Current, resistance, and electromotive force (emf): Chapter 25

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# Current, resistance, and electromotive force (emf): Chapter 25 - PowerPoint PPT Presentation

Current, resistance, and electromotive force (emf): Chapter 25. Charges (electrons) moving in a conductor Ohm’s Law &amp; resistance to flow of charge Energy and power in electrical circuits. C 2009 J. F. Becker. ELECTRON MOTION IN A CONDUCTOR WITH AND WITHOUT AN ELECTRIC FIELD.

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Charges (electrons) moving in a conductor

Ohm’s Law & resistance to flow of charge

Energy and power in electrical circuits

C 2009 J. F. Becker

A flashlight battery and some identical lamps

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.

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

Rank the brightness of the bulbs (bright to dim).

2. A wire is added as shown below.

• Does the brightness of bulb C increase, decrease, or remain the same?
• Does the brightness of bulb A increase, decrease, or remain the same?
• 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 (W).

C 1998 McDermott, et al., Prentice Hall

Resistance (R) is proportional to resistivity (r): R = r L / AThe resistivity (r) 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 (a): R(T) = R0 [ 1 + a (T-T0)]

r(T) = r0 [ 1 + a (T-T0)]

where R0 and T0 are the resistance and temperature at a standard temperature, usually room temperature or 20o C. (Measured in Lab #5)

C 2009 J. F. Becker

Current – voltage relations

• a resistor obeys Ohm’s Law

Constant slope = 1/R = I/ DV = or DV = I R

b) A vacuum tube diode

c) A semiconductor diode

CIRCUIT ENERGY and POWER

e I = rate of conversion of non-electrical (chemical) energy to electrical energy within the sourceI2 r = rate of electrical energy dissipation in the internal resistance of the source (battery)e I - I2 r = the rate at which the source delivers electrical energy to the load (headlight)

P = Vab I = I2 R = Vab2/ R

R

Review

See www.physics.edu/becker/physics51

C 2009 J. F. Becker