5.1a Electric fields and charge

THE IB SAYS

Essential idea: When charges move an electric current is created.

Understandings: Charge; Electric field

Applications and skills: Identifying two forms of charge and the direction of the forces between them

My vodcasts

5.1a(i) Introduction to field and charge

5.1a(ii) Drawing fields

An introduction to fields and charge

One of most important ad subtle ideas in all of physics is the concept of a field. This model suggests that all of space is filled with many coexisting volumes that are influenced by objects and processes inside that volume. The most familiar example of this is probably gravity. An analogy for this might be something like air temperature. Throughout the Earth's atmosphere we can determine a value for the temperature of the air, we can draw maps and plot charts and graphs of this temperature. This is a scalar field as temperature does not have a direction. The important fields in physics aren't limited to one planet but extend through all of space-time. And they are generally vector fields, including both a value and a direction. The example you might be most familiar with is a gravitational field. Throughout all of space we can place an object with mass and then measure the gravitational force (and direction) on the object. This force is determined by the combined effect of all other masses in the universe.

Charge

The idea of an electric field was developed through a recognition of electric charge. The ancient greeks noted that under certain conditions certain materials could be made to attract or repel each other. One of this materials, amber, is called elektron in the Greek language and this has led to the words electricity, electrified and electrical. By the 1700s it was realised there were two types of charges and that they exert a force on each other.

Objects are charged (positive or negative) or uncharged (neutral). In most cases neutrally charged objects are made from an equal proportion of positive and negative charges.
Total charge is a conserved quantity. Charges can be transferred but never completely appear or disappear.
The symbol for charges is Q or q ... and the SI unit for charge is the coulomb [C].

One coulomb is defined as the as the amount of charge that passes when one ampere of current flows for one second (which will make more sense when we look at current), but in 2019 the coulomb was redefined as being equivalent to 6 241 509 074 460 762 607.776 elementary charges, as one elementary charge (the charge on a single electron or a single proton) is set to 1.602 176 634 × 10⁻¹⁹ coulombs. Every directly observed charge is an exact multiple of elementary charges.

Electric Field Diagrams

As with gravitational fields we can plot two or three-dimensional diagrams showing the shape and extent of the electric field associated with charged objects. When the charges are effectively stationary, with no overall flow, we are studying electrostatics. Such diagrams have lines - these lines and their arrows depict the path taken by a SMALL POSITIVE TEST CHARGE in that electric field.

Additional videos

Covers electric charge and (the next lesson for you) Coulomb's Law.

More on electric fields and field diagrams.

Fantastic discussion at a conceptual level of what is a field / do fields exist?

Resources from textbooks

Oxford Physics: pages 169 - 172 (charge) 175 - 176 (mapping fields / electric field diagrams)

Hamper HL (2014): pages 210 - 213

Relevant questions from Dot Point Physics (6 Fields and forces)

pages 287 - 290 (charge), 296 - 298 (mapping fields / electric field diagrams)

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