Electricity

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• 2 types of charge: positive and negative

• 3 types of charge state

  • Positive
  • positron, proton, alpha particle, cation
  • Negative
    • electron, anion
  • Neutral
    • Neutron, anti-neutrino

• An object of charge will produce an electric field around it

• Charge: property that results in electrical phenomena

• An electric field is the region around an object possessing the property of charge, which extends to infinity, and within which another object of charge (or not neutral object possessing charge) will experience an attractive or repulsive force, the strength of which decreases with distance, as per the inverse square law

Electric Fields

Field line rules

1. They’re arrows, not lines (naming moment)
2. For point charges, field density is uniform, thus lines must be evenly distributed around surface
3. Field lines start/end at the surface
4. Field lines should start/finish perpendicular to surface
5. Number of field lines indicates field density
6. Minimum of 4 lines recommended (not required)
7. Direction of field lines are determined by hypothetical test charge
8. Field lines cannot touch or cross (segregation L)

Charge

• Electrical charge can be transferred (the physical transfer of charge)
• Usually by physical transfer of electrons
  • Comes about as the result of friction
• Referred to as charging by conduction
• This is due to different substances having different electron affinity
  • Triboelectric series shows the continuum of electron affinities of different substances

Charging by conduction

• Occurs when $e^{-}$ are unevenly distributed throughout the structure of an electrically neutral species
• A charged object is brought into contact with another, and attracts, repulses electrons, making the other object gain a net charge

Friction

• When insulating materials rub against each other, they may become electrically charged
• Electrons, which are negatively charged, may be ‘rubbed off’ one material and on to the other. The material that gains electrons becomes negatively charged. The material that loses electrons is left with a positive charge.
• The electrons gain enough energy to leave/enter the materials

Charging by Induction

• Occurs when $e^{-}$ are unevenly distributed throughout the structure of an electrically neutral species
• A charged object is brought near another

Definitions and Stuff

• Current: flow of electrons
• Potential difference/voltage: the difference in the amount of potential electric energy that charge carriers have between two points in a circuit
• E.m.f. (electromotive force): potential difference generated by one or more cells

Coulomb’s Law

$F_e=k\times \frac{q_1{q}_2}{r^2}$

• where $F_e$ is the electric force (N)
• q = electric charge (C)
• r = distance between the 2 charges (m)
• k = constant of proportionality $N{m}^2{C}^{-2}$, = $9.00\times 10^9$
• If it’s a negative number, it’s an attractive force
• If it’s a positive number, it’s a repulsive force

$F_g=\frac{G{m}_1{m}_2}{r^2}$

Practise Problems

If 2 charges of +13.8 TC and -16.4 kC are situated $53.0\times 10^{10}$ metres apart, what force is acting between them? Click for answer

An attractive force of $7.25\times 10^3$ Newtons

If 2 charges (in a vacuum) have charges of 7.41 $\text{micro}C$ and 8.90 $\text{micro}C$ and experience a repulsive force of 13.2 N, how far apart are they? Click for answer

0.212 metres apart

2 charges of equal magnitude but opposite charges are 3.47 m apart and experience a force of 17.6 N. What are the charges of the 2 particles? Click for answer

$\pm 1.53\times 10^{-4}$ C