A simple earthquake detector

Some of my physics students decided to construct an earthquake device. They suspended a cubic block of metal so that it hung between two plates of equal rectangular surface area. The two plates were then conected to a 12V battery so that effectively we had two capacitors in series with the gap between plates and the sides of the block equal(0.82mm). The thread which the block hung from was connected firmly to the earth so that any tremor would cause a movement of the block and therefore change the distance between the plates and the block. A separate voltmeter was connected from the centre of the block to one side of the circuit.
Initially since there is the same distance between plates and block, with equal capacitance, the voltage across each is 6.0 V. As soon as the earth tremors, the block moves and the distance between block and plates are not equal since capacitance and distance are inversely proportional, capacitance will increase or decrease and the side connected to the voltmeter will show this ( V = Q/C ) A decrease of about 0.1 mm will lead to a voltage drop of about 1 volt, so the slightest tremor will be detected.
With movement of people across a wooden floor affecting the apparatus, they linked the apparatus to the concrete foundations.
Next step is to link voltage change to the computer and finally to make comparisons with national seismographic data so they can convert voltage change to Richter scale.
They are great kids.

flash camera

A flash camera has a 2.5 V bulb with a constant resistance of 112 ohms and its in parallel with a 0.0077 Farad capacitor. The capacitor is in a loop with a 215 ohm resistor and a battery 2.50 V with internal resistance 2.15 ohms. The resistor loop and the bulb loop are have separate switches such that when the resistor switch is closed the capacitor charges up. From the total resistance ( 216.2) and the capacitance ( .0077F) we can calculate the time constant as 1.66 s. We can work out the current from Kirchoff's second law( closed loop V = 0) so 2.50 = 1.20xI + 215xI. This gives a current of 0.0116 amps.
Once the capacitor is fully charged(2.50V) we can open the resistor switch, close the bulb switch and the capacitor discharges through the bulb giving us a short flash as 63% discharges in 1.66 s and a further 63% in another 1.66s ( i/e1 is ~63%).
If we now close both switches the capacitor will recharge but it can be shown that the current is 0.00739 and therefore voltage across the 215 resistor is 1.64 V. Thus a voltage of only 0.85 across the capacitor means it cant make the bulb flash if you try to discharge it to the bulb. So the capacitor switch has to be set to close to get another flash

Trouton's Rule and Free Volume Theories

DHv = heat energy required to free the molecules of a liquid from the intermolecular forces without change of temperature. his can be measured by calorimetry, vapour pressure or by electrical energy required to vaporize.
Calculated values for molecular substances show DHv decreases with temperature - e.g.
for H2O @ 0 C = 44.8( kJ/mol) and at 200 C = 34.8 : for diethyl ether its 28.8 @ 0 C and 13.6 @ 160 C. From this arises DHv = f(T) and DHv -> 0 at the critical point ( liquid/equilibrium vapour merge)
dH/dT = DCp
In words- the rate of change of enthalpy of vaporization with respect to temperature at constant pressure equals the rate of change of heat capacity (DC) of the liquid at constant pressure.The pattern of energy level occupation may change, but essentially

DHv = f( Ep free moles- Ep bound moles) assuming ideality

Of course nett attraction in a liquid is multi-molecular but easier mathematically to consider in terms of just two molecules
Consider two polar molecules with dipole = u and distance r (r>>> +/-istance)

u(d-d) = -2/3{( u*4/4pie0*2)kT*1/2

Induction must also play a part ( a = dipole moment induced by unit electric field)

u ind. = -2au*2/4pie0r*6

A Chemical Oscillator

A mixture of the following three solutions leads to an interesting reaction -
Solution A - 200 mL of water + 200 mL hydrogen peroxide (27%)- dilute to 500 mL
Solution B - 21.5 g Potassium iodate + 400 mL water + 2.15mL conc. sulfuric-> 500 mL
Solution C - 8 g malonic acid + 1.7 g manganese II sulfate hydrate->250mL then add 10mL/ 1.5g sol. starch and dilute to 500mL

Add 50mL of A to 50 mL of B- swirl ( or use a magnetic fly) then add 50 mL solution C

The mixture will go amber then black then colourless and then keep repeating this colur sequence for some time.

The overall reaction is
IO3- + H2O2+ CH2(COOH)2 (in acid) -> ICH(COOH)2 + 2O2 + 3H2O

hypiodic acid is an intermediate and [I-] controls

Gravity never sleeps

Students initial struggle with projectile motion comes from not intuitively realizing that once a force has projected an object that force is no longer contributing to its motion and thus gravity is the only force acting on it: hence the parabolic path in the event of no resisting forces.

I find it pays to refer to the first law as " things like to stay as they are- so when you kick something into motion it will keep going in that direction forever if no other forces act on it. This is where " gravity never goes to sleep " comes in.

Once they accept this concept, I find they have little trouble in using trig to break the initial motion into its vertical and horizontal components.


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