ch 26 Flashcards
We often refer to the electricity at a typical household outlet as being 120 V. In fact, the voltage of this AC source varies; the 120 V is __________.
the minimum value of the voltage
the average value of the voltage
the peak value of the voltage
the rms value of the voltage
rms
In a transformer, energy is carried from the primary coil to the secondary coil by
the magnetic field in the iron core
A step-up transformer _____ the voltage and _____ the current.
raises;lowers
If you plug two devices into the two plugs in a standard household outlet, the devices are connected __________.
in parallel
A capacitor is connected to an AC supply. Increasing the frequency of the supply _________ the current through the capacitor.
increases
An inductor and a resistor are connected in a simple series circuit. If the inductance increases, the current in the circuit
decreases
An inductor is connected to an AC supply. Increasing the frequency of the supply _________ the current through the inductor.
decreases
In an RLC circuit connected to an AC voltage source, which quantities determine the resonance frequency? Choose all that apply.
capacitance and inductance
What is an Ac current defined by
An alternating emf that causes an alternating current
What is a DC
It is a circuit with a steady emf and a current in the same direction always
Because the AC circuit has a changing current and voltage, what is the current in a circuit element called
An instantaneous current
How does the current direction change every half cycle in an AC CIRCUIT
It changes direction
In an AC circuit, what is the relationship between the oscillations in current and voltage
They are in phase so the voltage will be at its max when the current is at its max
For every cycle of emf, how does the instantaneous power relate
It oscillates twice for every cycle of emf
For every cycle of emf, how does the instantaneous power relate
It oscillates twice for every cycle of emf
What is the purpose of a transformer
It is a device that takes an AC voltage as an input and produces either a higher or lower AC voltage as its output
What’s happening in the primary coil of a transformer
The AC voltage source creates an alternating current in the coil of wire. This creates an oscillating magnetic field that is transferred to the secondary coil by the iron core
whats happening in the secondary coil of a transformer
the oscillating magnetic field from the primary causes a change in the flux in the secondary
this produces a induced voltage that creates a current that dissipates power in the load
what is a step-up transformer
it is when N2>N1 and the voltage is increased
what is a step down transformer
it is when N1<N2 and the voltage is decreased
In a step up transformer, what happens to the current
it decreases
in a step down tramsformer, what happens to the current
it increases
what is the purpose of grounding
to provide a common reference point for potential difference
what is the wire connected to the ground called? what is the side connected to the voltage?
neutral and grounded
whats the purpose of a circuit breaker
when the amiplitude of a current exceeds a certain amount, the circuit breaker will switch open to disconnect the circuit
what makes electric sources dangerous
not the voltage but the amount of current flowing through a body
when do GFI circuits switch off
when they mesure a difference in current in the neutral and hot side, the current must be finding another way to the ground
in a capacitor AC circuit, how are the voltage, charge, and current related
the voltage and charge are in phase, the current is out of phase
in an AC circuit with a capacitor, does the current lead or lag the voltage
the current leads the voltage
what is the capacitative reactance
it controls the rate at which a capacitor charges and discharges
what happens to the reactive capacitance at high frequencies? At low frequencies?
at high frequencies the reactance decreases
at low frequencies the capacitance increases
when a current is increasing in a solenoid, what happens to the magnetic field and the induced emf
the magnetic flux increases and the induced emf will resist the change in the flux
where does the direction of the potential difference in an inductor point
an inductor resists changes in the current through it
when does an inductor develop a potential difference
it develops a potential difference if the current across it is changing
what is an inductance
the property of an electric conductor or circuit that causes an electromotive force to be generated by a change in the current flowing.
when is the voltage in an inductor high
when the current is rapidly changing
when is the voltage in an inductor low
when the current is changing slowly
does the AC current through an inductor lead or lag the inductor voltage
it lags the inductor voltage
what is inductive reactance
it is the resistance an inductor offers to the AC current
what is an LC circuit
A capacitor and an inductor
what happens to the current and charge in an LC circuit
the current in an LC circuit doesn’t stop, it continues until it has recharged the capacitor with the opposite polarization and it continues like this in an oscillation
what is an RLC circuit
the current oscillates from the capacitor but energy is lost through a resistor as thermal energy
what happens when we add an AC source to a RLC circuit
we create a driven RLC circuit
what is the resonance frequency
it is the frequency at which the current is at its maximum value in an RLC circuit
what happens to the emf of an inductor after some time has passed
Long after the switch is closed, the inductor generates no emf and acts like a wire with zero resistance.
when a switch is first closed, what happens to the inductor
it acts as an open circuit, no current will flow
when the switch is closed for a long time, what happens to the current
it gradually increases and then remains constant as there is no more change
If you stuck a paperclip in each of the two slots of a GFI-protected outlet, then grabbed one paperclip with your right hand and the other paperclip with your left hand, would the GFI protect you?
t depends on whether you are grounded or not. If you are, the outlet is likely to register it through a difference in the currents and disconnect the circuit
when does the inductor create a large emf
it is proportional to the rate of change in the current, so when the current is changing rapidly
when is there a max current in an AC circuit
at the max change in voltage
