Chapter 34 Electric Current
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?
Half as much current (I = V/R).
What is a typical "drift" speed of electrons that make up a current in a typical dc circuit? In a typical ac circuit?
In dc, less than 1 cm/s; in ac, zero drift speed since direction of current alternates.
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?
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.
In terms of heating, why are thick wires rather than thin wires used to large currents?
The thicker the wire the higher the current the less resistance.
Is electric resistance greater in a short fat wire or a long thin wire?
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.
Why is it that a bird can perch without harm on a high-voltage wire?
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!
What condition is necessary for the flow of heat? What analogous condition is necessary for the flow of charge?
There needs to be a temperature difference for the flow of heat; likewise a voltage difference is necessary for the flow of charge.
How many joules per coulomb are given to charges that flow in a 120-volt circuit?
120 V = 120 Joules / 1 Coulomb.
A diode converts ac to pulsed dc. What electric device smooths the pulsed dc to a smoother dc?
A capacitor.
Does charge flow through a circuit or into a circuit?
Charge flows through a circuit rather than into a circuit due to an electric field established by a potential difference.
What is Ohm's law?
Current = voltage/resistance.
Do an ampere and a volt measure that same thing, or different things? What are those things, and which is a flow and which is the cause of the flow?
Different quantities; the potential difference (or voltage) causes a flow of charge, which is the current.
What are the roles of a diode and a capacitor in an ac-dc converter?
Diode converts ac to dc, capacitor smooths current.
Distinguish between dc and ac. Which is produced by a battery and which is usually produced by a generator?
Direct current is produced by a battery and flows in only one direction, whereas alternating current is produced by a generator and alternates direction.
What is electric current?
Electric Current is defined as the flow of positive charges.
What is meant by the term potential? What is meant by potential difference?
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.
What is electric resistance?
Electric resistance is the property of materials that resists flow of charge, measured in ohms ().
How many amperes flow through a 60-watt bulb when 120 volts are impressed across it?
From P= IV, 60 W = I (120 V); I = 0.5 A.
From where do the electrons originate that flow in a typical electric circuit?
From the conductors themselves!
What is power?
Power = energy/time, rate of doing work or rate at which energy is expended measured in watts.
Which of these is a unit of power and which is a unit of electric energy: a watt, a kilowatt, a kilowatt-hour?
Power is measured in watts or kilowatts; electrical energy is often measured in the kilowatt-hour.
What is the function of the third prong in a household electric plug?
Serves as a ground for safety reasons.
What is an ampere?
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).
If the voltage impressed across a circuit is constant but the resistance doubles, what change occurs in the current?
The current would drop to half, in accord with Ohm's law (I = V/R).
What is voltage?
Voltage is defined as the PE given per unit charge (PE/q); it is the electric "pressure" that produces current.
Does voltage flow through a circuit, or is voltage established across a circuit?
Voltage is established across a circuit, causing charges to flow by establishing an electric field.
How does wetness affect the resistance of your body?
Wetness lowers skin resistance.
What happens to the brightness of light emitted by a light bulb with the current in it increases?
When the current increases the brightness of the light bulb increases.