Book Reference : Pages 94-95Book Reference : Pages 94-95

1.1. To understand what a capacitor isTo understand what a capacitor is

2.2. To understand their applicationsTo understand their applications

3.3. To understand how we define capacitanceTo understand how we define capacitance

A capacitor is a passive electronic device which A capacitor is a passive electronic device which stores charge.stores charge.

They consist of two or more conducting plates They consist of two or more conducting plates separated by an insulating material.separated by an insulating material.

+Q -Q

Capacitor Symbol

electrons

When connected to a battery, the When connected to a battery, the plate connected to the positive side of plate connected to the positive side of the battery loses electrons to the the battery loses electrons to the battery and the other plate gains battery and the other plate gains electrons from the battery.electrons from the battery.

We say that the capacitor has a charge We say that the capacitor has a charge Q when the positive plate is at +Q and Q when the positive plate is at +Q and the other plate is at -Qthe other plate is at -Q

We will see that capacitors effectively block We will see that capacitors effectively block direct current (DC) but allow alternating current direct current (DC) but allow alternating current (AC) to flow(AC) to flow

Smoothing circuits Smoothing circuits : Noise on power supply rails : Noise on power supply rails can be removed by conducting away to ground can be removed by conducting away to ground

This This processor on a computer motherboard processor on a computer motherboard wants nice smooth stable power supply lines. wants nice smooth stable power supply lines. The high speed clock (several GHz) cause The high speed clock (several GHz) cause unwanted noise on these power lines. A unwanted noise on these power lines. A suitable capacitor connected between the suitable capacitor connected between the power and ground will conduct away the AC power and ground will conduct away the AC noise and leave the DC supply unchangednoise and leave the DC supply unchanged

Store power Store power : A large capacitor can be thought of : A large capacitor can be thought of as a small battery. It can store power for backup as a small battery. It can store power for backup purposes, (for example in devices where you purposes, (for example in devices where you need to change batteries but not lose need to change batteries but not lose configuration). They can also suddenly dump a configuration). They can also suddenly dump a very large amount of energy in a short time very large amount of energy in a short time (camera flash gun)(camera flash gun)

Tuned Circuits Tuned Circuits : In traditional radios and TVs the : In traditional radios and TVs the thing you twiddle to change station is a variable thing you twiddle to change station is a variable capacitor. This makes the capacitor. This makes the tuned circuit tuned circuit sensitive sensitive to only the station you wantto only the station you want

Filter Circuits Filter Circuits : Remove unwanted frequencies: Remove unwanted frequencies

Timing and pulse producing circuits : Timing and pulse producing circuits : on and off on and off after a preset delay, on/off repeatedly after a preset delay, on/off repeatedly

Definition : Definition : Current : An electric current Current : An electric current measured in Amperes is defined as the amount measured in Amperes is defined as the amount charge. (Coulombs) flowing per second (I = Q/t) charge. (Coulombs) flowing per second (I = Q/t)

A

V

Switch

Variable Resistor

Microammeter

High Voltmeter

Close the switch, record Close the switch, record the voltage at given the voltage at given times and use the times and use the variable resistor to keep variable resistor to keep the current constant the current constant (e.g. 15(e.g. 15AA

[or use data logger][or use data logger]

For a constant current of 15For a constant current of 15A find the charge in A find the charge in the table below:the table below:

Time /s PD / V Q/C0 0.0020 0.2940 0.6260 0.9080 1.22100 1.50

Plot a graph of Plot a graph of Charge Q against Charge Q against Potential Potential Difference VDifference V

What can we find What can we find from the graph?from the graph?

Graph of Charge Q against Potential difference VGraph of Charge Q against Potential difference V

The gradient is the value of capacitance (1015The gradient is the value of capacitance (1015F)F)

A capacitor is charged by means of a constant A capacitor is charged by means of a constant current of 0.5current of 0.5A to a P.D. 5.0V in 55s. Calculate:A to a P.D. 5.0V in 55s. Calculate:

The charge storedThe charge stored

The value of the capacitorThe value of the capacitor

A 22A 22F Capacitor is charged by means of a F Capacitor is charged by means of a constant current of 2.5constant current of 2.5A to a PD of 12.0V. A to a PD of 12.0V. Calculate:Calculate:

The charge storedThe charge stored

The time takenThe time taken

A capacitor is charged by means of a constant A capacitor is charged by means of a constant current of 24current of 24A to a P.D. 4.2V in 38s. The A to a P.D. 4.2V in 38s. The capacitor is then charged from 4.2V by means of capacitor is then charged from 4.2V by means of a constant current of 14a constant current of 14A in 50s Calculate:A in 50s Calculate:

Charge stored at a pd of 4.2vCharge stored at a pd of 4.2v

The value of the capacitorThe value of the capacitor

The extra charged stored at a current of 14The extra charged stored at a current of 14AA

The new PD after the extra charged was storedThe new PD after the extra charged was stored