Charge in top plate changes inappropriately

[KatieWoe]

Test device:
Dell and Jordan
Operating System:
Win 10 and Mac 10.14
Browser:
chrome and safari
Problem description:
For phetsims/qa#306, may be connected to #234. When the sim is paused and discharging, moving the capacitors seems to make the amount of charge change. This should not be possible, and steping forward one step causes the charge to jump to the correct amount.
Steps to reproduce:

1. Charge the plates on the lightbulb screen
2. Turn on top plate charge bar graph
3. Pause the sim
4. Move the wires to connect to the bulb
5. Step forward and observe charge behavior
6. Move plates closer together or further away (while still paused)
7. Observe new charge bar
8. Step forward and observe

Screenshots:

Troubleshooting information (do not edit):

Name: ‪Capacitor Lab: Basics‬
URL: https://phet-dev.colorado.edu/html/capacitor-lab-basics/1.7.0-dev.7/phet/capacitor-lab-basics_all_phet.html
Version: 1.7.0-dev.7 2019-04-10 22:42:01 UTC
Features missing: touch
Flags: pixelRatioScaling
User Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/73.0.3683.103 Safari/537.36
Language: en-US
Window: 1536x722
Pixel Ratio: 2.5/1
WebGL: WebGL 1.0 (OpenGL ES 2.0 Chromium)
GLSL: WebGL GLSL ES 1.0 (OpenGL ES GLSL ES 1.0 Chromium)
Vendor: WebKit (WebKit WebGL)
Vertex: attribs: 16 varying: 30 uniform: 4096
Texture: size: 16384 imageUnits: 16 (vertex: 16, combined: 32)
Max viewport: 16384x16384
OES_texture_float: true
Dependencies JSON: {}

[arouinfar]

Nice find @KatieWoe!

Since the published version doesn't have a pause button, it is hard to tell if this bug exists there, but it doesn't look like it.

Definitely looks like charge isn't being conserved here @Denz1994. Moving the plates while the sim is paused should never affect the amount of charge. While discharging, the charge on the plates would exponentially decay.

[Denz1994]

@arouinfar I believe this issue persisted in the published version. You see the change when following the steps without pausing, but it is a lot less apparent.

The top plate charge only updates when the capacitor is connected to the battery. @arouinfar Please double check that this behavior is correct in master.

[arouinfar]

@Denz1994 master still looks off to me.

When the capacitor is disconnected from the battery, changing its capacitance results in a change in the voltage (Q is conserved and Q = CV). Here, decreasing C results in proportionally increased V.

Similarly, when connected to the bulb, changing C should create an instantaneous change in V. Here, decreasing C from 1.77 pF to 0.35 pF should have increased the voltage from 1.5 V to 7.5 V, resulting in a larger halo. Instead, the voltage remains 1.5 V, and the charge is instead reduced.

If you run into any questions, or would like to discuss over Zoom, please let me know.

[Denz1994]

this.capacitor.discharge( ) was is responsible for updating the inverse relationship between capacitance and voltage, when the light bulb was connected. This function was being called during step() so when the sim was paused this wasn't being handled.

The capacitor now discharges when the plate geometry changes which should maintain conservation of charge. @arouinfar could you confirm in master?

[arouinfar]

Looks great @Denz1994!

[KatieWoe]

May be connected to #259. As such, will leave up to @Denz1994 on whether this is fixed/whether to close

[Denz1994]

The changes in #259 should not have affected this, though closely related. This issue should be verified by QA to ensure it is resolved.

[arouinfar]

Charge is definitely not being conserved in master. For example:

1. Turn on Top Plate Charge
2. Charge capacitor (voltage irrelevant)
3. Disconnect capacitor, and take note of top plate charge.
4. Change plate separation or area, and the charge changes. This should not happen.

[KatieWoe]

I'm not always seeing as much as I saw in #256 (comment), and have some trouble reproducing to that degree for some reason. But even so I get a consistent one time jump when I start to move the plates.

[Denz1994]

The discharge model was simplified in the above commit. Also, the previousCapacitance was being changed as a side effect of discharging but used elsewhere (see updateDischargeParameters).

The steps in #256 (comment) should be verified by QA.

[KatieWoe]

Things are looking good in rc.8 so far. Closing for now, but will keep an eye out and reopen if something reappears.

[CliffordH123]

This has come back for phetsims/qa#616

Steps:

1. Charge plates
2. Pause Sim.
3. Switch plates to bulb
4. Do not un-pause
5. Change Plate distance
6. Observe top plate charge graph change

[arouinfar]

Good find @CliffordH123. Looks like yet another regression @jonathanolson, so I'm wondering if we should pause/cancel this dev test.

[jonathanolson]

@KatieWoe I believe this is fixed in master, can you verify?

[KatieWoe]

Adding this to issues to verify on QA board

[brooklynlash]

Looks fixed on master, Win10 Chrome.

[loganbraywork]

This appears fixed as of phetsims/qa#640 on Win 10 Chrome.