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IAL Physics > Electricity > Flashcards

Electricity Flashcards

1
Q

Electric current

A

The amount of charge passing a point
per second

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2
Q

Electric current is measured in…

A

Amps

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3
Q

Charge (current) =

A

current x time

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4
Q

Charge

A

is a property of some particles, e.g. protons and electrons

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5
Q

Unit for charge

A

Coulumbs (C)

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6
Q

Electrons have a charge of

A

1.6 x10^-19 C

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7
Q

Charge (electrons) =

A

number of electrons x electron charge

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8
Q

1C of charge =

A

6.25 x10^18 electrons

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9
Q

Conventional current

A

defined as moving from positive to negative terminal. This is what is marked on all diagrams and used in calculations.

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10
Q

Electric field lines show:

A
  • Which way a positive charge will move
  • The direction of the electric field
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11
Q

Objects have gravitational potential energy due…

A
  • to the gravitational force acting on mass
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12
Q

Objects have electric potential energy due…

A

to their position in an electric field, and the electric force acting

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13
Q

Potential difference (or voltage)

A

is the difference in electric potential energy between two points for 1C of charge

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14
Q

When current passes through components:

A

energy is transferred from the charges:
- The charge lose electric potential energy
- The potential difference drops

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15
Q

When current passes through batteries/cells:

A

energy is transferred to the charge:
- The charges gain electric potential energy
- The potential difference increases
- We say the batteries/cells provide an electromotive force e.m.f (it just means a positive change in p.d.).

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16
Q

P.D.

A

is the energy transferred per coulomb from
charge carriers to components.

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17
Q

E.M.F.

A

is the energy transferred per coulomb to
charge carriers from sources (batteries/cells)

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18
Q

Resistance:

A
  • Charge carriers collide with the materials
    they flow through, whether it’s a lattice or
    a liquid.
  • Each collision transfers KE from the
    carriers to thermal energy in the material
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19
Q

Ohm’s law:

A
  • The current through a conductor at
    constant temperature is directly
    proportional to the voltage supplied
  • For each conductor this constant is its
    resistance.
20
Q

Resistance and temperature

A
  • Resistance increases with temperature
  • As temperature of a material increases, its
    atoms/molecules vibrate more.
  • This causes more frequent and disruptive
    collisions with charge carriers, causing
    greater transfer of energy.
21
Q

Ohmic conductors:

A

Anything with a fixed resistance - which therefore follows Ohm’s law:
- Fixed resistors
- Wires
- Most metal components

22
Q

Non-ohmic conductors

A

Anything with a variable resistance - which therefore shows non-linear relationships on I-V graphs:
- Filament lamps
- Diodes

23
Q

Resistance of a wire depends on:

A
  • Length (more wire to travel through, more resistance)
  • Area (more charge carriers (or more pathways), less resistance)
  • Resistivity
24
Q

R =

A

ρL/A

25
Q

Resistivity

A

Measure of the intrinsic resistance of amaterial, due to electron density, electron structure and lattice structure

26
Q

Temperature dependance

A
  • Regular conductors (as resisitivity increases temp increases), increase in energy goes to vibrations in the lattice
  • Semiconductors (as resistivity increases temp decreases), increase in energy frees more electrons for current
27
Q

free electrons:

A
  • Free electrons behave like a gas inside
    conductors
  • The more free electrons, the more
    conductive a material
28
Q

electrons drift

A
  • Electrons drift through circuits at
    about 0.1 mm s-1.
  • Electrons bump into the lattice and
    each other – they don’t travel in
    straight lines, so their drift velocity
    varies.
  • Electric signals travel faster than this
    because once a potential difference is
    applied to a circuit, all electrons
    move at once
29
Q

current (drift velocity) =

A

I = nqvA

30
Q

Semiconductors

A
  • semiconductors are materials (including the elements on the right) that have a conductivity less than conductors but greater than insulators
  • as temperature rises lattice ions release electrons, increasing number density (n) and so decreasing resistivity
31
Q

Holes

A

the positive ‘holes’ left behind by free electrons themselves move (the opposite way to electrons), also increasing the current

32
Q

In semiconductors electrons absorb
energy, moving from the…

A

valence band to the conduction band of an atom, leaving behind a hole.

33
Q

valence band

A

is the last electron shell of atom

34
Q

conduction band

A

contributes to the sea of delocalised electrons that cause current

35
Q

Insulators (band theory)

A

the conduction band is quite empty and there is a large energy gap

36
Q

Semiconductors (band theory)

A

the conduction band is quite empty but the energy gap is small.

37
Q

Conductors (band theory)

A

the bands overlap, so the conduction band has lots of electrons

38
Q

Thermistor

A

Resistance decreases with temperature:
- Heat energy transferred to free electrons, increasing charge carrier density
- (Regular conductors have positive temperature coefficient: R ∝ T )

Used to monitor temperature, e.g. thermostats, air-conditioning, incubators, electrical devices, preventing overheating

39
Q

Light-dependent resistor (LDR)

A
  • Made from a semiconductor material in which incident light frees more charge carriers, increasing charge carrier density
  • Used for automatic lighting
40
Q

Diodes

A

Non-ohmic component formed of two types of semiconductor that allows current in one direction only

41
Q

Superconductors

A
  • For these materials at very low temperatures (<30K) resistance drops to zero.
  • They undergo a phase transition in which
    electrons form pairs (Cooper pairs) due to a quantum-mechanical attraction.
  • The electron pairs behave like a superfluid in the lattice, flowing without viscosity - not losing any KE to collisions.
42
Q

Why do electrical signs travel faster?

A
  • All e- move at once
  • e- pass charge through collisions to move charge quickly
43
Q

Potential dividers

A

A component (resistor) that divides the potential energy

44
Q

Potential dividers equation:

A

v1 / v2 = r1/ r2

45
Q

3 electrical power equations

A
  • P = V x I
  • P = I^2 x R
  • P = v^2 / R
46
Q

More brightness means

A

more power

IAL Physics (5 decks)

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