Module 6 Capacitors Flashcards
How is a capacitor constructed?
two parallel plates separated by an insulator so that no current can flow through
Describe the changes that take place in the circuit, in terms of movement of electrons, in a charging circuit with a capacitor
When switch is closed, electrons move from negative side of the power supply to a plate on the capacitor making it negative
An equal number of electrons leave the other plate on the capacitor and travel to the positive side of power supply
Plates have equal and opposite charges
What is the relationship between the charge and the voltage in a charging circuit?
directly proportional
What is the final PD across a charging capacitor?
same as EMF of supply Vo
How to calculate initial current when initial voltage is known?
Io=Vo/R R is circuit resistance
Define capacitance
charge stored per unit potential difference
What are the units for capacitance?
F - farads
Define a farad
One Coulomb per unit Volt
Explain how the formula for energy stored in a capacitor, W=1/2QV is derived
V changes from zero to final V as Q increases
Average PD is V/2 (if plotted on graph of Q/V)
W=QV generally
W=1/2QV the area under the graph
Explain the missing energy from the formula W=1/2QV
Energy supplied by power supply is W=QV
Half the energy is missing
Missing energy is due to current in the wires producing heat (P=I^2R)
Give a practical use for capacitors
used to provide small bursts of current for camera flashes
What decreases exponentially for a discharging circuit?
V , I and Q
voltage across capacitor
current
charge on capacitor
Explain how the formula for exponential decay can be manipulated to find time
ln (x/xo) X( -CR)
Define time constant
time taken for the charge, current, or voltage, to reach 1/e of any initial value
1/e = 0.37 0r 37%
What is the relationship between the value of the time constant and rate of exponential decay?
larger the time constant the slower the decay
What are the units for time constant?
seconds
What is the constant ratio property?
Relevant for exponential decay
For equal time intervals, the value of x changes by the same ratio for any initial value of x
What must be done to prove the constant ratio property for capacitors
Time intervals of τ (RC)
Value from 0 to τ of y axis must fall to 0.37 of original
Value of y axis from τ to 2τ must fall to o.37 of value at τ
Etc
Explain how the time constant can be found from an exponential decay graph
take 0.37 of the original value on the y axis (I, Q or V) read to the x axis, that is τ
Explain how the exponential decay equation can be altered to match y = mx+c
lnX = ( -1/CR ) x t + lnXo
When charging a circuit with a capacitor, how does the voltage across the resistor vary?
PD on the resistor decreases exponentially
How can a constant current in charging of a capacitor be achieved?
Using a variable resistor which will be gradually reduced to keep current constant as V falls
When do we know RC is not constant?
when current is constant, or charge and pd change at constant rates
When does the spreadsheet method or iteration method work?
when values of time interval t are much smaller than that of time constant
so charge or voltage assumed constant over time interval t
