1. Ch 34 Electric Current Notes

2. Chapter 17 Objectives Describe electric current.
Section 2 Electric Current and Electrical Energy
Chapter 17
Objectives
Describe electric current.
Describe voltage and its relationship to electric current.
Describe resistance and its relationship to electric current.

3. What is electricity? So electricity is…
movement or flow of charge round a circuit.
We call this electric current.

4. Chapter 17 Electric Current
Section 2 Electric Current and Electrical Energy
Chapter 17
Electric Current
Electric current is the rate at which charges pass through a given point. Electric current is expressed in units called amperes, or amps.
Making Charges Move When you flip a switch, an electric field is set up in the wire at the speed of light. The electric field causes the free electrons in the wire to move.

5. Electric Current, continued
Section 2 Electric Current and Electrical Energy
Chapter 17
Electric Current, continued
Commanding Electrons to Move This electric field is created so quickly that all electrons start moving through the wire at the same instant. Think of the electric field as a command to the electrons to charge ahead.

6. Flow of Charge

8. Amperes (A) I Charge, Current & Time
Electric current is given the symbol I
Current is the amount of charge flowing
per second and is given the unit
Amperes (A)

9. Electric Current, continued
AC and DC There are two kinds of electric current—direct current (DC) and alternating current (AC).

10. DC (Direct Current) AC (Alternating Current)
Charges flow in same direction
Charges constantly switch from one direction to the other

11. 5 Why is AC used in homes? More practical for transferring energy
An electrical power plant can produce up to 22,000 V of electric current, but the avg. home appliance in the US requires only 120 V
In the US, current changes direction 120 times per second, for a frequency of 60 cycles per second or 60 Hertz.
Normal outlet voltage in the US is volts, although some large household appliances run on volts. Most countries are on 220 V
The voltage is reduced at least three times as the electrical energy travels between the electric plant and your home!

12. Converting AC to DC
AC is converted to DC using devices called diodes, which allow charges to move in only 1 direction.

13. Chapter 17
Voltage
Voltage (V) is the potential difference between two points in a circuit. Voltage is expressed in volts (V).
Voltage and Energy Voltage is a measure of how much work is needed to move a charge between two points.You can think of voltage as the amount of energy released as a charge moves between two points in the path of a current.

14. Chapter 17 Voltage, continued
Section 2 Electric Current and Electrical Energy
Chapter 17
Voltage, continued
Voltage and Electric Current As long as there is a voltage between two points on a wire, charges will flow in the wire. The size of the current depends on the voltage.
Varying Nature of Voltage Different devices need different levels of voltage.

15. Resistance Symbol – R Unit - Ω
Section 2 Electric Current and Electrical Energy
Chapter 17
Resistance Symbol – R Unit - Ω
Resistance is the opposition to the flow of electric charge.
Resistance and Material Good conductors, such as copper, have low resistance. Poor conductors, such as iron, have higher resistance.
Resistance, Thickness, and Length Thick, short wires have less resistance than thin, long wires.

16. What Affects Resistance?

17. Chapter 17 Resistance, continued
Section 2 Electric Current and Electrical Energy
Chapter 17
Resistance, continued
Resistance and Temperature In general, the resistance of metals increases as temperature rises.

18. Factors That Affect Resistance
Section 2 Electric Current and Electrical Energy
Chapter 17
Factors That Affect Resistance
Click below to watch the Visual Concept.
Visual Concept

19. Ohm's Law
Georg Simon Ohm, a mathematician, proved a link between voltage (V), current (I) and resistance (R). The unit of resistance was named after him, the ohm. Ohm's Law states:
as long as temperature stays the same
the resistance of a conductor stays constant,
and the current is directly proportional to the voltage applied
R=V/I

20. The force or pressure behind electricity

24. R=V/I
R= 4V/0.5A
R= 8Ω
Resistance 8Ω

26. I=V/R
I=1.5V / 10Ω
I= 0.15 A
Current drawn 0.15 A

27. Generating Electrical Energy
Section 2 Electric Current and Electrical Energy
Generating Electrical Energy
Parts of a Cell A cell contains a mixture of chemicals called an electrolyte. Every cell also has a pair of electrodes made from conducting materials.
Kinds of Cells Two kinds of cells are wet cells and dry cells. The electrolytes in dry cells are solid or pastelike.

28. The zinc-carbon dry cell
The first mass-produced and widely used small-scale source of electrical energy.
Has changed very little in the last 100 years.
An electrolyte composed of a moist paste of zinc chloride and ammonium chloride plays the same role as the salt bridge.

29. Alkaline cells
The alkaline cell is optimised for performance and longevity.
The alkaline cell is designed for more high capacity use appliances than the zinc- carbon dry cell.

30. Wet Cells Lead storage batteries
A battery is a group of voltaic cells connected together.
A 12-V car battery consists of six voltaic cells connected together.
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31. Section 2 Electric Current and Electrical Energy
Chapter 17

32. Generating Electrical Energy, continued
Section 2 Electric Current and Electrical Energy
Chapter 17
Generating Electrical Energy, continued
Thermocouples Thermal energy can be converted into electrical energy by a thermocouple.
Photocells A photocell converts light energy into electrical energy.