Building an Electro-Magnet

[Len Lanetti]

I'm in the midst of helping David (son, 10) build an electro-magnet as a school project...most everyone is probably familiar with the "design"...a battery holder with a lead connected to a wire that is wraped around a 3 inch bolt and returns to the negative side of the battery holder.

I'm using 4 AA batterries as that is the only battery holder available in the house without making a run to the local Radio Shack. The batteries heat up a lot and very, very quickly. To the point where I don't want to let him put the batteries in for more than about 15 seconds. The text for building the project suggests using a single D cell battery. Are the 4 AA cells the issue?

The extent of my electric know how is really limited to knowing not much more past how to splice wires but I'm thinking that adding a switch prior to the winding around the bolt and a small light bulb after the winding around the bolt would help with the battery overheating (those parts are again available at home without having to run to Radio Shack).

As this board is loaded with engineers electrical and otherwise I would appreciate comments or other suggestions.

Thanks,

Len

 

[ScottM]

The four AAs are the issue. You have quadrupled the voltage and hence the current flowing through the wire is 4x as great and generates a lot of heat. Suggest you just go to the single D battery or try using 2-AAs.

You can solder the lead to one side and then just touch the other lead to the battery when you want it to work or solder both sides with a SPST switch in the circuit.

 

[Len Lanetti]

Scott,

Thanks.

Would changing to a larger diameter wire help?

Len

 

[jesse]

It doesn't really matter how you do it.. The heat is going to come.

But yeah. A D-battery is 1.5 volts...So now you've got your four AA batteries which has got you up to 6 volts...Anyways here is what I would do..and might get him some extra credit.

Put a resistor in the circuit..this will help with the heat..Another thing you could do is put a lightbulb which will also add some resistance.

I'd put a momentary switch in too..So you hold the switch the magnet is on and the lightbulb is on...you let go of the button..magnet is off ..object drops.

 

[ScottM]

Scott,

Thanks.

Would changing to a larger diameter wire help?

Len

Not really for this application. I was going to suggest resistor but I see Jesse has already done that, he also poitned out that a light bulb would make a good resitor and he is correct on that. A simple flashlight bulb with the 4 AAs would help reduce the heat.

Basically you are shorting accross the battery. There is a small amount of resistance due to the wire and back EMF from the coil around the nail but not a lot. The limiting factor is the internal resistance of the battery. YOu are going to get heat doing this.

 

[gismo]

Scott,

Thanks.

Would changing to a larger diameter wire help?

Len

No. You want a smaller gauge wire and more of it. If you use a couple hundred feet of 30 gauge wire (or 50 ft of 36 gauge) you will reduce the current to less than a half amp (easily supplied by AA batteries) and increase the magnetic field at the same time.

 

[gismo]

Not really for this application. I was going to suggest resistor but I see Jesse has already done that, he also poitned out that a light bulb would make a good resitor and he is correct on that. A simple flashlight bulb with the 4 AAs would help reduce the heat.

Basically you are shorting accross the battery. There is a small amount of resistance due to the wire and back EMF from the coil around the nail but not a lot. The limiting factor is the internal resistance of the battery. YOu are going to get heat doing this.

Adding external resistance will reduce the current but will also waste a lot of energy and more importantly decrease the strength of your electomagnet significantly. The magnet strength is proportional to the product of the current and number of turns so what you really need is more turns (longer wire) and more resistance (smaller gauge).

Also FWIW there is no "back EMF" in a DC electromagnet. The coil and battery's resistance are the only things limiting the current. For AA batteries you want the current to be less than half an amp and that will take lots of wire.

 

[Let'sgoflying!]

Just change the project to "glove warmer" or something instead of electromagnet.
I didn't think kids were still interested in such things anymore. We had small homemade electrical devices and backyard baseball; they have computer games and the mall.

 

[Steve]

Do you have to use the batteries?

I made an electromagnet for my 8th grade science fair and used a toy train transformer....a semi-"regulated" DC power supply.

 

[wsuffa]

What Lance said.

You really want a lot of wire, small gauge. That will offer its own resistance and generate more of a field.

Now if you really want to have fun, build a Tesla coil!!

 

[ScottM]

Also FWIW there is no "back EMF" in a DC electromagnet.

Doh! Right you are. I gues I was thinking of DC motors when the brushes move across contacts you will get a back EMF spark. It has been a lot of years since I played with thsoe types of circuits.

 

[Len Lanetti]

Thanks for all of the posts! Project went into school bag this morning for an on time delivery.

When I got home last night I found that my wife had stopped at Radio Shack and purchased a 2 x AA holder and a 1 x D holder....which turned out to be a good thing.

After normal homework was complete David and I went to work on the electromagnet project. I thought I had some switches in the garage (left over Lucas type from when I owned a GT6) but could only find one. I also found some blade connectors, alligator clips and several light bulbs with sockets and pigtails. I also found a nice sized breadboard. Jessie, we were thinking along the same lines.

