Electric Fields

• Charging by contact (friction) involves two kinds of charge.
• Point charges and spherical charges produce a radial electric field directed towards the centre of the negative charge and away from the centre of a positive charge.
• Static electricity studies the forces between stationary electric charges.
• Inside an atom, electrons carry a negative electric charge while protons carry a positive electric charge.
• Objects get charged mainly by gaining or losing electrons.
• Objects that have a positive charge have less electrons than protons.
• Objects that have a negative charge have more electrons than protons.
• If an object does not have an electrical charge, it is called electrically neutral.
• Objects that have opposite charges are attracted to one another.
• Objects that have the same charges are repelled from one another.
• When two charged objects touch, the charge will be distributed evenly between them because electrons will flow from one object to another.
• If two oppositely charged objects come in contact, some or all of their charges may cancel each other out.
• An electrical ground is something capable of accepting or donating large number of electrons without significantly affecting its own electrical state.
• When an object is grounded, the object becomes neutral because it gives or receives electrons from the ground.
• The ground is so called because the earth can be an electrical ground.
• When two objects transfer charge from one another, the total charge of their system must be conserved.
• The SI unit for electrical charge is the coulomb (C).
• All charges have discrete units.
• The smallest magnitude of electrical charge you can ever get normally is 1.6 x 10-19 C (this value is referred to as e).
• An object can have a charge of 2e or 3e, but it cannot have one of 2.5e.
• Physicists use the electronvolt because its size is convenient for expressing the energies involved in some chemical reactions between atoms.
• When charged particles move in an electric field, work is being done by the field on the particles.
• In this special case the work can be expressed in terms of potential energy which is associated with something called electric potential.
• The potential that the particle has to do work, and since it is in an electric field, electric potential seems like the most logical name for it.
• Potential is scalar because both potential energy and charge are scalar.
• The potential of 1 J/C is called 1 Volt (V) and is named after the Italian scientist Alessandro Volta (1745-1827).
• Just as in gravitational fields, you can draw equipotentials on a field diagram.
• Equipotentials are points in the field of equal potential.
• In all cases, the equipotentials are at right angles to the field lines.
• The separation of the equipotentials tells you about the field.
• Evenly spaced equipotentials indicate a uniform field.
• Increasing separation of equipotentials indicates a weakening field.
• The rate of change of spacing between equipotentials is called the “potential gradient” and it can be shown that Field strength, E = potential gradient.
• Gravitational fields are vector quantities that act on all particles with mass and are always attractive, with a force equation of F = mg where g is the gravitational field intensity/strength (force per unit mass).
• Gravitational equipotentials are lines of equal gravitational energy.
• The forces between point masses obey an inverse square law.
• A metal container connected to earth placed around a charge will shield other charges from its effect.
• The charge of an electron is -1.6 x 10-19 C.
• The charge on a proton is +1.6 x 10-19 C.
• You cannot have half an electron or half a proton.