Principle of Engineering Structure of matter charges electric

Principle of Engineering Structure of matter, charges, electric fields, current, electromotive force and potential difference. 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× 1018 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 m. A (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 12 V 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? Ammeter to measure current A/m. A A Current i COM + 1. 5 V _ motor

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

How does Current Feel? • What does it feel when your tongue touch 9 V battery terminals? • for limb-contact electrical shocks – – 1 -5 m. A is the level of perception 10 m. A is the level where pain is sensed 100 m. A severe muscular contraction occurs 100 -300 m. A electrocution occurs.

Potential Difference 電位差 /電壓 • An AA battery has about 1. 5 V 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? • 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? V

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 12 V light bulb. What happens if the voltage is increased? (Caution: do not use more than 12 V 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 = ___ 12 V • V 2 = ___ • Supply Voltage = _____ • Relationship between them? _________

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 12 V. • 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.