Electricity Flashcards
What is current defined as and what is it measured in?
Current (I) is defined as the flow of electric charges per second and it measured in amperes on an ammeter (A).
What is voltage defined as and what is its units?
Voltage (V) is defined as the energy given to move the charges around the circuit. When the energy is transferred by a component in the circuit, it is known as potential difference. It is measured in volts on a voltmeter (V).
What is resistance, what is it measured in and how does it effect parallel and series circuits?
Resistance (R) is a measure of how much a component resists the flow of electric current. A high resistance results in a lower current and vice versa. It is measured in ohms on an ohmeter.
In a series circuit, the total resistance is always bigger than the biggest resistor.
In a parallel circuit, the total resistance is always smaller than the smallest resistor.
What is Ohms law?
When the resistance remains constant, the potential difference V across the component increases so does current I. They are directly proportional.
V=IR
How do you measure current in series and parallel?
I = I = I = I
I = I + I + I
How do you measure voltage in series and parallel?
V = V + V + V
V = V = V = V
How do you measure resistance in series and parallel?
Formulas on formula sheet.
What is a potential divider?
A potential divider circuit is a series circuit where the voltage/ potential is split between two resistors.
It is also known as a voltage divider.
Use supply voltage formulas.
What is power?
Power (P) is defined as the rate of energy transferred per second.
It is measured in watts.
What is direct current?
D.c is where the current only travels in 1 direction e.g. a battery.
What is alternating current?
A.c. Is where the current is constantly changing direction e.g. the mains socket. This instantaneous value changes with time.
What is an oscilloscope?
An electric device which can be used to draw a graph of electrical current.
A.c is above and below x axis.
D.c is above or below the x axis.
What are the two dials on an oscilloscope?
Time base (x axis)
Y gain ( y axis)
What is the period of a wave and what is it measured in?
The period of a wave is the time taken for one complete wave to pass a point. It has the symbol T and is measured in s.
What is the frequency of a wave and what is it measured in?
The number of waves to pass a point in one second.
Measured in Hz.
What is peak voltage?
The absolute maximum voltage . It is always bigger than the R.m.s voltage.
The value quoted for the mains is 230v and is the r.m.s value.
How do you calculate the Y - gain of an oscilloscope?
The peak voltage divided by the amplitude.
Volts/div
How do you calculate the peak voltage from an oscilloscope?
Amplitude x y - gain
How do you calculate the period from an oscilloscope?
Horizontal distance between peaks x time base
How do you calculate the frequency from an oscilloscope?
By finding the period and using the formula.
What is electromotive force (e.m.f)
The e.m.f symbol (E) of a source is the electric energy supplied to each coulomb of charge which passes through the source.
What is internal resistance?
All real cells and batteries have internal resistance.
An electrical source is equivalent to a source of e.m.f with internal resistance.
What is e.m.f measured in?
Volts
What is the ‘lost volts’ ?
(Ir) The ‘lost volts’ increase with current.
What’s an easy way to define the formula E = V + Ir ?
Terminal potential difference + lost volts.
What would happen is a second resistor is placed in parallel with a load resistor, reading on voltmeter?
If two resistors are placed in parallel, Rtotal goes down, current goes up, lost volts go up, voltmeter reading goes down.
What is an experiment to find E and r?
See slide on power point.
What is a capacitor and what is it made from?
Capacitors are devices that can store charge in a circuit. The ability of a device to stiry charge is called its capacitance.
A capacitor is two conducting layers seperated by an insulator.
Learn circuit symbol.
Description of storing charge in a d.c circuit SUMMARY
Charge can be stored on parallel plates by connecting them to a d.c. circuit.
Electrons leave one plate at the same time and add to the other plate.
The energy to cause this transfer of charge from one plate to another is the work done.
The plates build up charge until the potential difference across the plates is equal to the potential difference of the source.
Large supply voltage = large charge in capacitor.
The charge (Q) on two parallel conducting plates is directly proportional to the V between the plates.
The charge lates store energy supplies to them in an electric field between the plates. When the source is disconnected, the charge and energy are stored.
True or false: Electrons flow from the negative terminal through the conductor to the positive terminal.
True.
What is capacitance and what is it measured in?
Capacitance (C) is the ratio of charge to p.d.
The unity of capacitance is the Farad (F).
Farads are ussually expressed in micro, nano, pico.
Work done in a capacitor.
Work done must be done against the electrostatic forces when pushing the electrons to the negative plate and pulling them off the positive plate. This work done becomes energy stored in the electric fields between the plates of the capacitor.
True or false: Once some charge is on the plate, this will repel more charge and so the current decreases until the electrons from the cell do not have enough energy to climb’ the potential gradient onto the plate.
True.
The charging then ceases. In this way the work is done in charging the capacitor.
How is the work done in a charging capacitor calculated by a graph?
It is given by the area under a graph of charge against p.d..
Summary of main points of energy in a capacitor.
• Work (energy) is required to charge a capacitor to move the electrons onto the plate against the force due to the other charges on the plate.
• This energy is found by the area under a QU graph or by any of the equations:
E = QV = ½ CV = z Qª/C
What is a CR/RC circuit?
It’s a d.c circuit which contains a resistor and capacitor in series.
Voltage and current for charging/is charging C graphs.
See PowerPoint slides.
True or false: the smaller the value of R, the shorter the time to discharge a capacitor.
False: the larger the value of R, the longer the time to discharge.
True or false: The larger the value of C, the longer the time to discharge.
True
What is time constant?
The product of CR.
True or false: Charge and voltage are directly proportional when the resistance remains constant.
True.
Experiment to investigate charge and p.d across the plates of a capacitor.
