Module 4 Resistance Characteristics Flashcards
Definition of resistance?
voltage per unit current
Definition of Ohm
Volt per Ampere
Method for finding IV/resistance characteristics for a specific component
Put an ammeter, variable resistor (a rheostat or variable power source may be used instead), and the component in series with a voltmeter in parallel
Use the variable resistor to change the resistance of the circuit and therefore the current and pd across the component
Use the ammeter and voltmeter to take readings of current and PD, note them and use this to draw a graph with I on the y and PD on the x
What is the advantage of using a potentiometer over a variable resistor when finding the characteristics of a component?
that the pd across the component can be varied from 0V to max (the output of the power supply)
What is an ohmic conductor?
a fixed resistor (obeys Ohm’s law) or a wire at constant temperature
What is Ohm’s law?
voltage is directly proportional to the current for a metal at a constant temperature
When plotting a graph for the IV characteristics of a fixed resistor, what characteristics can be observed?
straight line through the origin
As resistance is constant
Resistance can be found by using 1/gradient - gradient has one value, therefore so does resistance
How could you find negative side of the line of a graph of IV characteristics of a ohmic conductor?
switch the -ve and +ve of the power supply
Explain the IV characteristics of a
filament lamp
As more current passes through the filament, the temperature rises
Electrons collide more frequently with metal ions leading to an increase in resistance
How does a graph of IV characteristics show that a filament lamp does not obey Ohms law?
not a straight line through the origin
Describe an experiment to observe the variation of resistance with temperature of a thermistor
Place a thermistor in series with an ohmmeter in a water bath
Use the water bath to vary temperature
Record temperature using a thermometer and the resistance using the ohmmeter
Plot results on a graph with temp on the x and resistance on the y-axis
Describe and explain the IV characteristics of a thermistor
Not a straight line through the origin (does not obey ohms law)
As voltage increases so does temperature, more charge are released leading to a higher number density so current increases
Resistance decreases
In terms of in a circuit, what makes Diodes unique?
current only travels in one direction
diodes have a threshold or striking voltage (no current below this)
Describe and explain the IV characteristics of an LED
Not a straight line through the origin (does not obey ohms law)
Below the threshold voltage resistance is infinite so no current is detected
Above the threshold voltage, resistance decreases with increasing voltage
What are the two experimental ways which can be used to determine the point where a current is conducted through an LED?
Use a dark room to determine point where LED first lights up
Use an ammeter to determine when a current is conducted
