In the previous discussion about lumens (https://physbang.com/2025/12/30/what-are-lumens/) some numbers were quoted without any explanation. This post adds a bit more detail to those numbers. Firstly, the maximum theoretical output of a light source was stated to be 683 lumens-per-watt. Why that number? The answer is to be found in the definition of the candela, which … Continue reading What are lumens? (part 2)
What are lumens?
Modern light sources are rated in lumens or, more importantly, lumens per watt. The first figure expresses the brightness of the source whereas the second indicates its efficiency. But what, exactly, is a lumen? To answer that question it is useful to recall that the first man-made light sources relied on heat and were very … Continue reading What are lumens?
Charging a Capacitor (analysis)
Changes in charge, current and potential difference during the charging and discharging of a capacitor are all exponential-type behaviours. Specifically, the current that transfers charge to or from a capacitor (during charging or discharging respectively) is always greatest at first and declines to zero as time increases. During discharging, the charge loss and the drop … Continue reading Charging a Capacitor (analysis)
Uncertainties and Error Bars
Measurements are always uncertain: there is no such thing as a perfectly precise measurement. At the very least, the instrument used will limit the precision of the value but in many cases other factors can introduce even greater variability. It is always the greater of these two numbers (the resolution of the instrument and variability … Continue reading Uncertainties and Error Bars
How to determine voltages inside a resistor network
Resistor networks sometimes look scary but they can be solved by taking things one step at a time and applying basic circuit rules. The rules concerned are the calculations for combining series and parallel resistors, together with Kirchoff’s two laws for circuits. To combine resistors, we add their values if they are arranged in series … Continue reading How to determine voltages inside a resistor network
How to Use Logarithms
Although they can seem confusing at first sight, logarithms are just mathematical functions that allow calculations to be carried out quicker than would otherwise be the case. So although they may seen daunting, you should think of logarithms as a way to make calculations easier – once you have mastered the rules. There is quite a … Continue reading How to Use Logarithms
Hypothesis and Analysis
When carrying out a practical investigation it is always helpful to know what sorts of results are expected. In other words, before starting an experiment it is a good idea to consider what theory tells us should be the outcome. The predicted findings form the basis of an hypothesis, which is developed by considering an … Continue reading Hypothesis and Analysis
Discharge and Fluorescent lighting
The earliest ways of generating light all involved heat, whether that meant setting a fire, burning a pitch torch, lighting a wax candle or heating a mantle using a gas flame. When electricity arrived, thoughts turned from creating light by chemical means to the possibility of using an electric current. Initial approaches still involved the … Continue reading Discharge and Fluorescent lighting
Electron deflection tube
It is common to think of the electron deflection tube as a device that demonstrates the behaviour of electrons within an electric field but in fact there are two electric fields at work. The first field accelerates electrons that are released by a heated coil then the second field deflects those electrons as they travel … Continue reading Electron deflection tube
Electric Field & Electric Potential
It is tempting to regard the equations for electric fields and electric potentials as being separate things but they are so closely related that they are best viewed as different sides of the same coin. This is because electric field strength is the same as potential gradient. Or to put it another way, the electric … Continue reading Electric Field & Electric Potential
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