BRAINCAKECapacitors
Revision notes for Capacitors in OCR Physics A (H556). Read the start of any lesson here, then sign in free for the whole lesson, the R.E.C.I.P.E. recall steps and the quiz.
- A CapacitorA capacitor is a component that stores charge.
- Charging a Capacitor and Constant CurrentIn order to charge a capacitor at constant current a variable resistor is placed in the circuit to keep the current constant.
- CapacitanceThe capacitance C of a capacitor is defined at the charge stored per unit potential difference V The unit of capacitance is the farad F.
- Use of CapacitorsCapacitors are used in smoothing circuits e.g. when converting a.c. to d.c. camera flashes to store and then suddenly release charge in a short…
- Energy Stored in a Charged CapacitorWhen a capacitor is charged, energy is needed to remove electrons from the positive plate and push them onto the negative plate.
- Measuring the Energy Stored in a Charged Capacitor.The energy transferred to a lightlbulb when a capacitor discharges can be measured with a joulemeter.
- Energy stored in a ThundercloudIf the air is subjected to an electric field strength of more than 300kV-1 it ceases to be insulating.
- Discharging a Capacitor through a Fixed ResistorThe diagram below shows a capacitor charging-discharging circuit.
- Exponential Decay.When the charge on capacitor C decreases by ΔQ through fixed resistor R in time Δt, I = V/R where V = Q/C.
- The Time Constant RCGiven dQ/dt = -Q/RC, we get exponential equations for charge, current and time.
- Measurement with Oscilloscope. Capacitor SmoothingAn oscilloscope can be used to measure the potential difference across a resistor during the discharge of a capacitor.
- Charging a Capacitor through a fixed resistorA capacitor can be charged by connecting it to a source of constant potential difference.