1.10- Resistance and Resistivity

Question 1

State Ohms law

Answer 1

The current through a metallic conductor is directly proportional to the applied p.d PROVIDED the temperature is constant

Question 2

What is an ‘Ohmic’ conductor?

Answer 2

Materials that obey ohms law

Question 3

What would you observe in the graph of an Ohmic conductor?

Answer 3

A straight line through the origin

Question 4

P=IV substituting out I

Answer 4

P=V^2/R

Question 5

Equation for power substitung out voltage

Answer 5

P=I^2 X R

Question 6

What is an Ohmic material?

Answer 6

A current that obeys ohms law

Question 7

What is non-ohmic behaviour? Give an example when this would occur

Answer 7

A material that doesn’t obey ohms law
Filament lamp- As current increases temperature rises ( ohms law requires Constant temp )

Question 8

What is a diode?

Answer 8

A semiconductor which has very low resistance ( ideally 0 ) one way and really high ( ideally infinite) resistance another way

Question 9

What I-V characteristics curve would you obtain when connected across a diode

Answer 9

The diode will barely conduct until 0.6 volts is applied then the graph will go straight up

Question 10

When is a diode forward biased and when is it reverse biased. What do these terms mean?

Answer 10

Forward- when the positive terminal is connected to the positive end
Reverse- when the positive terminal is connected to the negative end

When reverse biased the diode conducts poorly so the current will remain small as voltage is increased

Question 11

What does ntc thermistor stand for

Answer 11

Negative temperature coefficient thermistor

Question 12

What happens to the resistance of an ntc thermistor as it heats up?

Answer 12

It decreases

Question 13

Describe the I-V characteristics curve you would expect from an ntc thermistor

Answer 13

Slowly increasing from the origin to a steep slope upwards

Question 14

KEY EXPERIMENTS

Answer 14

Pg 70, Pg 75,

Question 15

How and why do thermistors behave so differently than metals

Answer 15

In metals, as temperature increases resistance increases, this can be explained as the atoms vibrate more frequently than before, this reduces the average drift speed therefore increasing the time it takes them to reach the end of the circuit. This means that for a given P.D the current is reduced

In thermistors this same effect happens, However, only a small amount of energy binds the electrons to the termistor’s atoms therefore when the vibration is sufficient many electrons can break free giving a HUGE increase in current as more free charge carriers are present. This effect is much greater than the reduction in net drift so there is a net reduction in the resistance of a thermistor

Question 16

What can thermistors be used for

Answer 16

As thermometers
As part of electronic circuits which are switched on and off with changing temp

Question 17

Circuits

Answer 17

Pages 71-73

Question 18

What does the resistance of a metallic conductor ( at constant temp ) depend on?

Answer 18

The length
The cross sectional area
The material

Question 19

What the is the relation between length and resistance?

Answer 19

Directly proportional

Question 20

What is the relation between cross sectional area and resistance?

Answer 20

Inversely proportional, as the area of the cross section doubles, resistance is halved

Question 21

How could you obtain a straight graph of resistance and cross sectional area?

Answer 21

Plot R= k(1/A) where:
R= resistance
K=constant
A= CSA

Question 22

Equation for resistance

Answer 22

R=P(L/A) where:
R= resistance
P= resistivity
L= length
A= CSA

Question 23

Definition of resistivity

Answer 23

Numerically equal to the resistance of a sample of the material 1m long and of CSA 1m^2

Question 24

Around what resistivity would a good conductor have?

Answer 24

1X10^-8 ohms per meter

Question 25

around what resistivity would a good insulator have?

Answer 25

1X10^15 ohms per meter

Question 26

What is a superconductor?

Answer 26

A material which loses all its electrical resistivity to become a perfect conductor when below its critical temperature

Question 27

Outline some applications of superconductors

Answer 27

MRI scanners magnetic fields
Magnetic levitation transport to eliminate most friction between the train and the track