Charges (electrons) moving in a conductor Ohm’s Law & resistance to flow of charge Energy and power in electrical circuits Current, resistance, and electromotive force (emf): Chapter 25 C 2009 J. F. Becker

ELECTRON MOTION IN A CONDUCTOR WITH AND WITHOUT AN ELECTRIC FIELD

ANALOGY OF ELECTRON MOTION IN A CONDUCTOR 12 Volts 0 Volts

CONDUCTOR WITH CURRENT MOVING FROM HIGH ELECTRICAL POTENTIAL (VOLTS) TO LOW POTENTIAL

CHARGES DRIFTING IN A CONDUCTOR

Each lamp has the same amount of resistance to the flow of charge. Which Box (A, B, or C) has the most resistance to the flow of electric charge (current)? Current is the flow of charge past a point in the circuit per unit time interval. A flashlight battery and some identical lamps C 1998 McDermott, et al., Prentice Hall

Which network has the most resistance to the flow of charge? Rank the networks according to decreasing resistance. C 1998 McDermott, et al., Prentice Hall

1.Rank the brightness of the bulbs (bright to dim). 2. A wire is added as shown below. a)Does the brightness of bulb C increase, decrease, or remain the same? b)Does the brightness of bulb A increase, decrease, or remain the same? c)Does the current through the battery increase, decrease, or remain the same? C 1998 McDermott, et al., Prentice Hall

Rank the boxes according to their resistance to the flow of charge (maximum to minimum). Do not calculate the resistance! Now, calculate the resistance of each box if each bulb has a resistance of 10 Ohms (  ).

Resistance (R) is proportional to resistivity (  ): R =  L / A The resistivity (  ) depends on temperature and the physical properties of the material, so it has a different value for each material. Temperature dependence of resistance (and resistivity) is generally linear over limited temperature ranges and is characterized by the temperature coefficient of resistivity (  ): R(T) = R 0 [ 1 +  (T-T 0 )]  (T) =  0 [ 1 +  (T-T 0 )] where R 0 and T 0 are the resistance and temperature at a standard temperature, usually room temperature or 20 o C. (Measured in Lab #5) C 2009 J. F. Becker

Current – voltage relations a)a resistor obeys Ohm’s Law Constant slope = 1/R = I/  V = or  V = I R b) A vacuum tube diode c) A semiconductor diode

Electric potential (  V) rises and drops in a circuit (from previous circuit)

CIRCUIT ENERGY and POWER  I = rate of conversion of non- electrical (chemical) energy to electrical energy within the source I 2 r = rate of electrical energy dissipation in the internal resistance of the source (battery)  I - I 2 r = the rate at which the source delivers electrical energy to the load (headlight) R P = Vab I = I 2 R = Vab 2 / R

See Review C 2009 J. F. Becker