I'm a teacher and my students were presented with this as a constructed response on a district test.
(A question they need to answer). I'm exasperated b/c I feel like this is way above their level of knowledge and understanding. We studied basic electromagnets and made one in class already with a D Battery or AA or 1 9V. But they were never taught about putting 2 batteries together so I didn't think it was a fair question for them to have to answer but they tried their best.

THE QUESTION:
Given the following supplies:

• 2 9V Batteries,
• 21G Copper wire,
• an iron Bolt
• wire cutters
  How would you set this up as an ELECTROMAGNET giving it the ability to pick up the most paperclips?

Background Knowledge:
In class they had only ever made a regular electromagnet using a D battery or a AA battery.
How would you put it together. ???

 

Just as a matter of curiosity, how would YOU put it together?

 

What is 21G copper wire?

 

Interesting question.
What is the school subject and the school grade (or age of students)?
Can you show a diagram using only one of the two batteries?

As the previous poster asked, what is 21G copper wire?

 

I'm a teacher and my students were presented with this as a constructed response on a district test.
(A question they need to answer). I'm exasperated b/c I feel like this is way above their level of knowledge and understanding. We studied basic electromagnets and made one in class already with a D Battery or AA or 1 9V. But they were never taught about putting 2 batteries together so I didn't think it was a fair question for them to have to answer but they tried their best.

THE QUESTION:
Given the following supplies:

• 2 9V Batteries,
• 21G Copper wire,
• an iron Bolt
• wire cutters
  How would you set this up as an ELECTROMAGNET giving it the ability to pick up the most paperclips?

Background Knowledge:
In class they had only ever made a regular electromagnet using a D battery or a AA battery.
How would you put it together. ???

The critical piece of information that is missing is what does the strength of the magnetic field depend on?

1. Does it depend on Current?
2. Does it depend on Voltage?
3. Does it depend on the number of turns?

The answers are 1 & 3, and in fact the strength of the magnet is expressed in units of ampere-turns. That means you take the current and multiply it by the number of turns and that gives you the force of the magnet. It does not depend on the voltage. This where things get tricky because when you multiply two things together, the maximum value is achieved when one of them is infinite and the other is zero.

Back to your students. Some of them might appreciate that when two batteries are in series the voltages will add, but the currents will not. Rarely will it occur to them that when two batteries are in parallel the voltages are approximately the same, but the current is doubled. This assumes both batteries are fresh or have been recently recharged.

Now we come to the number of turns. If only a single layer is allowed, then whoever figures out how to pack the most turns onto the bolt will win. If multiple layers are involved you have to be careful about how you do the winding. The proper way to do it would be to twist multiple wires together and wind the package of wires along the bolt. If you go back and forth with single wire it will be like shooting yourself in the foot.

I'm in agreement with you that unless there was some foundational instruction this would be too difficult for students in middle school or the first two years of high school. A high school junior in a physics class should know this stuff.

Good luck fighting the ignorant administrators. I do not envy you one bit.

 

What is 21G copper wire?

I'm guessing 21 AWG copper wire. It is rare, but not unheard of.

 

And of course, there are other pieces of vital information missing.
How long is the wire given?
How many turns can one manage to wrap around the iron bolt?

 

We studied basic electromagnets and made one in class already with a D Battery or AA or 1 9V.

Since you already made one with a 9 volt battery connect the batteries in series or in parallel and compare which has a stronger pull.
Like this:

 

I'm guessing 21 AWG copper wire. It is rare, but not unheard of.

The whole point of asking is not to guess.

 

Without hearing the answers I will give you my thoughts.

You can do this as an experiment from just about any age, from about 10-15 years.
To gain an understanding of the principles involved, this is usually covered in a Physics class on Electricity & Magnetism for students 16-18 years. This is also covered in more detail at 1st and 2nd year university level in a Physics program.

If you have not been taught about the effects of batteries connected in series and parallel then it would be an unfair question to ask anyone.

To appreciate a bit more about this experiment, here are the important parameters.

1) The total number of turns wound around the iron bar is important, the more turns, the stronger the magnet.

2) Since you want to wrap as many turns as possible, you want the smallest diameter wire (up to a limit). Wire size is given a number such as 20 AWG (AWG = American Wire Gauge). The smaller wire has a higher number. 20 AWG is too fat for this experiment. 26 AWG to 32 AWG would be preferred. If it is too fine it will break easily as well as have other problems. (We never use the notation 20G to represent wire size.)

3) The higher the battery voltage, the more current will flow. Two 9V batteries in series will add to give 18V. Post #8 shows how to wire two 9V batteries in series as well as in parallel.

4) Even then, this is trick question. Even a professor teaching this stuff at university level could get the answer wrong. (I know because I have witnessed this myself.) The strength of the electromagnet thus created depends on the current flowing, not the voltage. Two 1.5V D-cells wired in series will likely create a stronger magnet than two 9V PP3 batteries wired in series or parallel. That is because the current capability of the battery needs to be taken into consideration.

In conclusion, the person who set the question needs to stick to what they know. It is obvious that Physics and how batteries work is not their forte.

 

Assuming 21G wire is 21 AWG wire. Wind as many turns as you can close to one end of the bolt. Use the wire cutters to cut some wire off to connect the two 9volt batteries in parallel. Use the end of the bolt you wound wire on to pick up your paper clips.

 

Since a 9V battery has low power (especially an old carbon-zinc one from "over-there") then modern 9V alkaline batteries will work in series (more voltage but also more voltage lost) or in parallel (more current but less voltage lost).

 

The issue here is the internal resistance of the battery (series or parallel connection) vs the resistance of the coil and connecting wires. 9V PP3 batteries have high internal resistance compared with that of the coil.

 

Without knowing the dimensions of the iron bolt and the amount of wire available, it's tough to know the resistance of the coil.

 

Without knowing the dimensions of the iron bolt and the amount of wire available, it's tough to know the resistance of the coil.

21 AWG wire? Less than 0.2Ω for 10 feet of wire.
The resistance is 0.012 ohms per foot.

18V / 0.2Ω = 90A
That is never going to happen with a 9V PP3 battery.

 

21 AWG wire? Less than 0.2Ω for 10 feet of wire.
The resistance is 0.012 ohms per foot.

Yes, but how much wire is available?

 

The whole point of asking is not to guess.

You're welcome to do you all the live long day.

 

Yes, but how much wire is available?

Can you wrap 10 feet of 21 AWG wire around an iron bolt? How big is your bolt? (Keep it clean!)

 

Yes, but how much wire is available?

Great question

 

Ok, this is a simple question. Without trying to re-derive Biot-Savart or get lost in the minutiae, focus on the simple practical answer. I'm assuming that this is for an elementary level or junior high level student.

• Wrap the wire around the bolt as many times as possible leaving a couple of short ends (enough to connect to the batteries).
• Connect the ends of the wire to the two 9-volt batteries, one end of the wire to both positive terminals and one end of the wire to both negative terminals.
• That produces the strongest magnet because the current is limited by the battery source resistance not the length of wire. It basically doubles the available current.
• Paralleling the batteries (as opposed to putting them in series) will produce more current, hence a greater magnetic field, hence more paperclips.

The OP didn't ask how strong the magnetic field was.