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Yr 12 Physics 07 - Charge And Current > Drift Velocity > Flashcards

Drift Velocity Flashcards

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1
Q

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

A

the material must contain suitable charge carriers

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2
Q

what are tow examples of suitable charge carriers

A
  • loosely bond electrons in metals (delocalised)

- or ions in electrolytes and gases

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3
Q

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

A
  • the ball would begin swinging back and forth

- touching one plate and immediately swinging to the plate

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4
Q

what is the ball doing in that circuit

A

it is transferring charge as it touches the metal plates

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5
Q

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

A
  • the left plate would have a positive charge

- whereas the right plate would have a negative charge

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6
Q

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

A
  • 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
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7
Q

what is the effect of the ball constantly transferring charge

A
  • the circuit becomes complete

- so it allows a current to flow through it

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8
Q

what is the balls role in this circuit

A

it acts as the charge carrier

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9
Q

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

A
  • 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
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10
Q

in a metallic conductor what are the charge carriers

A
  • loosely bound outer electrons

- known as delocalised electrons

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11
Q

how do the outer electrons move in a metal

A
  • 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
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12
Q

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

A
  • an electric field is created

- which exerts a force on the free electrons

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13
Q

what is the motion of the electrons specifically like

A

they begin to drift in the direction of the force

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14
Q

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

A
  • because they collide with regularly spaced atoms (or ions)
  • in the crystal lattice
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15
Q

why are the atoms actually ions

A
  • 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
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16
Q

what do these collisions do to the electrons

A
  • they cause an equal and opposite force to be exerted on the electrons
  • which by newtons law, continue with a constant drift velocity
17
Q

what does the constant drift velocity make

A

a constant current

18
Q

which terminal of the cell do electrons drift to and why

A
  • 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
19
Q

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

A
  • 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
20
Q

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

A
  • 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)
21
Q

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

A

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

22
Q

what is the formula for calculating the current for a conductor

A

I = nAvq

23
Q

what are the units for I = nAvq

A
  • 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
24
Q

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

A

0.01ms-1 to barely even 0.1ms-1

25
Q

what is the first obvious thing you notice about drift velocities

A
  • they are very slow

- to the point where an electron would probably not go around the whole circuit once before the cell ran out

26
Q

how do drift velocities compare in thick an thin wires

A

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

27
Q

why are the drift velocities quicker in thinner wires

A
  • 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
28
Q

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

A
  • 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
29
Q

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

A
  • 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
30
Q

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

A
  • the potential difference

- a thinner wire would have a larger ‘pressure’or potential difference

31
Q

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

A
  • 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
32
Q

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

A
  • 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
33
Q

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

A
  • 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

Yr 12 Physics 07 - Charge And Current (4 decks)

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