electricity Flashcards
electric current
rate of flow of charge
potential difference
energy transferred per unit charge
ohm’s law
current is directly proportional to potential difference, providing the temperature and other physical conditions remain the same
ohmic conductor
a conductor where I is directly proportional to V
non-ohmic conductor
a conductor where I is not directly proportional to V
resistance of a voltmeter and ammeter
voltmeter resistance is infinite
ammeter resistance is zero
ρ, R, A and L in resistivity equation
ρ is resistivity; R is resistance; L is length and A is the cross-sectional area
resistivity
ρ = RA/L
in metals, resitivity is proportional to
temperature as conducting electrons lose more energy in collisions with positive ions at a greater temperature due to greater vibrations
units of resistivity
Ω m
superconductor
a material with zero resistivity when cooled to or below critical temperature
use of superconductors
can carry large currents with no energy losses and produce strong magnetic fields
MRI scanners
Maglev trains (magnetic levitation)
Particle accelerators eg Large Hadron Collider
critical temperature
temperature at and below which a material has zero resistivity or resistance
emf
actual energy provided by the battery/cell per unit charge OR the terminal potential difference across the battery when no current is flowing
internal resistance
the hindrance to the flow of charge in the battery OR the loss of potential difference per unit current
‘Lost’ potential difference
potential difference dropped across the internal resistance of a cell
what is a potential divider
a combination of resistors in series connected across a voltage source designed to share the potential difference in a given ratio. it can be used with power source of fixed emf to provide variable power output
how resistance of an NTC thermistor and wire change with increasing temperature
NTC Thermistor : resistance decreases as temperature increases
Wire : resistance increases as temperature increases
How resistance of an LDR changes with increasing light intensity
resistance decreases with increasing light intensity
potential difference across a diode in forward bias
a potential difference of 0.6V is dropped across a diode in forward bias
potential difference across cells in series and parallel
in series, potential difference of each cell adds up
in parallel, potential difference is the same as each cell’s potential difference
how to find emf and internal resistance from graph of pd against current
emf is y-intercept
internal resistance = -gradient
resistance in series and parallel
series: R(total) = R(1) + R(2) + …
parallel: 1/R(total) = 1/R(1) +1/R(2) + …
power
rate at which energy is transferred
P=IV, P=V^2 /R, P=I^2 *R
charge and energy conservation
charge and energy are both conserved in d.c. circuits hence emf = I(R+r)
Kerchoff’s 1st law
total current entering a junction = total current leaving a junction
