Electricity Chapter 17.2

Section 2 Electric Current and Electrical Energy Electric current – is the rate at which charges pass a given point. The higher the current is, the greater the number of charges that pass the point each second.

Electric current is expressed in unit called Amperes – which are often shortened to amps. The symbol for unit ampere is A The symbol for current is the letter I

Why I is used for current? The conventional symbol for current is I, which originates from the French phrase intensité de courant, (current intensity). ... The I symbol was used by André-Marie Ampère, after whom the unit of electric current is named, in formulating Ampère's force law (1820)

Making Charges Move When you flip a switch, an electric field is set up in a wire at the speed of light. The electric field causes the free electrons in the wire to move. The energy of each electron is transferred instantly to the next electron. The electric field is created so quickly that all electrons start moving through the wire at the same instant.

AC and DC There are two kinds of electric current direct current (DC) and alternating current (AC) In direct current (DC) the charges always flow in the same direction. The electric current from the batteries used in a camera is DC In alternating current (AC), the charges continually shift from flowing in one direction to flowing in the reverse direction. The electric current from outlets in your home is AC.

Voltage Voltage – is the potential difference between two points in a circuit, (the push behind the current) Voltage is expressed in the unit volts (V). In equation, the symbol for voltage is the letter V. Voltage is a measure of how much work is needed to move a charge between two points. Think of voltage as the amount of energy released as a charge moves between two points in the path of a current. The higher the voltage is, the more energy is released per charge. As long as there is a voltage between two points on a wire, charges will flow in the wire.

Resistance Resistance – is the opposition to the flow of electric charge. Resistance is expressed in the unit ohms (Ω,the Greek letter omega). In equations, the symbol for resistance is the letter R. Resistance can be thought of as “electrical friction” The higher the resistance of a material – the lower the current in the material is. If voltage doesn’t change, as resistance goes up, current goes down.

Think About This… If you think of current as cars traveling on the road and the number of cars represent the amount of voltage. So if there is no “ROAD BLOCK” or resistance then the “TRAFFIC” or current increases in speed, but if there is resistance the current decreases. But the “NUMBER OF CARS” or Voltage is constant.

Thicker-less resistance Shorter-less resistance Colder-less resistance Resistance Varies An objects resistance varies depending on the object’s material, thickness, length, and temperature. As resistance increases, current decreases. (INVERSE RELATIONSHIP) Thicker-less resistance Shorter-less resistance Colder-less resistance

Generating Electrical Energy Energy cannot be created or destroyed. It can only be transformed or changed into other kinds of energy. Many things change different kinds of energy into electrical energy. Generators convert mechanical (kinetic) energy to electrical.

Generating Electrical Energy Cells and batteries convert chemical or radiant energy into electrical energy. The two kinds of cells are wet cells and dry cells. Wet cells have liquid electrolytes. A car battery is made of several wet cells that use sulfuric acids as electrolytes. Dry cells work similar to wet cells, but the electrolytes are solid or paste-like. Batteries used in small radios & flashlights are dry cells. Thermocouples – a device that converts thermal energy into electrical energy. Photocells – a device that converts light energy in electrical energy.