Electricity

Question 1

Electric charge

Answer 1

Two types: positive (proton) and negative (electron).
Unit: Coulomb (C)
One electron or proton = 1.6 x 10^⨦19

Like charges repel, unlike charges attract.

Question 2

Neutral object

Answer 2

An object with an equal number of protons and electrons (shows no electrical property).

Question 3

Negatively-charged objects

Answer 3

Objects with excess electrons.

Question 4

Positively-charged objects

Answer 4

Objects with excess protons.

Question 5

Can protons be transferred?

Answer 5

Only electrons can be transferred from one object to another.

Question 6

Law of Conservation of Charges

Answer 6

Charges can neither be created nor destroyed, but can be transferred from one object to another OR the total number of charges is always constant (see illustration).

Question 7

Electrical insulators

Answer 7

Have relatively fixed electrons (e.g. dry wood, rubber, paper).

Question 8

Electrical conductors

Answer 8

Have freely-moving electrons (in other words, electrons are delocalized): e.g. metal, water, humans.

Question 9

Methods of charging an object

Answer 9

Friction and induction.

Question 10

Friction (Rubbing)

Answer 10

Involves charging the two objects by rubbing them together (e.g. rubbing a balloon on your hair makes your hair positively-charged and the balloon negatively-charged).
Occurs mostly in insulators.
Conductors must have insulated handles to be charged by this method.

Question 11

What charges can attract neutral objects?

Answer 11

Both negatively-charged and positively-charged objects can attract neutral objects.

Question 12

Induction

Answer 12

Involves charging a conductor by bringing it near a charged object, while grounding (connecting a conductor to the earth [sink of electrons] so the electrons can flow to/from the earth) the conductor.
(See diagram) Charged object brought near a neutral conductor, conductor is grounded, some electrons flow to the earth, charged object is removed, conductor is now positively-charged and positive charge gets distributed uniformly over the sphere.
If a positive rod is brought closer, sphere acquires negative charge (in this case, electrons flow from the ground to the sphere).

Question 13

What happens when the net force on the free electron inside the metal is zero?

Answer 13

Distribution of charge ceases when the net force on the free electron inside the metal is zero and this happens instantly.

Question 14

Polarization

Answer 14

The process whereby charges are temporarily realigned, inducing charge on the surface of a neutral insulator.
This is the reason why charged objects can attract neutral objects.
Happens when a charged object is brought near a neutral object (w/o touching).
Electrons in the neutral object will either be attracted or repelled and will temporarily move (how much they move depends on the type of material [for insulators, they’ll move less, and for conductors, they’ll move more]).
Electrons in the neutral object spread back out once the outside object goes away.

Ex. Electrons repelled by electrons in negatively-charged rod, electrons in the electrically neutral metal sphere move away (still electrically neutral [electrons just distributed differently: if you move the rod away, they’ll spread back]): if we bring a positively-charged object, the electrons are attracted to it and move closer (protons don’t move).

So, a polarized effect (one side more negative, the other more positive [STILL electrically neutral b/c same amount]).

Question 15

Do protons move?

Answer 15

Electrons move, protons don’t.

Question 16

Coulomb’s Law

Answer 16

The force between two point (electrical charge considered to exist at a single point [no area/volume]: other things about the charge would get in the way/interfere [the law is only applicable to small]) charges is directly proportional to the product of the two charges and inversely proportional to the square of their distance apart.

Formula:
F∝q1q2/r^2
F = kq1q2/r^2
k is Coulomb’s constant, which we got when we replaced the ∝ with an = (8.99 x 10^9 Nm^2C^-2)

NOTE: 
In a vacuum, 
k = 1/4πε0
ε0 is the permittivity constant (8.85 x 10^-12C^2N^-1m^-2)
F = q1q2/4πε0r^2

*Sometimes, q and Q are used interchangeably.

Question 17

What does “-“ symbolize?

Answer 17

• symbolizes an electron (+ is not necessary to write).
• , w/ force, means attractive.

Also, remember symbol for micro.

Question 18

Manipulation of Coulomb’s Law

Answer 18

When q1 is doubled, 2F.
When q1 and are q2 doubled, 4F.
When r is tripled, F/9 ((3r)^2).
When r is doubled, F/4.

Question 19

Electric field

Answer 19

Region where a positive test charge feels a force.

Produced by charges.

Question 20

Electric field lines

Answer 20

Electric fields are represented using imaginary lines called field lines.
Conventionally, positive charges have their field lines pointing outwards, whereas those of negative charges point inwards.
Conventionally, you need at least six arrows (see diagrams).
For two positive charges, closer means more curvature (see diagrams for two positive and opposing): here, six (together) may not be enough (you have to show the curvature).
For two oppositely charged parallel rods, equal spacing in center (means electric field is constant [experimentally, it was discovered that anywhere between the two rods, the field is constant]), but at edge, edge effect.
NOTE: The closer the field lines, the stronger the field.
Inverse relationship between force and distance, so force weakens as distance increases (more spacing as we go farther away [radial]).

