chapter 9 - energy power and resistance

Question 1

current

Answer 1

rate of flow of charge
amps
ammeter ( ideally 0 R)
in series the same
in parallel it splits

Question 2

charge is made up of

Answer 2

electrons in wires
or ions in liquids

Question 3

kirchoffs first law

Answer 3

total current going into a junction = total current going out due to the conservation of charge
- explains why current splits / rejoins in parallel

Question 4

calculating current - in terms of charge use

Answer 4

I = Q/t
charge / time

Question 5

calculating current - in terms of electron use

Answer 5

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

Question 6

charge of an electron

Answer 6

1.6x10^-19 C

Question 7

number of free electrons per m3

Answer 7

number of electrons/volume

Question 8

using a micrometer to find the diameter and area

Answer 8

• 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

Question 8

number of electrons

Answer 8

charge flowing / charge of 1 electron

Question 9

measuring drift velocity in a wire

Answer 9

• 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

Question 10

voltage

Answer 10

Energy that charges are carrying/transfering

Question 11

2 types of voltage

Answer 11

• electromotive force (emf)
• potential difference (pd)

Question 12

electromotive force (emf)

Answer 12

• energy gained per unit charge from a battery/power supply (chemical > electrical)

Question 13

potential difference (pd)

Answer 13

energy given away per unit charge as they pass through a component (from electrical > other types)

Question 14

voltmeter

Answer 14

• 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)

Question 15

voltage in series vs parallel

Answer 15

SERIES
- supply voltage (emf) splits between components
PARALLEL
- each branch receives the same voltage

Question 16

kirchoffs 2nd law

Answer 16

the sum of the emf = the sum of the pd in any loop of a circuit due to conservation of energy

Question 17

voltage equation

Answer 17

V = E/Q

Question 18

resistance

Answer 18

• 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

Question 19

resistance is affected by:

Answer 19

• type of material/ RESISTIVITY
• LENGTH - longer wire = more resistance R ∝L
• AREA - thinner = more resistance R ∝1/A
• TEMP - see notes on lamp/thermistor

Question 20

resistance equations

Answer 20

R = pL /A
R = V/I (just a convenient way to calc R - V+I don’t actually affect R)

Question 21

resistors in series

Answer 21

INCREASES overall resistance
Rt = R1 + R2 + R3

Question 22

resistors in parallel

Answer 22

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…

Question 23

experiment to measure the resistivity of a material

Answer 23

• 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

Question 24

Fixer resistor

Answer 24

obeys ohms law “Voltage is proportional to current providing temp and dimensions are constant”

Question 25

Filament

Answer 25

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

Question 26

filament lamp graph

Answer 26

S shape graph for each increase in voltage current goes up by a smaller amound

Question 27

LED

Answer 27

- more efficient and robust than filament lamp - only allows current to flow if “forward biased” and above threshold voltage - otherwise has infinite R

Question 28

LED graph

Answer 28

I is zero until threshold voltage then straight line up

Question 29

thermistor / LDR

Answer 29

- 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

Question 30

thermistor / LDR graph

Answer 30

downward curve (resistance over temp / LI)

Question 31

component graphs watch out for

Answer 31

switched axes

Question 32

component graphs watch out for

Answer 32

switched axes

Question 33

explain the variation of resistance with pd for the filament bulb in terms of particle behaviour

Answer 33

- 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

Question 34

explain how the graph shows this is a diode

Answer 34

- conducts in one direction only - conducts when pd is beyond threshold voltage eg 0.2V - in reverse direction resistance is very high/ infinite

Question 35

how to get an IV graph for components

Answer 35

- 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)

Question 36

power

Answer 36

rate of transfer of energy - watts or J/s - for electrical power it’s a measure of how bright/hot / fast a component is

Question 37

power equations

Answer 37

P = E/t P = IV P = I^2 R P = V^2 /R

Question 38

power of lamps experiment

Answer 38

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

Question 39

energy (big)

Answer 39

if we leave a component in it transfers electrical energy to other types

Question 40

energy equation

Answer 40

E = ItV

Question 41

kiloWatt hour

Answer 41

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

Question 42

energy (small)

Answer 42

as one coulomb of charge passes through a 9V battery it gains 9J of charge

Question 43

1 volt in J/C

Answer 43

1 J/C - for every coulomb of charge that passes though a battery of x volts it gains x joules

Question 44

electron volts

Answer 44

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

Question 45

electron gun

Answer 45

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

Question 46

max power

Answer 46

when internal resistance = R