Download Chapter 34 Electric Current Review Questions

Chapter 34 Electric Current Review Questions
1. What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of
charge? (34.1)
There needs to be a temperature difference for the flow of heat; likewise a voltage difference is necessary for
the flow of charge.
2. What is meant by the term potential? What is meant by potential difference? (34.1)
Electric Potential is defined as the Potential Energy per unit Charge (EP = PE/q); Potential difference is the
CHANGE in Potential energy per unit Charge (EP = PE/q) between any two points of a circuit.
3. What condition is necessary for the sustained flow of water in a pipe? What analogous condition is necessary
for the sustained flow of charge in a wire? (34.1)
In order for water to flow in a pipe there must be a pressure difference; likewise for the flow of charge in a
wire there must be a potential difference.
4. What is electric current? (34.2)
Electric Current is defined as the flow of positive charges.
5. What is an ampere? (34.2)
The ampere is a unit of current defined as 1 Coulomb of charge flowing past a given point per second in a
circuit (1 Amp = 1 C / 1 sec).
6. What is voltage? (34.3)
Voltage is defined as the PE given per unit charge (PE/q); it is the electric “pressure” that produces current.
7. How many joules per coulomb are given to charges that flow in a 120-volt circuit? (34.3)
120 V = 120 Joules / 1 Coulomb.
8. Does charge flow through a circuit or into a circuit? (34.3)
Charge flows through a circuit rather than into a circuit due to an electric field established by a potential
difference.
9. Does voltage flow through a circuit, or is voltage established across a circuit? (34.3)
Voltage is established across a circuit, causing charges to flow by establishing an electric field.
10. What is electric resistance? (34.4)
Electric resistance is the property of materials that resists flow of charge, measured in ohms ().
11. Is electric resistance greater in a short fat wire or a long thin wire? (34.4)
There is greater resistance in a long thin wire than a short fat wire, just like it would be harder to push
water through a long thin pipe than a short thick one. Jumper cables for a car are a great example of
thick, low resistance wires in order to allow a lot of current to flow to the battery of a dead car.
12. What is Ohm’s law? (34.5)
Current = voltage/resistance.
13. If the voltage impressed across a circuit is constant but the resistance doubles, what change occurs in the
current? (34.5)
The current would drop to half, in accord with Ohm’s law (I = V/R).
14. If the resistance of a circuit remains constant while the voltage across the circuit decreases to half its former
value, what change occurs in the current? (34.5)
Half as much current (I = V/R).
15. How does wetness affect the resistance of your body? (34.6)
Wetness lowers skin resistance.
16. Why is it that a bird can perch without harm on a high-voltage wire? (34.6)
There is very little or no potential difference across the body of the bird on a high-voltage line. If the bird
straddled 2 different lines—well that would be a different story!
17. What is the function of the third prong in a household electric plug? (34.6)
Serves as a ground for safety reasons.
18. Distinguish between dc and ac. Which is produced by a battery and which is usually produced by a
generator? (34.7)
Direct current is produced by a battery and flows in only one direction, whereas alternating current is
produced by a generator and alternates direction.
19. A diode converts ac to pulsed dc. What electric device smooths the pulsed dc to a smoother dc? (34.8)
A capacitor.
20. What are the roles of a diode and a capacitor in an ac-dc converter? (34.8)
Diode converts ac to dc, capacitor smooths current.
21. What is a typical “drift” speed of electrons that make up a current in a typical dc circuit? In a typical ac
circuit? (34.9)
In dc, less than 1 cm/s; in ac, zero drift speed since direction of current alternates.
22. From where do the electrons originate that flow in a typical electric circuit? (34.10)
From the conductors themselves!
23. What is power? (34.11)
Power = energy/time, rate of doing work or rate at which energy is expended measured in watts.
24. Which of these is a unit of power and which is a unit of electric energy: a watt, a kilowatt, a kilowatt-hour?
(34.11)
Power is measured in watts or kilowatts; electrical energy is often measured in the kilowatt-hour.
25. How many amperes flow through a 60-watt bulb when 120 volts are impressed across it? (34.11)
From P= IV, 60 W = I (120 V); I = 0.5 A.