My initial thought was to splice in the switch and the light bulb. David got to use the wire crimper tool to add the female ends of the blade connectors which fit the switch perfectly. Sad to say the switch didn't work out too well. The switch was funky in that it remained closed no matter which position the toggle was placed. Probably due to the fact that it was a Lucas brand switch and about 40 years old.

At this point we also figured out that the 4 x AA batteries were pretty dead and we didn't have anymore AA batteries in the house (except for Dad's secret stash in his flight bag - which was not revealed). We switched to the D cell battery holder purchased by Karen (wife) earlier in the day.

So, switch comes out of the circuit and is deposited into the trash can (I was rather proud of myself for disposing of some of the junk that resides in the garage).

We replaced the switch with an alligator clip to close the circuit by connection to the blade connector that was on the battery lead. That was fool proof and worked out well.

Down stream of the coil around the bolt I had David connect in the lamp but that only proved Lance's comment above. With the light in the circuit the electromagnet wasn't. So out came the light.

Once all the connections were made we tested the device and it worked well. Not nearly as much heat and the device would pick up four or five paper clips concurrently.

David then used wire ties to connect the battery holder to the bread board.

Dave - re: do kids still like these things...my boys, David & Matthew, do. They have a few of the small electric project kits either given to them by an older kid in the neighborhood who out grew them and a few new ones we bought them. They also get a kick out of similar projects. I think all kids do if they are presented right. At David's 10th birthday last month I showed the kids how to build submarines out of plastic soda bottles, marbles, rubber bands, aquarium tubing, a straw and a balloon. They had a blast. I'm sure their parents thought "more junk" when they got to take their submarines home. I'm thinking we will build electromagnets for Matthew's birthday party this weekend.

Again, thanks all! I'll report back on what David gets for a grade.

Len

 

[AirBaker]

I remember using one of those 6v lantern batteries when I did this. It was probably because I didn't need the battery holder at all.

 

[Len Lanetti]

I remember using one of those 6v lantern batteries when I did this. It was probably because I didn't need the battery holder at all.

I have four 6v recharable batteries that came out of small electric cars...I thought about using them but figured I should save them for the rocket launcher I want to build. :<)

Edit...Oh, man...I almost forgot...I still have several huge capacitors from a '70's era printer...I guess I could have used them too...but I'm saving those for the electro-zapping wing deicer I want to build.

Len

 

[ScottM]

I have four 6v recharable batteries that came out of small electric cars...I thought about using them but figured I should save them for the rocket launcher I want to build. :<)

Len

I used a 12volt lawn mower battery for my launcher. But I was also launching some big stuff and needed the extra power. I was mostly shooting stuff with H and I motors in it. One of my rockets was a little over 6 feet tall and 4" in diameter. The funnest one was 6" but only about 3 feet tall. I had to put 1lb of clay in the nose to stablize it. On an I motor I would get it up to a little over 3000' AGL. And, yes, I needed to file an FAA NOTAM and get a a waiver.

Lost interest after 9/11. The rocket guys were really clamped down on hard with restrictions many of which still have not been lifted.

 

[AirBaker]

That's one heck of a rocket! Eventually you get lazy and just set off the engines without the rocket body. The things you can set in motion with a few of those...

 

[ScottM]

That's one heck of a rocket! Eventually you get lazy and just set off the engines without the rocket body. The things you can set in motion with a few of those...

FYI the engines for these rocket are 38mm in diameter and 4 to 12 inches long. I would not set them off without having able to go into a specific direction, they are very dangerous. The T-Shirt I am wearing is talking about a rocket launch event called LDRS. That standrds for Large and Dangerous RocketS

Just before I left that hobby a group of guy built and attempted to launch a 1/3 scale mercury/redstone rocket.

The "P" engine case fractured and the rocket exploded amid a very large fireball. The rocket itself was about 20 feet tall and almost 3 feet in diameter. These guys really went over the top! The funny part was that Discovery Channel was filming the launch and had inserted as payload a $50,000 camera into the rocket, it was destroyed.

 

[mikea]

FYI the engines for these rocket are 38mm in diameter and 4 to 12 inches long. I would not set them off without having able to go into a specific direction, they are very dangerous. The T-Shirt I am wearing is talking about a rocket launch event called LDRS. That standrds for Large and Dangerous RocketS

Just before I left that hobby a group of guy built and attempted to launch a 1/3 scale mercury/redstone rocket.

The "P" engine case fractured and the rocket exploded amid a very large fireball. The rocket itself was about 20 feet tall and almost 3 feet in diameter. These guys really went over the top! The funny part was that Discovery Channel was filming the launch and had inserted as payload a $50,000 camera into the rocket, it was destroyed.

Well that was a very accurate representation of what happened in the Mercury program before Alan Shepard flew.

 

[AirBaker]

FYI the engines for these rocket are 38mm in diameter and 4 to 12 inches long. I would not set them off without having able to go into a specific direction, they are very dangerous. The T-Shirt I am wearing is talking about a rocket launch event called LDRS. That standrds for Large and Dangerous RocketS
.

Yeah, thats a much bigger engine than I've played with. I think I'd have to agree with you.