CHAPTER 8 ELECTRIC CURRENT RESISTANCE SUBTOPIC Batteries and

CHAPTER 8 ELECTRIC CURRENT & RESISTANCE

SUBTOPIC • • Batteries and Direct Current Resistivity and Ohm’s Law Temperature Variation of resistance Electrical Energy and Power

Batteries & Direct Current Electric current is the flow of electric charge. A battery is a source of electric energy – it converts chemical energy into electric energy.

Batteries & Direct Current In a complete circuit, electrons flow from the negative electrode cathode to the positive one anode. A battery provides a constant source of voltage – it maintains a constant potential difference between its terminals. The potential difference between the battery terminals when the battery is not connected to anything is called the electromotive force (emf). Battery’s emf work done by the battery per coulomb of charge that passes through it (J/C @ V)

Batteries & Direct Current The actual terminal voltage of the battery is always less than the emf, due to internal resistance. Usually the difference is very small.

Batteries & Direct Current When batteries are connected in series, the total voltage is the sum of the individual voltages.

Batteries & Direct Current When batteries of equal voltage are connected in parallel, the total voltage does not change; each battery supplies part of the total current.

Batteries & Direct Current

Current & Drift Velocity Current is the time rate of flow of charge. SI unit of current: the ampere, A. The Ampere (A) in honor of the French physicist André Ampere (1775 – 1836), an early investigator of electrical and magnetic phenomena. André-Marié Ampere

Current & Drift Velocity The direction of current has been taken to be from positive to negative; this is opposite to the way electrons flow.

Current & Drift Velocity Electrons do not flow like water in a pipe. In the absence of voltage, they move randomly at high speeds, due to their temperature. When a voltage is applied, a very small drift velocity is added to thermal motion, typically around 1 mm/s; this is enough to yield the observed current.

Resistance & Ohm’s Law Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference or voltage across the two points, and inversely proportional to the resistance between them provided the temperature remains constant. Equation: V is the potential difference measured across the resistance in units of volts I is the current through the resistance in units of amperes R is the resistance of the conductor in units of ohms

Resistance & Ohm’s Law If there is a potential difference across a conductor, how much current flows? The ratio between the voltage and the current is called the resistance. SI unit of resistance: volts/ampere or the ohm, Ω. Ohm (Ω) in honor of the German physicist Georg Ohm (1789 – 1854), who investigated the relationship between current and voltage. Georg Simon Ohm

Resistance & Ohm’s Law An ohmic material is one whose resistance is constant. Ohm’s Law is NOT a fundamental law in the same as, for example – law of conservation of energy.

Resistance & Ohm’s Law Ohm’s law is valid only for ohmic materials: The resistance of a particular object depends on its length, cross-sectional area, material, and temperature. These are factors that influence the resistance.

Resistance & Ohm’s Law Resistor Variable resistance

Resistivity As expected, the resistance R is proportional to the length and inversely proportional to the cross-sectional area (why? ): The constant ρ is called the resistivity, and is characteristic of the material. Unit SI: ohm-meter (Ω. m)

Resistivity In this table, you can easily see the differences between the resistivity's of conductors, semiconductors, and insulators.

Resistivity For many materials, the temperature dependence of the resistivity is approximately linear, as long as the temperature change is not too large. Temperature variation of resistivity or The constant α is called the temperature coefficient of resistivity. The resistance of an object of uniform cross section varies with temperature Temperature variation of resistance Where; , and

Resistivity Some materials exhibit a curious phenomenon: at a very low temperature called the critical temperature, their resistivity drops abruptly to zero. These are called superconductors; they have a number of unique properties. However they are impractical for everyday home use, as they must be cooled to cryogenic temperatures.

Temperature variation of resistance • Most conductive materials change specific resistance with changes in temperature • A positive coefficient for a material means that its resistance increases with an increase in temperature. Pure metals typically have positive temperature coefficients of resistance. • A negative coefficient for a material means that its resistance decreases with an increase in temperature. Semiconductor materials (carbon, silicon, germanium) typically have negative temperature coefficients of resistance.

Temperature variation of resistance R = Conductor resistance at temperature T Rref = Conductor resistance at temperature Tref, usually 20 oc or sometimes 0 oc = Temperature coefficient of resistance for conductor material T = Conductor temperature Tref = Temperature reference that is specified

Electric Power, as usual, is the rate at which work is done. The power delivered to a circuit element depends on the element’s resistance, the current in it, and the volgate across it. In Electric term, power is the rate at which electrical energy is transferred by an electric circuit For ohmic materials, electric power can written as:

Electric Energy • The energy consumed by an electrical appliance depends on its power rating and the length of time it is operating. E = P. t • The joule is too small to measure electrical energy consumption. • More common are the mega joule (MJ), or the kilowatt. hour (k. W. h).

Electric Power So, where does this power go? It is changed to heat in resistive materials.

Electric Power Electric appliances are rated in watts, assuming standard household voltage.

Electric Power

Electric Power The electric company typically bills us for kilowatt-hours (k. W-h), a unit of energy. We can reduce our energy usage by buying efficient appliances.

EXTRA INFORMATION NOTES

PIER Chart

How to read Resistor

Exercise 1. Suppose there is a steady current of 0. 5 A in a flashlight bulb lasting for 2 minutes. (a) How much charge passes through the bulb during this time? (b) How many electrons does this represent? 2. Suppose an electric eel touches the head and tail of cylindrically shaped fish and applies a voltage of 600 V across it. If a current of 0. 8 A results, estimate the average resistivity of the fish’s flesh, assuming it is 20 cm long and 4 cm in diameter.

3. Find value for a aluminum resistor at 85 o. C, given that R at 20 o. C is 350 ohm. 4. A hair dryer is rate at 1200 W for a 115 V operating voltage. The uniform wire filament breaks near one end, and the owner repairs it by removing the section near the break and simply reconnecting it. The filament is then 10% shorter than its original length. What will be the heater’s power output after this ‘repair’?