HEAT
symbol: resistor rectangle, line through on diagonal with flat base
Variable resistor symbol
resistor rectangle, with arrow through
Superconductors
What?
How?
EG?
Loose all electrical resistance when BELOW a certain temp
Current can continue flowing for a very long time
EG Tin
[def] Superconducting Transition Temperature, Tc
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
Typical Superconducting Transition Temp?
Most metals show superconductivity, with transition temps a few degrees above absolute zero (-273 degC)
~special~ super conducting transition temps?
HIGH TEMPERATURE SUPERCONDUCTORS have transition temps above the boiling point of nitrogen (-196 degC)
therefore use liquid nitrogen to make cheaper
Kirchoff’s 1st Law
Sum of currents = 0 at a junction
Kirchoff’s 2nd Law
Sum of EMF = sum of IR in a closed loop
[voltage supplied = voltage used in a closed loop]
Kirchoff’s laws and x2 things to calculate circuits
Batteries (-ve to +ve): END ON +VE = +ve voltage
Resistor: Through resistor in same direction as current = -ve voltage
Current in a circuit: general,
series/ parallel
What is conserved?
Current splits and each branch depending on resistance
Series: same
Parallel: splits
CONSERVATION OF CHARGE
[def] Law of conservation of charge
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
Voltage in a circuit:
series/ parallel
What is conserved?
Series: Shared amongst components (sum= pd across supply)
Parallel: equal to pd across supply, same across all components
CONSERVATION OF ENERGY
Resistance in a circuit:
series/ parallel
Series: Rtotal = R1 +R2 +R3
Parallel: 1/Rtotal = 1/R1 + 1/R2 + 1/R3
Rtotal = product/sum (for only 2 resistors in parallel
[def] EMF
ENERGY CONVERTED from some other form (eg chemical) to ELECTRICAL POTENTIAL ENERGY,
per coulomb of charge flowing through the source
UNIT: V
Potential Divider
A combination of resistors in series connected across a voltage source to produce a required pd
Potential divider usag (x2)
to calibrate sensors
- -> Light (LDR) turns on in darkness
- -> Temperature sensor (from thermistor)
Use these items as one of the resistors
Internal Resistance equation
EMF (ε)
ε= IR +Ir
EMF cells:
series / parallel
Cells in series: EMF added together
Cells in parallel: EMF same
[LIKE VOLTAGE]
Electrons? Conventional current?
E- drift in opposite direction to conventional current
–> remember when drawing I=nAve diagram
Drift Velocity and temperature changes?
Drift velocity HIGHER at COLDER temps
- ->Ions vibrate less @lower temps
- -> so less freq collisions between e- and ions
Current and temperature changes?
Current DECREASES with HOTTER temps
- -> e- flow obstructed as e- collide with ions
- -> higher chance of collisions with more temperature because larger ion vibrations
Adding a component in parallel
V
R
I
Voltage across one will fall
- -> lower R in parallel bit
- -> Lower total R
- -> therefore increased current
Resistance across a voltmeter
assumed to be INFINITE
