Booklet 3 - Electricity (Factual) Flashcards
What is meant by alternating current?
An alternating current changes direction and instantaneous value with time.
Describe how to measure frequency using an oscilloscope.
Adjust the timebase until a number of complete waveforms can be seen on the screen.
Measure the distance between two neighbouring crests.
Multiply this distance by the timebase setting to get the period of the wave.
Use f = 1/T to calculate the frequency.
What is meant by the Root Mean Square Voltage? (Vrms)
It is the value of d.c. voltage that will deliver the same amount of power as an a.c. supply.
Which has a greater value? Vrms or Vpeak?
Vpeak is always greater than the Vrms
When calculating power should you use peak or rms voltage and current?
rms
What is meant by internal resistance?
This is the effective resistance within a power supply that can be used to model power losses within the supply itself.
What is an ideal supply?
A power supply with no internal resistance so that there are no power losses within the supply and it supplies a constant voltage.
What are the lost volts, Vlost?
This is the potential difference unavailable to the circuit because of the internal resistance of the supply.
It is difference between the emf and the tpd.
What is meant by electromotive force (EMF)?
The EMF is the voltage across the supply when no current is flowing and there is therefore no lost volts.
What is the terminal potential difference (tpd), V?
It is the potential difference across the load resistor when the circuit is complete and current flows.
In the following circuit what quantity will the voltmeter measure and why?
The e.m.f as NO current is flowing as the circuit is incomplete. When no current flows no voltage is lost across the internal resistance and tpd = emf.
In the following circuit what quantity will the voltmeter measure and why?
The tpd (V) as there is a complete circuit and current is flowing so, some voltage will be lost.
What is the short circuit current?
The maximum current a supply can give - this is achieved when the terminals of the supply are joined with a short thick wire (effectively zero external resistance) and all of the emf is applied across the internal resistance.
What is meant by an open circuit?
A circuit that is incomplete so that no current can flow.
Describe how you can measure the emf and internal resistance of a cell?
Set up the apparatus as shown.
Measure a range of voltages and currents, by changing the variable resistor.
Plot a graph of voltage versus current.
The internal resistance is -gradient.
The emf is the y-intercept.
From the graph, how can you calculate the internal resistance of the cell?
internal resistance = - gradient
From the graph, how do you find the emf ?
The y-intercept i.e. the voltage where current = 0A.
From the graph, how do you find the short circuit current?
The x-intercept i.e. the current when the tpd (V) = 0V.
Explain what happens to the reading on the voltmeter when the circuit is changed from figure 1 to figure 2.
Total resistance in the circuit decreases.
Total current in the circuit increases.
Lost volts will increase Vlost = Ir
Vtpd = E - Vlost
So the reading on the voltmeter will decrease as it measures the Vtpd.
What is capacitance?
The charge stored per unit voltage.
What is meant by 1 Farad?
A capacitor of 1 Farad will store 1 Coulomb of charge when the potential difference across it is 1 volt
Explain why work must be done to charge a capacitor.
Work must be done to charge a capacitor because any charge already stored on the plates will repel any further charge, requiring work to be done to overcome this force.
How is the energy stored in a capacitor calculated from a charge-voltage graph?
The energy stored in a capacitor is the area under a charge-voltage graph.
What could be altered in the circuit below to increase the charging time of the capacitor?
- Increase the resistance of the resistor
- Increase the capacitance of the capacitor
What could be altered in the circuit below to decrease the charging time of the capacitor?
- Decrease the resistance of the resistor
- Decrease the capacitance of the capacitor
Describe the shape of a graph of voltage across capacitor against time for charging a capacitor.
Starts from 0V. Increases to the supply voltage.
Describe the shape of a graph of voltage across capacitor against time for discharging a capacitor.
Starts from the supply voltage Decreases to 0V.
Describe the shape of a graph of current in a capacitance circuit against time for charging a capacitor.
Starts from a maximum value (Vs/R) and decreases to zero.
Describe the shape of a graph of current in a capacitance circuit against time for discharging a capacitor.
Starts from a maximum value (-Vs/R) and decreases to zero, current is in the opposite direction from the charging current.
Explain the shape of an I-t graph for charging and discharging a capacitor.
