Exploring Electricity: Lighting a Bulb Experiment

1. Electricity

2. Can you get a light bulb to light? • Using the equipment given to you, try to find as many ways as possible to get the light bulb to light. • Diagram four ways in which you were able to get the light bulb to light. Label the battery, the wire, and the bulb. • Diagram at least three ways in which you were not able to get the bulb to light. Questions: 1. From your observations, what conditions seem to be necessary in order for the bulb to light?

3. Successful Attempts

4. Producing Electric Current • The flow of charged particles is an electric current. • Charges flow from a conductor with a higher potential difference through a wire to a conductor with a lower potential difference. • Conventional current is defined as positive charges flowing from the positive (high potential difference) plate to the negative (low potential difference) plate. • The flow stops when the electric potential difference between the two plates is zero. • If the charge particles are pumped back to the original conductor (plate) by an outside energy source (battery), the flow of charges will continue.

5. Electric Circuits • Any closed loop or conducting path allowing electric charges to flow is called an electric circuit. • A circuit includes a charge pump and a device that reduces the potential energy of the charges as they flow from one plate to another. • The potential energy lost by the charges as they move through the connected device is usually converted to some other form of energy such as kinetic (motor), light (lamp), or thermal (heater).

6. Requirements that must be met to establish an electric current. • There must be a closed conducting path that extends from the positive terminal to the negative terminal. It is not enough that there is simply a closed conducting loop; the loop itself must extend from the positive terminal to the negative terminal of the electrochemical cell.

7. Requirements (continued) • The second requirement of an electric circuit that is common in each of the successful attempts is that there must be an electric potential difference across the two ends of the circuit. This is most commonly established by the use of an electrochemical cell, a pack of cells (i.e., a battery) or some other energy source.

8. In Summary • In conclusion, there are two requirements that must be met in order to establish an electric circuit. The requirements are 1)There must be an energy supply capable of doing work on charge to move it from a low energy location to a high energy location and thus establish an electric potential difference across the two ends of the external circuit. 2)There must be a closed conducting loop in the external circuit that stretches from the high potential, positive terminal to the low potential, negative terminal.

9. When a battery no longer works, which requirement is not met? 10 • An external energy supply to pump the charge • The external circuit must make up a closed conducting loop • Neither requirement is met.

10. When a light bulb is burned out and no longer works, which requirement is not met? 10 • An external energy supply to pump the charge • The external circuit must make up a closed conducting loop • Neither requirement is met.

11. Electric Current • Current is the rate at which charge flows past a point on a circuit. • Current flows from the positive terminal to the negative terminal. • Current (I) = q/t = Coulombs/sec=amperes (A) • Example: A 2 mm long cross section of wire is isolated and 20 C of charge is determined to pass through it in 40 s. I = _____amperes • Example: A 1 mm long cross section of wire is isolated and 2 C of charge is determined to pass through it in 0.5 s. I = ________amperes

12. Current is measured in units of ______. 10 • Amperes • Coulombs • Volts • Ohms

13. The direction that conventional current flows in the external circuit is from the ___ terminal to the ____ terminal. 10 • negative, positive • positive, negative • negative, negative • positive, positive

14. Electrons move very quickly through an electric circuit. 10 • True • False

15. If the current is measured as 6 A, then ____ C of charge flows past any point in the circuit in 3 s. 10 • 0.5 • 2 • 3 • 18

16. Power • Power (measured in watts, W) measures the rate at which energy is transferred. (1 W = 1J/s) • The energy carried by an electric current depends on the charge transferred and the potential difference across which it moves. • Current is the rate of charge flow (I=q/t). A flow of 1 C/s is called an ampere (A). • Power is equal to the current times the potential difference. • P = I V • Total energy transferred = P x t (W.s or J)

17. Practice Problems • A 6.0 V battery delivers a 0.50 A current to an electric motor connected across its terminals. • What power is delivered to the motor? • If the motor runs for 5.0 min, how much electric energy is delivered?

18. The Kilowatt- Hour • The electric company (power company) provides energy rather than power. • When a bill is paid, the customer is paying for the electric energy used. • Because of the large amounts of energy consumed, electric companies measure sales in a units of kilowatt-hours. • One kilowatt hour is equal to 1000 W being delivered for 3600 s (1 hr) or 3.6 x 106 J. • How many 100 W light bulbs would have to be used for 1 full hour to consume 1 KWh of energy?

19. Practice Problem • An electric heater draws 15.0 A from a 120 V source. It is operated, on the average, for 5.0 h each day. • How much power does the heater use? • How much energy in KWh does it consume in 30 days? • At $0.12 per KWh, how much does it cost to operate the heater for 30 days?

20. Problem • A TV has a power rating of 103 W. How much does it cost per month to operate it for 6 hours per weekday and 15 hours/day on the weekend? (Assume a 4 week month) • How much current does it draw?

