# What causes the physical resistance in a dynamo?

#### Pca

Joined Feb 10, 2020
17
I have a new old timey bicycle dynamo ( i think thats what it was built for) that when spun by contact friction from the bicycle tyre turns the DC generator and lights your light proportional to your pedal speed - standard old timey bicycle lighting system (thank you Mr Max Plank et al for inventing theories that invented LED,s TBH).

Anyhoo I have put the bike on rollers so it's stationary and putting lots of different light bulbs on it because reasons.

When I put lots of bulbs on the power required of my legs to light the bulbs obviously increases but i was amazed how hard it becones to pedal with the increased power requirements.

What physically makes it harder to pedal when trying to supply the power. It's like pedalling in quicksand.

#### Sensacell

Joined Jun 19, 2012
3,130

You are learning to never take energy for granted.

Pca

#### Pca

Joined Feb 10, 2020
17

You are learning to never take energy for granted.
Lol, I can honestly say I feel that guys pain even if my quads look like chicken legs in comparison.

#### Pca

Joined Feb 10, 2020
17

You are learning to never take energy for granted.
Lol, I can honestly say I feel that guys pain even if my quads look like chicken legs in comparison.

#### MrChips

Joined Oct 2, 2009
28,095
Back EMF.

Also something to do with the First Law of Thermodynamics.

#### crutschow

Joined Mar 14, 2008
31,477
The increase in bulbs requires more power, thus the dynamo requires more input power to deliver the required output power, which your legs have to supply.
There's no free lunch.

#### BobTPH

Joined Jun 5, 2013
6,268
The current in the windings on the dynamo produce a magnetic field. This field repels the magnets. More load means more current means stronger field means more force.

Bob

Pca

#### Glenn Holland

Joined Dec 26, 2014
703
It's a case of Lenz's law where the field induced in the output of the generator wingdings is opposite to the field of the magnets in the rotor. The field is proportional to the output current so as this current increases, the opposition is also increased.

Pca