Unit 11: Electric Current Many practical devices and applications are based upon the principles of static electricity. Electricity became an integral part of our daily lives when scientists learn to control the movement of electric charge.

Unit 11: Electric Current Electric Current is the rate at which electric charges pass through a given area. Current is measured in amperes which are equivalent to a Coulomb per second.

Unit 11: Electric Current Conductors transmit charges easily and are used for electric circuits. Conductor (freely flowing electrons) Insulator (electrons cannot easily flow)

Unit 11: Electric Current What makes Copper a good conductor? ◦ Copper has two electrons in the innermost shell, eight in the next shell, eighteen in the third shell, and one in the fourth shell  The first three shells each have as many electrons as they can hold  The fourth shell has one lonely electron  This one lonely electron can easily separate from the rest of the atom and go roaming around

Unit 11: Electric Current Batteries [pictures on next slide] provide a chemical reaction between two terminals (cathode and anode) in some form of medium (electrolyte) ◦ In an Alkaline Battery  Cathode: Manganese Dioxide  Anode: Zinc powder  Electrolyte: Potassium hydroxide The chemical reaction causes the build-up of electrons on one terminal, and a deficit of electrons on the other

Unit 11: Electric Current Equivalent pictures

Unit 11: Electric Current EMF (  ) – Electromotive Force ◦ The maximum potential difference (voltage) a power source can have  The potential difference between the terminals of a battery when it’s not connected to anything  Batteries maintain a constant voltage ◦ Not actually a force ◦ Measured in J/C (Volts)

Unit 11: Electric Current Each cell can produce a fixed amount of EMF Bigger cells can produce EMF for a longer time than smaller cells Batteries run out of EMF when all of the free electrons have been moved from the cathode to the anode Rechargeable batteries can reverse the chemical reaction to re-deposit negative charges back on the cathode

Unit 11: Electric Current Batteries convert chemical energy into electrical energy The actual voltage of a battery is always slightly less than the EMF ◦ Batteries have internal resistance ◦ Energy is lost as heat

Unit 11: Electric Current A basic circuit has three main parts: ◦ A source of energy  Sometimes called a “potential difference”. ◦ A closed path ◦ A device which uses the energy

Circuits are very similar to water in a pipe Pumps ◦ There is a pressure difference where the flow begins and ends ◦ A certain amount of flow passes each second Circuits ◦ There is a potential difference (voltage) were the charge begins and ends ◦ The amount of charge that flows per second is called current

Unit 11: Electric Current Resistance: The opposition presented to electric current by a material/device. ◦ Units are ohm (Ω) = volt/ampere. Ohm’s Law:

Unit 11: Electric Current Resistance depends upon length, area, material and temperature.

Unit 11: Circuits Electric Circuit: A set of electrical components such that they provide one or more complete paths for the movement of charges.

Unit 11: Circuits Ohm’s Law:

Unit 11: Circuits Series: describes two or more components of a circuit that provide a single path for current

Unit 11: Circuits Equivalent resistance in a series circuit is the sum of the resistances. In our picture below, R = R 1 + R 2 + R 3

Unit 11: Circuits Example: A 24.0 V battery is connected in series with R 1 = 2.0 Ω, R 2 = 5.0 Ω and R 3 = 5.0 Ω. Find the equivalent resistance and the current in the circuit.

Unit 11: Circuits Parallel: describes two or more components of a circuit that provide separate conducting paths for current because the components are connected across common points or junctions.

Unit 11: Circuits Equivalent resistance in a parallel circuit is the sum of the reciprocals of the resistances. In our example below,

Unit 11: Electric Power Batteries maintain a potential difference across their terminals by converting chemical energy to electrical potential energy. ◦ Electrons are moving and thus, they have kinetic energy. ◦ They collide with other charges, transferring energy as heat

Unit 11: Electric Power Sometimes batteries must be replaced or recharged (when all electrons move from the cathode to the anode). Generators do not run out of energy. ◦ They convert mechanical energy into electrical energy ◦ Generators are the source of current to a wall outlet in your home.

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Unit 11: Electric Power Calculating electrical power:

Unit 11: Electric Power Electric companies measure energy consumed in kilowatt-hours. Remember: Power = Energy/Time = J/s

Unit 11: Electric Power Some current, like in a battery, is Direct Current. Some current, like that which is generated by the power company, is Alternating Current.