Principle of Engineering Structure of matter, charges, electric fields, current, electromotive force and potential difference. Electricity Session 1 (2 hours) Electricity Session 1 (2 hours)

Structure of Matter Matter 物質 composed of Molecules 分子 Molecules composed of Atoms 原子 Structure of Atoms: electrons (- charge) 電子, nucleus: protons (+ charge) 質子, neutrons 中子

Charges Symbol: Q Unit of charges: Coulomb (C) 1 C = 6.24×10 18 times the charge on an electron

Electric Current Electric Current 電流 is the rate of flow of electrical charges. Symbol: I Unit: Ampere (A) 1 A = 1 coulomb of electrical charge moving past a specific point in one second. milli-Amp or mA (10 -3 A) micro-Amp or μA (10 -6 A)

Measuring Flow of Electric Current Ammeter measures electric current (part of Multimeter) How to use an ammeter? (Demonstration or actual practice using a 12V light bulb circuit.) Digital meter and analog meter (what they look like)

Exploration: Ammeter Measure the electric current when using an AA size battery Caution: misuse of ammeter will blow the fuse Re-do the experiment using a DC power supply. There is no need for ammeter this time as the power supply displays the current automatically. Why does the current larger when the motor starts? A + _ 1.5V Ammeter to measure current motor Current i A/mACOM

Accuracy A + _ 3V Ammeter to measure current motor Current i A/mA COM Measure the current with both the DC power supply and the multimeter. Assume the multimeter is accurate. % error forumula = ______________ % error measured = ______________

How does Current Feel? What does it feel when your tongue touch 9V battery terminals? for limb-contact electrical shocks –1-5 mA is the level of perception –10 mA is the level where pain is sensed –100 mA severe muscular contraction occurs – mA electrocution occurs.

Potential Difference 電位差 / 電壓 An AA battery has about 1.5V of potential difference. The battery is a kind of voltage supply used to drive an electric current through a electric component A voltmeter (part of a multi-meter) is used to measure potential difference between two contact points. Unit of potential difference: voltage 伏特 (V).

Voltage Supply or Power Supply

Exploration: Voltmeter Use a voltmeter to measure voltage of a battery. Use a voltmeter to measure voltage of a DC power supply at different settings. What are the % errors?

Exploration: Voltage of Loaded Battery? V Use a voltmeter to measure the voltage of the battery when a motor is connected in series. What is the loading voltage compared to unloaded voltage? Is a battery an ideal voltage supply?

Physical Effect of Increasing Supplied Voltage How to create a higher voltage with AA batteries? What happens to the motor at a higher voltage? Use the DC supply to connect to a 12V light bulb. What happens if the voltage is increased? (Caution: do not use more than 12V in the power supply. Why?)

Effect of Voltage on Current Connect the DC supply to a light bulb through an ammeter to measure the light bulb current Try different voltages by adjusting the knob. What happens to the current? DC power supply A

Division of Voltage Connect a 12 V supply to 2 bulbs in series Measure the potential differences V 1 between terminals of light bulb 1 Measure the potential differences V 2 between terminals of light bulb 2 V 1 = ___ V 2 = ___ Supply Voltage = _____ Relationship between them? _________________ 12V

Where did the Electrical Energy Go? Heat Light Mechanical Energy Form(s) of Energy generated by lamp ____________________ Form(s) of Energy generated by motor _____________________

Miniproject Set a DC power supply to 12V. Connect the power supply to 3 bulbs such that one is brighter than the other two. If one of the dimmer light bulbs are removed, it does not affect the brighter bulb.