Unit 3 - Electricity Flashcards
What is the rms value of an alternating voltage/current?
The value of an AC voltage which will deliver the same amount of energy as a d.c. voltage.
How do you calculate current in a short circuit?
V = Ir (notice small r)
What does the y-intercept, x-intercept and gradient equal?
y-intercept = E (EMF)
x-intercept = short circuit (max) current
gradient = -r (internal resistance)
Explain how a capacitor charges.
- Current is switched on, electrons flow onto one plate of the capacitor and away from the other.
- This results in one plate becoming negatively charged and the other plate positive.
- Eventually the current ceases to flow. This happens when the p.d. across the plates of the capacitor is equal to the supply voltage (Vs)
How does larger resistance and larger capacitance effect charge and discharge time?
They both increase the charge or discharge time. (think of the formula)
Describe the valence and conduction band in conductors.
In conductor, valence band overlaps the conduction band, making it simple for electrons to move between valence and conduction bands.
In conductor, highest occupied band is not completely full, allowing electrons to move through the material and conduct easily.
Describe the valence band and conduction band in insulators.
In an insulator the highest occupied band is full. This is called the valence band. Since it is full, electrons cannot move through this level.
The first unfilled band above the valence band is the conduction band. Ince it is empty there are no electrons to move through this level.
Describe the electrons in conductors.
The valence electrons are essentially free, depicted as an overlap of the valence band and the conduction band, so that a fraction of the valence electrons can move through the material.
Describe the electrons in insulators.
There’s a large forbidden energy gap between the valence band and the energy at which electrons can move freely through the material in the conduction band.
Describe the electrons in semiconductors.
The gap between the valence band and the conduction band is smaller, and at room temperature there is sufficient energy available to move some electrons from the valence band into the conduction band, allowing some conduction to take place. An increase in temperature increases the conductivity of a semiconductor as more electrons have enough energy to make the jump to the conduction band. This is the basis of a thermistor where an increase in temperature produces a lower resistance.
What is doping?
The process of adding impurities to semiconductors to make them more conductive.
What is an n-type semiconductor?
When an impurity is added to a lattice creating extra electrons. These extra electrons will be free to move and conduct. Since the conductive increases, the resistance decreases.
What is a p-type semiconductor?
When an impurity is added to a lattice creating a lack of electrons, a spot where an electron is ‘missing’. an electron from the next atom can move into the hole created. Conduction can then take plus bu the movement of positive holes.
When is a diode forward-biased and what is it?
When the negative end of a cell is connected to the n-type semiconductor. A potential difference of about 0.7 volts exists across the junction of an unbiased diode. To make a diode conduct, a potential difference greater than 0.7 volts must be applied across the diode in the opposite direction. If the junction Is forward biased then the majority charge carriers can flow across the junction and round the circuit.
When is a diode reverse-biased and what is it?
When the negative end of the cell is connected to the p-type semiconductor. If a potential difference is applied across the diode in the opposite direction we say the junction is reverse biased. The effect of this reverse potential difference is to increase the width of the depletion layer forming an even greater barrier to the flow of charge carriers. The diode scarcely conducts. There is a very small current, know as the reverse of leakage current.
