P4 - electric circuits

Question 1

how do you charge insulators

Answer 1

rub them with another insulator like a dry cloth

Question 2

why does rubbing an insulator charge it

Answer 2

electrons are transferred from one of the materials to the other

Question 3

what happens when two like charges are put near eachother

Answer 3

they repel

Question 4

what happens when you put two unlike charges near to eachother

Answer 4

they attract

Question 5

what is a non contact force

Answer 5

the force between two charged objects

Question 6

what happens when a field is too strong and why

Answer 6

sparking happens because some electrons are pulled out of air molecules by the force of the field

Question 7

what happens when you rub a polythene rod with a dry cloth (in terms of electrons)

Answer 7

electrons transfer from cloth to rod
rod becomes negatively charged

Question 8

what happens when you rub a perspex rod with a dry cloth (in terms of electron transfer)

Answer 8

electrons transfer from rod to cloth
rod becomes positively charged

Question 9

what is a cell used for

Answer 9

pushing electrons around a complete circuit

Question 10

what does a switch do

Answer 10

enables the current in a circuit to be switched on or off

Question 11

what does a bulb (indicator) do

Answer 11

emit light as a signal when a current passes through it

Question 12

what does a diode do

Answer 12

allow current through in one direction only

Question 13

what does a light emitting diode (LED) do

Answer 13

emit light when a current passes through it

Question 14

what does an ammeter do

Answer 14

measure electric current

Question 15

what does a fixed resistor do

Answer 15

limit current in a circuit

Question 16

what does a variable resistor do

Answer 16

allow current to be varied

Question 17

what does a fuse do

Answer 17

melt and ‘break’ the circuit if the current through it is higher than a certain amount

Question 18

what does a heater do

Answer 18

transfer energy from an electric current to heat surroundings

Question 19

what does a voltmeter do

Answer 19

measure potential difference (voltage)

Question 20

what is the size of an electric current

Answer 20

the rate of flow of electric charge
flow of charge per second

Question 21

what is the current at any point in a single closed loop

Answer 21

the same as the current at any other point

Question 22

what is the equation for charge flow

Answer 22

charge flow (Q) = current (I) x time taken (t)

Question 23

what is the current at any point in a series circuit

Answer 23

the same as any other point

Question 24

what is the equation for potential difference across a component

Answer 24

potential difference across a component = energy transferred / charge

Question 25

what is the equation for resistance

Answer 25

resistance = potential difference / current

Question 26

what is ohms law

Answer 26

the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor

Question 27

what does reversing the potential difference across a resistor do

Answer 27

reverse the current through it

Question 28

in a filament lamp, is current directly proportional to the PD?

Answer 28

no

Question 29

what happens in a filament lamp as the temperature increases

Answer 29

the resistance increases

Question 30

why does the resistance increase as temperature increases in a filament lamp

Answer 30

the atoms in the metal filament vibrate more as temperature increases so they resist passage of electrons through filament more

Question 31

what happens to the resistance if you reverse the potential difference in a filament lamp

Answer 31

it’s the same

Question 32

in a diode, is current directly proportional to PD?

Answer 32

no, the resistance changes as the current changes

Question 33

what happens when you change a diodes direction

Answer 33

reverse direction resistance is a lot higher than forward resistance

Question 34

what is a thermistor

Answer 34

a temperature dependent resistor

Question 35

what happens to a thermistors resistance if it’s temperature increases

Answer 35

it decreases

Question 36

what happens to an LDRs resistance if light intensity increases

Answer 36

it decreases

Question 37

what is the current in each component in a series circuit

Answer 37

the same as every component

Question 38

in a series circuit, what happens to the potential difference

Answer 38

it is shared between the components

Question 39

what is the total PD of cells in series the sum of

Answer 39

the potential difference of each cell

Question 40

what is the equation for total resistance in series

Answer 40

total resistance = R1 + R2

Question 41

what happens when you add more resistors in series

Answer 41

the total resistance increases current through resistors is reduced PD is unchanged

Question 42

what is the rule about current for components in parallel

Answer 42

total current through whole circuit is sum of currents through separate branches

Question 43

what is the rule about PD for components in parallel

Answer 43

PD across each component is the same

Question 44

in a parallel circuit, what happens to the current if the resistance increases

Answer 44

the current decreases

Question 45

what is the equation for current through a component

Answer 45

current = PD/component resistance

Question 46

what is the rule for resistors in parallel

Answer 46

total resistance of two (or more) components in parallel is less than resistance of the resistor with the least resistance

Question 47

why does adding for resistors in parallel decrease the total resistance

Answer 47

the total current through the resistors in increased PD across them is unchanged