electricity

Question 1

T in calculation

Answer 1

T=horizontal divisions x the time base setting (usually in ms)

Question 2

voltage is determined by

Answer 2

number of vertical divisions (amplitude x y gain)

Question 3

Vrms

Answer 3

the value that produces the same effect as an equivalent value a.c. supply

Question 4

the peak value is always higher than…

Answer 4

the rms value

Question 5

rms

Answer 5

root mean square

Question 6

a voltage divider

Answer 6

a circuit with 2 series resistors/bulbs etc.

Question 7

the size of the voltage across each resistor depends on…

Answer 7

the size of the resistors

Question 8

if the resistors are the same

Answer 8

then they both have an equal share of the supply voltage

Question 9

when no current flows

Answer 9

the t.p.d. = emf

Question 10

as the resistance of the variable resistors decreases

Answer 10

more current is drawn form the battery so the current and the t.p.d. increases

Question 11

lost volts

Answer 11

volts that are lost in the internal resistance of the battery itself as it heats up

Question 12

emf =

Answer 12

t.p.d. + lost volts

Question 13

in a graph, the gradient is

Answer 13

m= -r

Question 14

emf

Answer 14

the electric potential energy supplied to each coulomb of charge which passes through the source

Question 15

emf stands for

Answer 15

electromotive force

Question 16

the lost volts are the

Answer 16

voltage available to the circuit

Question 17

t.p.d.

Answer 17

the voltage available to the circuit, what is across the load resistor (sometimes more than one resistor)

Question 18

capacitors

Answer 18

two conducting layers separated by an insulator which can store charge

Question 19

charge can be stored

Answer 19

by connecting the capacitor to a d.c. source, electrons leave the positive plate and are added to the negative plate

Question 20

capacitance

Answer 20

a measure of the charge it can store for every volt across it

Question 21

the time taken for a capacitor to charge up and discharge depends on:

Answer 21

the size of the capacitor and the resistance of the circuit

Question 22

when a capacitor is fully charged

Answer 22

the voltage across it is equal to the supply voltage

Question 23

when a capacitor discharges

Answer 23

charge moves off the plates in the capacitors in the opposite direction to the way it moved onto the capacitor

Question 24

conductor

Answer 24

a material for which an applied voltage causes a current to flow, the current is proportional to the voltage (ohm’s law)

Question 25

insulator

Answer 25

a material for which an applied voltage causes very little current, the current remains very small until the voltage becomes very large

Question 26

semiconductor

Answer 26

really just an insulator, but where the voltage needed to drive a current is smaller than usual

Question 27

the two highest energy bands are known as

Answer 27

the conduction bands and the valence bands

Question 28

the valence band is

Answer 28

always completely full of electrons, these electrons are not free to move

Question 29

the conduction band is

Answer 29

the higher band, any electrons in this band are free to move and allow the material to conduct

Question 30

the gap between the valence band and the conduction band is called

Answer 30

the band gap

Question 31

if there are free electrons in the conduction band

Answer 31

the solid will conduct

Question 32

in an n-type semiconductor

Answer 32

the free charge carriers are negatively charged electrons

Question 33

in a p-type semiconductor

Answer 33

these holes are just above the valence band in the atoms

Question 34

at the junction

Answer 34

electrons from the n-type move across into holes in the p-type

Question 35

p-type in a junction

Answer 35

slightly negatively charged

Question 36

n-type in a junction

Answer 36

slightly positively charged

Question 37

the barrier potential

Answer 37

a small voltage across the junction, typically around 0.7V

Question 38

why would red LED show when the ammeter gives a +ve reading and the green LED show when the ammeter gives a -ve reading

Answer 38

LEDs will light when they are forward biased, the change in polarity of voltage changes the biasing

Question 39

the gap between the valence band and the conduction band in semiconductors is

Answer 39

smaller than in insulators

Question 40

use band theory to explain how an LED emits light

Answer 40

electrons move towards conduction band of p-type, then drop from conduction band to valence band and a photon is emitted

Question 41

photovoltaic effect

Answer 41

a potential difference is produced when photons enter the layer between the p-type and n-type materials

Question 42

why, in terms of photons, would the maximum output power increase if a lamp is moved closer to the solar cell

Answer 42

greater number of photons per second

Question 43

why may batteries not work with some LED colours, in terms of band theory

Answer 43

the electrons do not gain enough energy to move towards the conduction band of the p-type

Question 44

forward bias of LED

Answer 44

conducts and emits light

Question 45

band theory

Answer 45

electrons are excited from the valence band to the conduction band of the n-type semiconductor. the electron will conduct to the conduction band of the p-type, drop to the valence band as it loses energy in the form of a photon

Question 46

lost volts decrease with

Answer 46

current

Question 47

when frequency changes what happens to the current

Answer 47

nothing

Question 48

initial current only depends on

Answer 48

the EMF and resistance

Question 49

what effect does adding impurity atoms have on resistance of the material

Answer 49

decreases it

Question 50

an n-type semiconductor is formed by

Answer 50

adding impurity atoms to a sample of pure semiconductor material

Question 51

maximum discharge current

Answer 51

V=IR with total resistance and EMF

Question 52

use band theory to explain how a potential difference is produced when photons are incident on the photodiode

Answer 52

electrons gain energy from photons and move from the valence band to the conduction band. electrons move towards n-type semiconductor

Question 53

photovoltaic effect

Answer 53

when a photodiode produces a potential difference when photons are incident on it

Question 54

explain why the charging current decreases as the battery charges

Answer 54

the emf of the battery increases so the difference between the two emfs decreases

Question 55

explain why current decreases with time when a capacitor discharges

Answer 55

the voltage decreases

Question 56

use band theory to explain why semiconductors might conduct at room temperature

Answer 56

the band gap is small so some electrons have enough energy to move from the valence band to the conduction band

Question 57

alternating current

Answer 57

changes direction and value with time

Question 58

conduction in an n-type semiconductor is by

Answer 58

the movement of free negatively charged electrons

Question 59

solar cells cause the

Answer 59

photovoltaic effect

Question 60

LEDs are

Answer 60

forward biased p-n junction diodes that emit photons when electrons fall from the conduction band into the valence band of the p-type semiconductor

Question 61

why might the voltage reading decrease when the switch is closed (if it is originally short circuit)

Answer 61

there is now a current so voltage is less across the internal resistance

Question 62

short circuit current

Answer 62

maximum current

Question 63

doped semiconductor

Answer 63

a semiconductor that has impurities added