Physics Homework :(

Question

1. What is the voltage of an AA dry cell if the internal chemical action does 25 J of work while

supplying 11 C of charge to operate a small bulb?

2. The battery required by a smoke detector can provide 18 C of charge when the chemical

action in the battery expends 120 J of energy. How many dry cells would it take to operate the

smoke detector? (Note: For a dry cell that voltage is typically 1.5 volts)

3. If the potential difference across a battery is 9.0 V, how much work is done to move 10.0 × 10 2

C of charge through a circuit?

4. The work required to move an electric charge between two points in an electric field is 0.0055

J. If the potential difference between these points is 20 V, what amount of charge is moved?

5. If 10.8 × 10 -17 J of work is required to move an electron between two points in an electric field,

what is the electric potential difference between these points?

6. State the four factors which affects Resistance.

7. What is the resistivity of a piece of wire that is 2.0 m long, 1.0 mm in diameter, and having a

total resistance of 0.035 S?

8. Two wires are identical except that wire 1 has a cross-sectional area that is 2.0 times that of

wire 2. If the resistance of wire 1 is 1.5 S, what is the resistance of wire 2?

9. A small flashlight bulb draws 500 mA in a flashlight that has 3 three 1.5 V dry cells in series.

What is the resistance of the bulb filament?

10. If a circuit has 5A of current running through it and the total resistance in the circuit is 10Ω,

then what is the total voltage?

Answer 1

To do questions 1, 2, 3, 4, 5 you need the following:

There is some device operating over an unspecified period of time Δt.

During this time period some constant voltage U causes constant current A,

dissipating power P.

Over the period Δt the current carries a charge ΔC, performing work ΔW.

For example, in the first problem you are given

ΔW = 25J

ΔC = 11C.

For the first 5 problems you need the following identities

A = ΔC/Δt

P = ΔW/Δt

P = UA

For example, in problem 1 you can use those identities to solve for voltage U.

(The unknown time Δt cancels out, showing that its value is irrelevant.)

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For problems 7 and 8 you need the following relation among

resistance R,

resistivity ρ,

length l,

cross sectional area A:

ρ = RA/l

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For problems 9 and 10 you need Ohm's Law:

U = RA