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
Ohm’s Law
States that the current flowing through a conductor is directly proportional to the pd across it, provided temp (resistance) stays constant
Ohmic conductors
Devices that obey Ohm’s Law (ex. metals at constant temp)
In reality, almost any conductor you have in a circuit is non-Ohmic (except for wires at the beginning [before they heat up])
I vs. V—directly proportional
Non-Ohmic conductors
Devices that don’t obey Ohm’s Law (ex. filament lamp [technical name for bulb], diode [changes AC to DC], etc.)
For filament lamps, s passing through origin and always curving towards V (on different sides of y-axis (when axes flipped, just reflection across y = x [?])
For diodes, looks kind of exponential: passes through origin and asymptote (mw) just below the x-axis
NOTE:
Resistance (the slope) at any point on the Ohm’s Law graph = V/I
Just pick a point (in this case, don’t have to draw a tangent)
Power dissipation
In this case, dissipate means to “give off” (so, energy given off in a certain amt of time: if you touch something in the circuit and it’s warm, power is being dissipated)
Energy dissipated by a circuit per unit time
Unit: Js^(-1) or Watt (W)
Pd = ΔE/q
I = q/t
IPd = ΔE/t
P = VI
V = IR
P = I^(2)R
I = V/R
P = V^(2)/R
Power is just a combo of I and V
Electrical meters
Ammeter:
Used to measure current
Symbol: Ⓐ
Connected in series (just next to) at the point where the current needs to be measured
A perfect ammeter has zero resistance
Voltmeter:
Used to measure voltage/Pd
Connected in parallel (across) w/ component whose Pd is being measured (if you have it somewhere else, it’ll measure something else)
Symbol: Ⓥ
A perfect voltmeter has infinite resistance (if you want a max V, R has to be max)
Electromotive force (ε)/emf
Kind of has nothing to do w/ force?
Defined as the ratio of energy change to charge (so, same def)
Units: Jc^(-1) or volts
Whatever the battery supplies w/ closed current
If it’s a series circuit (one continuous loop), voltage is shared (every load shares what the battery provides)
Internal resistance (r)
Obstruction of electron flow in battery: the resistance of a cell (ex. battery)
When current introduced, conversion
Terminal voltage is what is shared
Also a type of resistance, but specifically for the battery
Causes the conversion of electrical energy to thermal energy inside a battery, leading to shortage in battery output (lower current?)
Therefore, the pd when there’s current in a circuit (terminal voltage) is less than the output of the battery w/o current (emf)
The voltage lost due to internal resistance is called voltage drop/lost voltage (voltage due to internal resistance of cell)
Lost volts = Ir
So, if battery is not connected in a circuit (no current flows), emf = terminal vo
ε = Vterm + Vlost
ε = IR + Ir
ε = I(R + r)
*See equations and apply y = mx + b
emf and power
ε = change in E/q
divide both numerator and denominator by t….
ε = total power/total current
total power delivered by battery = εI
*Alternative def is ratio of power and current
Kirchoff’s 1st Law
States that at every junction in a circuit, current in = current out (garbage in = garbage out)
sum in = sum out
sum = 0 (charge conservation law)
If in = out, if you move one to other side, 0
Applies to every circuit
Resistors in series
Has all components (resistors) connected in a continuous chain
At any point, Kirchoff’s 1st Law is true (current same everywhere)
But Pd is shared among the different resistors/components
Check ans by summing to see if same as voltage supplied by battery
Total resistance = R1 + R2 + R3 +…
Current same everywhere
Resistors in parallel
Has branches (more than one route for current flow)
Pd (voltage) stays the same b/c each has equal connection to battery
Current shared
Total current is sum
Pd same
1/RT = 1/R1 + 1/R2 + 1/R3 +…
More resistance = less current
No one resistor blocking another b/c not in same line
See diagram…
Sum first if in series (parallel first if like component in series) and then treat as parallel
No dots = in series (circles mean in parallel w/ whatever)
B/c parallel, current different in branches, so A measures for whatever’s next to…
Discharging in a cell
Once you complete a circuit, battery supplies the energy that pushes the electrons
After some time, energy depletes
Discharging is the process by which a cell delivers charges to an external circuit
The amt of charge a cell can deliver to ab external circuit is known as its capacity
The greater the current in the external circuit, the faster the discharge
During the discharge, the terminal pd loses its value quickly initially, stabilizes consistently for most of its lifetime, and then decreases rapidly to zero, as the discharge completes
*See graph…
*Battery we see is just a combo of different cells
*Remember charging…
Types of cells
Two types: primary and secondary
Primary cells
Everyday batteries that we know
Non-rechargeable
In other words, the chemical reactions that produce the pd use up the chemicals
Can be used in flashlights, remotes, toys, clocks, watches, etc.
ex. alkaline, lithium, zinc oxide batteries
gallium?
