8. Charge and current Flashcards
Define Electric Current
The rate of flow of charge
(must say flow)
What is the Equation and units for electric current?
I = ∆Q/∆t
I = electric current, amps A
Q = charge transferred, coulombs C
t = time, seconds s
What is the equation for electric charge?
∆Q=∆It
Define the Coulomb
The electric charge flowing past a point in one second when there is an electric current of one ampere
What is the elementary charge
e = -1.60x10^-19C
What is the total charge (equation)
Q = ±ne
Why is the charge on an object described as quantised?
Charge can only have certain values (an integer multiple of e)
What is current in terms of charge carriers?
The movement of electrons in metals
The movement of ions in electrolytes
How does charge flow through metals?
Metals have a regular crystal structure surrounded by a number of free electrons.
If you make one end of the wire negative and the other positive, the electrons will move from negative to positive.
How can you increase the current in a metal
Increase the number of electrons moving past a given point each second
or
Have the same number of electrons moving faster through the metal
What direction is conventional current?
Positive to negative
What are electrolytes?
Liquids that carry an electric current
How does charge flow in electrolytes?
If you place an anode and a cathode in an ionic solution,
the positive ions are attracted to and move towards the cathode,
the negative ions are attracted to and move towards the anode.
How do you measure electric current?
Place an ammeter in series at the point where you want to measure the current
Why should an ammeter have low resistance?
To minimise the effect it has on the current it is measuring
What is the conservation of charge?
Electric charge cannot be created nor destroyed
State Kirchhoffs first law
For any point in an electrical circuit, the sum of the currents in to that point is equal to the sum of the currents out of that point.
What is the equation for Kirchhoff’s first law?
∑I in = ∑I out
Define number density
The number of free charge carriers per unit volume
Why is a material with a higher number density a better electrical conductor?
The higher the number density the greater the number of free electrons per metre cubed.
This allows for more collisions between electrons to occur
So the material is a better electrical conductor
(the gist)
What is the number density of conductors?
10^28 m^-3
What is the number density of semiconductors?
10^17 m^-3
Describe the speed and motion of electrons
Electrons move slowly
Free electrons repeatedly collide with the positive metal ions as they drift throughout the wire towards the positive terminal.
What is the equation for mean drift velocity?
I = Anev
I - current/A
A - cross-sectional area/m^2
n - number density / m^-3
v - mean drift velocity / ms^-1
Define mean drift velocity
The average velocity of electrons as they move through a wire
