Drift Velocity

Question 1

what condition has to be met in order tor a current to flow in a material

Answer 1

the material must contain suitable charge carriers

Question 2

what are tow examples of suitable charge carriers

Answer 2

• loosely bond electrons in metals (delocalised)

- or ions in electrolytes and gases

Question 3

a circuit has a high voltage supply with two metal
plates facing each other (not touching) and an ammeter in series. if a ball coated in aluminium paint were to be dangling in between the plates, what would you see happen to the ball

Answer 3

• the ball would begin swinging back and forth

- touching one plate and immediately swinging to the plate

Question 4

what is the ball doing in that circuit

Answer 4

it is transferring charge as it touches the metal plates

Question 5

what would the charges of the plates be if the current was flowing from left to right

Answer 5

• the left plate would have a positive charge

- whereas the right plate would have a negative charge

Question 6

what is actually happening to the ball in this circuit now that you know the charges of the plates

Answer 6

• the ball is being attracted to the positively charged plate (or the negative whichever is first)
• when it comes into contact with the plate it transfers its electrons and becomes positively charged
• when it becomes positive it repels with the positive plate so swings in the other direction
• and because the other plate is negatively charged it is attracted to it
• where it will come into contact with it and become negatively charged

Question 7

what is the effect of the ball constantly transferring charge

Answer 7

• the circuit becomes complete

- so it allows a current to flow through it

Question 8

what is the balls role in this circuit

Answer 8

it acts as the charge carrier

Question 9

why is it conventional to take the direction of an electric current to be in the direction in which a positive charge would move

Answer 9

• because if the charge carriers were negative like the electrons in a connecting wire
• the charge carriers actually move in the opposite direction to the current

Question 10

in a metallic conductor what are the charge carriers

Answer 10

• loosely bound outer electrons

- known as delocalised electrons

Question 11

how do the outer electrons move in a metal

Answer 11

• they move with random thermal motion
• back and forth within the crystal lattice of the metal
• at speeds approaching one-thousandth the speed of light

Question 12

what happens when a potential difference is applied to a circuit which causes the electrons to move

Answer 12

• an electric field is created

- which exerts a force on the free electrons

Question 13

what is the motion of the electrons specifically like

Answer 13

they begin to drift in the direction of the force

Question 14

if the electric field is constant, why dont the delocalised electrons accelerate at a constant rate as newtons second law says

Answer 14

• because they collide with regularly spaced atoms (or ions)
• in the crystal lattice

Question 15

why are the atoms actually ions

Answer 15

• because the electron(s) on their outer shell is delocalised and is moving
• leaving the atoms with a few less electrons and therefore a positive charge

Question 16

what do these collisions do to the electrons

Answer 16

• they cause an equal and opposite force to be exerted on the electrons
• which by newtons law, continue with a constant drift velocity

Question 17

what does the constant drift velocity make

Answer 17

a constant current

Question 18

which terminal of the cell do electrons drift to and why

Answer 18

• they drift to the positive terminal of the cell
• as they are negatively charged charge carriers
• so they will be attracted to the positive end

Question 19

why is the way in which we draw the direction of current (conventional current) wrong

Answer 19

• we draw current flowing from the positive terminal to the negative terminal
• when it should be from the negative to the positive as they are drifting towards it

Question 20

why did we get the direction of current flow wrong in the first place

Answer 20

• when it was drawn scientists didnt know it was the drift of electrons that caused a current to flow in a metallic conductor
• so they just defined the flow in the wrong direction (positive to negative)

Question 21

what does this therefore mean in terms of the relationship between conventional current and the flow of charge

Answer 21

conventional current is in the opposite direction to the actual drift of electrons

Question 22

what is the formula for calculating the current for a conductor

Answer 22

I = nAvq

Question 23

what are the units for I = nAvq

Answer 23

• I = current
• A = area of cross section
• n = number of charge carriers per cubic meter
• q = charge on each carrier
• v = drift velocity of charge carriers

Question 24

what range would you typically expect the calculated drift velocity to be in

Answer 24

0.01ms-1 to barely even 0.1ms-1

Question 25

what is the first obvious thing you notice about drift velocities

Answer 25

- they are very slow | - to the point where an electron would probably not go around the whole circuit once before the cell ran out

Question 26

how do drift velocities compare in thick an thin wires

Answer 26

the drift velocities are much faster in thinner wires than in thicker ones

Question 27

why are the drift velocities quicker in thinner wires

Answer 27

- the current is the same throughout a circuit - in other words, the number of electrons passing a given point in a certain amount of time - in as thinner wire you have less electrons - and as current has to remain constant, the electrons in the thinner wire have to speed up - in order for the 'number of electrons passing a given point for a certain amount of time' to remain the same

Question 28

what are the changes in drift velocity in relation to the thickness of the wire synonymous to

Answer 28

- the flow of water in a pipe speeding up when it meets a constriction - its like when you put your finger over a tap - the amount of water flowing out is constant - but because there is a smaller exit (wire) the pressure increases - which leads to the water flowing out faster

Question 29

why is the water analogy a good one when it comes to explaining drift velocity

Answer 29

- because it correctly implies that the current (amount of water flowing out) doesnt actually change - it is just the 'opening' in which the current can flow through - and because the flow of current cant slow down as that will go against the conservation of charge - the water simply has to flow out quicker (faster drift velocity) - for the amount of water poured out over a given time to remain the same

Question 30

what would the 'pressure' actually be in the circuit and how would the pressure change in relation to the thickness of the wire

Answer 30

- the potential difference | - a thinner wire would have a larger 'pressure'or potential difference

Question 31

a filament lamp has a thinner wire than the rest of the circuit. why would the drift velocity in the filament lamp be quicker in terms of the potential difference

Answer 31

- a larger potential difference is produced across the filament lamp - almost all of the p.d. provided by the cell - which actually applies the necessary force to speed up the electrons

Question 32

when you turn on a light, why does the light turn on instantly when the drift velocity is so slow

Answer 32

- although each electron is moving slowly - the electric field that is exerting a force on them and causes them to move travels at nearly the speed of light - this means that the electron start moving virtually instantly

Question 33

even though the electrons begin moving instantly, how does the filament lamp receive enough energy in order to instantly turn on when the drift velocity is so slow

Answer 33

- despite the slow drift velocity - there are simply a huge number of electrons - about 10^29 per m^3 - so therefore the charge flowing per second equates to a significant current