Exam 25: Electric Current and Direct-Current Circuits

Use the diagram for the next problem: -The switch S is initially at position a for a long time.It is then switched to position b.Describe what happens to the light bulb as a function of time when the switch is flipped from a to b?

A resistor carries a current I.The power dissipated in the resistor is P.What is the power dissipated if the same resistor carries current 3I?

Use the following figures to answer the next problem. -An uncharged capacitor and a resistor are connected in series to a battery as shown. If $\varepsilon$ = 15 V,C = 20 $\mu$ F,and R = 4.0 $\times$ 10⁵ $\Omega$ ,the time constant of the circuit is approximately

Wire A,which is of the same length and material as wire B,has twice the diameter of wire B.If the resistance of wire B is R,what is the resistance of wire A?

A heating element made of nichrome wire (temperature coefficient of resistivity, $\alpha$ = 0.400 $\times$ 10^-3 K^-1)is connected to a source.When it has just been turned on and its temperature is 20ºC,it dissipates 960 W of power.What power is dissipated when its temperature has risen to 300ºC?

A potential difference V = 25 V is applied across a wire of length L = 10.0 m and uniform diameter.The electric field throughout the wire is

A resistor develops heat at the rate of 20 W when the potential difference across its ends is 30 V.The resistance of the resistor is approximately

A storage battery with emf 6.4 V and internal resistance 0.080 $\Omega$ is being charged by a current of 15 A.The power loss in internal heating of the battery is

By what percentage does the resistance of a copper wire ( $\alpha$ = 3.9 $\times$ 10^-3/K)increase when its temperature increases from 20ºC to 100ºC?

A wire of length L,cross sectional area A and resistance R is now drawn out by pulling such that its new resistance is 25R.What is its new cross sectional area? (Assume the density remains constant.)

Tony charges a capacitor and then discharges it through a resistor.He notices that,after two time constants,the voltage across the capacitor has decreased to _____ of its value just prior to the initiation of the discharge.

When two identical resistors are connected in parallel,the equivalent resistance,compared with that of the same two resistors connected in series,is

A 0.12- $\mu$ F capacitor,initially uncharged,is connected in series with a 10-k $\Omega$ resistor and a 12-V battery of negligible internal resistance.Approximately how long does it take the capacitor to reach 90% of its final charge?

Use the diagram below for the next problem: -Two copper wires have the same volume,but wire 2 is 10% longer than wire 1.The ratio of the resistances of the two wires R₂/R₁ is

The drift velocity of an electron in a wire varies

Which of the following statements is true concerning an ideal ammeter and an ideal voltmeter?

A sensitive galvanometer has a resistance of 180 $\Omega$ and requires 2.40 $\mu$ A of current to produce a full-scale deflection.The series resistance needed to construct a voltmeter that gives full-scale deflection for a potential difference of 10.0 V is approximately

A wire of length L = 3.0 m.When an electric field of E = 0.5 V/m is applied to the wire,the current is 1.5 A.The resistance of the wire is

Doubling the resistance in an RC circuit

An energy efficient light bulb uses 15 W of power for an equivalent light output of a 60 W incandescent light bulb.How much money do you save each month by using the energy efficient light bulb instead of the incandescent light bulb for 6 hours a day? Assume that 1 kW.hr costs 14 cents and there are 30 days in one month.