Student’s Guide

Photographic FlashThe Impressive Properties of Capacitors

Olivier Tardif-ParadisMathieu Riopel

Cégep GarneauCentre de démonstration en sciences physiques (support for the development of demonstrations)

APP/Student Guide 1

Impact of a drop of water photographed with a very short exposure made possible by the use of a flash

Source: http://commons.wikimedia.org/wiki/File:2006-02-13_Drop-impact.jpg

Immortalizing the World in ActionBackground

The first photographs date back to the early 19th century. Despite the success of the first photographic procedures for immortalizing the image of a still subject, it was impossible to take an image of a subject in motion. At that time, the main obstacle to “instant” photos was the long exposure required: an incredible amount of light was needed to make the sensitive surface of the photographic plate react, which demanded a lot of time, from several minutes to several hours. As technological improvements were made, exposure times became shorter and shorter. By 1880, exposure times of 1/100 of a second could be achieved using certain shutters and materials that were more light-sensitive. Even at these very short exposure times, a lot of light had to reach the recording surface. The need arose for a brief but extremely intense source of light.

It was not until 1887 that photographers could make use of the invention of the first photographic flashes. At that time, they used magnesium powder, which was burnt in an open recipient. This manual procedure was relatively dangerous because of the extreme flammability of magnesium. In 1930, a safer procedure came to the market: the flashbulb. These bulbs contained an aluminium wire in an oxygen environment. The metal was ignited by an electric current. After a single use, with the metal burnt, the bulb was thrown away. Flashbulbs were eventually abandoned by the public during the 1960s when a new affordable and less voluminous technology appeared: the electronic flash. The electronic flash uses a reusable xenon bulb and capacitors. When the shutter’s synchronization contact closes the electric flash circuit, the capacitors rapidly release a large amount of electric energy at a high voltage, ionizing the xenon in the bulb. The ionized gas becomes a conductor, releasing an intense light. A photographic flash can produce an intense light for a very short period of time (about 1/1000 of a second). This flash is generally used to illuminate a scene that is too dark or to take photos of rapid movement.

To understand the role of capacitors in the function of an electronic camera flash, you will design a device that creates a luminous flash comparable to photographic flash, using a source of continuous voltage, capacitors and an electric bulb.

PBL/Student’s Guide 2

Figure 1: In 1880, these Huron-Wendat, in Wendake, Québec, had to stay very still while their photo was taken. Notice the woman in the centre, who is blurry because she moved during the exposure.

Designing an Effective Flash

The system of an electronic flash is relatively complex. It includes capacitors, resistances, diodes, transformers, batteries and a discharge bulb. For this problem, we will simplify the system, considering only the following basic components: capacitors, a bulb and a source of electricity.

Keeping this simplification in mind, we are asking you to design an electronic flash that produces maximum luminous intensity but minimizes the length of the discharge, to achieve the current standard of less than 1/1000 of a second.

You will need the following materials:

A source of electric voltage that can provide a maximum electrical potential difference of 100 volts.

Four capacitors with a capacity of 200 F, to the terminals of which a maximum electrical potential difference of 120 volts can be applied.

o If the electrical potential difference applied to the capacitor’s terminals is higher than 120 volts, the electric field may attain the value of the disruptive electric field of the dielectric between the two surfaces of the capacitor. If this occurs, the dielectric will become a conductor; an intense electric current will travel between its walls, leaving nothing but a useless, burnt-out capacitor.

An incandescent electric bulb with a resistance of 16 ohms.

You need a maximum power of 2500 watts at the bulb terminals for it to light with optimal intensity without burning out. If the power supplied to the bulb is higher than this limit, the bulb will likely burn out and stop working.

You need a minimum power of 1300 watts at the bulb terminals for it to light properly. Below this power, the bulb will not emit enough light for the requirements of the flash.

PBL/Student’s Guide 3

Figure 2: Electronic camera flash, 2005 model

Three-step Cycle

List all the relevant information you gathered when you read the problem. Based on this information, state what you need to know to solve the problem. As you discover new information, you should summarize and update the relevant information you have gathered and ask new questions.

List the Following:

What We Know To Determine Summary

PBL/Student’s Guide 4

Questions

1) What configuration will you use to charge your system from the source of voltage, to maximize the energy stored in each capacitor? Calculate the total energy stored in the chosen circuit.

A few questions to head you in the right direction.a. What parameters influence the energy stored in a capacitor?

b. How is the voltage at the terminals of each capacitor affected by the choice of a series, mixed or parallel charge circuit?

PBL/Student’s Guide 5

2) Assuming that the capacitors have been charged by the circuit selected in question 1, you must now configure the discharge circuit. What discharge circuit configuration uses the fewest capacitors while still allowing the system to be functional without burning out the capacitors?

A few questions to head you in the right direction.a. How does limiting the power of the electric bulb affect the quantity of capacitors required in the

circuit?

b. What physical parameters do you have to limit to avoid burning out a capacitor when mounting the discharge circuit?

PBL/Student’s Guide 6

3) What configuration will you use to discharge your system into the bulb, knowing that the flash will have to release as much energy as possible without exceeding the maximum power of the bulb and without exceeding a duration of 1/1000 of a second?

A few questions to head you in the right directiona. What do you need to maximize in the discharge circuit if you want to maximize the energy in the

flash?

APP/Guide de l’élève 7

b. What equation describes the graph of the power consumed by the electric bulb as a function of time in an RC circuit?

c. Without exceeding the discharge time of 1/1000 of a second, how does a given configuration of capacitors maximize the energy dissipated by the flash? In this case what will be the duration of the flash?

Did You Know?

A xenon flash can only function at an electrical potential difference of between 200 and 400 volts. But it can only light if it is subjected to a spike in electrical potential difference of more or less 3000 volts. When the shutter release is pressed, the 300 volts from the capacitor pass through a transformer with a turns ratio of 10, producing an electrical potential difference spike of 3000 volts, allowing the flash to light. The concepts related to how transformers work are explained in college-level physics books.

PBL/Student’s Guide 8