Questions and Answers
I am creating an electromagnet for my school's science fair project. Does the shape of the iron core make a difference? Is solid copper wire better than stranded wire? Is copper wire best, or is another wire better? Should the wire be insulated or bare? What can I do to get the most current out of a battery?
Yes, the shape of the iron core does matter. The more complete the circuit formed by the iron, the more field that you will get for a given coil and current. The best way to do a simple magnet is to have an iron core shaped like a "C". The gap formed by the "C" should be as small as possible and still be useful to you. Just wrap the wire around the iron and you have a magnet. The iron should be "soft iron" rather than a hard steel, like a bolt. If you have to use a bolt or threaded rod, make sure it is ordinary steel and not stainless or extra high strength steel. Stainless is nonmagnetic (will not work) and high strength steel gives less magnetic field than soft steel. If you buy steel then try to get an alloy called "1010". If you are in a good high school with a good metal shop, your metal shop teacher may be able to soften a piece of steel for you by annealing. This just means that you heat it up and soften the steel. After you bend the steel into the "C" shape, reanneal it to get the softness back. Bending steel hardens it and degrades the magnetic properties. The mathematical formula that describes the function of a magnet is called Ampere's Law. It says that the strength of the field times the gap is equal to the number of turns in the coil times the current times a constant called Mu.
B * gap = Mu * N * I
|B is in Tesla (10,000 gauss)|
|gap is in meters (This is the opening of the "C ".)|
|Mu = 4π * 10-7|
|N is an integer equal to the number of turns in your coil|
|I is the current in Amps|
Solid copper wire is better because it can usually carry more current. It is best to have a large amount of copper to keep the resistance down. It is also good to have a lot of turns to make better use of the available current. Copper has the lowest resistance at room temperature, so its a great choice. It's also easy to solder together and you can buy copper wire anywhere. It is very important to insulate the wire otherwise it will short out the turns and the magnet will be ineffective. You get the most current from a battery when its fully charged. Also, its a good idea not to withdraw the current at too high a rate. You have to find out the optimum discharge current to get the best battery life. Also, you have to optimize your magnet design, especially the coil, to match your battery. The above equation tells you for a given current and number of turns how much magnetic field you will get. Another famous law, called Ohms law, tells you the relationship between amps, volts and resistance.
Volts = Amps * resistance (measured in Ohms)
Your coil should be made so it has a voltage drop equal to what your battery can put out at the best rate of discharge. A piece of copper has resistance which can be determined form the following formula.
Resistance = Rho * L / A
|Resistance is in Ohms|
|Rho is a constant for each metal and for copper it is 1.6*10-6 ohm-centimeters|
|L is the length of the wire used in centimeters|
|A is the cross section area of the copper wire in centimeters squared|
So Ampere's law tells you how much field you get for a given current and turns count. Ohms law tells you about the voltage drop at the current you want for a given resistance and the formula for resistance tells you the resistance of the wire that you choose. Now you have to optimize your magnet design. The trick is to make the coil so it discharges the battery efficiently at its rated voltage. If you have a 12 volt deep cycle battery, then you can discharge these at 12 volts and 5 amps for a long time. The popular ones are rated 120 amp-hours. That means that they will run for 24 hours at 5 amps. It also means that they will run at 20 amps for 6 hours. However, remember that the battery will wear out faster if you discharge at a high current and it will definitely not last long if you take over 50% of the available charge. So you pick a copper wire, match the voltage drop at say 5 amps to the available 12 volts and you have your coil. Check how many turns you get and that's your magnet. If the field is too low then you need more turns or higher current. Just adjust the size of wire to match the required amps and the voltage at the required turns to what the battery can do. For the 120 amp-hour battery it is best to withdraw only 60 and then stop and recharge. It is a good rule of thumb for all deep cycle batteries to not take out more than half the rated charge. This way they last a very long time. Other common batteries that you might want to consider are the "D" cells. They are rated at 1.5 volts. They are only good for currents of a few milliamps. You can get more current by putting these batteries in parallel. If you need more voltage, you can put them in series. If this is a home science fair project and your parents are helping, then I'd use something like a garden tractor battery. They are rated at 12 volts, they are cheap and they are rechargeable. You may already have one at home you can use.
Paul Brindza, Experimental Hall A Design Leader (Other answers by Paul Brindza)