- When charging, a large current flows initally (limted only by the resistance in the circuit, I = Vs/R), but as charge builds up on the plates this repels any further charge trying to flow on to the plates, reducing the current.
The capacitor is fully charged when the repulsion equals the forward ‘push’ of the supply (i.e. Vc = -Vs) and the current reduces to zero. - When discharging, a large current flows initially in the opposite direction (negative values) because of the repulsion of all the charge on the plates. As some charge leaves, there is less charge left to repel further charge so the current reduces.
The capacitor is fully discharged when all the charge has left (i.e. Vc = 0) and no current flows.
Explain the shape of an Vc-t graph for charging and discharging a capacitor.
- For a capacitor voltage and charge are directly proportional.
- When charging, a large current flows initally giving a large increase in charge and voltage (Vc), but as charge builds up on the plates this repels any further charge trying to flow on to the plates, reducing the increase in charge and voltage.
The capacitor is fully charged when the repulsion equals the forward ‘push’ of the supply (i.e. Vc = -Vs). - When discharging, the initial voltage Vc = Vs but as charge leaves the voltage reduces meaning the current decreases and charge leaves more slowly.
The capacitor is fully discharged when all the charge has left (i.e. Vc = 0) and no current flows.
Describe the effect of changing the resistance in a capacitor charging circuit on a graph of voltage versus time.
A smaller resistance will make the capacitor charge up more quickly, larger resistance more slowly.
Describe the effect of changing the capacitance on a graph of voltage versus time when charging a capacitor.
A capacitor with smaller capacitance will charge up more quickly, larger capcitance more slowly.
Describe the effect of changing the capacitance on a graph of current versus time when charging a capacitor.
A smaller capacitance will charge up more quickly, larger capacitance more slowly.
Describe the effect of changing the resistance in a capacitor charging circuit on a graph of current versus time.
A smaller resistance will make the initial current greater and result in the capacitor charging up more quickly, larger resistance smaller initial current and slower charging.
NB Since resistance doesn’t affect the total charge stored in the capacitor, both graphs will have the same area under graph since this is the total charge.
From this energy level diagram, what type of material is being represented?
Insulator
Using this energy level diagram, explain why an insulator is a poor conductor.
The valence band is full and the conduction band is empty. The large band gap makes it unlikely that electrons will have sufficient energy to jump across to the conduction band and contribute to conduction. (This can happen in extreme conditions such as lightning.)
From this energy level diagram, what type of material is being represented?
Semiconductor
Explain how a pure semiconductor conducts.
There is a small band gap. At room temperature the electrons in the valence band can gain enough energy to jump the gap into the conduction band. These electrons are free to move and so conduction takes place.
From this energy level diagram, what type of material is being represented?
Conductor
Using this energy level diagram, explain conduction in conductors?
In a conductor the conduction and valence bands overlap and is only partially filled, which allows free electrons to move into accessible empty states and therefore conduct easily.
How is a p-n junction connected in forward bias?
Connect n-type to negative of supply connect p-type to positive of supply
How is a p-n junction connected in reverse bias?
Connect n-type to positive of supply Connect p-type to negative of supply
Explain why a forward biased p-n junction conducts.
- The applied voltage (Va) raises the energy levels of the n-type semiconductor
- This reduces the effect of the internal electric field (Vi).
- The electrons in the conduction band of the n-type now gain enough energy to overcome the internal electric field and pass into into the conduction band of the p-type.
- The p-n junction will conduct.
Explain why a reverse biased p-n junction will not conduct.
- The applied voltage (Va) raises the energy levels of the p-type semiconductor
- This increases the effect of the internal electric field (Vi).
- The electrons in the conduction band of the n-type do not have enough energy to overcome the larger potential difference of the electric field.
- Electrons cannot pass into the conduction band of the p-type so the p-n junction will not conduct.
Explain how an LED produces light.
- The electrons from the n-type move towards the conduction band of the p-type across the junction.
- Electrons fall from the conduction band to the valence band.
- The electron gives out energy as a photon of light.
Explain how a photovoltaic cell produces a potential difference.
- A photon is absorbed by an electron in the valence band of the junction.
- If this energy is high enough the electron can jump into the conduction band, leaving a hole in the valence band.
- The separation of the electron and hole generates a potential difference.
