Ohm’s law ties the three together. V is volts, I is amps, and R is ohms. Some books write E for volts instead of V.
Cover a letter on the triangle to see how to find it. Cover V and you see I × R. Cover I and you see V ÷ R. Cover R and you see V ÷ I.
Voltage pushes current through the heater. Resistance, measured in ohms, works against it. Amps measure the current you get.
Voltage is always measured between two points. That is why the meter touches both of the heater's screws.
The clamp meter reads amps by going around one wire. Both wires carry the same amps. The heater does not use up any current.
The dots show how much current flows. More dots passing each second means more amps. The dots do not show how fast electrons move. Real electrons drift only a fraction of a millimeter per second.
The battery is DC, so the dots run one way. They show current going from + to −, the way electricians draw it. Electrons move the other way.
Wall power is AC. It runs at 60 cycles a second, so the current reverses 120 times a second. The dots move back and forth in place, slowed way down so you can see it. The meters show the usual effective (RMS) value.
The wire inside the heating element is the resistance. Its ohms come from the heater's nameplate, using ohms = volts × volts ÷ watts. At 240 V, a 4,500 W element has 12.8 Ω. A 1,000 W heater has 57.6 Ω, and a 500 W heater has 115.2 Ω. Heater wire has nearly the same ohms hot or cold.
The 4 ft heater makes twice the heat of the 2 ft one, so it is built with half the ohms.
All three heaters are made for 240 V, so no setting puts a heater over its rating. On 120 V a heater makes a quarter of its heat. On 12 V it makes a tiny part. Running a heater on 12 V or 120 V is a teaching setup, not a normal install.
A water heater element must sit in water. In air it burns out, so the picture shows it in its tank.
The wires are drawn as perfect. Real wires have a little resistance, so they drop a few volts.
On 240 V, both wires are hots. A two-wire cable has a black wire and a white wire. The white wire is taped red at both ends to mark it as a hot, per 200.8(C)(1). The heater does not use a neutral.
All three heaters count as continuous loads, per 422.13 and 424.5. A continuous load needs a breaker rated at least 125 percent of its amps, per 210.20(A).
On 240 V the water heater draws 18.75 amps, so it needs at least 23.4 amps of breaker. That means a 30 amp breaker and 10 AWG copper wire, per 240.4(D). The baseboards need only 15 amp breakers and 14 AWG wire.
Cable jacket colors show the wire size by maker custom, not by code. Cream is 14 AWG, yellow is 12 AWG, and orange is 10 AWG.
The panel, breaker, and ground wire are left out. A real heater's metal case is grounded. Test before you touch. That is standard trade practice.