Secondary cells
Rechargeable
Done by passing current through the current in direction opposite that of electron flow (regenerates the chemical reactions)
During discharging process, current goes in one direction (when recharge, other direction)
Used in mobile phones, cars, laptops, tablets, ?PS,
SLI (starting on [switching on], lighting [ex. headlights?], ignition [engine])
ex. lead acid batteries (for cars), lithium batteries, nickel ion batteries, etc.
Potential divider/Potentiometer (POTs)
A circuit arranged in a way that two or more resistors divide the supplied pd
To use pot formula, have to reduce circuit to series
Recall,
I = V/R
Therefore, is total resistance = R1 + R2, then T = V/(R1 + R2)
The pd across R2 = Vout
Since V = IR
Therefore, Vout = Vin/(R1 + R2) x R2
NOTE: R2 symbolizes the resistor whose voltage (Vout) is being calculated
Applications of POTs
Used as sensors
Two types of sensors: LDR (light-dependent resistor) and thermistor
LDR (light-dependent resistor)
A variable resistor
Special type of resistor where resistance depends on the amt of light shining on it
An increase in light results in a decrease in the resistance (voltage) and vice versa
ex. sunlight-dependent bulb (uses electronic switch)
Resistance can go all the way down to 0, so v will be 0 as well (switch won’t turn on [opposite true at night?])
*see circuit
Thermistor
Used in fire alarms
Detects presence of fire
Variable resistor
When it gets hot, its resistance decreases, resulting in decreased voltage (therefore, the voltage of the resistor attached to the alarm switch decreases, turning on the alarm)
In series, so have to sum up to the battery’s (so, if voltage of one decreases, other increases)
*see circuit, note symbol
Kirchoff’s 2nd Law (Loop Law)
States that in any closed loop, total emf (cells) = total voltage (pds of all resistors)
or
total v = 0
NOTE: V = IR
Rules of summing emfs and voltage
emf is positive if loop is from negative to positive of cell (and negative if opposite)
V is positive in loop foes along current direction (negative if opposite)
Steps to follow when solving problems
- Draw lines to show loop direction (clockwise/counter—doesn’t matter as long as you follow the rules)
- Draw arrows across each cell to show current direction (*conventional?)
- Name your currents (if junctions [parallel situation] are involved)
- Apply 1st and 2nd Laws
Magnetic field (B)
A region where a magnet/charge/CCC (current-carrying conductor [depending on strength of curr) feels a force
Magnets feel force when they enter a magnetic field
If they can make a circuit w/ high-capacity batter or to AC (mains) of a , if a heavy object contains a lot of iron, can use
In every field, you have a force
Current produces electric field (also charge b/c no current w/o charge)
Mass produces gravitational field
Only iron and other things that contain iron (atoms that align themselves)
White will point north
Only true test is repulsion (means both have fields): a magnet can attract something that’s not a magnet
CCW = current-carrying wire
W/ the presence of current, a conductor will be able to behave like a magnet
Magnets are attracted to southernmost pole of other magnets
red end points to geographic north of earth (magnetic south)
like w/ iron shavings b/c attracted, compasses same way
line red called north seeker
Magnetic field lines
Magnetic fields are represented using field lines, known as flux
Magnetic field lines are real (not imaginary like electric field lines)
Field pattern of a bar magnet
See…
Field strength decreases, so spacing getting bigger
A compass aligns itself to direction of magnetic field (pointing south)
Magnetic field of the earth
See…
Magnetic field pattern around a CCC
See…
*Distance also increases
An electric current produces a magnetic field
NOTE:
1. Field lines around a CCC always circular
2. Direction of magnetic field can be predicted using the right-hand grip rule (if thumb is direction of current, direction of field is natural fingers circle pencil)
Magnetic field pattern around a solenoid
See…
A solenoid is when you wrap a wire around a conductor (whole system known as a solenoid [more you )
Same as that of a bar magnet
NOTE:
1. Poles of a solenoid (can’t tell in practice) can be predicted using the right-hand grip rule, but in a different way (direction the fingers curl is current direction and direction of thumb is that of the magnetic field)
Magnetic force of a CCC in a magnetic field
When a CCC is placed in a magnetic field, it feels a force
This force is perpendicular to both the current’s direction and the magnetic field’s direction *have to be able to sketch)
This force (F) is directly proportional to: the magnitude of current (I), the strength of the field (B), the length of the CCC (L), and the sine of the angle between the field and the current
F = BILsin theta (if angle not involved, just F = BIL
B = F/ILsin theta
Unit for B: NA^(-1)m^(-1) or Tesla
see formula, use 90 degrees?
NOTE:
1. Force is max when current and field lines are perpendicular
2. Force goes down if the theta between current and field lines decreases
3. Force is 0 if both CCC and field lines are parallel
The direction of the force of CCC is determined using the following: Flemming’s left hand rule (force is thumb, current is middle, and field is first/fore/index [?]) and the right hand palm rule
*Rsr…
Mre
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