4.3 Resistance and Resistivity Flashcards
(30 cards)
How does voltage exist in series?
Voltage = sum of pd across each resistor V = V1 + V2...
How does current exist in series?
Constant
How do you measure resistance of a series circuit?
total resistance = sum of each resistor
R = R1 + R2…
How does voltage exist in parallel?
constant
How does current exist in parallel?
total current = sum of current across each resistor
How do you measure resistance of a parallel circuit?
1/R = 1/R1 + 1/R2
What is resistance?
the pd across the component divided by the current passing through it
What affects resistance at a given temperature?
material of the wire
length of the wire
cross sectional area of the wire
What is resistivity?
product of the resistance and cross-sectional area of a wire divided by its length
What does a metal consist of?
a lattice of metal ions and free moving electrons
What is the cause of resistivity?
moving electrons being obstructed by vibrating ions
Why does temperature increase resistivity?
ions vibrate more and obstruct the electrons more as there are more frequent collisions between the electrons and ions
How do impurities affect resistivity?
increase it, obstruct movement of electrons
What does the metal get hotter?
when electrons collide with metal ions they lose some of their kinetic energy and drift velocity decreases. the ions gain energy and vibrate more, making the metal hotter
How do semi-conductors differ from conductors?
their resistivity decreases with temperature
Why does a semi-conductors resistivity decrease with temperature?
more electrons can escape from their atoms and become conducting electrons, making them a better conductor by increasing number density of electrons
What is Ohm’s law?
the electric current through a conductor is proportional to the pd across it, as long as the temperature remains constant
What is an ohmic component?
one which obeys ohm’s law. Has constant resistance
What is a non-ohmic component?
does not obeys ohm’s law. no constant resistance
What is the resistance when there is 0 current?
infinite
Give 3 examples of non-ohmic components:
filament bulb
diode
thermistor
Why is a filament bulb a non-ohmic component?
as I increases, temp increases. V increases more than I at higher V so R increases
In which direction does the arrow of a diode have to face?
towards conventional current
In a diode R remains infinite until
voltage reaches threshold voltage
