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Module 4 - Capacitance Section 3.4.4

Section 3.4.4 - Capacitance: Recomended interactive site

Multiple choice Structured questions
Syllabus Extract You should be able to:

Definition of capacitance

Recall and use of

Revise this section after electric fields
Carry out calculations using this equation.

Explain the effect of varying 2 of the variables on the 3rd.
Recognise the circuit symbols for polarised and non-polarised capacitors.

Explain the practical differences between the two and suggest possible practical applications for each.

Practical: You have been shown electrolytic and non-electrolytic capacitors of widely varying capacitances (we have some very large caps. retrieved from an old amplifier!)

See animation on a variable capacitor's design.

See the animation on charging a capacitor.

Energy stored by capacitor

- derivation and use of

So E = ½ Q V = ½ C V2 = ½ Q2/ C

and interpretation of area under a
graph of charge against p.d.

Derive the equation using the Q/V graph as a basis.
Explain why the energy stored by a capacitor is half that provided by the supply in charging it (ie ½ QV compared to QV)

Refer to integral as area under graph and show that the units are consistent with work done ie energy.

Graphical representation of charging and discharging of capacitors through resistors

time constant = RC

Calculation of time constants including their determination from graphical data

Calculate what % discharge has occurred after time RC.

Derive the straight line form of the exponential decay equation (not on syllabus but good for the soul.)

Find values of R or C from graphical data.

Quantitative treatment of capacitor discharge

Candidates should have experience of the use of a voltage sensor and datalogger to plot discharge curve for a capacitor

Practical: Should know how to describe an experiment where you set up a capacitor discharge circuit using an analogue V.

Task: plot a straight- line graph to find accurate value for C.

Choose your own resistor value (need to be able to interpret resistor colour codes) by applying the RC value.

Then use datalogger with V blue box to plot ongoing exponential discharge graph and then use software to plot best fit line and find value of C.