, MODULE 6
CAPACITANCE = amount of charge that can be stored per unit volt of potential difference across the
capacitor measured in farads (F)
- C = Q/V
Charging and discharging process in terms of forces and flow of electrons:
- initially during the charging, there is a force from the EMF source pushing electrons around the
circuit. However, as electrons build up on the capacitor, a force to oppose the emf source builds,
decreasing the rate at which the charge can flow arounf the circuit.
- during discharging : there is initially a large electrostatic force to push electrons of the capacitor
from the other electrons stored. As the number of electrons on the plates reduces, the force decreases
and therefore the rate of flow of electrons decreases.
Capacitor in series : 1/Ctotal = 1/C1 + 1/C2 + …
Capacitors in parallel: Ctotal = C1 + C2 + ….
Energy stored in capacitor from graph, when potential difference is constant ( straight line through
origin) - W = QV
- when potential difference is changing - W = area under Q vs V graph = W = 1/2QV
Energy stored in capacitor from equations:
E = 1/2QV = 1/2CV^2 = (Q^2) / 2C
Where capacitors wouls be used as a store of energy:
- capacitors are useful when energy needs to be stored/discharged at a very fast rate ( for high power
applications)
- capacitors are used in F1 ERS systems to store energy when braking and release it to boost
acceleration
In terms of Ohms law the current in charging and discharging circuits:
- charging -
Initially the potential difference across the resistor is equal to the EMF of the power source as the
potential on the capacitor is zero so the largest current is recorded
- as the potential on the capacitor increases, the potential difference across the resistor decreases
and so the current decreases
- when the potential on the capacitor is equal to the poetential of the EMF source, the potential
difference = 0 and current = 0
- discharging-
Initially the potential difference across the resistor = starting potential difference across the capacitor
- as the capacitor discharges, its potential difference decreases and so therefore does the current
- when the capacitor is totally discharged, its potential difference = 0 , current = 0
Procedures to plot charging / discharging curves for a capacitor:
Connect the circuit shown with ammeter and voltmeter connected to a datalogger to record the
current and potential difference over time. The data can then be used to plot pd and current vs time
graphs
- by integrating the current equation ( finding area under graph) the charge stored / discharged by
capacitor can be found and plotted
CAPACITANCE = amount of charge that can be stored per unit volt of potential difference across the
capacitor measured in farads (F)
- C = Q/V
Charging and discharging process in terms of forces and flow of electrons:
- initially during the charging, there is a force from the EMF source pushing electrons around the
circuit. However, as electrons build up on the capacitor, a force to oppose the emf source builds,
decreasing the rate at which the charge can flow arounf the circuit.
- during discharging : there is initially a large electrostatic force to push electrons of the capacitor
from the other electrons stored. As the number of electrons on the plates reduces, the force decreases
and therefore the rate of flow of electrons decreases.
Capacitor in series : 1/Ctotal = 1/C1 + 1/C2 + …
Capacitors in parallel: Ctotal = C1 + C2 + ….
Energy stored in capacitor from graph, when potential difference is constant ( straight line through
origin) - W = QV
- when potential difference is changing - W = area under Q vs V graph = W = 1/2QV
Energy stored in capacitor from equations:
E = 1/2QV = 1/2CV^2 = (Q^2) / 2C
Where capacitors wouls be used as a store of energy:
- capacitors are useful when energy needs to be stored/discharged at a very fast rate ( for high power
applications)
- capacitors are used in F1 ERS systems to store energy when braking and release it to boost
acceleration
In terms of Ohms law the current in charging and discharging circuits:
- charging -
Initially the potential difference across the resistor is equal to the EMF of the power source as the
potential on the capacitor is zero so the largest current is recorded
- as the potential on the capacitor increases, the potential difference across the resistor decreases
and so the current decreases
- when the potential on the capacitor is equal to the poetential of the EMF source, the potential
difference = 0 and current = 0
- discharging-
Initially the potential difference across the resistor = starting potential difference across the capacitor
- as the capacitor discharges, its potential difference decreases and so therefore does the current
- when the capacitor is totally discharged, its potential difference = 0 , current = 0
Procedures to plot charging / discharging curves for a capacitor:
Connect the circuit shown with ammeter and voltmeter connected to a datalogger to record the
current and potential difference over time. The data can then be used to plot pd and current vs time
graphs
- by integrating the current equation ( finding area under graph) the charge stored / discharged by
capacitor can be found and plotted
