4.2 energy, power and resistance Flashcards
ammeter
voltmeter
cell
light bulb
diode
light-emitting diode (LED)
resistor
variable resistor
thermistor
light-dependant resistor
heater
electric motor
e.m.f definition
work done per unit charge, transferring energy to electrical energy.
e.m.f equation
W is the energy transferred
p.d definition
work done per unit charge transferring energy from electrical energy into other energy stores
p.d equation
W is energy transferred
unit for p.d
Volt = 1 J C-1
how must voltmeters be connected in a a circuit
in series
resistance definition
measure of the difficulty of making current pass through it
resistance equation + unit
unit = Ω
ohm’s law states:
the current through a conductor is directly proportional to the potential difference across it
resistivity of a material definition
the resistance of a 1m length of the material that has a cross-sectional area of 1m2
resistivity equation + unit:
Ωm
what can resistivity depend on?
temperature and light intensity
what does a positive temperature coefficient mean?
resistance increases with temperature
why does resistance increase with temperature in a metal
- at higher temps, the atoms and ions vibrate more, increasing the chance of collisions
- slows the charge carriers down, so there is a lower current per volt
- R = V/I so R increases
what happens to the resistance of a negative temperature coefficient thermistor as temperature increases
decreases
why does the R of a NTC decreases as T increases
- because its made from a semiconductor
- n in a semiconductor increases with T
- higher current/p.d
- R = V/I so R decreases
resistance of an LDR as light intensity increases
decreases
current voltage graph axis
V on x-axis
I on y-axis
I-V graph for an ohmic conductor
- current proportional to p.d
- straight line through origin
- R = 1/grad
I-V graph for a filament lamp
- gradient decreases because R of lamp increases
- R = V/I at any point
I-V graph for diode or LED
- shows diode only lets current flow in one direction
- shows p.d must be above threshold before any current can flow
- R = V/I at any point
I-V graph for a NTC
- shows resistance decreases as gradient increases
- R=V/I at any point
power equations + unit
- P = IV
- P = I2R
- P = V2/R
- watt
energy transferred
W = IVt
energy unit
J or kWh
how to calculate energy in kWh and J
energy (kWh) = power (kW) x time (h)
1kWh = 1000W x 60 x 60 = 3.6MJ
how to calculate cost of electricity
multiplying energy in kWh by the cost of 1kWh in pence
what happens to the charged particles between two points of p.d
the p.d will accelerate the particles
energy transferred to a charged particle equation:
E = eV
e is charge
V is difference in p.d
