Chapter 20: Problem 4: Suppose you have a 19 m long piece of copper wire with a 2.224 mm diameter and a resistivity of 1.72 × 10^-8 Ω⋅m.   a) What is the resistance of the copper wire, in ohms?

When a straight wire is heated, its resistance changes according to the equation R = R0 [1 + (T T0)] (Eq. 17.7), where is the temperature coefficient of resistivity. (a) Show that a more precise result, which includes the length and area of a wire change when it is heated, is R=R0[1+(TT0)][1+(TT0)][1+2(TT0)] where is the coefficient of linear expansion. (See Topic 10.) (b) Compare the two results for a 2.00-m-long copper wire of radius 0.100 mm, starting at 20.0C and heated to 100.0C.

A child's electronic toy is supplied by three 1.58-V alkaline cells having internal resistances of 0.0200 inseries with a 1.53-V carbon-zinc dry cell having a 0.100- internal resistance. The load resistance is 10.0 . (a) Draw a circuit diagram of the toy and itsbatteries, (b) What current flows? (c) How much power is supplied to the load? (d) What is the internal resistance of the dry cell if it goes bad, resulting in only 0.500 W being supplied to the load?

Electric current I enters a node with three resistors connected in parallel (Fig. CQ18.5). Which one of the following is correct? (a) I1 = I and I2 = I3 = 0. (b) I2 I1 and I2 I3. (c) V1 V2 V3 (d) I1 I2 I3 0. Figure CQ18.5

Currents of approximately 0.06 A can be potentially fatal. Currents in that range can make the heart fibrillate (beat in an uncontrolled manner). The resistance of a dry human body can be approximately 100 k . (a) What voltage can cause 0.06 A through a dry human body? (b) When a human body is wet, the resistance can fall to 100 . What voltage can cause harm to a wet body?

(a) Can the circuit shown in Figure P27.21 be reduced to a single resistor connected to a battery? Explain. Calculate the currents (b) I1, (c) I2, and (d) I3. Figure P27.21

(a) Can the circuit shown in Figure P18.29 be reduced to a single resistor connected to the batteries? Explain. (b) Find the magnitude of the current and its direction in each resistor. Figure P18.29

(a) A defibrillator sends a 6.00-A current through the chest of a patient by applying a 10,000-V potential as in the figure below. What is the resistance of the path? (b) The defibrillator paddles make contact with the patient through a conducting gel that greatly reduces the path resistance. Discuss the difficulties that would ensue if a larger voltage were used to produce the same current through the patient, but with the path having perhaps 50 times the resistance. (Hint: The current must be about the same, so a higher voltage would imply greater power. Use this equation for power: P=I2 RP = .)

Review. When a straight wire is warmed, its resistance is given by R = R0,[1 + a(T T0)] according to Equation 27.20, where a is the temperature coefficient of resistivity. This expression needs to be modified if we include the change in dimensions of the wire due to thermal expansion. For a copper wire of radius 0.100 0 mm and length 2.000 m, find its resistance at 100.0C, including the effects of both thermal expansion and temperature variation of resistivity. Assume the coefficients are known to four significant figures.

The resistance between terminals a and b in Figure P27.36 is 75.0 . If the resistors labeled R have the same value, determine R. Figure P27.36