4.2 Energy, Power and Resistance

Question 1

define:
potential difference
electromotive force

Answer 1

p.d. = work done by charge carriers per unit charge
emf = work done to charge carriers per unit charge

Question 2

recall the equation for electron acceleration

Answer 2

eV=1/2mv^2
where V is the final speed of the electron

Question 3

recall ohms law

Answer 3

R=V/I
for an ohmic conductor at constant temperature

Question 4

describe and explain the IV characteristic for:
an ohmic conductor
a filament lamp

Answer 4

straight line, constant resistance

’s’ shape, small current=ohmic, larger current=resistance^

Question 5

describe and explain the IV characteristic for:
a diode
a thermistor

Answer 5

current flows in one direction, threshold voltage, semiconductor

negative temperature coefficient, temp ^=res v

Question 6

define resistivity and provide an equation for it

Answer 6

a physical quantity of a metal at a set temp (room temp=25 dc)

p=RA/L

Question 7

describe a PAG to determine resistivity

Answer 7

• determine diameter using gauge micrometer (take average of diff. points)
• calculate CSA in m^2
• set up circuit using set length of wire, ammeter & voltmeter
• record current & p.d., calc res
• increase length & repeat
• plot graph of resistance against length
• calc gradient and multiply by CSA

Question 8

define power and provide some equations

Answer 8

rate of energy transfer
P=Itv P=W/t P=I^2R

Question 9

define work done by a component and relate it to power

Answer 9

power x time component is used for
kWh used instead of J for industrial/domestic purposes