7.7 - Capacitor Charge & Discharge Flashcards
What is the property of the area under the graph of a potential difference, charge graph?
The electric potential energy stored.
How do you know when a capacitor is fully charged in a circuit?
Current is 0.
How do you investigate what happens when you charge a capacitor?
set up a circuit.
D.C power supply, high resistance resistor, switch and capacitor (all in series). connect to data logger.
turn on D.C supply and let data logger record voltage and current over time.
What does a plotted I-t graph look like?
An exponential decay curve.
What does a plotted V-t Q-t graph look like?
They are the same. They both start at 0 and move to a maximum value.
What is the area under an I-t graph?
I = Q/t
Q = I x t
therefore are under graph is charge.
Describe a capacitor charging?
1) as switch closes current starts to flow. Electrons start to accumulate on the negative plate of the capacitor
2) The other plate becomes more positively charged and the potential difference between the plates increases.
3) As charge builds up on the plates, electrostatic repulsion makes it harder and harder (electrons oppose electrons) for more electrons to be deposited. When the p.d across the capacitor = p.d across power supply, the capacitor is charged.
How do you correctly log the discharge of a capacitor?
- Remove the power supply from the circuit and close the switch.
- Let the capacitor discharge whilst the data logger records the p.d and current over time. - When the current and p.d across the plates are 0, the capacitor is fully discharged.
What does the discharge of a capacitor look like graphically? (for all I-t, V-t and Q-t graphs)
They all decay exponentially at the same rate.
(graphs are identical).
What is the definition of the time constant?
For discharging: The time taken for the charge, current or voltage of a discharging capacitor to decrease to 37% of its original value. 37% = 1/e
What is the equation for the time constant?
t = RC
R = resistance of the resistor (ohms)
C = capacitance of the capacitor (F)
What is the time constant of a charging capacitor?
The time taken for a charge or voltage of a charging capacitor to rise to 63% of its max value.
Why is the time constant RC, how does resistance and capacitance effect the circuit?
R - the resistance effects the current in the circuit
C - capacitance affects the amount of charge that can be transferred at a given potential difference.
What is the half life of a discharging capacitor?
The time taken for the charge, current or voltage of a discharging capacitor to reach half its initial value.
t1/2 = ln(2)RC
What is the capacitor charging equation? (when calculated with charge)
Q = Q₀ (1 - e⁻ᵗ/ᴿᶜ)
Q = charge on the capacitor plates (C)
Q₀ = fully charged capacitor (C)
t = time since charging began
RC = resistance x capacitance (the time constant)
