ELECTRICAL Currents & Energy

1. ELECTRICAL Currents & Energy

2. Remember that an electrical charge is when there are more or less electrons than protons in an atom. The electrical force between any two objects obeys an inverse-square relationship withdistance. This relationship is explained by Coulomb’s law and has a nice formula whichwe’ll save for high school. The SI unit of charge is the coulomb “C”. One C is equal to 6.24 billion billion(6.24 x 1018) electrons.

3. 6.24 billion billion electrons might seem like agreat number of electrons, but it represents only the amount of charge that passes through a common 100-W light bulb in about one second. A continuous flow of charge is called an electric current. Electric current is measured in amperes, “A”. An ampere is the flow of 1 coulomb of charge per second. A current carrying wire doesn’t have a net electriccharge. The same # of electrons enter as the same # electrons exit. Net Charge = 0

4. Electric currents can be produced in many ways.One way is through chemical reactions in a battery. A battery is a container consisting of one or more cells, in which chemical energy is converted into electricity and used as a source of power. A cell is a device that produces an electriccurrent.

5. Every cell contains a mixture of chemicals thatconducts a current. The mixture is called anelectrolyte. Chemical reactions in here convertchemical energy into electrical energy. Every cell also contains a pair of electrodes madefrom 2 different conducting materials that are incontact with the electrolyte. The electrode is the part of a cell through which charges enter or exit.

6. Cells are divided into 2 groups: Wet cells &Dry cells. Wet Cells contain liquid electrolytes.

7. Dry Cells contain electrolytes that are solid orpastelike.

8. Why does an electric current exist between the two electrodes? The electric current exists because a chemical reaction causes a difference in charge between the two electrodes. The difference in charge means that an electriccurrent – a flow of electric charges – can be produced by the cell to provide energy. The energy per unit charge is called the potentialdifference and is expressed in volts (V).

9. Current Revisited: Current is the continuous flow of charge, but itis more precisely defined as the rate at which charge passes a given point. There are two different types of current. Direct Current (DC) are where the charges alwaysflow in the same direction. The electric current produced by batteries and cells is DC.

10. In Alternating current (AC) the charges continuallyswitch from flowing in one direction to flowing in the reverse direction. Alternating current is usedin homes today because it is more practical fortransferring electrical energy. In the U.S., the AC provided to households changes directions 120times each second. https://www.youtube.com/watch?v=xyQfrzBfnDU

11. In addition to voltage, resistance also determinesthe current in a wire. Resistance is the oppositionto the flow of electric charge. It is expressed inOhms (Ω). In equations, the symbol for resistanceis the letter R. You can think of resistance a “electrical friction.”The higher the resistance of a material, the lowerthe current is in it. Therefore, as resistance increases, current decreases if the voltage is keptthe same. An object’s resistance varies depending on theobject’s material, thickness, length, and temperature.

12. How you should be thinking about electric circuits: Voltage: a force that pushes the current through the circuit (in this picture it would be equivalent to gravity)

13. How you should be thinking about electric circuits: Current: the actual “substance” that is flowing through the wires of the circuit (electrons!)

14. How you should be thinking about electric circuits: Resistance: friction that impedes flow of current through the circuit (rocks in the river)

15. The relationship between voltage difference, current, and resistance in a circuit is known as Ohm’s Law I = V / R I = Current (Amperes) (amps) V = Voltage (Volts) R = Resistance (ohms) Georg Simon Ohm (1787-1854)

16. Other Ways to produce electricity: A Photocellis the part of a solar panel that converts light into electrical energy. Thermal energy can be converted to electrical by a Thermocouple. The temperature differencewithin the loop causescharges to flow.

17. Would This Work?

18. Would This Work?

19. Would This Work?

20. The Central Concept: Closed Circuit

21. circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires

22. Simple Circuits • Series circuit • All in a row • 1 path for electricity • 1 light goes out and the circuit is broken • Parallel circuit • Many paths for electricity • 1 light goes out and the others stay on

23. measuring current Electric current is measured in amps(A) using an ammeter connected in series in the circuit. A

24. measuring current This is how we draw an ammeter in a circuit. A A PARALLEL CIRCUIT SERIES CIRCUIT

25. measuring voltage The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V

26. measuring voltage This is how we draw a voltmeter in a circuit. V V SERIES CIRCUIT PARALLEL CIRCUIT

27. measuring current SERIES CIRCUIT 2A • current is the same at all points in the circuit. 2A 2A PARALLEL CIRCUIT 2A 2A • current is shared between the components 1A 1A

28. fill in the missing ammeter readings. ? 3A 3A ? 4A ? 1A ? 4A 4A 1A ? 1A

29. The circuit is no longer complete, therefore current can not flow The voltage decreases because the current is decreased and the resistance increases.

30. The current remains the same. The total resistance drops in a parallel circuit as more bulbs are added

31. Series and Parallel Circuits • Series Circuits • only one end of each component is connected • e.g. Old Christmas tree lights • Parallel Circuits • both ends of a component are connected • e.g. household lighting

32. measuring voltage Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. Unlike an ammeter, a voltmeter is connected across the components Scientist usually use the term Potential Difference (pd) when they talk about voltage.

33. measuring voltage V V V V

34. series circuit • voltage is shared between the components 3V 1.5V 1.5V

35. parallel circuit • voltage is the same in all parts of the circuit. 3V 3V 3V

36. measuring current & voltage copy the following circuits on the next two slides. complete the missing current and voltage readings. remember the rules for current and voltage in series and parallel circuits.

37. measuring current & voltage a) 6V 4A A V V A

38. measuring current & voltage b) 6V 4A A V A V A

39. answers a) b) 6V 4A 6V 4A 6V 4A 4A 2A 3V 3V 4A 6V 2A

40. Voltage, Current, and Power • One Volt is a Joule per Coulomb (J/C) • One Amp of current is one Coulomb per second (6.24 x10^18 electrons/second). • If I have one volt (J/C) and one amp (C/s), then multiplying gives Joules per second (J/s) • this is power: J/s = Watts • So the formula for electrical power is just: • More work is done per unit time the higher the voltage and/or the higher the current P = VI: power = voltage  current