Electric Charges

Electricity Unit

Exploring Static Charges

• Electricity is a form of energy that results from the interaction of charged particles, such as electrons or protons.

• Static charge (or static electricity) is an electric charge that tends to stay on the surface of an object, rather than flowing away quickly.

• Static = stationary or not moving

• Static charges build up as different materials rub together, such as in a clothes dryer.

• This is called charging by friction = a process in which objects made from different materials rub against each other, producing a net static charge on each.

• When objects become charged by friction, one material has a stronger material, and therefore pulls electrons off the material that has the weaker attraction for electrons.

• As a result, both materials become charged due to an excess or a deficit (shortage) of electrons.

• Remember back to the Bohr-Rutherford Model of the Atom, in which electrons move in the outer parts of the atom, relatively far from the nucleus. Electrons are negatively charged.

• Charged objects exert an electric force on each other.

• Objects with like charges repel each other; objects with opposite charges attract each other.

• This is called the Law of Electric Charges.

The B-R model of the atom explains the following:

• Particles that carry electric charges can be neither created nor destroyed.

• Any net charge on a solid object, whether it is positive or negative, results from the transfer of electrons from one object to another.

• Compared with a neutral object, an object with an excess of electrons (more electrons than protons) has a negative charge.

• Compared with a neutral object, an object with a deficit of electrons (fewer electrons than protons) has a positive charge. Remember, protons have a positive charge.

• Different materials hold on to their electrons with different strengths.

See the Electrostatic Series.

• An electrostatic series is a list of materials that have been arranged according to their ability to hold on to electrons.

• If we rub together two materials that are far apart in an electrostatic series, we can accurately predict the charge on each that will result.

• Electrons can move each from one atom to another.

• When electrons move, they transfer electric charges.

• Protons do not move from one atom to another.

• Neutral objects have an equal number of protons and neutrons.

• When a neutral object gains electrons, it becomes negatively charged.

• When a neutral object loses electrons, it becomes positively charged.

• Electric force increases with an increase in electric charge and decreases with increasing distance between the objects.

Moving Charges

• A conductor is a material that transmits thermal energy or electrical energy easily. – Metals such as copper wire are considered good

conductors.

• An insulator is a material, such as ceramic, which prevents the flow of electrons. – Most non-metals are insulators.

• Electrical conductivity is a material’s relative ability to conduct electricity and allow electrons to flow through it.

• Different materials fall along a range that based on increasing levels of conductivity.

Grounding: removing static charges

• The simplest way to remove the net static charge on an object is to put it in contact with a ground = an object that can supply a very large number of electrons to, or can remove a very large number of electrons from, a charged object, thus neutralizing the object.

• In many situations, static charges are a serious hazard to both people and electrical equipment.

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