electricity

Question 1

direct current

Answer 1

flow of charge in one direction only; negative to positive

Question 2

alternating current

Answer 2

the terminals alternate their roles continuously, meaning the electrons effectively oscillate in place, back and forth

Question 3

timebase and voltage gain

Answer 3

timebase is x-axis
voltage gain is y-axis

Question 4

peak and r.m.s equations

Answer 4

Vrms = Vpeak/root 2
Irms = Ipeak/root 2

Question 5

potential difference definition

Answer 5

the energy given to each unit of charge by a power supply

Question 6

current definition

Answer 6

rate of flow of charge

Question 7

power definition

Answer 7

(electrical) the rate at which electrical energy is converted from one form into another

Question 8

resistance definition

Answer 8

the opposition of flow of charges.

Question 9

Ohm’s Law

Answer 9

V = IR
(voltage = current x resistance)

Question 10

internal resistance

Answer 10

resistance exhibited by power supplies

Question 11

EMF

Answer 11

• stands for electromotive force
• energy supplied to each coulomb of charge passing through the supply, before the effects of internal resistance are taken into account

Question 12

lost volts

Answer 12

the voltage ‘lost’ due to internal resistance and therefore not available to the rest of the circuit, generally converted into heat energy

Question 13

t.p.d

Answer 13

• terminal potential difference
• the voltage actually available to the circuit
  = EMF - lost volts

Question 14

what would an ideal supply have?

Answer 14

no internal resistance

Question 15

short circuit

Answer 15

occurs when a power supply is directly connected to itself, there is no internal resistance and current becomes very high.

Question 16

open circuit

Answer 16

occurs when there’s no connection between the terminals of the power supply (switch opened).
- no current
- no lost volts
- t.p.d and EMF equal

Question 17

what is a capacitor?

Answer 17

component that can be used to store energy, consists of two metal plates facing each other, separated by an insulator.

Question 18

how does a capacitor work?

Answer 18

a cell or battery does work to move charges away from one plate of the capacitor to the other, meaning one plate is positively charged and the other negatively. if it is connected to a different component, the capacitor will discharge and the charges will redistribute until both plates are neutral again.

Question 19

what is capacitance?

Answer 19

the measure of how much charge can be stored in a capacitor per volt connected across it.
measured in Farads

Question 20

capacitance equation

Answer 20

C = Q/V
( capacitance = charge/voltage)

Question 21

effect of varying capacitance

Answer 21

increasing capacitance increases the time taken to charge, initial current is not affected

Question 22

effect of varying resistance

Answer 22

increasing resistance increases the time taken to charge, initial current is lower

Question 23

band theory

Answer 23

energy bands exist in a very similar way to energy levels in individual atoms, when a material has very many atoms, it makes less sense to think about each possible energy level and more sense to treat the group as a band

Question 24

conduction band

Answer 24

outermost band, has the highest energy.
when it’s partially filled, the material can conduct

Question 25

valence band

Answer 25

band below conduction band

Question 26

properties of insulators

Answer 26

no electrons in conduction band, band gap too big for electrons to be promoted from valence band.
no conduction takes place

Question 27

properties of semiconductors

Answer 27

normally no electrons in conduction band but band gap is small enough that electrons could be promoted from valence band.
conduction will take place under correct circumstances

Question 28

properties of conductors

Answer 28

no gap between bands, they overlap so electrons are already in conduction band, this allows movement of electrons. conduction will take place

Question 29

production of n-type semiconductors

Answer 29

• group 5 element is introduced, this will result in a “free” electron, allowing a flow of charge within the material
• not electrically negative as each electron is still accompanied by a proton

Question 30

production of p-type semiconductors

Answer 30

• group 3 element introduced, which leaves a “hole” that can be occupied by an electron. provides a stepping stone-like path for electrons, allowing a flow of charge within the material
• not electrically positive as each electron is accompanied by a proton in the nucleus

Question 31

formation of p-n junctions

Answer 31

when a substrate is doped with both a group 3 and 5 element, an interface called a p-n junction forms
- the electrons from n-type will migrate to fill holes in the p-type
- this results in the n-type becoming slightly positive and the p-type slightly negative, the region between them is called a depletion layer

Question 32

forward bias

Answer 32

• diode is connected to d.c power supply in order to allow the flow of charge (n-type to negative terminal, p-type to positive terminal)
• increasing voltage causes depletion layer to shrink and when it’s overcome, the material will conduct

Question 33

reverse bias

Answer 33

• diode is connected to d.c power supply the opposite way round (n-type to positive terminal and p-type to negative terminal)
• power supply effectively increases depletion layer, so increasing voltage does not overcome depletion layer

Question 34

how do LEDs work?

Answer 34

• electrons in n-type move across to p-type
• this transition results in a decrease in electron energy which, in turn, causes a photon of the same energy to be emitted

Question 35

how do solar cells work?

Answer 35

• light incident on the junction within the solar cell causes an electron to be promoted to conduction band
• electrons migrate from p-type to n-type, creating a voltage across the solar cell
• this production of electricity is known as a photovoltaic effect

Question 36

band theory of electron energy