Electricity Flashcards
(30 cards)
Electric charge
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.
Neutral object
An object with an equal number of protons and electrons (shows no electrical property).
Negatively-charged objects
Objects with excess electrons.
Positively-charged objects
Objects with excess protons.
Can protons be transferred?
Only electrons can be transferred from one object to another.
Law of Conservation of Charges
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).
Electrical insulators
Have relatively fixed electrons (e.g. dry wood, rubber, paper).
Electrical conductors
Have freely-moving electrons (in other words, electrons are delocalized): e.g. metal, water, humans.
Methods of charging an object
Friction and induction.
Friction (Rubbing)
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.
What charges can attract neutral objects?
Both negatively-charged and positively-charged objects can attract neutral objects.
Induction
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).
What happens when the net force on the free electron inside the metal is zero?
Distribution of charge ceases when the net force on the free electron inside the metal is zero and this happens instantly.
Polarization
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]).
Do protons move?
Electrons move, protons don’t.
Coulomb’s Law
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.
What does “-“ symbolize?
- symbolizes an electron (+ is not necessary to write).
- , w/ force, means attractive.
Also, remember symbol for micro.
Manipulation of Coulomb’s Law
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.
Electric field
Region where a positive test charge feels a force.
Produced by charges.
Electric field lines
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 (?).
Electron field strength (E)
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
Potential difference (or voltage)
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
Electric current (I)
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.
Direct current
Current that flows in only one direction.
Source: battery
