Capacitance

What Capacitance covers

Capacitance is examined in Component 2, Electricity and the Universe. It covers the parallel plate capacitor and stored charge, capacitance and the farad, the capacitance of a parallel plate capacitor, dielectrics, the uniform field between the plates and E = V/d, energy stored in a capacitor, capacitors in series and in parallel and charging and discharging through a resistor.

The 8 subtopics

Each one has its own page.

• The parallel plate capacitor and stored chargeConstruction of a simple parallel plate capacitor, net charge on a charged capacitor as a whole and charge stored on a capacitor.Open →
• Capacitance and the faradCapacitance of a capacitor and defining equation for capacitance with every symbol and its unit.Open →
• The capacitance of a parallel plate capacitorRecall questions on C = ε₀A/d with every symbol, the permittivity of free space, how capacitance depends on the plate area and separation, worked calculations, and why a practical capacitor is rolled from long strips of foil.Open →
• DielectricsTypical relative permittivity for a plastic film dielectric and relative permittivity of a vacuum.Open →
• The uniform field between the plates and E = V/dRecall questions on how a uniform field is produced and what uniform means, E = V/d, the two equivalent units for field strength, worked calculations of field strength and of the force on a charge, and why the field distorts at the edges.Open →
• Energy stored in a capacitorRecall questions on E = ½QV and its equivalent forms, why the factor of a half appears, the energy as the area under a charge–voltage graph, the fourfold rise when the voltage doubles, and worked energy calculations.Open →
• Capacitors in series and in parallelEquation for the combined capacitance of capacitors connected in parallel and equation for the combined capacitance of capacitors connected in series.Open →
• Charging and discharging through a resistorRecall questions on the shapes of the charge, voltage and current curves, the exponential decay equations, the time constant RC and the fraction remaining after one, the logarithmic plot that straightens the curve, and the experiment to determine the time constant.Open →

What you need to know

Answer each one before you reveal it.

Take Capacitance further

The same content, four other ways.

Questions from this topic

Each one with the model answer underneath.

1. 1

   Describe the construction of a simple parallel plate capacitor.

   Model answer

   It consists of a pair of equal parallel metal plates separated by a vacuum or by air.

2. 2

   Explain what happens to charge when a capacitor is connected across a battery.

   Model answer

   The battery transfers electrons from one plate through the circuit to the other, so one plate becomes positively charged and the other equally negatively charged.

3. 3

   What is the net charge on a charged capacitor as a whole?

   Model answer

   The net charge is zero, because the two plates carry equal quantities of opposite charge.

4. 4

   What is meant by the charge stored on a capacitor?

   Model answer

   The charge stored means the magnitude of the charge on one plate, not the sum of the charges on both.

5. 5

   Does charge flow across the gap between the plates of a capacitor while it charges? Explain your answer.

   Model answer

   No charge crosses the gap, because the plates are separated by an insulating vacuum or air. The charge travels round the external circuit from one plate to the other.

6. 6

   Why does the current in the charging circuit fall to zero as a capacitor charges?

   Model answer

   The accumulating charge builds up a potential difference between the plates that opposes the supply, and when the two are equal there is nothing left to drive charge round the circuit.

What is inside Levo

Six ways to work the same material.

Capacitance revision questions

What does Capacitance cover in Eduqas A-Level Physics?

8 subtopics: the parallel plate capacitor and stored charge, capacitance and the farad, the capacitance of a parallel plate capacitor, dielectrics, the uniform field between the plates and e = v/d, energy stored in a capacitor, capacitors in series and in parallel and charging and discharging through a resistor. Each one has its own page with questions and model answers.

How is Capacitance organised?

By the Eduqas A420QS specification, subtopic by subtopic, so it matches the order you are taught it in.