Try these "busters" to exercise your brain ... they should help you grasp the concepts underlying electrical circuits, etc. To gain the maximum effect you should attempt to answer them before looking at the answers!
[1] The circuit below consists of two identical bulbs burning with equal brightness and a single 1.5V battery. When the switch is closed, what happens to the brightness of bulb Y?
[2] If the bulbs in the circuit below are identical, what happens when the switch is closed?
[3] When an ordinary household light bulb is switched on, the tungsten filament becomes white hot and its resistance increases. So, what happens to the power; does it increase, decrease or remain the same? Explain your answer.
[4] Two materials have different resistivities. Two wires of the same length are made, one from each of the materials. Is it possible for each wire to have the same resistance? Justify your answer.
[5] One electrical appliance operates at a voltage of 120V, while another operates at 240V. Based on this information alone, is it correct to say that the second appliance uses more power than the first? Give your reasoning.
[6] The power rating of a 1000W heater specifies the power consumed when the heater is connected to a voltage of 120V. If two such heaters are connected in series across the same voltage explain why the total power consumed is not 2000W.
[7] You have four identical resistances, each of value R. You are asked to connect all four resistances together in such a way that the equivalent resistance is R. How many ways can you find to do it?
[8] There is something distinctly odd (amusing, surprising, ...) about these four circuits. Can you figure out what it is?
[9] You are given two bulbs, one rated at 40W at 120V and the other rated at 100W at 120V. Contrast their brightness if they are put:
[10] Sometimes you hear someone say that a particular appliance "uses up" electricity. What does the appliance actually use up?
[11] A simple lie detector consists of an electric circuit, one part of which is your body - perhaps from one finger to another. A sensitive meter is used to indicate the current that flows when a small voltage is applied. Can you think how such an arrangement might indicate that a person is lying?
[12] Only a small fraction of the electrical energy fed into a common, household light bulb is transformed into light. What happens to the rest?
[13] In a typical day, about 20kW.hr of electrical power is consumed in the average household. How many electrons does the electric company provide then to a household in a typical day?
[14] Two 1.5V batteries and a bulb are connected together in three different ways, as shown below:
In which arrangement is the bulb the brightest? And in which arrangement is it the least bright? Explain the differences between the circuits.
[15] ( Not as easy as it may seem at first glance!) Twelve, identical resistances are connected to form a cube, as shown below.
How would you go about determining the effective resistance between the point A and the point Z?
[16] In the circuit below the bulbs are identical. Which bulb (or bulbs) is (are) the brightest, and which is (are) the dimmest, or are they equally bright?
[17] (Here's a good one!) In the chapters on electrostatics we saw that there was no electric field inside a conductor, so how come we can define them and use them now in this chapter on direct currents?
[18] If you determine the work done by a battery, with emf E, in charging a capacitor, with capacitance C, to a full charge, you'll find it is equal to CE2. However, if you look at your class notes, you'll find that the energy stored in the capacitor is only (1/2)CE2! What's going on ... what's happened to the other (1/2)CE2? Is this a violation of the law of conservation of energy? Do you have an answer?
[19] A capacitor is being discharged through a resistor. If it takes T seconds for the charge on the capacitor to drop to 1/3 of its initial value, how long does it take for the energy to drop to 1/3 of its initial value?
[20] You have a large electrical circuit that you know operates with potential differences of 10,000 volts. You also know it contains capacitors with capacitance values up to a maximum of about 10mF and resistors with a maximum resistance of 100,000W. Estimate how long you should have to wait until you could "safely" touch the various components after switching everything off.