Chapter 18 Electric Currents

Question 0

Develop an analogy between blood circulation and an electrical circuit. Discuss what plays the role of the heart for the electric case, and so on.

Answer 0

Blood Circulation:   Electrical circuit: 
• heart                      • battery 
• blood                     • current 
• blood cells             • electrons 
• organs                   • resistance 
• blood pressure      • voltage 
• veins/ arteries       • wires

Question 1

Car batteries can be rated in amo here-hours (A•h). What quantity is being rated?

Answer 1

The current of the electrons is being rated.
P = IV
I = Q/t
Q/t • t = Q (charge held in battery)

Question 2

Can a copper wire and an aluminum wire of the same length have the same resistance? Explain.

Answer 2

Yes, if the cross sectional areas are adjusted so that the resistance are the same.
R = resistivity (length) / cross sectional area

Question 3

If a rectangular solid made of carbon has sides a, 2a, 3a, how would you connect the wires from a battery so as to obtain (a) the least resistance, (b) the greatest resistance?

Answer 3

R = resistivity (length) / cross sectional area
Greatest resistance: 
a x 2a = cross sec. 3a = L
Least resistance: 
2a x 3a = cross sec. a = L

Question 4

Explain why light bulbs almost always burn out just as they are turned on and not after they have been on for some time.

Answer 4

When the bulb is off the temperature and resistance are low, so the current is high. Once the bulb is turned on, the increase in current causes a surge, blowing the bulb.

Question 5

Electric power is transferred over large distances at very high voltages. Explain how the high voltage reduces power losses in the transmission lines.

Answer 5

P (loss) = I^2 • R. P (delivered) = IV
P (delivered) / V = I
P (loss) = (P (delivered) / V)^2 • R
P (loss) oc I / V^2
High voltages require a greater resistance to maintain the same power.
The greater the voltage the less power lost because they are inversely, exponentially proportional.

Question 6

Which draws more current, a 100-W lightbulb or a 75-W bulb? which has the highest resistance?

Answer 6

P = I • V
P = V^2 / R 
The greater the power the greater the current and lower the resistance. 
100-W has the greater current
75-W has the higher resistance

Question 7

The heating element in a toaster is made of Nichrome wire. Immediately after the toaster is turned on, is the current (I [rms]) in the wire increasing, decreasing, or staying constant? Explain.

Answer 7

The current decreases because as the toaster heats up, resistance increases, decreasing the current.
R oc T • L / cross sectional area
V = IR
I = 1 / R

Question 8

Is current used up in a resistor?

Answer 8

No, the electrons/ energy in the current is turned into another form of energy (Thermal). Energy is conserved, and can never be destroyed.