MOTION MOUNTAIN

24 1 electricity and fields
TA B L E 2 Values of electrical charge observed in nature.
Observation
Smallest measured non-vanishing charge
Charge per bit in computer memory
Charge in small capacitor
Charge flow in average lightning stroke
Charge stored in a fully charged car battery
Charge of planet Earth
Charge separated by modern power station in one year
Total charge of positive (or negative) sign observed in universe
Total charge observed in universe
Charge
1.6⋅10 −19 C
down to10 −15 C
10 −7 C
1 C to100 C
0.2 MC
−1 MC
3⋅10 11 C
10 60±1 C
0 C
Ref. 7
electric charge of a physical system changes, the reason always is that charge is flowing
into or out of the system. This can be checked easily with two metal pots connected
to two electrometers, as shown in Figure 8. Charge thus behaves like a fluid substance.
Therefore we are forced to use for its description a scalar quantityq, which can take
positive, vanishing, or negative values on a physical body.
Describing charge as a scalar quantity reproduces the behaviour of electrical charge
in all everyday situations. However, as in the case of all previously encountered classical
concepts, some of the experimental results for electrical charge in everyday situations
from Table 1 will turn out to be only approximate. More precise experiments will require
a revision of the idea of continuous change of charge value. Nevertheless, no counterexampletochargeconservation
has as yet been observed.
In summary, electric charge is a scalar quantity that describes the origin of electric
fields. Electric charge is conserved. There is on way to destroy or create electric charge.
We mentioned above that objects without electric charge are called neutral. Also neutralbodiesareinfluencedbyelectricfields.Thishappensbecauseachargedobjectthatis
brought near a neutral body polarizes it. Electrical polarization is the separation of the
positive and negative charges onto different regions of a body. For this reason, neutral
objects, such as hair or a water stream, are usually attracted to a charged body, such as a
rubbed comb. Both insulators and conductors can be polarized; and polarization occurs
for single molecules, everyday bodies and whole stars.
Electric field strength
Charges produce attraction and repulsion on other charges. Equivalently, charges change
momenta; charges exert forces on other charges. This happens over large distances.Experimentsthatexploreenergyandmomentumconservation
show that the best descriptionoftheseinteractionsisastoldsofar:achargeproducesafield,thefieldthenactson
asecondcharge.
Experiments show that the electric field forms lines in space. As a consequence, the
electricfieldbehaves like a small arrow fixed at each pointxin space. Electric fields are
described by a direction and a magnitude.The local direction of the field is given by the
local direction of the field line – the tangent of the field line.The local magnitude of the
Motion Mountain – The Adventure of Physics copyright © Christoph Schiller June 1990–November 2015 free pdf file available at www.motionmountain.net