Ch. 6 Electricity (Unit 5b)

Electric Current Electric Current: the net movement of electric charges in a single direction Measured in amperes (A) 1 ampere = 1 coulomb of electric charge flowing past a point per second (1 C/s) A voltage causes charges to move, which in turn produces a current. A circuit must be closed for a current to be produced.

Open vs Closed Circuit Closed circuit allows electron to flow, lights on Open circuit does not, lights off

A charged object has electric PE due to its position in an electric field. Potential difference or voltage is the difference in electrical PE per unit charge. SI unit for pot. diff. are volts (V). 1 volt = 1 Joule/coulomb Ex. 12V vs. 9V

Conventional electric current flows from higher voltage to lower voltage Voltage sources (like batteries or generators) provide force that causes electric charges to flow Charge Flow _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ Current Low Voltage High Voltage

Electric circuit: a closed path that electric current follows

Current and Flow Direction The direction of conventional electric current is always from a higher voltage to a lower voltage, but … The electrons in a circuit actually flow from a lower voltage to a higher voltage This is due to a historical error that stuck

Resistance Resistance: the tendency for a material to resist the flow of electrons and to convert electrical energy into other forms of energy Measured in ohms ( ) Caused by internal friction which slows the movement of charges through conducting material Resistor: a special type of conductor used to control current

Resistance Good conductors have low resistance Insulators or poor conductors have high resistance Thicker wires have lower resistance Thinner wires have higher resistance

Water works analogy Water is analogous to charge (electrons) Water flow (total volume passing a point per second) is analogous to current

Water works analogy How high the water starts relative to how low it ends up is analgous to voltage (potential difference)

Water works analogy (alternatively, a voltage source (like a battery) can be thought of as a pump. How much pressure the the pump generates is analogous to voltage)

Water works analogy The narrowness of pipe channels, which reduces the total flow (current), is analogous to resistance

Water works analogy

Water works analogy

Putting it together: Ohm’s Law V is voltage, in volts (V) I is current, in amperes (A) R is resistance, in ohms (Ω) V = I·R What it means: For a given voltage, if there is lower resistance, there is more current (flow) For a given voltage, if there is higher resistance there is less current

Alternating Current/Direct Current (AC/DC) Many power sources produce alternating current that is continually reversing flow This is the kind of current supplied to US households Current reversal occurs 60 times per second (60 Hz) Appliances like toasters, fans, and lights often designed to use AC

Alternating Current/Direct Current (AC/DC) Current that has a constant flow in a single direction is called direct current Many electronic devices need DC, so in order to work when plugged in, an AC adaptor is required AC adaptors allow conversion from ACDC

Batteries: Ex. DC Source of Voltage Dry – Cell Batteries Wet – Cell Batteries Consist of 2 electrodes surrounded by a material called an electrolyte Electrolyte enables charges to move from one electrode to another Chemical reaction occurs when the 2 terminals are connected in a circuit Voltage difference between these 2 terminals causes a current through a closed circuit Example: flashlight battery Contains 2 connected plates made of different metals or metallic compounds in an electrolyte The electrolyte is a conducting liquid solution Example: car battery

Dry - Cell Wet - Cell

Semiconductor Semiconductors in pure state are insulators. However, as atoms/impurities are added, or become compounds, these materials begin to be able to conduct electric charge.

Schematic Diagram or drawing used to build a circuit. There are two types of circuits. Series: one path for electrons, if one bulb goes out they all do Parallel: more than one path for electrons, if one bulb goes out the rest can still shine

Series and Parallel Schematics

Schematic Symbols Battery Switch Wire Ammeter Resistor Voltmeter Light Bulb

Series Circuit

Parallel Circuit

Ammeters & Voltmeters Ammeters read current (in A), placed in series with circuit Voltmeters read V, placed in parallel with device/resistor

Circuit Breakers and Fuses Too many appliances on at the same time in a home cause the overall resistance of the circuit to be lowered. Result is a circuit carrying more electrical current than is safe it is Overloaded. Fires can result when this happens. Circuit Breakers act as a switch and open the circuit. Fuses physically break and open the circuit

Power Formula Remember, power is Energy per Time Unit for Electric Power (P) = Watts (Watts) --- Formula: Power = Current x Voltage --- With symbols: P= I·V Remember, power is Energy per Time Using higher wattage appliances will use more energy in less time, and be more costly