Physics 2: Physical Science & Technology (PHYC10004)
Week 5
Discussion Questions
1
A capacitor is charged and left connected to the battery. A dielectric is then inserted, filling the capacitor. What happens to the quantities below when the dielectric is inserted?
• the electric field in the capacitor
• the voltage across the plates
• the charge on the plates
• the energy stored in the capacitor
2
The same capacitor is charged and disconnected from the battery. The same dielectric is again inserted, filling the capacitor. What happens to the quantities listed in Question 1 when the dielectric is inserted?
3
As more identical resistors R are added to the parallel circuit shown, does the total resistance between P and Q increase, decrease or remain the same?
4
The light bulbs in the circuit are identical. When the switch is closed,
a) both go out.
b) the intensity of light bulb A increases.
c) the intensity of light bulb A decreases.
d) the intensity of light bulb B increases.
e) the intensity of light bulb B decreases.
f) some combination of 1–5 occurs.
g) nothing changes.
5
Two light bulbs A and B are connected in series to a constant voltage source. When a wire is connected across B as shown, bulb A:
a) burns more brightly.
b) burns as brightly.
c) burns more dimly.
d) goes out.
Questions 1 – 3 Copyright © 1997 Eric Mazur; all rights reserved. Reproduced with permission.
6
The globes in the circuit are identical; the battery is ideal.
ai) Rank bulbs A, B and C in order of brightness.
aii) Rank the voltages across the bulbs.
aiii) How do voltages across bulbs A and B relate to the battery voltage?
aiv) Is the voltage across bulb A greater than, less than, or equal to one half the battery voltage?
A student now cuts the wire between bulbs A and C, as shown.
bi) Rank bulbs A, B and C in order of brightness.
bii) Rank the voltages across the bulbs.
biii) How do voltages across bulbs A and B relate to the battery voltage?
biv) Is the voltage across bulb A greater than, less than, or equal to one half the battery voltage?
c) Analyse the following discussion between two students regarding the change in the circuit when the wire in the circuit is cut:
• Student 1: “I think that bulb B will get brighter. Bulb B used to share the current with bulb C, but now it gets all the current. So bulb B will get brighter. ”
• Student 2: “I don’t think so. Now there aren’t as many paths for the current, so the resistance in the circuit has increased. Since the resistance in the circuit has increased, the current in the circuit decreases. Bulb B will get dimmer. ”
Reproduced from Tutorials in Introductory Physics, LC McDermott et al. © Prentice-Hall 2002
Problem solving questions
7
A parallel-plate capacitor is filled with a dielectric for which k = 5.5. The area of each plate is 0.034 m2, and the plates are separated by 2.0 mm. The capacitor will short out if the electric field between the plates is > 200 kN/C. In light of the energy density, what is the maximum energy that the capacitor can store?
Halliday, David. Fundamentals of Physics Extended, 10th Edition. John Wiley & Sons, 2013.
8
A standard AA battery can deliver about 2.0 W-hr of energy.
a) If a standard battery costs $1.00, what is the cost of operating a 100 W lamp for 8.0 hr using such batteries?
b) Calculate the operating cost if energy is provided at 25 c per kW-hr 9
Find the current in each resistor and the potential difference between points a and b in the figure. Put R1=100 Ω and R2=50 Ω.
From Fundamentals of Physics, 6/e, Resnick, Halliday and Walker, © John Wiley 2001.
10
A battery with internal resistance (assumed constant) and unknown emf is connected to a 6.0 Ω resistor, and a current of 0.50 A flows through it. The resistor is then swapped for a 3.0 Ω resistor, and a current of 0.80 A flows. Calculate the internal resistance and emf of the battery.
11
A capacitor with plate area 2.0 m2 and plate separation of 0.10 mm and an air dielectric ( k = 1) is charged from a 24 V battery. It is then filled with a castor oil dielectric (k = 4.7). Calculate the difference in energy stored in the capacitor between the two arrangements.
Past examination questions
12
A linear accelerator produces a beam of electrons that is pulsed. The current in each pulse is 0.5 A, and each pulse has a duration of 0.10 μs.
a) How many electrons are accelerated in each pulse?
b) What is the average current for an accelerator operating at 500 pulses per second?
c) If the electrons are accelerated to an energy of 50 MeV, what is the average power and peak power of the accelerator?
13
A circuit is set up with three resistors and two ideal batteries as in the diagram below. One of the batteries has an emf 10 V, but the emf of the second battery is unknown. An ammeter shows that 2 A is flowing through the 3 Ω resistor from left to right.
a) Using Kirchhoff’s junction and loop laws, determine the emf, ε, of the second battery.
b) In the circuit above the 3 Ω resistor is replaced with a globe while a current of 2 A is still measured by the ammeter. The voltage across the globe is measured to still be 6 V and therefore the resistance of the globe is 3 Ω according to the equation V = iR. Can we assume the globe is ohmic? Explain your answer.
14
Four resistors are arranged as shown in the circuit diagram. R1, R2 and R3 each have fixed resistance and R4 is a variable resistor. A sensitive ammeter whose internal resistance is RA is connected between points A and B in the circuit.
(a) By applying Kirchhoff’s loop rule and junction rule, show that the ammeter will register no current flow when the variable resistor is adjusted so that its resistance satisfies
(b) If E = 5.0 V, R1 = 3 Ω, R2 = 2 Ω, R3 = 1 Ω and R4 = 1.5 Ω, what is the potential at A and B? (Assume the potential at the negative terminal of the cell is zero.)