Episode 129: Discharge of a capacitor: Q = Q_oe^-t/CR  

Students will have already seen that the discharge is not a steady process (Episode 125), but it is useful to have graphical evidence before discussing the theory.

You need to build up your students’ understanding of exponential processes, through experiments, and through graphical and algebraic approaches, all related to the underlying physical processes

Charging a Capacitor

Purpose: To study the charging and discharging of a capacitor.

Equipment: 800 mF Capacitor, 22 kW Resistor, Voltmeter, Shorting Bar, Stop Watch, Galvanometer, Power Supply.

Discussion: In this lab we will verify the forms of the expressions for the current and the charge as functions of time for charging and discharging capacitors. We will use the RC circuit shown to the right. As a capacitor charges, the

Charging & Discharging a Capacitor

Charging & Discharging a Capacitor

If the circuit below is set up we can examine the charging and discharging characteristics of a capacitor when it is connected in series with a resistor across a 10 V D.C. supply.

CHARGING

The switch is initially open (down) at t = 0, so that the power supply is short-circuited. At time t = 0.1 s (see Graph 1) the switch is pushed up to the position shown below and current begins to flow whilst the capacitor is charging. The value of the resistor in series with the capacitor controls the charge per second (current) that can flow on to the plates of the capacitor.

NOTES FOR CAPACITOR

3.1d  Capacitors

Capacitance is the ability (or capacity) to store charge. A device that stores charge is called a capacitor.

Practical capacitors are conductors separated by an insulator. The simplest type consists of two metal plates with an air gap between them. The symbol for a capacitor is based on this:

QUESTION IN CAPACITOR

When two parallel plates are connected across a battery, the plates will become charged and an electric field will be established between them.