Unit 2.1 - Conduction of electricity Flashcards
What are electrical currents caused by?
The movement of charged particles (electrons)
What does the movement of electrons cause?
Electrical currents
What type of charges is it that always moves in currents?
Negative
Unit of charge
Coulomb (C)
Coulomb unit
Unit of charge
Symbol for charge
q or Q (mostly capital Q for WJEC)
What’s Q the symbol for?
Charge
Charge of 1 electron
-1.6x10^-19C
(In data book)
How many electrons in 1 coulomb?
1
—
1.6x10^-19
= 6.25x10^18 electrons
How many protons in one coulomb and why?
6.25x10^18 (the same as electrons) as they’re the same size as electrons, just positively charged
How is an object that’s lost electrons charged?
Positively
How is an object that’s gained electrons charged?
Negatively
Current
The rate of flow of charge
The rate of flow of charge
Current
Conductors
Materials where he charges are in constant motion inside them
Give an example of a material that are all conductors
Metals
How can metals conduct electricity?
-have ions arranged in a crystalline lattice
-regular “corridors” along the structure in all directions
-outer electrons are freer than inner electrons
-move from atom to atom throughout the lattice
-“Corridors” - free to drift throughout the lattice
What type of process is metals conducting electricity and why?
Continuous process
Due to the thermal energy of the metal
Why is a metal conducting a current continuous?
Due to the thermal energy of the metal
Why does a piece of metal feel cold when you touch it?
Your hand is providing kinetic energy to the free electrons where the metal is being touched - electrons move away, taking thermal energy from the hand (thermal conduction)
Thermal conduction when touching a metal
A hand provides kinetic energy to the free electrons in a metal when touched, causing the electrons to move away, taking the thermal energy from the hand
2 examples of electrical fields
Potential difference and voltage
How is an electrical field set up?
Across the 2 ends of a metal
What happens when electrons are in an electrical field?
Random motion of free electrons pushed to 1 direction, towards the positive end
When electrons move towards the positive end when in an electrical field, how do they travel?
Don’t zip in a straight line, just have a general drift in one direction
Electrical current
When electrons drift towards 1 direction when in an electrical field
When electrons drift towards 1 direction when in an electrical field
Electrical current
Unit of electrical current
Amperes (A)
What is the Ampere the unit of?
Electrical current
Ampere definition
A current of 1 ampere flows if 1 coulomb of charge passes in 1 second
Equation for calculating electrical current
I = ∆Q
——
∆t
What does the symbol I (capital i) stand for?
Current (A)
Current symbol
I (capital i)
Equation for calculating a current in words
Current (A) = rate of charge (c)
————————
Time (s)
Which of the following is a base unit - ampere or coulomb?
Ampere
Derivation of coulomb unit
As
What is current measured as? What is this known as?
Flow from + to -
Conventional current
Conventional current
Flow from + to -
What’s the difference between conventional current and electrical current?
Electrical current involves the movement of negative electrons, so they move from a - to a + direction
(In the opposite direction)
How can we calculate charge on a current-time graph?
Area underneath
The area underneath of which type of graph can give us important data for this unit?
Are underneath a current-time graph = charge
How can we calculate charge on a graph and what type of graph would this be?
Area underneath a current-charge graph
What do electrons do in a wire when a potential difference is applied?
Move a distance l along the wire in a certain time
What can l be replaced with for the purpose of the I=nAve equation and how come?
vt, due to the equation for velocity, where l is the displacement
What equation can be used to calculate the drift velocity of electrons?
I = nAve
What are the steps in deriving the equation for current in terms of drift velocity of electrons (I = nAve)?
- Volume of cylindrical wire
A x vt (remember, using the velocity equation, the displacement l is not vt) - Number of free electrons in the wire
n x Avt
where n = the number of free electrons per m^3 - Total charge (Q)
Q = nAvt x e
where e = the charge of 1 electron (1.6x10^19) - Current (I)
I = ∆Q. =. nAvte. = nAve
——. ———
∆t. t
What is different for al metals in the I = nAve equation?
The value of n (the number of free electrons per m^3)
How do we calculate n, the number of free electrons per m^3?
Number of free electrons
————————————. (m^-3)
Volume of conductor
Compare how free electrons move before and after a potential difference is applied
Before
Randomly
No overall velocity
Free electrons move very quickly
After
Free electrons accelerated = overall velocity
Drift velocity is small
If we have twice as many free electrons in one wire compared to another (the same volume), how does the current, drift velocity and amount of free electrons per m^3 differ?
Same current
Half the drift velocity
Double the amount of free electrons per m^3
If one wire compared to another of the same length has twice the diameter of the other, how does its current, drift velocity and umber of free electrons per m^3 differ?
Same current
4x lower drift velocity
Same number of free electrons per m^3
Drift velocity definition
Resultant of the velocity vectors of the electrons
What does “number density” stand for?
“n” in I=nAve
What type of wire is more likely to overheat and why?
Thin, as there’s more collisions in the wire transferring more K.E. into the vibrational energy of the metal (heat energy)
What do we do if using a prefix and the original value is squared?
Multiply them together to get the new prefix
E.g 1.2mm^2 is not 1.2x10^-3 but 1.2x10^-6
What type of graph can we calculate resistance on and how?
Current vs potential difference
gradient
What are the free electrons in a conductor often referred to as?
“electron gas”
Is the velocity of each individual electron dependant on the temperature?
no, it’s independent on it
Draw a triangle relating charge, electrons and electron charge
Charge
e I electrons
What’s roughly the speed of electrons before a potential difference is applied?
1x10^6 ms-1
current
the rate of flow of charge
the rate of flow of charge
current
the rate of flow of charge
current
