6.1 Capacitors

What is a capacitor?

an electrical component that stores charge of two separated metallic plates, with an insulator/dielectric placed between the pates to prevent charge from travelling across the gap

What is capacitance?

charge stored per unit potential different across the two plates

What happens when a capacitor is connected to DC power supply e.g. cell or battery?

there is a brief current as the power supply draws electrons from one plate and deposits them on the other plate, leaving first plate with charge +Q and the second with charge -Q - equal and opposite due to conservation of charge, current will flow until the potential difference between plates is equal to electromotive force of power supply

What is the rule for capacitors in series?

as VT=V1+V2, so 1/CT=1/C1+1/C2

What is the rule for capacitors in parallel?

as IT=I1+I2, and QT=Q1+Q2 (time is constant) so CT=C1+C2

What work is done on a capacitor?

depositing negatively charged electrons on a negatively charged plate (they repel), removing electrons from positive plate as negative charges are attracted to positive plate

What are the applications of capacitors?

stores and discharges lots of energy in short time - useful for camera flashes (short pulse) and backup power supplies, converting AC to DC (with a diode)

How does a capacitor discharge?

electrons packed onto negative plate repel and flow round the circuit, stops once no potential difference across capacitor

What is the time constant equation?

τ=CR

What happens to the time constant when the resistance is lower?

it is lower as current is higher so charge on plate will fall to zero in faster time as Q=It

What has happened to charge when one time constant has gone by?

decreased to approximately 37%

What is the discharging equation for charge?

Q=Q0e^-τ/CR

What is the discharging equation for voltage?

V=V0e^-τ/CR

What is the discharging equation for current?

I=I0e^-τ/CR

What is the charging equation for charge?

Q=Q0(1-e^-τ/CR)

What is the charging equation for voltage?

V=V0(1-e^-τ/CR)

What is the charging equation for current?

I=I0e^-τ/CR

After how many time constants can you say the capacitor is fully discharged?

5

Describe an experiment to investigate the charge and discharge of a capacitor-resistor circuit

What direction is conventional current?

positive to negative

What is direction of electron flow?

negative to positive

What is the area under a p.d. charge graph for a capacitor?

energy

Describe an experienet to investigate capacitors in both series and parallel combinations