Q: Each second, $$1.0 \times 10$$^{17}$$ electrons pass from right to left past a cross section of a wire connecting the two terminals of a battery. What are the magnitude and direction of the electric current in the wire?

$$1.6 \times 10$$^{-2}$$ A, left to right

Question 1

A cell membrane can be modeled as a parallel-plate capacitor. If the membrane has a surface area $$A$$ and thickness $$d$$, and the dielectric constant of the membrane is $$\kappa$$, what is the expression for its capacitance?

Accepted Answer

$$C = \frac{\$$varepsilon_0$$ \kappa A}{d}$$

Question 2

A capacitor stores 10 J of energy with a potential difference of 300 V. To store 90 J of energy with the same capacitor, what potential difference is required?

Accepted Answer

900 V

Question 3

If a parallel-plate capacitor is isolated and the plates are pulled apart, what happens to the charge on the plates?

Accepted Answer

The charge remains constant

Question 4

A capacitor is connected to a battery. Then, while remaining connected, a dielectric is inserted between the plates. What happens to the charge stored on the plates?

Accepted Answer

The charge increases

Question 5

Which of the following real-world devices relies on the rapid release of energy stored in a capacitor?

Accepted Answer

Camera flash

Question 6

The energy stored in a capacitor is given by $$U = \frac{1}{2}CV^2$. If the voltage across a capacitor is tripled, by what factor does the stored energy increase?

Accepted Answer

9

Question 7

In a cell membrane, the sodium-potassium pump creates a potential difference by moving ions. Which ions are moved and in what direction?

Accepted Answer

Three Na$$^+$$ out, two K$$^+ in$$

Question 8

A camera flash capacitor stores 10 mJ of energy with a potential difference of 300 mV. To store 90 mJ of energy with the same capacitor, what potential difference is required?

Accepted Answer

900 mV

Question 9

What is the SI unit of electric current, and how is it defined?

Accepted Answer

Ampere, defined as 1 C/s

Question 10

Each second, $$1.0 	imes 10$$^{17}$$ electrons pass from right to left past a cross section of a wire connecting the two terminals of a battery. What are the magnitude and direction of the electric current in the wire?

Accepted Answer

$$1.6 	imes 10$$^{-2}$$ A, left to right

Question 11

In which of the following situations do you expect a lightbulb connected to a battery to light up?

Accepted Answer

When there is a closed loop connecting the battery and bulb

Question 12

What is the physical origin of resistance in a metal wire?

Accepted Answer

Electrons collide with ions in the crystal lattice

Question 13

Ohm’s law relates current, voltage, and resistance. Which equation correctly expresses Ohm’s law?

Accepted Answer

$$V = IR$$

Question 14

If a circuit contains a battery, two resistors, and ideal wires, what is true about the current in a single-loop circuit?

Accepted Answer

The current is the same through all elements

Question 15

A parallel-plate capacitor is charged and then disconnected from the battery. If a dielectric is inserted between the plates, what happens to the potential difference across the plates?

Accepted Answer

It decreases

Question 16

Which equation gives the energy stored in a capacitor in terms of charge $$Q$$ and capacitance $$C$$?

Accepted Answer

$$U = \frac{Q^2$}{2C}$$

Question 17

In a circuit diagram, what is the resistance of an ideal wire?

Accepted Answer

Zero

Question 18

A defibrillator uses a capacitor to deliver a rapid pulse of energy to a patient. If the capacitor is charged to a higher voltage, what happens to the energy delivered?

Accepted Answer

It increases quadratically with voltage

Question 19

If the cross-sectional area of a parallel-plate capacitor is doubled while keeping the plate separation and dielectric constant the same, what happens to the capacitance?

Accepted Answer

It is doubled

Capacitance, Current, and Resistance: Study Notes for Physics 202

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