*Same distance, same q2 (?).

Question 21

Electron field strength (E)

Answer 21

Electricity is the flow of electrons.
Defined as the force per unit test positive (in Physics, conventionally, when testing the strength of a field, we use a positive charge) charge (we use a test charge to gauge the strength of the electric field of another charge) placed in a field.

Formulae:
E = F/q2
q2 is the test charge
*Force is a vector, to E is a vector (once force is involved, as long as the other variable is a scalar, the thing involved will be a vector [build up from fundamentals to know]: vector x vector = scalar)
Recall, F = kq1q2/r^2
E = kq1/r^2
q1 is the producer of the field
Unit: N/C

Question 22

Potential difference (or voltage)

Answer 22

Defined as work (other term for energy [?]) done per unit charge to move the charge from one point
Formula: V = w/q (work/charge)
Change in energy per unit charge when a charge moves from one point to another
Formula: V = ΔE/q
Unit: J/C or volts
Scalar
Source: battery
Measured using voltmeter

Question 23

Electric current (I)

Answer 23

Flow of charges per unit time.
Formula: I = q/t (number of flow changes [?]/time taken)
Unit: C/s or Ampere (A)
Measured using ammeter.

Question 24

Direct current

Answer 24

Current that flows in only one direction.

Source: battery

Question 25

Alternate current

Answer 25

Current that constantly changes direction.
Source is mains (power station).
Measured using ammeter or galvanometer.

Question 26

Why conductors heat up during current flow

Answer 26

Delocalized electrons move in presence of current
Carrying KE: when bump into the metal ions (lattice ions [called in Chem: don’t move much]), ions gain some KE (and temp is directly proportional).
In wires, once you connect to battery, circuit complete, electrons flow in one direction.
As the conduction (b/c found in conductors) electrons (delocalized) move, the collide with the metal atoms/fixed lattice (positive) ions.
Leads to a transfer 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 the conductor up.

Question 27

Resistance (R)

Answer 27

When the delocalized electrons bump into the cations, the lose energy, so the cations are resisting them
The ratio of potential difference and current (technically defined in terms of potential difference and current).
Formula: R = V/I (R ∝ V, R ∝ 1/I)
Tells us how much resistance they experience.
This means that a conductor with a very high resistance needs a large potential difference to get current to flow across it.
This results in an increase in temp across it (more collision).
Unit is Ohms (Ω [omega]).
1 Ohm = 1 VA^-1
*Don’t use /.

Question 28

Factors that affect resistance (of a conductor)

Answer 28

Three key factors:
The cross-sectional area (A): decrease b/c more space for electrons to move.
Length of conductor (L): takes larger for the electrons to travel through (more cations, more resistance).
The type of material it’s made of (resistivity 𝜚 [varies with the conductor—depends on the metal used]).

𝜚 (rho [basically just a p or e in italics]) is a constant called resistivity (of the conductor), and it represents how many cations you have in the wire.
Formula:
R ∝ LA
R = 𝜚L/A
This means that current flows faster through a short, fat conductor (fewer atoms [more current can pass through])
So, long and slim means more resistance.
NOTE:
Every piece of equipment in a circuit (path for transmitting electric current: includes a device that gives energy to the charged particles constituting the current [ex. battery or generator]; devices that use current [ex. lamps, electric motors, or computers]; and the connecting wires or transmission lines) is a resistor (they all have positive ions).
There are standard resistors (specifically made with specific values).
Symbol for resistor (in diagrams): see diagram.
Ohmmeter/Multimeter measures resistance.

Question 29

Ohm’s Law

Answer 29

States that the current flowing through a conductor is directly proportional to the potential difference across it, provided the temp stays constant (true only under this condition).
*Voltage is P.d (potential difference) b/c two ends of the battery suppl different amounts of energy (potential energy [potential difference is short for potential energy difference for short, and the source is a batter]).
Formula:
V ∝ I {if temp is constant [resistance]}
V = IR
*Resistance is the constant
Stops being true after say a light heats up (wires much more dependable).

Question 30

Power Dissipation

Answer 30

Power is energy per unit time (J/s or watt).
Energy (thermal energy) dissipated by a circuit per unit time.
Unit: Js^-1 or Watt (W)
Lower is better (won’t heat up too fast).
Formula:
Recall,
P.d = ΔE/q
and
Current = q/t
Multiply and you’ll get ΔE/t, so power is a product of P.d and current (biggest equation [Power = VI])
Also, Power = I^2R
And:
I = V/R
So,
Power = V^2/R