Electric Current

Electric fish were known before the time of Christ. (2000 BC)

Benjamin Franklin and Current Electricity Benjamin Franklin (1752) was an early experimenter with electricity. He thought (incorrectly) that electricity was a fluid (something that flowed like a gas or liquid). He called a surplus of electricity positive (+) and a loss or deficit of electricity negative (-). Franklin thought that electricity moved from + (a surplus) to – (a deficit).

Do Positive Charges Actually Move? (No!) Ben Franklin (1752) was guessing that positive charges moved from positive to negative because he could not see or detect anything. It was not until the early 1900s that scientists could actually detect that current electricity was actually a flow of negative electrons which moved from negative poles to positive poles.

Two Ways of Describing Electrical Current For over 140 years since Ben Franklin, scientists and engineers had taught that electricity was positive charges that moved from positive to negative. Millions of books had been written about electricity and electrical devices, all describing electricity this way. Electromagnets, electrical meters, motors, transformers, microphones and all other electrical devices were adequately explained by thinking of electricity as a flow of positive charges.

Two Ways of Describing Electrical Current Scientists decided to keep the convention of electricity as something that is a flow of positive charges from + to – and called this the conventional flow theory. The real flow of electricity as negative charges flowing from – to + was (is) called the actual flow theory. All electricians in the world are trained in the conventional flow theory. High school science courses in many places are taught according to the actual flow theory but in British Columbia we teach according to the conventional flow theory since students going on in electrical trades will be taught this way also.

In This Course, Electrical Current is Positive Charges Moving from + to – (Conventional Flow Theory). A power source gives positive charges energy (voltage) and they flow from a positive region (pole) to a negative region (pole). Moving positive charges can do work, like turning motors, generating heat (toaster/stove), making light (bulbs) or producing sound (speakers).

Ben Franklin’s Kite Experiment Ben Franklin thought that lightning was a form of electricity, flowing from the clouds to the ground. To prove this he is said to have flown a kite in a storm so that the kite wire could conduct and store the lightning in a Leiden jar. He supposedly protected himself by holding the kite wire with a silk insulator. He noticed that a key on the wire glowed. Franklin was lucky he wasn’t killed since the lightning could just as well have passed through him as into the Leyden jar.

Energy – Causes Work to be Done Energy is defined as the ability to do work (applying a force over a distance). E or W = F x d. Energy is the ability to move things. Energy is measured in Joules (J). 1 N of force acting over 1 m of distance is 1 J of work

Forms of Energy All energy forms can do work (can exert a force over a distance). Moving objects have kinetic energy.

Potential and Kinetic Energy (Joules – J) Kinetic energy is the energy moving objects have because of their motion (the work they can do because of their motion). Potential energy is the energy stored in an object (the work that can be done when the stored energy is released).

Gravitational Potential Energy Lifting objects gives them Gravitational Potential Energy. The more massive the object, the more energy it has and the higher it is lifted, the more energy it has.

Electrical Charge : A Coulomb (C) A coulomb of electrical charge is the charge that -6.24 x 1018 electrons or +6.24 x 1018 protons carry. ... …

Voltage (V) : Energy per Coulomb (J/C) Voltage is the amount of energy carried by a charge (electrons or protons). Precisely, 1 volt (V) = 1J/C . Charge carrying less energy or having less “push” has lower voltage. Lower voltage Higher voltage

Electrical Potential Energy The total electrical potential energy depends on voltage and the amount of charge that can be separated. This has less EPE (electrical potential energy) This has more EPE

Voltage, Current, Coulombs and Resistance The voltage is the push or energy moving the electron current. The number of electrons is measured in coulombs (6.28 x 1018 electrons is 1 coulomb). The resistance is what opposes the electron current through a substance (the amount its atoms block the electron flow).

Electrical Potential Energy The text uses the analogy that total electrical potential energy is like the total gravitational potential energy in lifting an object to the top of a stairs. The GPE depends on the total mass that is lifted to the top of a stairs and the height of the stairs.

Energy Transforms from One Form to Another Energy is never created or destroyed but it can change from one form to another. Melt 2 TBSP of butter in a frying pan. Add the bread crumbs and stir until

Ways of Producing Electrical Current Chemical reactions inside electrochemical cells cause electrons to be removed from one place (which becomes positive) to another place (which becomes negative). The positive place on a cell or battery is called the positive electrode or terminal. The electrical symbol for a cell is a long line and a short line. The long line is the positive electrode. A connection of two or more cells is called a battery. Electrochemical cells convert chemical energy into electrical energy.

The Parts of an Electrochemical Cell All that is needed to make an electrochemical cell is two different metals which then are inserted into an electrolyte. Compared to each other, one metal will become positive and the other negative. The electrolyte is a liquid or paste which has positive and negative ions. Acids (sour substances), bases (bitter substances) and salts all produce positive and negative ions and therefore are good electrolytes.

Ways of Producing Electrical Current There are two groups of “batteries” (technically they should be called cells): dry cells and wet cells.

Ways of Producing Electrical Current Electrochemical cells produce electricity by using chemical reactions to take electrons from one place (which becomes positive) and adding electrons to another place (which becomes negative). To work an electrochemical cell needs two electrodes (terminals) made from DIFFERENT materials and an electrolyte, a liquid or moist paste which has charged + and – ions.

Ways of Producing Electrical Current A wire coil moving in a magnetic field changes kinetic energy (energy of motion) into electrical energy (an electric current).

Ways of Producing Electrical Current Certain crystals like quartz will send out electric current when moved in or out. Electricity sent into these same crystals makes them move in or out. These piezoelectric crystals convert movement into electricity or vice versa.

Ways of Producing Electrical Current Heat energy can be converted electrical energy (current electricity) with a thermocouple. In a thermocouple, two different wires are connected together and the two connections are kept at different temperatures. The bigger the temperature difference in the connections, the larger the flow of electricity.

Ways of Producing Electrical Current Photovoltaic cells convert light energy into electrical energy (current electricity). Solar panels (often installed on roofs) have banks of solar cells that convert sunlight into electricity.

Alessandro Volta In 1799, Volta invented the “voltaic pile” a series of alternating zinc and copper disks with salt water soaked fabric between them. This was the world’s first battery.

Voltage or Potential Difference (Measured in Volts) The voltage or potential difference (or EMF – electromotive force) is the amount of energy carried by (associated with) each moving charge. Voltage indicates the amount of energy per unit of charge or the work that a unit of charge can do. V = E/Q. If Electrical current is compared to a flow of water, then voltage in a current is like the pressure behind the flow of water.

Voltage is shown by the rise in the charge from ground level.

End of Presentation