Electricity If you rub a piece of amber With the fur of a rabbit It will attract bits of stuff (paper, leaves, etc) • Equal charges repel. • Opposite charges attract. The force between two charges: • Proportional to the magnitude of the charges • Decreases as the square of the distance (a) electrons are transferred to the metal sphere by rubbing the negatively charged rod (b) when the rod is removed, electrons distribute themselves uniformly over the surface Five Styrofoam balls are suspended from insulating threads. Several experiments are performed on the balls and the following observations are made: I. Ball A attracts B and repels C. II. Ball D attracts B and has no effect on E. III. A negatively charged rod attracts both A and E. What are the charges, if any, on each ball? A B C D E a. + - + 0 + b. + - + + 0 c. + - + 0 0 d. - + - 0 0 e. + 0 - + 0 Answer: The first statement indicates that A attracts B and repels C. This indicates that both A and C are charged, but B could be charged by induction. The second statement indicates that D attracts B, but has no effect on E. This means that D and E must not be charged because if either were charged and the other not, then they would by attracted to each other by induction. A negative rod attracts A and E. We know then that A is positively charged. This then tells us from statement one that B is negatively charged. C is also positively charged. Therefore, choice (C) is the correct answer. COULOMB’S LAW The magnitude of the electrostatic force exerted by one point charge on another point charge is directly proportional to the magnitude of the charges and inversely proportional to the square of the distance between them. q1 q2 F k 1 r2 k 4 o 8.99 109 N m 2 C 2 8.85 1012 C2 N m2 mp 1.6731027 kg mn 1.675 1027 kg me 9.1110 31 kg 19 e 1.60 10 C An atom contains a small, positively charged nucleus, about which negatively charged electrons move. The orbits shown are only symbolic. In reality electrons, occupy a region of space that encircles the nucleus. In nature, atoms are normally found with equal numbers of protons and electrons, so they are electrically neutral. By adding or removing electrons from matter it will acquire a net electric charge with magnitude equal to e times the number of electrons added or removed, N. q Ne An electrical conductor is a material in which electrons move easily and are readily conducted. metals: silver, copper, gold, aluminum, tungsten, iron, etc. An electrical insulator is a material in which electrons are poorly conducted. insulators: mica, rubber, teflon, wood, paper semi-conductors: silicon, germanium, carbon (semi-metal) DEFINITION OF ELECRIC FIELD The electric field that exists at a point is the electrostatic force experienced by a small test charge placed at that point divided by the charge itself: F SI Units of Electric Field: newton per coulomb (N/C) E qo Electric fields from different sources add as vectors. The Electric Fields from Separate Charges May Cancel THE PARALLEL PLATE CAPACITOR Parallel plate capacitor WAB EPE A EPE B WAB EPE A EPE B qo qo qo The potential energy per unit charge is called the electric potential. EPE V qo SI Unit of Electric Potential: joule/coulomb = volt (V) Work, Potential Energy, and Electric Potential The work done by the electric force as a test +2.0x10-6 C charge moves from point A to point B is + 5.0x10-5J. (a) Find the difference in EPE between these points. (b) Determine the potential difference between these points. WAB EPE A EPE B 5 EPE B EPE A WAB 5.0 10 J 5 WAB 5.0 10 J VB VA 25 V qo 2.0 10 C -6 Batteries Within a battery, a chemical reaction occurs that transfers electrons from one terminal to another terminal. The electric current is the amount of charge per unit time that passes through a surface that is perpendicular to the motion of the charges. q I t One coulomb per second equals one ampere (A). If the charges move around the circuit in the same direction at all times, the current is said to be direct current (dc). If the charges move first one way and then the opposite way, the current is said to be alternating current (ac). A Pocket Calculator The current in a 3.0 V battery of a pocket calculator is 0.17 mA. In one hour of operation, (a) how much charge flows in the circuit and (b) how much energy does the battery deliver to the calculator circuit? (a) q I t 0.17 103 A 3600 s 0.61 C Energy (b) Energy Charge 0.61 C 3.0 V 1.8 J Charge The resistance (R) is defined as the ratio of the voltage V applied across a piece of material to the current I through the material. OHM’S LAW The ratio V/I is a constant, where V is the voltage applied across a piece of material and I is the current through the material: V R constant or V IR I SI Unit of Resistance: volt/ampere (V/A) = ohm (Ω) A Flashlight The filament in a light bulb is a resistor in the form of a thin piece of wire. The wire becomes hot enough to emit light because of the current in it. The flashlight uses two 1.5-V batteries to provide a current of 0.40 A in the filament. Determine the resistance of the glowing filament. V 3.0 V R 7.5 I 0.40 A Solar Array ELECTRIC POWER When there is current in a circuit as a result of a voltage, the electric power delivered to the circuit is: P IV SI Unit of Power: watt (W) Many electrical devices are essentially resistors: P I IR I 2 R V V2 P V R R The Power and Energy Used in a Flashlight In the flashlight, the current is 0.40A and the voltage is 3.0 V. Find (a) the power delivered to the bulb and (b) the energy dissipated in the bulb in 5.5 minutes of operation. P IV 0.40 A3.0 V 1.2 W E Pt 1.2 W330 s 4.0 102 J 100 watt bulb What is the resistance and how much current flows through a 100 W bulb? Note: The wattage on a bulb is its power output and assumes that you will use it in the US where the voltage in 110 V. 60 W bulb Redo the calculations for a 60 W bulb.
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