Power point slide.
How do you calculate the internal resistance and E.M.F from a graph
- Gradient of the line is internal resistance.
- intercept on y axis is emf
(On a V I graph)
What is a conductor?
A conductor is a material with many free electrons which can flow through the material. E.g. all metals, semi metals like graphite and arsenic.
What is a semiconductor?
Materials which are insulators when pure, but will conduct by the addition of impurities. E.g. silicon, geranium.
What is an insulator?
Materials which have very few free electrons, which can not move easily. E.g. plastic, glass and wood.
What is band theory in relation to conductors, semi conductors and insulators?
The electrons in an isolated atom occupy discrete energy levels. When atoms are close together to each other, these electrons can use the energy levels of their neighbours. When the atoms are all regularly arranged in what is called the crystal lattice of a solid, the energy levels became grouped together in a band. This is a continuous range of allowed energies rather than a single level. There will also be groups of energies that are not allowed, in what is known as a band gap. Similar to the energy levels of an individual atom, the electrons will find the lower bands first. The Fermi level gives us a rough idea of which levels electrons will generally fill up to, but there will always be some electrons with individual energies above this.
CONDUCTORS
In metals, the highest occupied band is not completely full and this allows the electrons to move and therefore conduct. The band is known as the conduction band.
INSULATORS
In an insulator, the highest occupancy band (the valance band) is full. The first infilled band above the valance band is the conduction band. For an insulator, the gap between the valence band and the conduction band is large and at room temperature there is not enough energy available to move electrons from the valance band into the conduction band where they would be able to contribute to conduction. There is no electrical conduction in an insulator.
SEMICONDUCTOR
In a semiconductor, the gap between the valence band and the conduction band is smaller and at room temperature 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 by giving more electrons the energy to move from the valence band to the conduction band. As the resistance is decreased.
What is doping?
When we add impurities to a pure semiconductor this is called doping.
Doping reduces the resistance of a semiconductor (therefore making it more conductive to electrical current).
Semiconductors in silicon (doping)
The silicon atom has 4 valence electrons electrons in the outer shell which means they have 4 electrons available for bonding.
In a pure crystal? Each atom is bonded to another atom, meaning there are very few free electrons.
The material insulates
At higher temperatures, a few electrons escape from their atoms.
They leave behind a “hole”.
Both electrons and holes increase conductivity and reduce resistance.
N-type semiconductors
Arsenic is an impurity with five outer electrons. If it is added to the silicon, it leaves one spare electron. These free electrons can form a current. This is called a n-type semiconductor.
P-type semiconductors
Indium is an impurity with three outer electrons. If it is added to the silicon, it leaves a ‘hole’ i.e. a missing electron. This hole can move through the lattice, carrying a positive charge. This is called a p-type semiconductor.
What’s the p-n junction diode?
When a semi conductor is grown so that one half is p-type and the other type is n-type, the resulting product is known as a p-n junction diode.
There is an electric field in the p-n junction. The electrical properties of this p-n junction are used in a number of devices.
Biasing (applying voltage)
If the cell is connected positive end to p-type and negative end to n-type, this is forward biased.
If the cell is connected negative end to p-type and positive end to n-type, this is reversed biased.
Forward bias reduces the electric field; reverse bias increased the electric field in the p-n junction.
The LED in relation to p-n junctions
The LED has a p-n junction very close to the surface. It is used in forwards biased mode.
The forward bias potential difference across the junction causes the electrons to move from the conduction band of the n-type semiconductor towards the conduction bands of the p-type semiconductor.
Photons are emitted when electrons ‘fall’ from the conduction band into the valence band either side of the junction).
The photodiode in relation to p-n junctions
A p-n junction coated in a transparent coating will react to light. Solar cells are p-n junction designed to that a potential different is produced when photons are absorbed. This is known as the photovoltaic effect
The absorption of photons provides energy to raise electrons from the valence band of the semi conductor towards the conduction band. The p-n junction causes the electrons in the conduction band to move towards the n-type semi conductor and a potential difference is produced across the cell.
P-N JUNCTION SUMMARY
p-n junction diode - Forward biased, conducts. Reverse biased, does not conduct.
LED - Forward bias, conducts and emits light. Reverse bias, does not conduct and does not emit light.
Photodiode - Not bias - photovoltaic effect - acts like a solar cell.
What’s the Fermi level?
What does RMS stand for?
Root mean square
What happens when a lamp moves closer to a solar cell?
The maximum output power from the solar cell increases as a greater number of photons strike the solar cell per second.
What happens when you connect larger thicker wires in a circuit?
Wires with a larger diameter have a low resistance, so it prevents overheating and prevents the wires melting.
What happens when a charging current decreases? (Two battery’s)
The battery charges as the end of the battery increases and the differences between the emfs decrease.
What happens when the switch is closed and there is two lamps on a voltmeter?
The reading on the voltmeter (voltage across the lamps) decreases and the total resistance deadens, current increases and lost volts increase.
What happens when a switch is closed to connect an EMF to a circuit?
The voltmeter reading decreases as there is a current in the circuit and the voltage is dropped across the internal resistance.
What happens when a smaller capacitance is connected to a lamp?
The lamp will stay lit for a smaller time and there is less energy/charge stored in a smaller capacitor.
True or false: Less resistance = larger initial charging current of capacitor
True
True or false: Capacitance = supply voltage
True
How do you obtain the value of a variable resistor in an experiment?
You collect range of values for V and I and plot a graph to get the gradient of -r.
True or false: lost volts decrease with current
False
True or false: emf = supply voltage
What happens to the tpd when resistance decreases?
The current increases, lost volts increase, tpd decreases
