Equations will be in one of the images below.

Ohm's Law

Statement: The resistance value of a resistor is equal to the quotient of the potential difference across the resistor at any instant and the current caused by that current. A resistor is said to be "Ohmic" if the quotient is the same regardless of the potential difference.

1. What current is produced in a 6 Ω resistor by a potential difference of 2 V?

I = ∆V/R = 2 V/6Ω = .33 A

2. What resistance is required if it is desired that a 12.6 V potential difference should produce .85 A?

R = ∆V/I = 12.6 V÷.85 A = 14.8 Ω

3. What potential difference will produce a 350 mA current in a 220 Ω resistor?

∆V = I•R = .35 A • 220Ω = 77 V

Ammeter-Voltmeter determination of resistance

4. Assume that you've accurately set up a circuit with a voltmeter and an ammeter. The voltmeter is connected to the knob that reads 1.5 and the ammeter is connected to the knob that reads 500 (these are the very meters that you have been using in the lab). Look at the images below and determine the resistance of the object being tested. Show your work.

The voltmeter reads 1.35 V and the ammeter 145 mA so:

1.35 V ÷ .145 A = 9.31 Ω

Resistance and Resistivity

**Resistance of a wire is directly proportional to its length and inversely proportional to its cross-sectional area.

5. A coil of wire 5 meters long is found to have 12 Ω of resistance. How much resistance does each meter of wire have?

12Ω ÷ 5 m = 2.2 Ω/m

6. A one meter long piece of wire with a radius of 0.001 meters (1 mm) has a resistance of 4 Ω. How much resistance does a 1 meter wire of the same metal but a radius of 0.0005 m (.5 mm) have?

halving the radius will reduce the area to 1/4th that it had before so we'll have 4 times the resistance: 4Ω • 4 = 16 Ω

Electric Power

Three equations for power can be found below.

7. How much power is produced in a 12 Ω resistor with 2 A of current?

P = I²R = 2² • 12 = 48 W

8. How much power is produced in a 5 Ω resistor at 120 V?

P = V²/R = 120² ÷ 5 Ω =2,880 W (120 V is normal house voltage -- but you wouldn't ordinarily place a resistance that small in a circuit in your house)

9. How much power is produced by 220 V with a 15 A current?

P = I∆V = 15 • 220 = 3300 W.

Wheatstone Bridge

No questions about the Wheatstone Bridge today. But to be ready for the first class following Thanksgiving you should look at the Wikipedia article on the subject and Google the term to find out as much as you can.