P2 electricity

Question 1

what is current

Answer 1

current is the rate of flow of charged particles in a circuit

Question 2

what is the unit of current

Answer 2

amps (A)

Question 3

what is potential difference

Answer 3

the driving force that pushes the charge around of circuit

Question 4

what is the unit of potential difference

Answer 4

volts (V)

Question 5

what is the equation for charge

Answer 5

charge = current x time

Question 6

what is the symbol for charge

Answer 6

Q

Question 7

what is the symbol for current

Answer 7

I

Question 8

what is the symbol for time

Answer 8

T

Question 9

what is the equation triangle for charge

Answer 9

Q
I T

Question 10

what is resistance

Answer 10

anything in the circuit that slows the flow down

Question 11

what is the unit for resistance

Answer 11

ohms

Question 12

what is the equation for potential difference

Answer 12

potential difference = current x resistance

Question 13

what is the symbol for potential difference

Answer 13

V

Question 14

what is the symbol for resistance

Answer 14

R

Question 15

what is the equation triangle for potential difference

Answer 15

V
I R

Question 16

what are some characteristics of an LDR

Answer 16

• dependant on the intensity of light
• in bright light resistance falls
• in darkness resistance is highest

Question 17

what are some characteristics of a thermistor

Answer 17

• temperature dependant resistor
• in hot conditions resistance drops
• in cold conditions resistance increases
• useful temperature detectors

Question 18

what is a sensing circuit used for

Answer 18

to turn on or increase the power to components depending on the conditions they are in

Question 19

how is potential difference distributed in a sensing circuit

Answer 19

the potential difference of the power supply is shared out between the thermistor and the loop made up of the fixed resistor and the fan according to their resistances - the bigger a components resistance the more of the potential difference it takes.

Question 20

what happens to the resistance in a sensing circuit when the room gets hotter

Answer 20

the resistance of the thermistor decreases and it takes a smaller share of the potential difference from the power supply do the potential difference across the fixed resistor and the fan rises making the fan go faster

Question 21

describe the features of a series circuit

Answer 21

-in series circuits, the different components are connected in a line end to end.
- if you disconnect or remove a component the circuit is broken and they all stop.

Question 22

how is potential difference distributed in series circuits

Answer 22

the total potential difference of the supply is shared across the varies components so the potential differences round a series circuit always add up to the source potential difference

Question 23

how is current determined in a series circuit

Answer 23

the size of the current is determined by the total potential difference of the cells and the total resistance of the circuit.

Question 24

how is current distributed in a series circuit

Answer 24

current is the same everywhere

Question 25

how is resistance distributed in a series circuit

Answer 25

resistance adds up - in series circuits the total resistance of two components is the sum of their resistances..

Question 26

why does resistance get added in a series circuit

Answer 26

because by adding a resistor in series the two resistors have to share the total potential difference. the potential difference across each resistor is lower so the current through each resistor is lower as well. in a series circuit the current is the same everywhere so the total current in the circuit is reduced when a resistor is added.this means the total resistance of the circuit increases. the bugger a components resistance the bugger its share of the total potential difference

Question 27

describe the features of a parallel circuit

Answer 27

- in parallel circuits each component is separately connected to the +ve and -ve of the supply - if you remove or disconnect one component it will hardly affect the others at all

Question 28

how is potential difference distributed in parallel circuits

Answer 28

potential difference is the same across all components. this means that identical bulbs will all be the same brightness

Question 29

how is current distributed in parallel circuits

Answer 29

current is shared between branches. in parallel circuits the total current flowing around the circuit is equal to the total of all the currents through the separate components. in a parallel circuit there are junctions when the current either splits or rejoins, the total current going into a junction is equal to the total current leaving it. if two identical components are connected in parallel then the same current will flow through each component

Question 30

what happens if you add a resistor to a parallel circuit

Answer 30

the total resistance will be reduced. when you have two resistors in parallel their total resistance is less than the resistance of the smallest of the two resistors. in more detail - in parallel both resistors have the same potential difference across then as the source - this means the pushing force making the current flow is the same as the source potential difference for each resistor that you add - but by adding another loop the current has more than one direction to go in - this increases the total current that can flow around the circuit. an increase in current means a decrease in total resistance of the circuit

Question 31

is the mains supply ac or dc

Answer 31

ac

Question 32

what are the three different wired in a plug

Answer 32

neutral wire -blue - the neutral wire completes the circuit live wire - brown - the live wire provided the alternating potential difference from the mains supply earth wire - green and yellow - the earth wire is for protecting the wiring and for safety, it stops the appliance casing from becoming live and it does not usually carry a current.

Question 33

what is the national grid

Answer 33

a giant system of cables and transformers that cover the uk and connect power stations to consumers

Question 34

how does the national grid work

Answer 34

-to transmit the huge amount of power needed you need either a high potential difference or a high current - the problem with high current is that you loose loads of energy as the wires heat up and energy is transferred to the thermal energy store of the surroundings - it is much cheaper to boot the potential difference up really high and keep the current relatively low - for a given power, increasing the potential difference decreases the current which decreases the energy lost by heating the wires and the surroundings. this makes the national grid and efficient way of transferring energy

Question 35

how is potential difference changed by a transformer

Answer 35

- to get the potential difference to 400000V to transmit power requires transformers as well as big pylons with huge insulators but its still cheaper - the transformers have to step the potential difference up at one end for efficient transmission and then bring it down to safe unstable levels at the other end - the potential difference is increased by a step up transformer - it is then reduced again for domestic use by a step down transformer - transformers are almost 100% efficient so you can assume that the input power is equal to the output power.

Question 36

what is build up of static caused by

Answer 36

friction

Question 37

how are electrons transferred through static electricity

Answer 37

-when certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other - this will leave the materials electrically charged with a positive static charge on one and an equal negative static charge on the other - the direction in which the electrons are transferred depends on the two materials involved

Question 38

what is a positvie static charge caused by

Answer 38

electrons moving away elsewhere. the material that loses electrons loses some negative charge and is left with an equal positive charge.

Question 39

how are sparks caused

Answer 39

- an electric charge builds on an object. the potential difference between the object and the earth increases - if the potential difference gets large enough electrons can jump across the gap between the charged object and the earth - this is the spark - they can also jump to any earthed conductor that is nearby which is why you can get static shocks when getting out of a car. a charge builds up on the cars metal frame and then you tough the car the charge travels through you to earth

Question 40

what is an electric field

Answer 40

an electric field is created around any electrically charged object. the closer to the object you get the stronger the field is and the further you are from it, the weaker the field is. - you can show an electric field around an object using field lines.