Electricity Flashcards
Electric charges
Two types: positive (proton), negative (electron)
Unit: Coulomb (C)
One electron/proton = +/- 1.6 x 10^(-19) C
Like charges repel, unlike charges attract
Only electrons can be transferred from one object to another (can move around freely, but protons are relatively fixed)
Charged and neutral objects
Neutral: equal # of protons and electrons (shows no electrical property)
Charged: excess # of electrons (negatively-charged)/protons (positively-charged)
NOTE
Friction (rubbing), polarization…
Permittivity
Manipulating…
Assume all test charges positive
Delocalized electrons are the charge carriers in a metal
Speed due to current supplied by (originally, moving randomly, but once c
Conversion of thermal energy b/c of resistance
All ions, ofc, charged (when you dissolve metals in liquids
Emf is voltage—only diff is if it’s connected to a circuit or not (when no current in circuit, voltage is called emf)
Conductor = wire
Depending on how the wires are connected, I and V can be negative
Ammeter, multimeter, voltmeter
1.5 V each
Way you connect determines whether you’ll find…
Voltage is shared: for everything to allow current to flow [wires, switched, etc.], need energy)
Circles show battery terminals
Load = device = component = resistor
arrow = variable
*V = IR to calculate voltage
*If multiple between, measures for all…
*Encircled M is electric motor
*Arc thing is a lightbulb
*If resistance of loads is the same, they’re identical
Variable, so value of resistor can change depending on where you place your slider to divide
Law of Conservation of Charges
States that charges can neither be created nor destroyed, but can be transferred from one object to another (OR the total # of charges is always constant)
Electrical insulators and conductors
Insulators: have relatively fixed electrons (ex. dry wood, rubber, paper, plastic, etc.)
Conductors: have freely-moving (delocalized) electrons (ex. metals, water, people, etc.)
Coulomb’s Law
States that the force between two point charges is directly proportional to the product of the two point charged and inversely proportional to the square of their distance apart (see formulae [and constant in vacuum])
Coulomb’s constant…
Law applies also to spheres, but the distance starts at the center
Distance has to be in m!
Electric field strength (E)
Defines as force per unit test positive charge placed in a field (see formulae [use Coulomb’s to get second])
Unit: NC^(-1)
Vector
Overall electric field strength is the difference in the field strength of the individual charges
Electric field
A region where a test charge feels a force
Vector—direction can be represented using field lines
No two field lines can cross each other b/c they represent the direction of the charge (a charge cannot go in two directions at the same time)
W/ positive, away, and w/ negative, towards (if sphere, lines don’t start at center)—planes, w/ lines continuing beyond
Field strength decreases as you move away from the source of the field (distance between lines increases)
For same, opposite, and between two parallel oppositely-charged rods, see note (remember edge effect [field strength same/uniform in the middle—distance between lines is equal])…
Addition of electric fields is done using either a calculation or scale diagram
Potential difference (Pd) or voltage (v)
Work done per unit charge to move the charge from one point to another (V = work/charge [work/energy])
OR
Change in energy involved when a charge moves from one point to another (V = change in E/q)
Unit: volts or JC(-1)
Scalar
Source is battery or mains supplies (?)
See diagram (long is positive [high potential energy?] and short is negative [low potential energy])
Measured using voltmeter
Electron volt (eV)
Energy gained by an electron moving through a Pd of 1 volt
It’s a unit of energy at the atomic level
See working for formulae, but change in E = Vq
change in E = Pdq
1 eV = 1 volt(1.6 x 10^(-19) C)
1 eV = 1.6 x 10^(-19) VC or J
To convert from eV to J, multiply by 1.6 x 10^(-19)—do the opposite for J to eV
Energy difference in an electric field
Change in electric Pe = force x distance
but E = F/q
f = Eq
EPE = Eqd
Gain in KE = loss in PE ([1/2]mv^[2] = Eqd)
Also F = ma
F = qE
ma = qE
Electric current (I)
The rate of flow of electric charges
= # of charges fl./time takes
= change in q/change in time
Unit: Ampere (A)
1 amp = 1 coulomb/1 sec
Other unit of I = Cs^(-1)
Direct current (dc)
Current that flows in only one direction
Source is the battery
Alternating current (ac) is current that constantly changes direction
Source is mains (?)
Measured using an ammeter or galvanometer
Representation of direction of current
Conventionally, the flow of current is from the positive to the negative terminal (this is the same as the electric field due to applied pd), but in reality, current actually flows from the negative to the positive terminal (the actual direction of electron flow is opposite to that of the conventional current/applied electric field)
Direction of flow of electrons in negative direction of flow of current (“confused human beings who refused to adapt to change”): direction of electric field goes w/ that of current
See most basic unit…
Drift velocity
Velocity of the electrons due to current
Vey quick (at the level of particles)
approx. 10^(-3) mms^(-1)
Already moving, so current adds to it
Actual random velocity (w/o current) is 10^5 mms^(-1)
Metallic/Conduction electrons
Delocalized electrons
See diagrams…
*Have to know how to derive all the formulae on page…
Why conductors heat up during current flow
Cord warm if, for a long time, cha
*Lattice ions just protons in metals
As the conduction electrons move, they collide w/ the metal atoms/fixed lattice (positive) ions (KE when moving, colliding)
Leads to a transfer of some of their KE to the metal atoms/ions, resulting in an increase in the KE of the metal atoms
This increases the temp of the metal atoms and eventually heats up the conductor (as they receive more KE)
Heat builds up
Resistance (R)
Ratio of potential difference and current
R is directly proportional to V and to 1/I (formula cuts across…)
A conductor w/ a very high resistance needs a large pd to get current to flow across it
Results in increase in temp
Unit: Ohms (omega symbol) or VA^(-1)
Current is about flow: more voltage means they’ll move faster and thus collide more
More collisions, more KE transferred, higher temp
Factors that affect resistance
Cross-sectional area (A), length of conductor (L), type of material it’s made up of (resistivity [rho], a measure of how much resistance electrons meet [lattice ions—some metals have more])
R directly proportional to L/A
R = rho x L/A
w/ L, more lattice ions
w/ A, more space to move
Every metal is different, so no two
Current flows faster through a short, fat conductor
Every equipment in a circuit is a resistor (all conductors)
There are standard resistors specially made w/ specific values (for convenience)
See symbols for resistor