Ohm's Law and Resistivity

[Mickey]

Can someone please help me with these questions.

1. The resistance of a bar heater increases as the heating element becomes hotter.
a Does the current through the heater decrease or increase as the element heats up?
b. Explain what is happening inside the heating element that would cause it’s resistance to increase as its temperature rises.

2. Electricians use three-core general purpose domestic cable when wiring a house. The manufactures design the cabling for use with 240V but it is still safe at 415V. The standard for such cable states that one kilometre of any three conductors must have a resistance of not more than 18.1 ohms.
a. Calculate the maximum resistance of one of the conductors in the cable if it runs 36m from the meter board to a power outlet in a house.
b. Calculate the current in the cable at its maximum voltage.

3. A lie detector measures the sweat rate of subjects by measuring changes in their skin resistance. Persons under stress normally produce more sweat so lie-detector operates assume subjects will sweat more when they tell lies.
a. Would a lie detector register increased or decreased skin resistance as a typical subject starts to tell a lie?
b. The detector shows this change as a needle movement on paper. What would this resistance change cause to happen to the current driving the needle motor?

4. Why do manufactures coil the filaments in their incandescent globes?

5. Explain why you expect low electrical resistance in blood and other body fluids.

6. If you examine the cables leading to starter motor of a car you will notice they have a large diameter compared with the cables going to the stereo unit. Explain why.

7. You find two wires in an old building, one iron and the other copper. Both have the same cross-sectional area. You need to find out which wire is longer. You decide you can find out by measuring the resistance of each wire. You find they both have the same resistance. Which is longer?

[Moseley]

Hi there, Mickey, welcome to the forum. Dunno what your background is but you seem to have a range of questions. I am not going to do them for you but will try and give you some tips.

1. V=IR is the most important equation in electrical systems. As R increases, I decreases for steady voltage. I belive the rise in R is due to induced back emfs. You could perhaps read about those to confirm it.

2. 1km has max resistance of 18.1 ohms. Cable resistance = (36/1000)*18.1 then use V=IR with V at 415 for max current.

3. This question seems a bit silly but presumably lying causes increased perspiration and thus produces more electrolyte on the skin surface so resistance will decrease. Not sure what a needle motor is.

4. Dunno - may be trying to get more length of conductor inside the globe.

5. Disolved salts.

6. The starter draws much higher current.

7. R = pla where p is the resistivity (rho) - easy to look up the relative resistivities of iron and copper - suspect copper is better conductor/has lower resistivity.

Hope that helps.

[J. Wensveen]

1. a. Assuming V is constant and since V=I*R : If R goes up, I would go down to keep V constant.

1.b. If a current (electrons) move through a material, they move from one molecular bond to the next in the crystalline like structure of the material. These electrons collide with atoms making them vibrate more (= heat) therefore it becomes harder for them to move through the structure since they colide more often due to the increase in vibration of the material. Since heat = molecular/atom vibrations, if you cool down a material up to a point where they hardly move (close to 0 °K) the material will become super conductive if its molecular structure allows for this.

2: see above.

3: V=I*R so, a change in R with a constant V will give a change in I, and I is what determines the position of the needle in a needle motor. (magnetic induction and Forces)

4: P=U*I, = I^2 * R. If a current goes through a wire, it heats up, if it heats up enough, it will start to glow. if it glows, it emits light. The amount of light emitted is dependent on the current, but also on the amount of wire. More wire = more surface that gives light. By coiling your wire, you can put in alot of surface in a light bulb to give off light.

5: Dissolved salts. their molecular bondage is loosened therefore they can more easily conduct current (electrons)

6: Lower resistance in thicker copper cables. You need a high current to run through them to get a spark from your plugs. Thin cables at that current have a too high resistance and will try to act like a glow bulb (they melt)

7: the copper wire. The resistance per meter of Copper is much lower than the resistance per meter of Iron. since they have the same width, the length of the copper wire must be alot longer to get to the same resistance value as the iron one.