Electricity

define electric current and units

rate of flow of charge, A

how does conventional current flow

from positive to negative

define potential difference and units

work done per unit charge, V

1V means..

1J per coulomb is transferred

define a coulomb

the amount of charge that passes in 1 second if the current is 1 ampere

define resistance

a measure of an electrical component’s opposition to current

property that impedes the flow of current

what is resistance caused by

repeated collisions between charge carriers passing through component

positive ions in component vibrating in fixed positions

1 ohm is equivalent to..

1 volt per ampere

define ohm’s law

for a metallic conductor, the potential difference across it is directly proportional to the current through it, providing other conditions remain constant

ohmic conductor graphically

straight line through origin on current voltage graph

resistance on filament lamp

I against V, gradient = 1/R
V against I, gradient = R

describe shape of filament lamp graph

v increase with i

current heats the filament so temp increases

pd and current dont increase proportionally

particle vibrations impede flow of electrons, so R increase

graph - mirror image in negative quadrant because negative V and I produce same effect

details of semiconductor diode

potential difference <0.7V, resistance is very high (effectively infinite) and current is very small (zero)

potential difference >0.7V, resistance is very low (effectively zero) and current flows but pd remains close to 0.7V

diode current flow direction

diodes let current flow in the direction that the triangle in the circuit symbol points

when current passes through diode..

need 0.7V for current to pass through diode. once current is flowing, pd across diode remains approx constant

define resistivity

a property that describes the extent to which a material opposes the flow of electric current through it

resistivity formula

rho=RA/L

resistivity graphically

R on y axis, L on x axis, gradient = rho / A

function of thermistor

resistance is dependent on its temperature
resistance decreases as the temperature increases (non-linear)

function of light dependent resistor

as intensity of light increases, resistance decreases

application of thermistor

temperature sensors to control heating, e.g. mobile phones

define superconductivity

a property of materials with 0 resistivity at and below crit temp which depends on the material

current flows through material, no electrical energy is transferred to heat, so none is wasted

applications of superconductor

reduction of energy loss in transmission of electrical power

produce strong magnetic fields

electromagnets that don’t need a constant power source

MRI scanners and Maglev trains

total resistance in circuit

in series: RT = R1 + R2 + …

in parallel 1/RT = 1/R1 + 1/R2 + …

define power and units

rate of energy transfer, W

energy power time equations

power = energy / time

energy = power x time = IVt

how does higher current increase temperature

more current means more electrons passing a point every second

more collisions

more vibrational energy in wire - higher temperature

series circuit rules

same current flows through all parts of circuit

voltage shared across components

bc total work done on the charge by the battery = total work done by the charge on the components

parallel circuit rules

pd is same across all components

total current is shared between branches

total current going into junction = total current leaving (conservation of charge)

conservation of energy

energy per coulomb supplied by battery = energy per coulomb transferred in external resistance + energy per coulomb wasted in internal resistance

use of potential divider

to supply constant or variable potential difference from a power supply
use with variable resistors, thermistors and LDR

explain process of using potential divider

as temp/light intensity decreases, resistance of thermistor/LDR increases

total resistance in circuit increases

V 0 remains constant so I decreases as I = V/R

then, pd across fixed resistor decreases as I has decreased

so pd across thermistor/LDR increases since V 0 = Vout + V R and V R has decreased and V 0 is constant

why does resistivity of metals increase as temp rises

because positive ions vibrate more with increasing temperatures, making collision with electrons more likely

define electromotive force

potential difference across terminals when there is zero current

emf equations

emf = E/Q
emf = I(R+r)

finding internal resistance graphically

V against I

y intercept = emf

gradient = -r

define internal resistance + describe

resistance within the cell

work is done inside cell to overcome this

when current flows through cell, voltage is lost across the internal resistance

cells facing the same way

total emf = emf1 + emf2

total internal resistance = r1 + r2

cells not facing the same way

total emf = difference between emf1 and emf2

total internal resistance = r1 + r2

cells in parallel

overall emf is same as emf of one cell, assuming both have same emf

total internal resistance 1/r = 1/r1 + 1/r2

bulbs and power

power determines light and heat produced

increase power = brighter

digital voltmeter adv disadv

adv - less error than reading a pointer

disadv - gives continuous scale

graph of how current changes over time through thermistor when constant voltage is applied

i against t

straight line initially - low resistance (not heated) so current high

peak then curve down and flatten - resistance/temp increasing

flat - equilibrium. energy in = energy loss to surroundings

why is the terminal potential difference of a cell less than its emf when current flows

when current flows through a real cell, some energy is lost due to internal resistance inside the cell

why isn’t the current exactly zero for a semiconductor diode at voltages below 0 V?

because of reverse leakage current

tiny negative current flows below 0 V

small positive current flows near 0 V due to minority carriers

so current is very small but not zero