Why do aerials have direction?

The pictures come from the museum at Croome. I remember studying this for my degree but I've lost the detail now. So I was wondering why you get the lobes for electromagnetic waves and don't get equal intensity all the way round. It turns out that it is to do with interference. There are angles where there is destructive interference. I don't yet fully understand why. But notice that the smaller wavelength is more uni-directional. That means that the lobe doesn't touch the ground. In the top picture the radar can only detect planes in front that are as far horizontally as the plane is vertically above the ground, because the ground reflects waves and it detects the ground as well.

The Flame Probe

I suspended a foil covered football from the ceiling and charged it up to +1000V. I used a flame probe to measure the electrical potential in the space around the ball. The flame probe is a hypodermic needle attached to a gas tap with a tiny flame on top. The tip of the needle is attached by wire to the plate of a gold leaf electroscope.  The large + voltage on the ball pulls free electrons up the wire from the plate of the gold leaf electroscope onto the tip of the needle. That leaves the electroscope positive so the gold leaf is repelled upwards. The angle is a measure of potential energy because work has to be done by the electric field of the ball against the attraction of the electrons to the nucleus. Energy is stored when work is done against an attractive force.

 There has to be a flame on the tip because the flame creates ions which discharge the free electrons. If they built up on the tip, they would cause a field that would distort the field being measured by the probe.

The closer the probe is to the ball, the stronger the field and hence the bigger the angle. We projected the gold leaf so that we could take a more accurate measurement of the angle.

Keeping the milk fresh part 2

Last week I used ice to keep milk fresh but that was not an option when we spent 3 days away from electricity. This time we used a bucket of water. All we were doing was increasing the mass of liquid that would have to be heated. The water and the milk have to be in thermal equilibrium - ie they will settle at the same temperature. Thermal energy from the air will thus be spread through far more molecules of water than if we'd just left the milk. Each molecule gets a smaller share of the thermal energy so the internal energy of each molecule only goes up by a small amount. Temperature is a measure of the internal energy.

Estimating the walkie talkie frequency

On camp for the D of E Silver, we sat debating the frequency of the walkie-talkies. I estimated the aerial at perhaps 20cm. Aerials like this are half wavelength. To make the mental maths easier, I took the wavelength as 0.50m. Using the wave equation, frequency = speed of electromagnetic waves/wavelength = 300 000 000/0.50 = 600 MHz. Same make walkie-talkies on the Internet are claiming 400 - 480 MHz so that not a bad estimate!

Solar phone charger discharged my phone!

I borrowed a solar phone charger. I set it up in bright sunshine and all was going well. After sunset last night it was still connected for about an hour and nearly discharged the phone. The solar panel and the phone battery must act as emfs in parallel. When the sun in shining, the emf of the panel will be higher than that of the battery and so will send charge that way. However when the sun sets, the emf of the phone battery will be higher. At any point in time, charge will move in such a way as to try to equalise the emfs.The higher emf will always send energy to the lower. The difference is that the solar panel is constantly having its energy replaced by an external source.

More electric field patterns

The theory says that field lines leave and join surfaces at 90 degrees. I went on to have a point with a plane electrode, as shown in the top picture. The semolina lines are very close to the the theory. A uniform field has parallel field lines. Near the surface of the plane electrode the field approximates to uniform. The same could be said in the middle of the gap between the two point charges shown in the bottom picture,

Electric field lines with semolina and cooking oil

 I put cooking oil into a Petri dish and sprinkled on some semolina. I used some electrodes made from bent coat hanger and connected it up to 2000V. The pieces of semolina touch the electrodes and gain a charge. They are then pulled along the electric field lines. I suspect that they join in clear lines from one side to the other because each charged piece attracts the next. That probably means that one side of the piece of semolina becomes positive and the other side negative - charge separation. Notice that no pattern is shown outside the gap between the electrodes presumably because there is no possibility of a continuous chain of + and - all the way to the other plate.