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Physics 1 > Electricity > Flashcards

Electricity Flashcards

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1
Q

Electric charges

A

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)

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2
Q

Charged and neutral objects

A

Neutral: equal # of protons and electrons (shows no electrical property)

Charged: excess # of electrons (negatively-charged)/protons (positively-charged)

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3
Q

NOTE

A

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

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4
Q

Law of Conservation of Charges

A

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)

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5
Q

Electrical insulators and conductors

A

Insulators: have relatively fixed electrons (ex. dry wood, rubber, paper, plastic, etc.)

Conductors: have freely-moving (delocalized) electrons (ex. metals, water, people, etc.)

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6
Q

Coulomb’s Law

A

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!

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7
Q

Electric field strength (E)

A

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

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8
Q

Electric field

A

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

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9
Q

Potential difference (Pd) or voltage (v)

A

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

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10
Q

Electron volt (eV)

A

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

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11
Q

Energy difference in an electric field

A

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

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12
Q

Electric current (I)

A

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)

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13
Q

Direct current (dc)

A

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

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14
Q

Representation of direction of current

A

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…

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15
Q

Drift velocity

A

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)

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16
Q

Metallic/Conduction electrons

A

Delocalized electrons

See diagrams…

*Have to know how to derive all the formulae on page…

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17
Q

Why conductors heat up during current flow

A

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

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18
Q

Resistance (R)

A

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

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19
Q

Factors that affect resistance

A

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

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20
Q

Ohm’s Law

A

States that the current flowing through a conductor is directly proportional to the pd across it, provided temp (resistance) stays constant

21
Q

Ohmic conductors

A

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

22
Q

Non-Ohmic conductors

A

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)

23
Q

Power dissipation

A

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

24
Q

Electrical meters

A

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)

25
Q

Electromotive force (ε)/emf

A

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)

26
Q

Internal resistance (r)

A

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

27
Q

emf and power

A

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

28
Q

Kirchoff’s 1st Law

A

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

29
Q

Resistors in series

A

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

30
Q

Resistors in parallel

A

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…

31
Q

Discharging in a cell

A

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…

32
Q

Types of cells

A

Two types: primary and secondary

33
Q

Primary cells

A

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?

34
Q

Secondary cells

A

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.

35
Q

Potential divider/Potentiometer (POTs)

A

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

36
Q

Applications of POTs

A

Used as sensors

Two types of sensors: LDR (light-dependent resistor) and thermistor

37
Q

LDR (light-dependent resistor)

A

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

38
Q

Thermistor

A

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

39
Q

Kirchoff’s 2nd Law (Loop Law)

A

States that in any closed loop, total emf (cells) = total voltage (pds of all resistors)

or

total v = 0

NOTE: V = IR

40
Q

Rules of summing emfs and voltage

A

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)

41
Q

Steps to follow when solving problems

A
  1. Draw lines to show loop direction (clockwise/counter—doesn’t matter as long as you follow the rules)
  2. Draw arrows across each cell to show current direction (*conventional?)
  3. Name your currents (if junctions [parallel situation] are involved)
  4. Apply 1st and 2nd Laws
42
Q

Magnetic field (B)

A

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

43
Q

Magnetic field lines

A

Magnetic fields are represented using field lines, known as flux

Magnetic field lines are real (not imaginary like electric field lines)

44
Q

Field pattern of a bar magnet

A

See…

Field strength decreases, so spacing getting bigger

A compass aligns itself to direction of magnetic field (pointing south)

45
Q

Magnetic field of the earth

A

See…

46
Q

Magnetic field pattern around a CCC

A

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)

46
Q

Magnetic field pattern around a solenoid

A

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)

47
Q

Magnetic force of a CCC in a magnetic field

A

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…

48
Q

Mre

A

*

Physics 1 (13 decks)

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