Resistance Characteristics

Question 1

Resistance

Answer 1

Voltage per unit Current
Ohm: Volt per Ampere
1Ω = 1VA^-1

Question 2

Base Units: Volt

Answer 2

1V = 1JC^-1
1J = 1Nm
1N = 1kgm^-2
F = ka
1V = 1kgms^-2C^-1
1C = 1As
1V = 1kgms^-2A^-1
1Ω = 1kgm^-3A^-2

Question 3

Experimental Method for IV Characteristics

Answer 3

PD across component must vary
Variable resistor/power supply or potentiometer
Collect pairs of values for voltage and current
Potentiometer: Varied from 0 to Power supply voltage
Variable: Cannot achieve 0V
Variable Power Supply: No other resistor
Voltmeters have infinite resistance (so no current flows)
Ammeters have negligible resistance (so no potential difference)
Draw graph of I on Y-axis and P.D. across component on X-axis

Question 4

Ohmic Conductors

Answer 4

Ohm’s Law: Voltage is directly proportional to current for a metal at constant temperature
IV characteristic is a straight line through the origin
Resistance is constant for a resistor of ohmic conductor
Resistance is 1/Gradient
Non-Directional

Question 5

Filament Lamp

Answer 5

The filament is a thin coil of wire that glows
More current means increased temperature
Electrons collide more frequently with metal ions which are vibrating with a greater amplitude
Resistance is V/I at any point
Non directional so the IV characteristic is the same for -ve values
Not a straight line through the origin so does not obey ohm’s law

Question 6

Thermistor Resistance / Temperature Characteristics

Answer 6

Semi-Conductors (Not Metals)
As the temperature increases for an NTC (Negative temperature coefficient) the resistance of the thermistor decreases
At lower temperatures, there is a large change in R for a change in T (1 degree Celsius)
Makes them more sensitive in fridges than in ovens

Question 7

Experimental Method - Thermistor Characteristics

Answer 7

To obtain the change in resistance with temperature experimentally
Use a water bath to vary the temperature
Record the resistance for each temperature
Use either (Ammeter and Voltmeter) or Ohmmeter
Use V = IR to calculate all resistance characteristics

Question 8

IV Thermistor Characteristics

Answer 8

Not a straight line through the origin -> Does not obey Ohm’s law
As voltage is increased, the temperature increases
The number density of charge carriers increases leading to a higher current
Resistance decreases
Select two points on the curve, use R = V/I at each and compare the resistance values

Question 9

Diode or Light Emitting Diode

Answer 9

Current only flows in one direction
For a diode / LED to work normally, it must be connected in the flow of conventional current flow
Arrow pointing towards the -ve terminal

Question 10

IV Characteristics of Diode or LED

Answer 10

Not a straight line through the origin, so the diode / LED does not obey Ohm’s Law
For voltages less than the threshold voltage, current is 0 (diode does not conduct) - The resistance is ∞
Above the threshold voltage, the diode conducts - The resistance decreases with increasing voltage
Compare two points on the curve, calculate R = V/I and compare differences