Basic Concepts and Resistance Flashcards
Movement of charge is
an electrical current
Charge of a electron and proton
+- 1.6x10-19C
Unit of charge
C
If a battery is rated at 120 amp hours it is
able to provide flow of 120 amps for one hour - way of describing charge as Q =It
E=
VQ=VIt= energy in J
P=
Power in watts W, It=Et = I^2 R = VI = V^2 /R
Recommend to use I^2 *R
kWh
Unit of power 1kWh is equal to power of 1000W being delivered for a period of 1 hour
Which side of a cell diagram is positive
longer slide current leaves the longer side and returns to the shorter side
What is an ideal voltage supply
Delivers a prescribed voltage across its terminals independent of the current flowing through it, no internal resistance
What is an ideal current supply
Delivers a prescribed current through its terminals independent of the voltage across them
A resistance is an
energy sink, converts electrical energy into thermal energy
Ohms Law
V=IR
Calculating the resistance of a material
R = rho * L/A where rho is resitivity in ohms per metre
What is conductivity
conductivity is the reciprocal of the resitivity ie 1/rho
it has unit siemens per metre S/m and given symbol sigma
What is conductance
conductance is 1/R it has symbol G units siemens S or ohms to the minus 1
How does resistance vary with temperature
as you increase temperature resistance increases
What is Ro
Resistance of a material at 0 degrees Celsius
If after several hours the current has reached a certain point what does this mean
The temperature has gotten to a steady point as the heat generated due to the resistance is equal to the heat lost to the surroundings
When using P=VI or P=V^2R
make sure that you are using voltage across the resistor of interest equation does not work otherwise
In series how do you add resistance
simply some Rt=R1 + R2 + R3 +….
At a node what is the rule for current
a node is a join, what goes in must come out current in = current out
in parallel how do you add resistance
1/Rt=1/R1 + 1/R2 + 1/R3 + ….
What is internal resistance
Resistance of a battery (will be in series)
what is the open circuit or no load voltage
When you have a non ideal battery it is the voltage supplied by the battery before the internal resistance
it would be the voltage across the terminal if no voltage was connnected
When does maximum power output occur and what does this mean
RL=Rint (load resistance) = (internal resistance), that maximum possible efficiency is 50% as equal power dissipation, can greater efficiency at lower power ouputs
Prove that max power output occurs at RL = Rint
setup basic internal resistance circuit with load resistance
calculate current flowing through circuit
Calculate the power going to the load resistor
differentiate with respect to the load resistance and equate to zero
Whats the difference between an ideal and a practical current source
a practical source will have an internal resistance parallel to it
Where does the waste power go in a practical voltage supply
it heats up the source
maximum power output to any load occurs when
its resistance is equal to that of the rest of the circuit
