1. Ch. 6 Electricity (Unit 5b)

2. 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.

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

4. 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

5. 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

6. Electric circuit: a closed path that electric current follows

7. 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

8. 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

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

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

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

12. 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)

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

14. Water works analogy

15. Water works analogy

16. 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

17. 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

18. 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

19. 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

20. Dry - Cell
Wet - Cell

21. 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.

22. 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

23. Series and Parallel Schematics

24. Schematic Symbols Battery Switch Wire Ammeter Resistor Voltmeter
Light Bulb

25. Series Circuit

26. Parallel Circuit

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

28. 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

29. 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