Electricity, Resistance, DC Circuits, Capacitors, Materials

Question 1

[def] Electric Current

Answer 1

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

A= ΔQ/ Δt

Question 2

Charge through conductors?

Answer 2

Conduction in metals from drift of free e-

Question 3

I=nAve define

Answer 3

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

Question 4

Derive I=nAve (x6 lines)

Answer 4

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

Question 5

vt graph for e- in wire explained

x3

Answer 5

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

Question 6

Why light switches on immediately?

x3

Answer 6

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

Question 7

[def] Potential Difference (pd), V

Answer 7

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]

Question 8

[def] Ohm’s law

Answer 8

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

V=IR

Question 9

[def] Electrical Resistance, R

Answer 9

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

Question 10

Filament lamp IV graph

explained

Answer 10

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

= shallower grad (when I =y axis)

Question 11

Metal wire at constant temp graph

Described

Answer 11

Linear, through origin

Question 12

LED graph
Described
Explained

Answer 12

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

Question 13

Resistance Factors (x3, one explained)

Answer 13

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

Question 14

Why resistance?

Answer 14

Collisions between free electrons and large +ve ions

Question 15

Conductors and resistance?

Answer 15

Conductors = more free e- = less resistance

Question 16

Resistance and temp?

Answer 16

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

Question 17

Resistance variation trend in metals

Answer 17

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

Question 18

LDR

Answer 18

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

symbol: resistor rectangle, in circle, two arrows towards

Question 19

Thermistor

Answer 19

resistance DECREASES with HEAT

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

Question 20

Variable resistor symbol

Answer 20

resistor rectangle, with arrow through

Question 21

Superconductors
What?
How?
EG?

Answer 21

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

Question 22

[def] Superconducting Transition Temperature, Tc

Answer 22

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

Question 23

Typical Superconducting Transition Temp?

Answer 23

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

Question 24

~special~ super conducting transition temps?

Answer 24

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

Question 25

Kirchoff's 1st Law

Answer 25

Sum of currents = 0 at a junction

Question 26

Kirchoff's 2nd Law

Answer 26

Sum of EMF = sum of IR in a closed loop [voltage supplied = voltage used in a closed loop]

Question 27

Kirchoff's laws and x2 things to calculate circuits

Answer 27

Batteries (-ve to +ve): END ON +VE = +ve voltage Resistor: Through resistor in same direction as current = -ve voltage

Question 28

Current in a circuit: general, series/ parallel What is conserved?

Answer 28

Current splits and each branch depending on resistance Series: same Parallel: splits CONSERVATION OF CHARGE

Question 29

[def] Law of conservation of charge

Answer 29

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

Question 30

Voltage in a circuit: series/ parallel What is conserved?

Answer 30

Series: Shared amongst components (sum= pd across supply) Parallel: equal to pd across supply, same across all components CONSERVATION OF ENERGY

Question 31

Resistance in a circuit: | series/ parallel

Answer 31

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

Question 32

[def] EMF

Answer 32

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

Question 33

Potential Divider

Answer 33

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

Question 34

Potential divider usag (x2)

Answer 34

to calibrate sensors - -> Light (LDR) turns on in darkness - -> Temperature sensor (from thermistor) Use these items as one of the resistors

Question 35

Internal Resistance equation

Answer 35

EMF (ε) | ε= IR +Ir

Question 36

EMF cells: | series / parallel

Answer 36

Cells in series: EMF added together Cells in parallel: EMF same [LIKE VOLTAGE]

Question 37

Electrons? Conventional current?

Answer 37

E- drift in opposite direction to conventional current --> remember when drawing I=nAve diagram

Question 38

Drift Velocity and temperature changes?

Answer 38

Drift velocity HIGHER at COLDER temps - ->Ions vibrate less @lower temps - -> so less freq collisions between e- and ions

Question 39

Current and temperature changes?

Answer 39

Current DECREASES with HOTTER temps - -> e- flow obstructed as e- collide with ions - -> higher chance of collisions with more temperature because larger ion vibrations

Question 40

Adding a component in parallel V R I

Answer 40

Voltage across one will fall - -> lower R in parallel bit - -> Lower total R - -> therefore increased current

Question 41

Resistance across a voltmeter

Answer 41

assumed to be INFINITE