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Getting a Feeling for Energy

Have you heard of GravityLight?

It is a gravity-powered lamp designed as an alternative for off-grid families who would otherwise use kersene lamps. It is basically only a 12kg weight, lifted and put on the gravity light. When the weight goes down again it pulls a cord. This cord makes an electric motor which generates electricity for LEDs. If you lift the weight 1.83m, the light lasts for about 20 minutes.

I wondered how much weight I would need to lift (assuming 100% efficiency) to power my computer for 8 hours.

According to the power supply unit, my laptop can consume up to 65 Watt. That is astonishingly low. I think my big one is at about 600-800 Watt.

\begin{align} 65 W \cdot 8 h &= \frac{65 kg \cdot m^2 \cdot 8h \cdot 60 \frac{min}{h} \cdot 60 \frac{s}{min}}{s^3}\\ &= 1872 \cdot 10^3 \frac{kg \cdot m^2}{s^2} \end{align}

You might also remember from your physics courses that potential energy is \(E_{pot} = m \cdot g \cdot h\) where \(m\) is the mass (in kg), \(g = 9.80 \frac{m}{s^2}\) is the gravitational acceleration and \(h\) is the height in meters.

\begin{align} m \cdot h &= \frac{E_{pot}}{g}\\ &= \frac{1.872 \cdot 10^6 \frac{kg \cdot m^2}{s^2}}{9.80 \frac{m}{s^2}}\\ &= 191.0 \cdot 10^3 kg \cdot m \end{align}

This means I would have to lift 191 000 packages one liter of milk to a height of 1 meter. Every day. Just to let my small laptop run.

Or lets view it from another angle. I think lifting about 5 packages of milk to a height of about 1.8m each hour would not be too exhausting. This would generate about \(E_{pot} = 5kg \cdot 1.80m \cdot 9.80 \frac{m}{s^2} / (1h \cdot 60 \frac{min}{h} \cdot 60 \frac{s}{min}) = 0.0245 \frac{kg \cdot m^2}{s^3} = 0.0245 W\). Lets think what you can power with 0.0245 Watt...

It is amazing about how much energy we have today.


Published

Mai 19, 2015
by Martin Thoma

Category

Cyberculture

Tags

  • Energy 1
  • Physics 5
  • Politics 15
  • Poverty 1

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