Homework Help: How to work out mechanical energy used to stroke a key on a keyboard

1. May 18, 2010

knatedogg86

I am working on a project to use piezoelectric ribbons printed on rubber underneath a laptop keyboard to charge its battery. I have the conversion rate of the ribbons from mechanical energy to electric energy.

I need to work out how much mechanical energy is used (and what is the formula) to stroke a key on a laptop keyboard with your finger as in if your finger was on top of the key, how much would it require to press it down.

2. May 18, 2010

phyzguy

I'll take a stab. On my keyboard it seems to take about 50g of weight to depress the key, which then travels ~ 2mm. So the total energy would be 50E-3kg*10nt/kg*2E-3m = 1E-3 Joules of energy.

3. May 18, 2010

knatedogg86

Thanks,

At the risk of sounding completely dense, where does the 10nt/kg come from??

4. May 18, 2010

phyzguy

It's the acceleration of gravity (really 9.81 nt/kg, but I rounded off to 10). An object with a weight of 1 kg exerts a downward force of 9.81 nt, due to the Earth's gravity.

5. May 18, 2010

knatedogg86

Sorry to bother you again but i cant get my head around this project.

If i know the mechanical energy applied to the ribbons will be say 1E-3 Joules and my laptop on average uses say about 80w and the specific piezoelectric ribbons are capible of converting 80% of mechanically energy into electrically energy, is it now possible to calculate how many watts the piezoelectric are capible of producing to find out how many keystrokes are required to run the laptop for instance?

6. May 18, 2010

knatedogg86

Piezoelectric Project

I am working on a project that will use piezoelectric ribbons printed on rubber underneath a laptop keyboard to charge its battery. The conversion rate of the ribbons from mechanical energy to electric energy is 80%.

If I know the mechanical energy applied to the ribbons will be say 1E-3 Joules and my laptop on average uses say about 80w, is it now possible to calculate how many watts the piezoelectric are capable of producing to find out how many keystrokes are required to run the laptop for instance?

7. May 18, 2010

phyzguy

Sure! If the piezoelectric is 80% efficient and you need 80W, that means you need to generate 100W of mechanical energy. 100W is 100 J/s, and if each keystroke generates 1E-3 J, you need 100,000 keystrokes/second. Happy typing!

8. May 18, 2010

Gokul43201

Staff Emeritus
Re: Piezoelectric Project

Energy per keystroke = 1e-3 J
Efficiency = 80%
Power required = 80W

How much mechanical energy do you need to supply each second to run the laptop?

9. May 18, 2010

diazona

10. May 18, 2010

nickjer

Hmm... That only works out to be 1.2 million words per minute. I can almooooost type that fast :)

11. May 18, 2010

phyzguy

I believe this estimate because I know how hard it is to put out 100W of mechanical energy on a bicycle. Lance Armstrong can put out about 400W. A mere mortal like me can put out about 250W, and this with the lungs burning and the sweat running in my eyes. To put out 100W from typing on a keyboard - no way.

12. May 18, 2010

Gokul43201

Staff Emeritus
Re: Piezoelectric Project

Yes it is.

13. May 19, 2010

Staff: Mentor

(Two threads merged and moved to Intro Physics)

14. May 19, 2010

JaWiB

Eh, my laptop has a power cable rated at 90W but uses probably 20-30W usually.

If the laptop uses 20W, and maybe you can push the energy per keystroke to a factor of 10 better, then you're looking at about 2,500 keystrokes/second. Even if you can hit 250 keystrokes per second you'd increase battery life by 10%. Not so bad, really...Good finger exercise I bet

15. May 20, 2010

knatedogg86

yea, thats what i was afraid of....so in conclusion, at the moment using piezo-electric materials isn't an effiecent enough way to harvest electrical power?

16. May 20, 2010

phyzguy

No I don't think this is the right conclusion at all. Th efficiency of the piezoelectric converter isn't the issue. The conclusion is that there just isn't enough mechanical energy there to use to power you laptop, even if the piezoelectric conversion efficiency is 100%. If you want to power your laptop with human power, hook a generator to your exercise bike and pedal away. Then you might have a shot.

17. May 20, 2010

mheslep

Time of power output of course will effect max. power. For ~6 minute periods I know olympic athletes can produce 600W of mechanical work. I imagine the 100m sprinters exceed 1.5 kW.

[*] 0.5*90kg*(10m/s)^2 = 4500J, if produced evenly in the first 3 secs = 1.5 kW.