21. 10 Power is measured in units called • amperes • joules • kilowatt-hours • watts

22. 10 Current is measured in units called • amperes • kilowatt-hours • potential • watts

23. 10 If a 9.0 V battery delivers 0.50 A of current to a heater, the power consumed by the heater is • 0.056 W • 4.5 W • 9.0 W • 18 W

24. 10 If a light bulb is rated at 50 W, the amount of energy consumed in 30 minutes is • 2 kWh • 30 kWh • 30 J • 90,000 J

25. 10 The kilowatt-hour is a unit of • current • energy • potential • power

26. 10 A 60 W light bulb illuminated day and night for 30 days consumes ___ of energy, • 43.2 J • 43.2 kWh • 1800 kWh • 1.5 x 1012 kWh

27. 10 If a family’s electric bill is $74.00 per month and the cost of electricity is $0.12 per kWh, how much electricity does the family use per month? • 8.9 kWh • 270 kWh • 620 kWh • 62000 kWh

28. Resistance • Resistance (R) is a property that determines how much current will flow. • Resistance is measured in units called ohms (Ω)

29. Factors Impacting Resistance • Length Resistance increases as length of wire increases. • Cross-sectional area Resistance increases as cross-sectional area decreases. • Temperature Resistance increases as temperature increases. • Material Resistance varies with the material used. silver<copper<gold<aluminum<iron<platinum (in order of increasing resistance)

30. Ohm’s Law • Resistance is equal to potential voltage divided by current. • V = I R • According to the law, 1 ohm is the resistance permitting an electric charge of 1 A to flow when a potential difference of 1 V is applied across the resistance.

31. 10 For a wire that obeys Ohm’s Law, the resistance of a wire depends on the ___ the wire. • current in • length of • power delivered by • voltage across

32. 10 A battery with a voltage of 9.0 V is connected to a lamp. The current flowing in the circuit is 0.30 A. The resistance of the lamp is • 0.030 Ω • 2.7 Ω • 27 Ω • 30 Ω

33. 10 What factor impacting resistance explains why light bulbs tend to burn out more frequently when they are first switched on rather than while they are operating? • Length • Cross-sectional area • Temperature • Material

34. 10 Which wire conducts electricity with the least resistance? • One with a large cross-sectional area • One with a small cross-sectional area

35. World’s Longest Burning Light • Livermore's Centennial Light

36. Resistors • Wires used to complete circuits have low resistance. • A wire 1 m in length that is typically used in a physics lab has a resistance of 0.03 Ω. • Wires used in home wiring offer as little as 0.004 Ω of resistance for each 1 m of length. • Resistors are devices with specific resistances that are used to control current in circuits or parts of circuits. • A variable resistor (potentiometer) is used when a smooth, continuous variation of the circuit is desired (example: volume, brightness, contrast, hue, and tone controls on a TV) • The human body acts as a variable resistor. (Dry skin has high resistance-wet skin has lower resistance)

37. 10 A device that can be used to change the current in a circuits in a continuous way is a • potentiometer • battery • motor • lamp

38. 10 If a 200 Ω resistor is connected to a 5 V battery, the current in the circuit will be • 0.025 A • 2.5 A • 40 A • 1000 A

39. Types of Circuits • There are three types of circuits: Series circuits-all current travels through each device. Parallel circuits-a circuit in which there are several different current paths Combination series-parallel circuit- circuits which includes both series and parallel branches.

40. Series Circuits • In a series circuit, the current is the same throughout. Itotal= IA =IB = IC = … • The equivalent resistance (total resistance) of the circuit equals the sum of the individual resistances of the resistors. Rtotal = RA + RB + … • The total voltage is the sum of the individual voltages across each device. Vsource = VA + VB + … • If the voltage source does not change, adding more devices in series will decrease the current. • Because the current flows through one pathway, if one light goes out, they all go out.

41. 10 The current is ___ in a series circuit. • higher at the beginning of • the same everywhere • lower at the beginning • variable in

42. 10 In a series circuit, the equivalent resistance is ___ any single resistance. • larger than • determined by • equal to • smaller than

43. 10 If the battery voltage does not change, adding more devices in the series ___ the current. • sometimes decreases • always decreases • sometimes increases • always increases

44. 10 To find the current through a series circuit, first calculate the ____. • voltage • equivalent resistance • power • equivalent voltage

45. Parallel Circuits • A parallel circuit is one in which there are several paths. • The total current is the sum of the currents through each path. Itotal = IA + IB + Ic + … • Placing two or more resistors in parallel always decreases the equivalent resistance of a circuit. 1 = 1 + 1 + 1 + … R RA RB RC • The potential difference across each path is the same. Vtotal = VA = VB = VC = …

46. Safety Devices • A short circuit occurs when a circuit with very low resistance is formed. • Parallel circuits are susceptible to short circuits. • Safety devices are incorporated into the circuit to prevent a circuit overload. 1. Fuse-a piece of metal melts when too much current flows and breaks the circuit. 2. Circuit breaker-an automatic switch that opens when the current reaches a value greater than the rated value in the circuit. 3. Ground-fault interrupter-an outlet that prevents injuries that are caused when an electric appliance falls into water. It contains an electric circuit that detects an extra current path and opens the circuit.

47. A 16.0 Ω and a 20.0 Ω are connected in parallel. 40.0 V is applied to the combination. • What is the equivalent resistance? • What is the total current in the circuit? • What is the current in the 16.0 Ω resistor? • What is the power dissipated by the 16.0 Ω resistor?

48. A heating coil has a resistance of 4.0 Ω and operates on 120 V. • What is the current in the coil? • What is the power rating of the coil? • What energy is supplied to the coil if it is operated for 30 min per day for 30 days? • At $.10 per kWh, how much does it cost to operate the coil for 1 month?

49. A 20.0 Ω, 50.0 Ω, and 10.0 Ω resistor are connected in parallel to each other. 5.5 A of current flows through the 20.0 Ω. Resistor. • What is the voltage across the 20.0 Ω resistor? • What is the current through the 50.0 Ω resistor? • What is the current through the 10.0 Ω resistor? • What is the total current through the circuit?

50. A person has a $5 and a 200 W stereo. • At $0.15 per kWh, how many kWh can the person use before the money runs out? • How many hours can the person operate the stereo at this rate? • If the stereo is plugged into a 120 V outlet, what is the current flowing through the stereo when it is operating? • What is the resistance of the stereo?