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Phys 5B > ch 26 > Flashcards

ch 26 Flashcards

1
Q

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

A

rms

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1
Q

In a transformer, energy is carried from the primary coil to the secondary coil by

A

the magnetic field in the iron core

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2
Q

A step-up transformer _____ the voltage and _____ the current.

A

raises;lowers

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3
Q

If you plug two devices into the two plugs in a standard household outlet, the devices are connected __________.

A

in parallel

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4
Q

A capacitor is connected to an AC supply. Increasing the frequency of the supply _________ the current through the capacitor.

A

increases

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5
Q

An inductor and a resistor are connected in a simple series circuit. If the inductance increases, the current in the circuit

A

decreases

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6
Q

An inductor is connected to an AC supply. Increasing the frequency of the supply _________ the current through the inductor.

A

decreases

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7
Q

In an RLC circuit connected to an AC voltage source, which quantities determine the resonance frequency? Choose all that apply.

A

capacitance and inductance

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8
Q
A
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9
Q

What is an Ac current defined by

A

An alternating emf that causes an alternating current

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10
Q

What is a DC

A

It is a circuit with a steady emf and a current in the same direction always

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11
Q

Because the AC circuit has a changing current and voltage, what is the current in a circuit element called

A

An instantaneous current

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12
Q

How does the current direction change every half cycle in an AC CIRCUIT

A

It changes direction

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13
Q

In an AC circuit, what is the relationship between the oscillations in current and voltage

A

They are in phase so the voltage will be at its max when the current is at its max

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14
Q

For every cycle of emf, how does the instantaneous power relate

A

It oscillates twice for every cycle of emf

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15
Q

For every cycle of emf, how does the instantaneous power relate

A

It oscillates twice for every cycle of emf

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16
Q

What is the purpose of a transformer

A

It is a device that takes an AC voltage as an input and produces either a higher or lower AC voltage as its output

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17
Q

What’s happening in the primary coil of a transformer

A

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

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18
Q

whats happening in the secondary coil of a transformer

A

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

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19
Q

what is a step-up transformer

A

it is when N2>N1 and the voltage is increased

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20
Q

what is a step down transformer

A

it is when N1<N2 and the voltage is decreased

21
Q

In a step up transformer, what happens to the current

A

it decreases

22
Q

in a step down tramsformer, what happens to the current

A

it increases

23
Q

what is the purpose of grounding

A

to provide a common reference point for potential difference

24
Q

what is the wire connected to the ground called? what is the side connected to the voltage?

A

neutral and grounded

25
Q

whats the purpose of a circuit breaker

A

when the amiplitude of a current exceeds a certain amount, the circuit breaker will switch open to disconnect the circuit

26
Q

what makes electric sources dangerous

A

not the voltage but the amount of current flowing through a body

27
Q

when do GFI circuits switch off

A

when they mesure a difference in current in the neutral and hot side, the current must be finding another way to the ground

28
Q

in a capacitor AC circuit, how are the voltage, charge, and current related

A

the voltage and charge are in phase, the current is out of phase

29
Q

in an AC circuit with a capacitor, does the current lead or lag the voltage

A

the current leads the voltage

30
Q

what is the capacitative reactance

A

it controls the rate at which a capacitor charges and discharges

31
Q

what happens to the reactive capacitance at high frequencies? At low frequencies?

A

at high frequencies the reactance decreases

at low frequencies the capacitance increases

32
Q

when a current is increasing in a solenoid, what happens to the magnetic field and the induced emf

A

the magnetic flux increases and the induced emf will resist the change in the flux

33
Q

where does the direction of the potential difference in an inductor point

A

an inductor resists changes in the current through it

34
Q

when does an inductor develop a potential difference

A

it develops a potential difference if the current across it is changing

35
Q

what is an inductance

A

the property of an electric conductor or circuit that causes an electromotive force to be generated by a change in the current flowing.

36
Q

when is the voltage in an inductor high

A

when the current is rapidly changing

37
Q

when is the voltage in an inductor low

A

when the current is changing slowly

38
Q

does the AC current through an inductor lead or lag the inductor voltage

A

it lags the inductor voltage

39
Q

what is inductive reactance

A

it is the resistance an inductor offers to the AC current

40
Q

what is an LC circuit

A

A capacitor and an inductor

41
Q

what happens to the current and charge in an LC circuit

A

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

42
Q

what is an RLC circuit

A

the current oscillates from the capacitor but energy is lost through a resistor as thermal energy

43
Q

what happens when we add an AC source to a RLC circuit

A

we create a driven RLC circuit

44
Q

what is the resonance frequency

A

it is the frequency at which the current is at its maximum value in an RLC circuit

45
Q

what happens to the emf of an inductor after some time has passed

A

Long after the switch is closed, the inductor generates no emf and acts like a wire with zero resistance.

46
Q

when a switch is first closed, what happens to the inductor

A

it acts as an open circuit, no current will flow

47
Q

when the switch is closed for a long time, what happens to the current

A

it gradually increases and then remains constant as there is no more change

48
Q

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?

A

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

49
Q

when does the inductor create a large emf

A

it is proportional to the rate of change in the current, so when the current is changing rapidly

50
Q
A
51
Q

when is there a max current in an AC circuit

A

at the max change in voltage

Phys 5B (13 decks)

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