9.1 - 9.11

Question 1

which side of a cell is positive

Answer 1

the positive terminal is the longer side of the symbol

Question 2

define potential difference

Answer 2

the energy transferred from electrical energy to other forms per unit charge

Question 3

define the volt

Answer 3

the P.d across a component when 1 joule of energy is transferred per 1 coulomb of charge passing through the component

Question 4

how can voltage be measured

Answer 4

connecting a voltmeter in parallel. ideally, it should have infinite resistance so that no current passes through it.

Question 5

define electromotive force (e.m.f)

Answer 5

the energy transferred from chemical (or other forms) to electrical energy per unit charge

Question 6

how does an electron gun work?

Answer 6

it needs a source of electrons. usually, through thermionic emission. then it’s placed in a vacuum and a high P.d is applied across the filament and an anode. the electrons accelerate towards the anode gaining kinetic energy. if the anode has a hole in it some electron pass through it creating a beam of electrons with a specific kinetic energy

Question 7

what is thermionic emission

Answer 7

the emission of electrons from the surface of a hot metal filament

Question 8

define resistance

Answer 8

a property of components calculated by dividing the P.d across it by the current in it

Question 9

define the ohm

Answer 9

the resistance of a component that has one Volt of P.d per ampere of current

Question 10

what is Ohm’s law

Answer 10

the P.d across a conductor is directly proportional to the current as long as the temperature remains constant

Question 11

how does temperature affect resistance?

Answer 11

as temperature increases, resistance increases. The positive ions have greater internal energy so they vibrate with a greater amplitude about their mean position. charge carriers then collide with them more frequently causing more work to be done

Question 12

what is the I-V characteristic of resistors

Answer 12

a straight-line graph with a constant gradient that goes through the origin. this means it has a constant resistance

Question 13

what is the I-V characteristic of filament lamps

Answer 13

it’s not a straight line but goes through the origin, it curves off horizontally. this means it has non-constant resistance that increases with P.d

Question 14

what is a diode

Answer 14

a semiconductor component that only allows current to flow in one direction

Question 15

what is the I-V characteristic of diodes

Answer 15

the current is zero for all negative P.d and a small part of the positive P.d. it curves upwards into a straight line. this means the resistance is not constant.

Question 16

what is the threshold P.d relating to diodes

Answer 16

the minimum P.d at which a diode begins to conduct. the resistance begins to drop sharply beyond this point

Question 17

what three factors affect resistance

Answer 17

the material,
the length of the wire,
cross-sectional area

Question 18

what’s the difference between resistance and resistivity

Answer 18

resistance may be different for components all made from copper due to different lengths or areas. resistivity is an electrical property that is constant for a material at a given temperature

Question 19

how is resistance affected by wire length

Answer 19

for any current, increasing the length of the wire increases the P.d across it. doubling the length doubles the P.d. V=IR, therefore resistance must have doubled. R∝L (directly proportional)

Question 20

how is resistance affected by the cross-sectional area

Answer 20

for any P.d, doubling the cross-sectional area doubles the current. V=IR so the resistance must have halved. therefore R∝ 1/A (inversely proportional)

Question 21

how can resistance be obtained from resistivity

Answer 21

by combining R∝L and R∝1/A you get R∝L/A. ρ (resistivity) is the constant of proportionality. therefore R= ρL/A

Question 22

define resistivity

Answer 22

property of materials. its the resistance of a component made of the material multiplied by its cross-sectional area divided by its length. measured in Ωm

Question 23

whats a negative temperature coefficient (NTC) thermistor

Answer 23

for an NTC thermistor, as the temperature increases the resistance decreases. this is because as the temperature increases the number density of charge carriers increases.

Question 24

what is the I-V characteristic of thermistors

Answer 24

it’s not a straight line but goes through the origin, it curves off vertically. this means it has non-constant resistance that decreases with P.d

Question 25

what is an LDR

Answer 25

light-dependent-resistor. its resistance decreases as light intensity increases. this is because the number density of charge carriers increases as light intensity increases.

Question 26

define electrical power

Answer 26

the rate of energy transfer by a component.

Question 27

how can you derive the formula for electrical power

Answer 27

power = Work done / time. Voltage = work done/charge

therefore power = (voltagecharge)/time. since current = charge/time therefore power = voltagecurrent

Question 28

what factors affect the electrical energy transferred to a device

Answer 28

the power of the device,

its operating time,

Question 29

what is a kilowatt-hour

Answer 29

a unit of energy used to deal with large numbers of joules. It’s the energy transferred to a device with a power of 1kW operating for 1 hour