1. Physics Section 17.3 Apply the properties of electric current Electric current is the rate at which charge flows through a conductor. The charges can be either electrons or positive ions. I = Q I = current(ampere A) t Q = charge (C) t = time (s) An ampere is equivalent to a coulomb of charge passing through a cross-sectional area in one second. 1A = 1C/1s http://videos.howstuffworks.com/hsw/18447-electricity-and-magnetism-electric-current- video.htm

2. example The current passing through a light bulb is.75 A. How long would it take a total charge of 1.5 C to pass through the bulb?

3. example The current in a circuit is 80.0 μA. How long does it take 20.0 x 10 16 electrons to pass a given point?

4. A potential difference (electric field) causes charges to flow. Conventional current is defined as the direction that positive charges flow. Conventional current flows opposite the direction of electrons in a conductor. Drift velocity is the average velocity of an electron in a conductor. It is only a few millimeters per second. The velocity that the electric field is established in a conductor is nearly the speed of light. (300,000,000 m/s)

5. Resistance is the opposition to electric current by a material or a device. R = V R = resistance (ohm Ω) I V = potential difference (V) I = current (ampere) One ohm equals one volt per ampere. Ohm’s Law: V = IR For a constant resistance, current is directly proportional to voltage. For a constant voltage, current varies inversely as the resistance.

6. http://phet.colorado.edu/en/simulation/battery-resistor-circuit Materials with a constant resistance over a wide range of potential differences are ohmic materials. Materials whose resistance varies are nonohmic. A diode is a semi-conducting nonohmic device. It has a low resistance for current in one direction and a high resistance for current in the opposite direction. The resistance of a conductor varies directly as its length and temperature and inversely as its cross- sectional area. The structure of the atoms in a material also affect the resistance.

7. Resistors are used to control the amount of current in a conductor. A rheostat is a device that acts as a variable resistor. It can be used to vary the amount of current in a circuit. The human body has resistance close to 500,000 ohms.

8. example A light bulb has a resistance of 15 Ω. Find the current in the circuit when it is connected to a potential difference of 24 V.

9. example The current in a radio is.50 A. Find the potential difference across the circuit if the resistance of the radio is 250 Ω.

10. Assignment Page 616 problems 1 - 7