Voltage, Current, Resistance Flashcards
(22 cards)
What is a free electron?
Electron that is not attached to an ion, atom or a molecule
Describe the properties of an electrical conductor.
- Good Conductivity
- Large number of free electrons
- Usually less than 4 valence electrons
- Copper, Aluminium
Describe the properties of an electrical insulator.
- Poor Conductivity
- Very few free electrons
- Usually more than 4 valence electrons
- Oxygen, Nitrogen
Describe the properties of an electrical semiconductor
- Neither good or poor conductivity
- Has fewer free electrons than conductors
- Usually 4 valence electrons
- Silicon, Germanium
What is a valence electron?
Electrons in the outermost shell of an atom
How many electrons are carried in 1 Coloumb?
6.25 x 10^18 electrons
What is voltage?
- Energy needed to move charges from 1 point to another
- V = W / Q
What is a voltage source?
Electromotive force that moves electric charges through a circuit.
What is current?
- Current is the rate of movement of electric charges between a potential difference.
- When a voltage is applied, electrons move in an orderly fashion
- I = Q / t
Whats the difference between conventional current and electron current?
- Conventional current moves from high to low potential in a conductor (anode to cathode).
- Electron current moves from low to high potential in a conductor (cathode to anode).
What is electrical resistance?
- Electric resistance is a hindrance to current flow.
- R = V/ I
How to find resistance of wire?
R= ρℓ/ A
What is a wirewound resistor?
- Thin metal alloy (such as nichrome) wire wound onto a ceramic former in a spiral helix
- Low resistance value, 0.01Ω to 100 kΩ
- High precision value
- Able to withstand high current and high power
What is a composition resistor?
- Made of carbon dust or graphite paste.
- Good for high frequency applications
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Large tolerances, typically ±20%.
100 Ω ± 20% = 80 Ω to 120 Ω
What is a film resistor?
- Carbon film, metal film, metal oxide film resistors
- Resistance value is controlled by varying the thickness of the deposited film and by cutting the film in a helical manner along its length.
- Low tolerance, as low as ±0.1% for some metal film resistors.
How to read resistor with a 4-Band colour code?
- First and second band indicate two digits of resistance respectively.
- Third band is the multiplier.
- Fourth band specifies the tolerance.
How to read resistor with a 5-Band colour code?
- First ,second and third band indicate three digits of resistance respectively
- Fourth band is the multiplier
- Fifth band specifies the tolerance
How to remember colour codes and their magnitudes?
- Big Boys Race Our Young Girls But Violet Generally Wins
- Black is 0 and white is 9
Why is it hard to make resisistors with exact resistance?
- It is neither economical nor practical to produce resistors with exact resistance values.
- A more practical solution is to produce resistors of nominal values with small but predictable deviation
What is conductance and its formula?
- Conductance of an object is a measure of how well the object conducts current.
- G = 1 / R (S, siemen)
State tolerance values for gold and silver band.
- Gold: ±5%
- Silver:±10%
State apparatus to measure voltage, current, resisitance
- Voltmeter
- Ammeter
- Ohmmeter
