chapter 9 - energy power and resistance Flashcards
current
rate of flow of charge
amps
ammeter ( ideally 0 R)
in series the same
in parallel it splits
charge is made up of
electrons in wires
or ions in liquids
kirchoffs first law
total current going into a junction = total current going out due to the conservation of charge
- explains why current splits / rejoins in parallel
calculating current - in terms of charge use
I = Q/t
charge / time
calculating current - in terms of electron use
I = nAve
n - number of free electrons per m3
A - cross sectional area of the wire m2
v - average drift velocity of electrons - m/s
e - charge of an electron - C
charge of an electron
1.6x10-19 C
number of free electrons per m3
number of electrons/volume
using a micrometer to find the diameter and area
- zero the micrometer
- gently clamp the wire between the jaws to get diameter in mm
- in case wire is not uniform repeat in 3 places and average
- convert to m and use pir2
number of electrons
charge flowing / charge of 1 electron
measuring drift velocity in a wire
- set up a circuit with a cell, ammeter and the wire
- measure diameter using micrometer to find area
- measure I
- know n and e
- use v = I/nAe
voltage
Energy that charges are carrying/transfering
2 types of voltage
- electromotive force (emf)
- potential difference (pd)
electromotive force (emf)
- energy gained per unit charge from a battery/power supply (chemical > electrical)
potential difference (pd)
energy given away per unit charge as they pass through a component (from electrical > other types)
voltmeter
- measures voltage
- connected across a component in parallel
- it reads out the difference in energy being carried by the coulombs of charge going in vs out
- an ideal voltmeter has infinite R (prevents charge passing through)
voltage in series vs parallel
SERIES
- supply voltage (emf) splits between components
PARALLEL
- each branch receives the same voltage
kirchoffs 2nd law
the sum of the emf = the sum of the pd in any loop of a circuit due to conservation of energy
voltage equation
V = E/Q
resistance
- restricts the flow of current
- high R = low I > low R = high I
- measured in Ohms which can be defined as Volt per Amp
- all components have some resistance
resistance is affected by:
- type of material/ RESISTIVITY
- LENGTH - longer wire = more resistance R ∝L
- AREA - thinner = more resistance R ∝1/A
- TEMP - see notes on lamp/thermistor
resistance equations
R = pL /A
R = V/I (just a convenient way to calc R - V+I don’t actually affect R)
resistors in series
INCREASES overall resistance
Rt = R1 + R2 + R3
resistors in parallel
DECREASES overall reistance (more routes to take)
- if two identical resistors Rt is half one of them
- two non identical use “product over sum” Rt = R1R2/R1+R2
- any resistors use 1/Rt = 1/R1 + 1/R2 + 1/R3…
experiment to measure the resistivity of a material
- measure d with a micrometer (zero it, 3 times and average)
- calculate area using pi r^2
- measure length with a ruler (eye level) (use low I to avoid heating)
- measure R with an Ohmeter
- repeat for several diff lengths
- plot R and L - R = p/A L
- gradient = p/A
- p = gA
Fixer resistor
obeys ohms law
“Voltage is proportional to current providing temp and dimensions are constant”
Filament
resistance increases due to temp increasing (current has a heating effect due to electrons colliding with atoms so they vibrate faster)
ions in the filament vibrate faster causing more freq collisions with electrons
filament lamp graph
S shape graph
for each increase in voltage current goes up by a smaller amound
LED
- more efficient and robust than filament lamp
- only allows current to flow if “forward biased” and above threshold voltage
- otherwise has infinite R
LED graph
I is zero until threshold voltage then straight line up
thermistor / LDR
- heat/light energy releases more electrons per m^3 in the semiconducting material (n increases) so more current will flow (I = nAve) and therefore resistance will decrease as R = V/I
thermistor / LDR graph
downward curve (resistance over temp / LI)
component graphs watch out for
switched axes
component graphs watch out for
switched axes
explain the variation of resistance with pd for the filament bulb in terms of particle behaviour
- as V ^ velocity of electrons ^
- greater energy transfer in collisions
- increases the temp of filament ions
- amplitude of vibrations increases
- collision rate between ions/electros ^
- due to R ^ current doesn’t increase in proportion to V
explain how the graph shows this is a diode
- conducts in one direction only
- conducts when pd is beyond threshold voltage eg 0.2V
- in reverse direction resistance is very high/ infinite
how to get an IV graph for components
- measure I and V for a range of values
- vary V across the component
- reverse the supply or polarity (if its a diode graph)
- repeat and average
(circuit with a variable power supply, ammeter, component, voltmeter)
power
rate of transfer of energy
- watts or J/s
- for electrical power it’s a measure of how bright/hot / fast a component is
power equations
P = E/t
P = IV
P = I^2 R
P = V^2 /R
power of lamps experiment
connect 2 diff lamps (A and B) in series and parallel
measure I and V across each lamp
use P= IV
- in series the higher R lamp recieved more V so is brighter
- in parallel the higher R gets smaller I so is dimmer
energy (big)
if we leave a component in it transfers electrical energy to other types
energy equation
E = ItV
kiloWatt hour
used instead of J as for household appliances J is too big
(use E = Pt with power in ke and t in hours)
1kWh = 3 600 000 J
energy (small)
as one coulomb of charge passes through a 9V battery it gains 9J of charge
1 volt in J/C
1 J/C
- for every coulomb of charge that passes though a battery of x volts it gains x joules
electron volts
energy transferred to/ from an electron when it passes through a pd of 1V
eg if an electron passes though the 9V battery it gains 9eV
1eV = 1.6x10^-19 J
electron gun
speeds up electrons with an accelerating pd (see diagram)
- thermionic emission causes electrons to escape the coil
- electrons are attracted to the anode
KE = 1/2mv^2
so V*e = 1/2mv^2
max power
when internal resistance = R
