Capacitance Flashcards
What is the equation for capacitance in terms of charge and voltage?
C=Q/V
What is the effect of a larger plate on the value of Capacitance?
Larger plate = more electrons for a given voltage, so higher capacitance
What happens to the effectiveness of a capacitor as charge increases?
Increases, as plates attracted to each other, which attracts the charges, which means more charge can fit on the plates
What is the time constant for a capacitor?
Time for a capacitor’s charge to fall to 1/eth its original value.
How do you prove an exponential decay?
Constant ratio - old value/new value constant for all adjacent pairs.
What is Q(0) and how is it calculated during capacitor charging?
Q(0) is the maximum charge when the voltage is equal to the supply voltage, V(0).
Q(0)=CV(0)
What is the relationship between current-time and charge-time graphs for a charging capacitor?
I-t graph exponentially decreases,
Q-t graph increases. Looks like a log graph.
What is RC and what is its significance in the circuit?
RC is the time constant. It represents the time taken for the charge in the capacitor to fall to 1/e of its original value.
When a capacitor is charging, what happens to the voltage, current and charge over time?
Charge increases, voltage increases logarithmically to the cell voltage, current decreases exponentially to zero when no current flows.
When discharging a capacitor, what affects the time taken?
Resistance of the circuit, which affects the current.
What is the approximate time taken to charge a capacitor
Around 5RC
What is the shape of a current time graph during charging?
Exponential decrease tending to zero
What is the shape of a charge time graph during charging?
Inverted exponential increase tending to CV(0)
What is the shape of a charge time graph during discharging?
Inverted exponential decrease tending towards CV(0)
What is the shape of current time graph during discharging?
same as for charging - exponential decrease tending towards 0.
