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ebike 101

EBikes Physics 101

by Electric bike guru on August 2, 2010

First, Some Basic Physics
Work is defined as the transfer of energy.  In physics, they say that work is done on an object when you transfer energy to that object.

For example, if a golfer uses a club and gets a stationary golf ball moving when he or she hits the ball, the club does work on the golf ball as it strikes the ball. Energy leaves the club and enters the ball. This is a transfer of energy.  And, before the ball was struck, the golfer did work on the club. The club was initially standing still, and the golfer got it moving when he or she swung the club.  So, the golfer did work on the club, transferring energy into the club, making it move, and the club did work on the ball, transferring energy into the ball, getting it moving..

Formula For Work
In the previous golf example the club places a force on the ball, and this force acts on the ball over the short displacement, or distance through which the club and the ball are in contact as the ball is being hit. Energy is transferred as the force acts over this displacement.  The amount of work is calculated by multiplying the force times the displacement. That formula looks like this:
W = F * d

Work is measured in a unit known as the joule.  One joule is the amount of energy required to move an object one meter (about three feet) using a force of one Newton, or about 0.225 pounds.  (Work can also be measured in foot-pounds.  One foot-pound is the energy required to move an object one foot using a force of one pound.)

Ampere’s Law of Magnetism
On an EBike, the force originates from an electric motor.  In an electric motor, coils of wire pushing against each other push a moving central hub around in a continuous circle.  Although the parts in a motor move in a circle, the physics is the same as what has been described above.
How do these coils of wire generate the force to do work by pushing an object, such as your Ebike?  Well, to answer this question, we need to go a little deeper down the rabbit hole.

an electrical current generates a magnetic field

an electrical current generates a magnetic field

Around 1820, Hans Christian Oersted discovered that an electrical current generates a magnetic field encircling it.  Then, in 1826, Andre-Marie Ampere published his Circuital Law, relating the magnetic field around a closed loop to the electric current passing through the loop.

Right hand rule magnetic force

Right hand rule magnetic force

This is often summarized as the “right-hand rule,” where the fingers of the right hand are curled, and the thumb is extended.  When a current travels around a loop in the direction of the curled fingers, a magnetic force is generated in the direction of the extended thumb.

To amplify this effect and make it practical to use in a motor, the wire “loop” is replaced by a coil, or multiple coils, each of which may contain several hundred such loops.  These coils are then rotated next to a magnet or magnets, and the two magnetic fields interact, producing forces of attraction and repulsion.

But magnets are heavy, and coils of wire are light.  So couldn’t the magnets be replaced by coils of wire which have a current passing through them?  Excellent suggestion!  And, in fact that’s just how modern, light-weight motors for EBikes are manufactured.

BionX Electric Drive Motor (cutaway view)

BionX Electric Drive Motor (cutaway view)

The intensity of the magnetic field produced by the loop coils is proportional to the electric current passing through them.  More current produces more a stronger field, and a stronger field produced more force.  But wait a minute.  Where does this current come from?  Well. the current for these coils comes, of course, from the battery which powers your EBike.  The two main types of batteries are SLA (Sealed Lead-Acid) and Li (Lithium Ion).

But we still haven’t explained just exactly what an electrical current actually consists of?  That’s also a good question, but unfortunately, we have used up our available space for this article.  To avoid keeping you in suspense unnecessarily, the answer to the question above is, in a word, “electrons.”  And in the next article, we will explore a little more about what is an electron, and find out how batteries manage to get these little things moving around in a circuit (and doing work.)  We will also see how the formula for work above (W = f *d) can be used to measure the amount of work done by an EBike motor, and to predict the maximum range of the combination of motor and battery that are on your EBike.