Chapter 18- Electric currents Flashcards
What is an electric current?
When such a circuit is formed, charge can move (or flow) through the wires of the circuit, from one terminal of the battery to the other, as long as the conducting path is continuous.
Current I enters a resistor R as shown in Fig. 18–13. (a) Is the potential higher at point A or at point B? (b) Is the current greater at point A or at point B?
A—R—B
I–>
a) Positive charge always flows from ± to –, from high potential to low potential. So if current I is conventional (positive) current, point A is at a higher potential than point B.
Current I enters a resistor R as shown in Fig. 18–13.(b) Is the current greater at point A or at point B?
A—R—B
I–>
(b) Conservation of charge requires that whatever charge flows into the resistor at point A, an equal amount of charge emerges at point B. Charge or current does not get “used up” by a resistor. So the current is the same at A and B.
Suppose a wire of resistance R could be stretched uniformly until it was twice its original length. What would happen to its resistance? Assume the amount of material, and therefore its volume, doesn’t change.
If the length l doubles, then the cross-sectional area A is halved, because the volume (V = Al) of the wire remains the same. From Eq. 18–3 we see that resistance would increase by a factor of four (2 ½ = 4)
Why can the temperature coefficient for semiconductors be negative?
temperature coefficient for semiconductors can be negative. It seems that at higher temperatures, some of the electrons that are normally not free in a semiconductor become free and can contribute to the current. Thus, the resistance of a semiconductor can decrease with an increase in temperature.
What does short circuit mean?
means that two wires have touched that should not have (perhaps because the insulation has worn through) so the path of the current is shortened through a path of very low resistance.
What are two reasons to why a circuit can burn out?
there are two possibilities: there may be too many devices drawing current in that circuit; or there is a fault somewhere, such as a “short.”
A dangerous extension cord. Your 1800-W portable electric heater is too far from your desk to warm your feet. Its cord is too short, so you plug it into an extension cord rated at 11 A. Why is this dangerous?
1800 W at 120 V draws a 15-A current. The wires in the extension cord rated at 11 A could become hot enough to melt the insulation and cause a fire.
What is a direct current?
When a battery is connected to a circuit, the current moves steadily in one direction.
What is an alternating current?
An alternating current reverses direction many times per second and is commonly sinusoidal, Fig. 18–21. The electrons in a wire first move in one direction and then in the other.
