Capacitors in circuits

Capacitors, capacitance.

Circuits with capacitors

A parallel plate capacitor consists of two metal plates, one carrying charge +q andthe other carrying charge –q.

It is common to fill the region betweenthe plates with an electrically insulatingsubstance called a dielectric.

Quick lab: charge the capacitor with a hand generator and measure potential difference across it.

Make a circuit with: capacitor, light bulb, switch and A-meter. Keep the switch open. Get the timer ready to measure the time it will take for the current to die out. Complete the circuit, measure the time and sketch a graph I(t) in the space below (use your actual numbers to mark the significant points).

THE RELATION BETWEEN CHARGE AND POTENTIALDIFFERENCE FOR A CAPACITOR

The magnitude of the charge in each place of the capacitor is directly proportional to the magnitudeof the potential difference between the plates.

CVq The capacitance C is the proportionality constant.

SI Unit of Capacitance: coulomb/volt = farad (F)Charge the capacitor again, use V-meter to determine the charge on either of the plates of the capacitor

THE DIELECTRIC CONSTANT

If a dielectric is inserted between the plates of a capacitor, the capacitance can increase markedly.

Dielectric constant E

Eo

THE CAPACITANCE OF A PARALLEL PLATE CAPACITOR

Three ways to increase capacitance:

A 5.0 F capacitor is connected to a 6.0V battery and then to a 12.0V battery. If C = Q/V, will the capacitance increase or decrease?

Example 12 A Computer Keyboard

One common kind of computer keyboard is based on the idea of capacitance. Each key is mounted on one end of a plunger, the other end being attached to a movable metal plate. The movable plate and the fixed plateform a capacitor. When the key is pressed, the capacitance increases. The change in capacitance isdetected, thereby recognizing the key which has been pressed.

The separation between the plates is 5.00 mm, but is reduced to 0.150 mm when a key is pressed. Theplate area is 9.50x10-5m2 and the capacitor is filled witha material whose dielectric constant is 3.50.

Determine the change in capacitance detected by the computer.

F100.19 12C

ENERGY STORAGE IN A CAPACITOR

Produce two other formulas, excluding:a) Cb) V

Parallel capacitors 321 CCCCP

VCCVCVCqqq 212121

212121

11

CCq

C

q

C

qVVV

Series capacitors 321

1111

CCCCS