'Limit of DC Traction' Northbound at Farringdon - Overhead lines at 25,000V AC used on the Northern Thameslink network. The wires connected to a car battery are much thicker than most other wires you're familiar with. Just as you need a fat pipe to supply water to a whole town, so you need fat wires to carry a heavy current. A small solar cell like the ones on solar garden lights can only produce perhaps a few tens of milliamps of current, but a car battery might deliver 100 amps (written 100A) to turn the starter motor. On the other hand, some 2,400 cubic metres of water per second flow over Niagara Falls on average. A dripping tap represents a very small flow of water - it might take hours to fill a litre jar. Remember, since electrons don't like piling up, the current has got to be the same at every point around a simple circuit.Īgain, we can use the analogy of water. This is the current, measured in Amps, milliamps (mA - thousandths of an amp) or microamps (μA - millionths of an amp). just how many electrons pass a given point per second. Mains electricity comes at 240v, which is enough to do useful work but not so much as to jump across a switch or break out anywhere else it's not meant to go.Īs well as the pressure (voltage), we'll also be interested in how heavy the flow of electricity is, i.e. So a kilometre long lightning flash implies a voltage of 3 billion volts! It takes roughly 3,000V (3kV) to create a 1mm spark. And the bulb probably wouldn't last long either! If you replaced the battery in our circuit by a source of a few thousand volts, the pressure would be so great that the electricity would form a spark in order to jump over the open switch. We measure the pressure in volts, of which an AA cell will give us around 1.5, and so it will be labelled 1.5V. That might be an AA cell, which doesn't push very hard. In the circuit we looked at above the current is driven round the circuit by a battery. Imagine trying to stop that with your thumb! The Jet d'Eau at Geneva uses an enormous pressure to create a fountain several hundred feet high. If you try the same at the kitchen tap, probably fed directly off the water main, you're likely to get very wet because the pressure is much higher. If you put your thumb over the bathroom tap and turn it on, you can probably stop the flow because you're only holding back the pressure of the water from the tank in the loft just a few feet above your head. Positive and negative charges attract one another, which is what keeps the electrons bound in an atom. Normally, the positive and negative charges cancel exactly. Electrical Circuits - doing the roundsĪn atom consists of a very dense nucleus carrying a positive electric charge surrounded by a cloud of electrons, each having a negative electric charge. Mains-powered appliances such as vacuum cleaners, washing machines and power tools contain powerful electric motors which can easily cause injury to fingers. Uncontrolled, it can cause serious burns or start a fire. An excessive current flowing in a wire will make it hot. Nevertheless, if the jolt causes you to fall off a ladder or cause some other accident, it might kill you that way instead. Much higher voltages, for example from static build-up, can give you a nasty jolt but may be incapable of killing you if they are unable to sustain sufficient current.

In normal circumstances anything less will give you no more than an unpleasant tingle. (Don't forget to mark them!).In the worst case scenario (wet hands and standing in the bath) you can kill yourself with 50V. Now that you know the resistance value of each type of coil, you can quickly grade each rotor. 24-volt rotors draw approximately 2.15 amps 24 voltsĮnter 12 volts and 4.60 amps into the calculator and it will show the coil resistance as 2.61 ohms.Įnter 24 volts and 2.15 amps into the calculator and it will show the coil resistance as 11.16 ohms.12-volt rotors draw approximately 4.60 amps 12 volts.Use the Delco-Remy 1G-188 chart to look up the voltage and amp draw of the 27SI rotor coils. Delco-Remy published only field-current values (amps), not resistance (ohms). You want to use an ohmmeter to test each rotor-but you don't know the resistance value (ohms) for each type of rotor coil. You have a box of 27SI rotors and need to determine which are 12 volt and which are 24 volt. Ohm's Law Example: Calculate Ohms from Voltage and Amps

Amps to watts