AP physics EM finals T3

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Transformer Turns Ratio Formula

• N_P​: Number of turns in the primary coil

• N_S: Number of turns in the secondary coil

• V_P​: Voltage in the primary coil

• V_S​: Voltage in the secondary coil

Given this formula, how do we find the power dissipated in the resistor?

What is the unit of power?

W (watt)

What is the unit of resistance?

Ω (ohm)

How would increasing the number of turns of wire on the secondary coil of a transformer impact the currents and voltages in the primary and secondary circuits?

Overall, the whole thing will behave as a step-up transformer

• There would be no effect on the primary components

• V_s would increase

• I_s (secondary current) would decrease

Describe the impact on the currents and voltages in the primary and secondary circuits of a transformer of increasing the number of turns of wire on the primary coil of a transformer.

Overall, the whole thing will behave as a step-down transformer

• I_p (primary current) will decrease

• V_s will decrease

• I_s will increase

What is the general behavior of a stepdown transformer?

Stepdown: N_p > N_s (Voltage goes DOWN)

V_s < V_p

I_s > I_p

What is the general behavior of a step-up transformer?

Step-up: N_s > N_p (Voltage goes UP)

V_s > V_p

I_s < I_p

For an ideal step-down transformer, which of the following is/are the same in the primary coil and in the secondary coil?

• Frequency

• Maximum voltage

• Maximum current

• Average Power

Frequency and Average power

Describe the relationship between current and voltage when X_L>X_C?

X_L>X_C
ELI

• E = VOLTAGE leads current

• L = More INDUCTIVE than capacitive

• I = phase angle ϕ is POSITIVE

Describe the relationship between current and voltage when X_L< X_C?

X_L< X_C
ICE

• I = CURRENT leads voltage

• C = more CAPACITIVE than inductive

• E = phase angle ϕ is NEGATIVE

Describe the relationship between current and voltage when X_L= X_C?

• voltage and current are IN PHASE

• purely resistive (resonance)

• phase angle ϕ is 0

When does resonance occur?

When X_L= X_C (inductive reactance is equal o inductive capacitance)

What is the SI unit of capacitive reactance AND inductive reactance?

ohm (Ω)

What is the formula of angular frequency?

ω = angular frequency in rad/s

f = frequency in Hz

Instantaneous Voltage in AC Circuit

v = instantaneous voltage

V_max = peak (maximum) voltage

ω = angular frequency

t = time

sin(ωt) = describes how the voltage varies sinusoidally with time

How do we find V_max from the RMS value?

(multiply root(2) with the RMS)

How do we find the RMS value from V_max?

Q9a) A circuit containing a capacitor has a source of time-varying EMF that provides a voltage given by V = VCsinωt. What is the current in the circuit when the potential difference across the capacitor is largest?

I_C = 0

Q9b) A circuit containing a capacitor has a source of time-varying EMF that provides a voltage given by V = VCsinωt. What is the potential difference across the capacitor when the current in the circuit is the largest?

V_C = 0

What is formula of X_L?

X_L = ωL

What is formula of X_C?

What is the impedance formula?

How do we find I_max from impedance (Z) and V_max?

How do we find the phase constant?

(according to Q12 of the structure?)

Intensity of unpolarized light after passing through the first polarizer

I₀ = Initial intensity

I₁ = New (first) intensity

Intensity of polarized light after passing through the second polarizer

I₂ = New (second) intensity

θ = angle

What’s the unit of light intensity?

W/m²

Q14) Unpolarized light is passed through a Polaroid filter. What happens to the vibration directions and the intensity of the transmitted light?

Vibration direction: in one direction only (in a single plane along the filter's axis)

Intensity: half the original intensity (halved)

Q15) How do we find the frequency of a radio wave?

f = Frequency of the wave

c = speed of light 3×10⁸

λ = wavelength in meters

Who is leading who?

What is the electrical component doing this?

(hint: look at the beginning of the graph and which one is higher on the Y axis)

Voltage is leading current

this is the INDUCTOR

Who is leading who?

What is the electrical component doing this?

(hint: look at the beginning of the graph and which one is higher on the Y axis)

Voltage and current are in phase

This is the RESISTOR

Who is leading who?

What is the electrical component doing this?

(hint: look at the beginning of the graph and which one is higher on the Y axis)

Voltage lags current (or) Current leads voltage

This is the CAPACITOR

Why does polarized light end up having greater intensity?

because it starts with a higher intensity before reaching Polarizer B

Why doesn’t the polarized light lose its intensity after passing through polarizer A?

its polarization direction is parallel to the polarizer’s axis.

Q19c) What would be the effect of increasing the resistance of the resistor on the resonant frequency and maximum current in a series RLC circuit?

1. As the resistance increases, the maximum current decreases.

2. Resonant frequency remains constant

3. The graph will be smaller and broader

Q19d) What would be the effect of decreasing the resistance of the resistor on the resonant frequency and maximum current in a series RLC circuit?

1. As the resistance decreases, the maximum current increases.

2. Resonant frequency remains constant

3. The graph will be larger and sharper

Q17) What type of transformer is ‘transformer A’ and what does it do?

A: Step-up transformer increases the voltage to reduce power loss.

Q17a) What type of transformer is ‘transformer B’ and what does it do?

B: Step-down transformer decreases the voltage for home applications.

Q17d) Why do we use high alternating voltage to transmit electrical power over large distances?

How does a step-up transformer look like?

REMEMBER: more secondary turns

How does a step-down transformer look like?

REMEMBER: more primary turns

Q13) How do we find the angle between 2 polarizers?

IMPORTANT: PLEASE SET YOUR CALCULATOR IN DEGREES!!!