P9 Flashcards
(31 cards)
pd
energy transferred per coulomb of charge
i.e when electrical energy is transferred to other forms e.g thermal energy
work is done by charge carriers so essentially they LOSE energy as they pass through components
emf
total energy gained by charge carriers per coulomb of charge
work is done ON the charge carriers e.g gaining energy when passing through battery
more emf means more energy per coulomb transferred from chemical to electrical energy
emf equation
E = W/Q
Q = charge
W = energy transferred by charge
usually total pd = total emf
W = VQ or W = EQ
How do electron guns work
Small metal filament heated by electric current
electrons gain ke, some gain enough to escape from metal surface - thermionic emission
placed in vacuum and high pd between filament and anode then filament is cathode, so electrons accelerate to anode gaining ke
if anode has small hole then electrons can pass through creating a beam with specific ke
equation linking current energy to kinetic energy
eV = 1/2mv^2
work done on electron/ charge carriers = gain in kinetic energy
what is resistance
all components resist the flow of charge carriers through them
energy is required to push electrons through a component, and resistance shows how much energy is required to do so (probably energy per coulomb of charge)
Ohms law
for a metal conductor at constant temperature the current in wire is directly proportional to pd across ends
Why does resistance vary with temperature
More heat means positive ions in wire have more internal energy and vibrate higher amplitude around mean positions.
Collisions between charge carriers and positive ions increase so charge carriers do more work - transfer and ‘lose’ more energy as they travel through the wire, hence more resistance
to investigate IV characteristics circuit set up
connect battery ammeter component and variable resistor in series, and voltmeter parallel to component
Fixed resistors (including e.g wires) IV graph
Direct proportion line - ohmic conductor as obeys law
behaves same way regardless of polarity
wires can be seen as resistors with very low resistance
Filament lamp IV graph
Increases with decreasing rate of increase (opposite for 3rd quadrant)
non-ohmic, unaffected by polarity
because it heats up
Diode
only allows current in one direction
LED
emit light when they conduct
of a single specific wavelength i.e colour
different threshold pd related to colour
take very little energy to run so often use as indicators of current direction - light up shows that part of circuit is live
LED IV graph
flat in 2nd quadrant then exponential rise after about x = 0.7 (for silicon, thresholds vary slightly)
resistance very high (aka infinite) when voltage is wrong direction
non-ohmic, non-constant resistance
resistivity
electrical property of a material
4 factors that affect resistance in wire
temperature
material
length
CSA
what is resistivity
electrical property of a material - which affects resistance
equations for resistance and resistivtt
R = pL/A
or
p = RA/L
(p is resistivity)
Experiment to find resistivity
Find I, V and then R of a wire with different lengths then draw a graph where the gradient will be p/A (because p = gradient x A)
so times gradient by CSA to get resistivity
thermistor function
semiconductor component with a negative temperature coefficient i.e resistance drops as temperature increases
as more electrons are freed so number density of charge carriers increases - less R more I
uses of thermistors
thermometers and thermostats
monitor and measure temp - device components/ engines from overheating, electrical homeware devices
how to detect thermistor resistance setup
put thermistor in parallel with ohmmeter then measure resistance at different temperatures using a water bath
usually resistance change will be biggest over a small range of temperatures
thermistor IV graph
positive correlation, unaffected by polarity, increases with an increasing rate of increase
similar to lamp but curved other way
LDR function
higher light intensity means more electrons freed so higher number density - less R more I