Electricity Flashcards

Question

What’s an easy way to define the formula E = V + Ir ?

Answer 1

Terminal potential difference + lost volts.

Answer 2

If two resistors are placed in parallel, Rtotal goes down, current goes up, lost volts go up, voltmeter reading goes down.

Answer 3

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.

Answer 4

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.

Answer 5

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.

Answer 6

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.

Answer 7

True. The charging then ceases. In this way the work is done in charging the capacitor.

Answer 8

It is given by the area under a graph of charge against p.d..

Answer 9

• 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

Answer 10

It’s a d.c circuit which contains a resistor and capacitor in series.

Answer 11

See PowerPoint slides.

Answer 12

False: the larger the value of R, the longer the time to discharge.

Answer 13

The product of CR.

Answer 14

- Gradient of the line is internal resistance. - intercept on y axis is emf (On a V I graph)

Answer 15

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.

Answer 16

Materials which are insulators when pure, but will conduct by the addition of impurities. E.g. silicon, geranium.

Answer 17

Materials which have very few free electrons, which can not move easily. E.g. plastic, glass and wood.

Answer 18

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.

Answer 19

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).

Answer 20

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.

Answer 21

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.

Answer 22

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.

Answer 23

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.

Answer 24

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).

Answer 25

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.

Answer 26

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.

Answer 27

Root mean square

Answer 28

The maximum output power from the solar cell increases as a greater number of photons strike the solar cell per second.

Answer 29

Wires with a larger diameter have a low resistance, so it prevents overheating and prevents the wires melting.

Answer 30

The reading on the voltmeter (voltage across the lamps) decreases and the total resistance deadens, current increases and lost volts increase.

Answer 31

The voltmeter reading decreases as there is a current in the circuit and the voltage is dropped across the internal resistance.

Answer 32

The lamp will stay lit for a smaller time and there is less energy/charge stored in a smaller capacitor.

Answer 33

You collect range of values for V and I and plot a graph to get the gradient of -r.

Answer 34

The current increases, lost volts increase, tpd decreases

Electricity Flashcards

(64 cards)