o Aim of the lecture Appreciation of Current Density Drift Velocity Kirchhoff’s Laws Voltage Loop Law Current Node Law RC Circuits Response to step Voltages Charge and discharge o Main learning outcomes familiarity with Kirchhoff’s Laws and application to circu Typical current densities and drift veloc Calculation of RC time constant Charging and discharging Lecture 6
30
Embed
O Aim of the lecture Appreciation of Current Density Drift Velocity Kirchhoff’s Laws Voltage Loop Law Current Node Law RC Circuits Response to step.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
o Aim of the lecture Appreciation of
Current Density Drift Velocity
Kirchhoff’s Laws Voltage Loop Law Current Node Law
RC Circuits Response to step Voltages Charge and discharge
o Main learning outcomes familiarity with
Kirchhoff’s Laws and application to circuits Typical current densities and drift velocities Calculation of RC time constant Charging and discharging
o Electrons are made to drift in an electric field caused by an external voltage. They loose energy in collisions with the fixed atoms They therefore do not accelerate They drift at constant speed
o These are effectively energy conservation charge conservation
o Applied to circuits
Current Law
Charge cannot be destroyed, so the sum of currents flowing into a node is equal to the sum of currents flowing out ( hence it is vital to understand that capacitors do NOT store charge)
This is the reason we have been so ‘determined’ thatcapacitors should not be thought of as ‘storing’ charge - if they could then IIN would not necessarily be Ia+Ib.
o Finally, these are differential equations.o To find particular solution requires boundary conditionso For step voltages (switches for example)
determined from the conditions at t=0Need to evaluate voltages and currents just after switch movedImportant:
The voltage across a capacitor cannot change instantaneously Because E=CV2/2 so if the voltage change instant, implies infinite power
o The voltage across a capacitor CANNOT change instantlyo The current through a capacitor CAN change instantlyo The voltage and the currents for a resistor can BOTH change instantly