Exam 2 Physics 2

For heat to be transferred, there must be a temperature differential between the two ends of an object. For water to flow, there must be a pressure difference between the two ends of a pipe. What is the analogous "drive" behind the flow of charge?

You need a potential difference across the end of a "current pipe" aka wire. This creates an electric field which pushes charges along the wire.

Why are electrons rather than protons the charge carriers in a wire?

Protons are tightly bound in the nucleus and therefore difficult to remove. Valance electrons move as a "sea" inside conductors.

What is the error in saying that electrons in a common battery-driven circuit travel at about the speed of light?

Electrons can’t travel at the speed of light, they move but they drift. Lights turn on right away because electrons exist everywhere in the circuit.

As the electrons begin to move, the energy that they dissipate as they run into the conductors is what causes the lights to turn on, meaning they don’t have to move at the speed of light from the switch to the lightbulb in order to turn on.

What happens to the brightness of a lightbulb when the current through it increases? What causes this change?

The brightness increases. More current means more electrons running into more atoms causes more energy transfer. More energy transfer means more light radiated.

An electron moving in a wire collides repeatedly with atoms and travels at an average distance between collisions called the mean free path. If the mean free path is less in some metals, what can you say about the resistance of these metals?

If an electron can not go very far without running into another atom, as implied by a "shorter mean free path" that would mean electrons are running into things more often, making it a bad conductor. That would mean the resistance must be large

{This doesn’t mean that the resistance is large, it just mean the resistivity would have to be larger}

Will a lamp with a thick filament draw more or less current than a lamp with a thin filament?

A thick filament means a large cross sectional area. Resistance is inversely proportional to cross sectional area. So larger area means less overall resistance. And less resistance means less current. This means a thick filament will draw more current than a thin filament.

Explain the difference between resistance and resistivity.

Resistivity is the property of the metal conductor and does not change unless you change materials. It is a measure, essentially, of how often electrons run into other objects in the wire. Resistance can be adjusted by increasing or decreasing the length or cross-section of the wire

If you double the diameter of a wire, by what factor does the resistance change? Did it get larger or smaller?

If you double the diameter, the radius also doubles. This means when calculating the area as πr², you would square this new factor of 2, so the area increased by a factor of 4. Since resistance and area are inversely proportional, that means the resistance decreased by a factor of 4.

When birds stand on a wire, why do they not experience a large shock? What if they stood in between two different wires?

A bird sitting on the wire with its two feet spread out slightly experiences a very small potential difference. The resistance of the bird is very large, so very little current passes through the bird, thus no real shock. If it put another foot on another wire, the potential difference could be very large, which might provide more current, causing a larger shock.

You note two bulbs connected in series in a circuit with a battery. One of the bulbs glows less brightly than the other. What can you say about the relative resistance of the two bulbs?

The resistance of the brighter bulb must be larger than the resistance of the dimmer bulb. P = I²R and I is the same. 

Consider two copper wires of equal cross-sectional area. One wire is 3 times as long as the other. How does the resistivity of these two wires compare?

Both have the same resistivity.

You are given a copper bar of dimensions 3 cm x 5 cm x 8 cm and asked to attach leads to it in order to make a resistor. If you want to achieve the smallest possible resistance, you should attach the leads to the opposite faces that measure

5 cm x 8 cm

In a series of bulbs, which would glow the brightest when the switch is closed?

The largest resistor would glow the brightest because in a series P=I²R, the brightest bulb is the biggest resistor

In a parallel circuit of bulbs, which would glow the brightest when the switch is closed?

The smallest resistor because in a parallel circuit P and R are in an inverse relationship. P=V²/R which means the smaller the resistor, the brighter the bulb.

Are car headlights in series or in parallel?

They must be parallel because when one goes out the other is unaffected. If they were in series; if one were to go out, the other would too.

The figure shows conventional current passing through a resistor. The arrow shows the direction which the conventional current is flowing. Which of the following statements is true?

The electric potential at point b is the same as it is at point a. the electric potential at point b is lower than at point a

For the graph shown in the figure, what physical quantity does the slope of the graph represent for an ohmic material?

1/(resistance)

The following statements are true about a voltmeter

It has a small internal resistance, it is used to measure voltage, and it must be placed in parallel with a resistor to measure the voltage across the resistor

The following statements are true about an ammeter

It is used to measure current, it has a large internal resistance, and it must be placed in parallel with a resistor to measure the current through the resistor

DC stands for

direct current