Exam 25: Electric Current and Direct-Current Circuits

You charge a capacitor and then discharges it through a resistor.You notice that,after four time constants,the voltage across the capacitor has decreased to _____ of its value just prior to the initiation of the discharge.

A galvanometer has an internal resistance of 200 $\Omega$ and requires a current of 5.0 mA for full-scale deflection.What resistance should be connected in series with this galvanometer to make a voltmeter that reads 25 V at full-scale deflection?

Use the diagram below for the next problem: -The diagram shows the current versus voltage for a diode.The region which has the highest resistance is

A wire of length L = 6.0 m has a uniform diameter d = 3.0 mm.When an electric field of E = 0.25 V/m is applied to the wire,the current is 1.5 A.The resistivity of the wire is

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 current as a function of time for this circuit is

If it takes you 5 minutes to dry your hair using a 1200-W hairdryer plugged into a 120-V power outlet,how many Coulombs of charge pass through your hair dryer?

Suppose instead of using a 12-gauge (diameter = 2.05 mm)solid copper cable you made a hollow copper cable of outer diameter 2.59 mm (10-gauge)and inner diameter 1.29 mm (16-gauge).Calculate the length of hollow cable that would give the same resistance as 1 m of 12-gauge solid cable.Use $\rho$ (Cu)= 1.7 $\times$ 10^-8 $\Omega$ /m.

Two resistors are connected in parallel across a potential difference.The resistance of resistor A is twice that of resistor B.If the current carried by resistor A is I,what is the current carried by B?

You can pay a lot of money for fancy connecting cables between your audio/video equipment components.Some of the more expensive ones use silver wire rather than copper.Calculate the difference in resistance between a silver and a copper cable with the same 12-gauge diameter of 2.05 mm and length of 3 m (hence total length = 6m).(Use $\rho$ (Cu)= 1.7 $\times$ 10^-8 $\Omega$ /m and $\rho$ (Ag)= 1.6 $\times$ 10^-8 $\Omega$ /m.Unfortunately (or fortunately),there are many other factors that go into making an interconnect cable than simple resistance considerations.)

Which of these five terms is most unlike the others?

You connect two identical resistors in series across supply lines maintained at a constant potential difference.When you later connect them in parallel across the same supply lines,the total current furnished by the supply lines,compared with the former current,is

Two pieces of copper wire have the same length,but wire A has a square cross section of width s whereas wire B has a circular cross section of diameter s.Which of the following statements is true?

A 0.120- $\mu$ F capacitor,initially uncharged,is connected in series with a 10.0-k $\Omega$ resistor and a 12.0-V battery of negligible internal resistance.The charge on the capacitor after a very long time is approximately

A 20- $\mu$ F capacitor is charged to 200 V and is then connected across a 1000- $\Omega$ resistor.The time constant for this circuit is approximately

The seemingly instantaneous propagation of electric current in a wire when a switch is closed can be understood in terms of

A wire of length L and resistance R is now drawn out by pulling such that its new resistance is 15R.What is its new length? (Assume the density remains constant.)

Which of the following statements is false?

In your four years at college you dry your hair everyday for 5 minutes with a 1200-W hairdryer.If the average price of electrical energy was $0.085 per kilowatt-hour,how much did it cost you during this time? (Assume 1 year = 365 days.)

An electric motor drives a pump that lifts water from a well that is 11.9 m deep and then delivers the water through a hose at a rate of 180 L/min and a speed of 21.0 m/s.If we assume 100% efficiency and ignore viscosity,the current drawn by the motor from a 220-V source is

The forward current for a pn-diode is given by where I₀ and V₀ are constants.If I₀ = 4 $\mu$ A and V₀ = 1.5 V,what is the ratio of the current for V = 2 V to 0.5 V?