1 of 8 Current Concept Tests CTR-1. Current is flowing through wires which are part of a circuit, as shown. What is the current in the wire at the bottom right? A) 1 A to the left B) 1 A to the right C) 3 A to the left D) 3 A to the right E) none of these 1 A 2 A I ? Answer: 1 A to the right CTR-2. Consider the following four situations. i) An ion (charge +Q) moves to the right. ii) A neutral hydrogen atom (proton, +e, and light electron, -e) moves right. iii) A beam of electrons in a TV shoots right iv) In an ionic solution, massive positive ions flow right, light negative electrons flow with equal speed to the left. In which is the net current to the RIGHT? A: i only B: iii only C: iv only D: i and iv only E: None of these Answer: i and iv only M. Dubson, Phys1120 ©University of Colorado at Boulder2 of 8 CTR-3. A copper cylinder is machined to have the following shape. The ends are connected to a battery so that a current flows through the copper. A C B Which region A, B, or C has the greatest magnitude current density J? A B C D) all three have the same J. Which region has the greater conductivity σ? A B C D) all three have the same σ. Which region A, B, or C has the greatest magnitude electric field E? A B C D) all three have the same E Answers: Region B has the largest current density J = I/A. All 3 regions have the same current I, so the region with the smallest A has the largest J. All three regions have the same conductivity σ. Conductivity σ is a property of the composition, not of the shape of the sample. Region B has the largest E-field. J = σ E, J is largest is region B (from the 1st part of this question), σ is constant for all regions, so E must be max where J is max, in region B. M. Dubson, Phys1120 ©University of Colorado at Boulder3 of 8 CTR-4. Salt water contains n sodium ions (Na+) per cubic meter and n chloride ions (Cl– )per cubic meter. A battery is connected to metal rods that dip into a pipe filled with salt water. The cross-sectional area of the pipe is A. + - A ammeter pipe full of salt water The magnitude of the drift velocity of the Na ions is v+ and the magnitude of the drift velocity of the Cl ions is v– Assume v+ > v– What is the magnitude of the ammeter reading? A) |enAv+| – |enAv–| B) |enAv+| + |enAv–| C) enAv+ D) enAv– E) zero Answer: enAv+ + enAv– Notice that v+ and v- are the magnitudes of the drift velocities, so they are both positive by definition. A current of (+) charges to the right is equivalent to a current of (–) charges to the left, so the currents due to the 2 types of ions add. They do not cancel. M. Dubson, Phys1120 ©University of Colorado at Boulder4 of 8 CTR-5. What is the correct way to connect the battery to the light bulb to make it light up? Answer: Circuit (B) is the only one that will work. CTR-6. The instructor goes crazy and grabs the terminals of a fully-charged 12V battery. He will .. A) immediately be killed. B) not be killed, but he will start hollering because it really hurts. C) not feel anything. This action is harmless. Answer: He will not feel anything. His body's resistance R is very large (mega-ohm), so the current through him (I = V/R) is very small. + - B) A) + - + - C) + - D) metal insulator hollow metal E) None of these will work M. Dubson, Phys1120 ©University of Colorado at Boulder5 of 8 CTR-7. Which graph of voltage vs. current shows the behavior of an "ohmic" resistor (a resistor obeying Ohm's Law). Answer: Graph B. The slope of this graph is V/I = R. This graph shows constant slope, so it shows constant resistance R. Ohm's Law says that R = constant. CTR-8. Two cylindrical resistors are made of the same material (same resistivity ρ). Resistor 2 is twice as long and has twice the diameter of resistor 1. What is the ratio RR21? (The arrow shows the direction of current flow.) A) 2 B) 4 C) 1/2 D) 1/4 E) 1 I V (A) I V (D) I V V (C) I (B) (E) None of these 2 1 M. Dubson, Phys1120 ©University of Colorado at Boulder6 of 8 Answer: The ratio is 1/2. 2222111121LRALA112LRLA4Aρ== =×=ρ2. The area is proportional to (radius)2 or (diameter)2. When the diameter is doubled, the area increases by a factor of 4. CTR-9. A battery with emf or voltage V is attached to a resistor of resistance R. The circuit diagram is shown below. The point A is at zero volts. B C The correct voltages at the points B, C, and D are: A) VB = 0V, VC = 5V, VD = 0V. B) VB = 5V, VC < 5V, VD > 0V. C) VB = 5V, VC < 5V, VD = 0V. D) VB = 5V, VC = 5V, VD = 0V. E) None of these. Answer: D. In a circuit diagram, a line represents a wire of zero resistance. There is no voltage change along a zero-resistance wire because ∆VIR Iwire wire=⋅=⋅=00. So the voltage at B is the same as at C, and the voltage at D is the same as at A. CTR-10. Two light bulbs are connected to a battery in series (in a chain, one after another). How does the current in upper light bulb A compare to the current in lower light bulb B? A) IA > IB B) IA < IB C) IA = IB D) answer depends on relative size of the two resistors A V = 5 V R = 10 Ω DIbatIARAV IBRBM. Dubson, Phys1120 ©University of Colorado at Boulder7 of 8 Answer: IA = IB Current is not "used up" in a resistor. Just like water is not "used up" when it flows through your garden hose. Every drop of water that enters your garden hose from the faucet, eventually exits the other end. And if the hose has no leaks and no bubbles, the rate at which water enters the hose from the faucet (in gallons per minutes) is exactly the same as the rate at which it leaves. This is because water is incompressible: in order to make room for a new drop of water from the faucet, one drop of water has to leave the other end. This is how electrons flow through a resistor. The flow of charges into the resistor is exactly equal to the flow of charges out of the resistor. CTR-11. A light bulb is attached to a battery with constant voltage V. The light filament has resistance R. The circuit diagram is shown below. When the light bulb is first turned …
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