1. Chapter 18 Electric Currents

2. Why study electric current? Most electrical devices depend on electric current. For example… ◦ Light bulbs ◦ Heating elements in stoves or heaters ◦ TVs and computers

3. History of the battery ~1800 Alessandro Volta Experimented with combinations of metals connected by a moist conductive fluid Found that when these metal discs were in a “pile” or “battery” they produced an “electromotive force”, which we call voltage This voltaic pile or battery was the first electric battery

4. 18.1 The Electric Battery The simplest batteries were created when dissimilar metals called electrodes were connected by a conductive solution called an electrolyte This is a simple electric cell. Several connected together is a battery The purpose of the battery is to produce a potential difference, or voltage, and make charges move

5. 18.1 Electric Battery A battery transforms chemical energy into electrical energy Chemical reactions within the cell create a potential difference or voltage between the terminals Voltage between the terminals depends on;  what the electrodes are made of  their ability to be dissolved or give up electrons When one or more of the electrodes is used up the cell is “dead”

6. 18.2 Electric Current A complete circuit is a conducting path between battery terminals where current can flow This flow of charge is called an electric current

7. Electric current is the rate of flow of charge through a conductor I = electric current in amperes (A) named after French physicist Andre Ampere 1A=1C/s Δ Q = amount of charge passing through conductor at any location during the time interval Δt 18.2 Electric Current

8. In order for current to flow, there must be a complete circuit from one battery terminal to the other In an open circuit there is a break in the conducting path and no current flows Which circuit(s) are complete and which are open?

9. Ground In many real circuits wires are connected to a common conductor called a ground Ground is a common return electrical path, a connection to the Earth or car frame (for example) that limits the dangerous build up of static charge, and allows excess voltage to be directed away from electronics and users

10. By convention, current is defined as flowing from + to – and is called conventional current Electrons actually flow in the opposite direction, but not all currents consist of electrons

11. 18.3 Ohm’s Law: Resistance & Resistors To produce an electric current in a circuit a potential difference or voltage is required The greater the voltage the greater the current ~1800 Georg Simon Ohm conducted experiments to describe electrical current relationships http://www.pbs.org/wgbh/amex/edison/sfeature/acdc.html

12. 18.3 Ohm’s Law: Resistance and Resistors Electrical current depends on the voltage but also the resistance Resistance slows the flow of electrons along the wire and decreases the current Resistors are electronic devices used to control the amount of current Ohm’s Law ◦ V=potential difference applied across the wire in volts (V) ◦ I=current through the wire in amperes (A) ◦ R=resistance in ohms ( Ω) or 1V/A

13. 18.3 Ohm’s Law: Resistance and Resistors All devices have resistance to some extent In a circuit diagram wires with minimal resistance are a straight line and a resistor in the circuit is indicated with a jagged line

14. 18.3 Ohm’s Law: Resistance and Resistors  Resistors are manufactured for use in electric circuits  They are color-coded to indicate their value and precision.

15. Some clarifications: Batteries maintain a (nearly) constant potential difference; the current varies. Resistance is a property of a material or device. Current is not a vector but it does have a direction. It is always parallel to the wire Current and charge do not get used up. Whatever charge goes in one end of a circuit comes out the other end.

16. 18.4 Resistivity  The resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area: (18-3)  The constant ρ, the resistivity, is characteristic of the material and depends on purity, heat treatment, temperature, etc. Unit is Ω·m  L of wire is measured in m  Cross-sectional A of wire is measured in m 2

17. 18.5 Electric Power  Electric energy is useful because it can be transformed into other types, such as thermal energy ( Electric heaters, stoves, hair dryers, etc)  Power (measured in watts, W) is the energy transformed by a device per unit time  Since current (I) = Q/t this can be simplified

18. 18.6 Power in Household Circuits Wires used in homes to carry electricity have very low resistance If the current is high enough the wires can become hot enough to start a fire To avoid this, we use fuses or circuit breakers, which disconnect when the current goes above a predetermined value.

19. 18.6 Power in Household Circuits Fuses are one-use items – if they blow, the fuse is destroyed and must be replaced. Circuit breakers are switches that will open if the current is too high; they can be reset.

20. 18.7 Alternating Current Current from a battery flows steadily in one direction and is called direct current (DC) Electric generators produce alternating current (AC). It reverses direction many times per second

21. References  Zitewitz. Physics: Principles and Problems. 2004  Giancoli, Douglas. Physics: Principles with Applications 6th Edition. 2009. www.pbs.org/wgbh/amex/edison/sfeature/ac dc.html www.pbs.org/wgbh/amex/edison/sfeature/ac dc.html http://commons.wikimedia.org/wiki/File:Volt aBattery.JPG http://commons.wikimedia.org/wiki/File:Volt aBattery.JPG http://www.pbs.org/wgbh/amex/edison/sfeat ure/acdc.html