Electric Resistance – The Physics Hypertextbook
only exhibit a marked drop in their resistance at very high temperatures. They remain good insulators over all temperatures they are likely to encounter in use. Class practical Investigating the changing resistance of a wire as it heats up. with the coil in air, and then plot the current/potential difference characteristic. While the electric potential difference established between the two terminals lists resistivity values for various materials at temperatures of 20 degrees Celsius.
Only some electrons are free to migrate however. Others within each atom are held so tightly to their particular atom that even an electric field will not dislodge them.
The current flowing in the material is therefore due to the movement of "free electrons" and the number of free electrons within any material compared with those tightly bound to their atoms is what governs whether a material is a good conductor many free electrons or a good insulator hardly any free electrons. The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher the temperature the more violently the atoms vibrate.
In a conductor, which already has a large number of free electrons flowing through it, the vibration of the atoms causes many collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms jostle around in the material, the more collisions are caused and hence the greater the resistance to current flow. In an insulator however, there is a slightly different situation.
The power dissipated in a resistor goes into heating the resistor; this is know as Joule heating. In many cases, Joule heating is wasted energy.
Temperature Coefficient of Resistance
In some cases, however, Joule heating is exploited as a source of heat, such as in a toaster or an electric heater. The electric company bills not for power but for energy, using units of kilowatt-hours. It does add up, though. The following equation gives the total cost of operating something electrical: Try this at home - figure out the monthly cost of using a particular appliance you use every day.
Possibilities include hair dryers, microwaves, TV's, etc. The power rating of an appliance like a TV is usually written on the back, and if it doesn't give the power it should give the current.
Temperature Coefficient of Resistance
Anything you plug into a wall socket runs at V, so if you know that and the current you can figure out how much power it uses. The cost for power that comes from a wall socket is relatively cheap. On the other hand, the cost of battery power is much higher.
Although power is cheap, it is not limitless. Electricity use continues to increase, so it is important to use energy more efficiently to offset consumption.Resistance variation with temperature
Appliances that use energy most efficiently sometimes cost more but in the long run, when the energy savings are accounted for, they can end up being the cheaper alternative.
Direct current DC vs. If the circuit has capacitors, which store charge, the current may not be constant, but it will still flow in one direction.
The current that comes from a wall socket, on the other hand, is alternating current. With alternating current, the current continually changes direction. This is because the voltage emf is following a sine wave oscillation. For a wall socket in North America, the voltage changes from positive to negative and back again 60 times each second.
You might think this value of V should really be - volts. That's actually a kind of average of the voltage, but the peak really is about V.
This oscillating voltage produces an oscillating electric field; the electrons respond to this oscillating field and oscillate back and forth, producing an oscillating current in the circuit.
The graph above shows voltage as a function of time, but it could just as well show current as a function of time: Root mean square This average value we use for the voltage from a wall socket is known as the root mean square, or rms, average.
Because the voltage varies sinusoidally, with as much positive as negative, doing a straight average would get you zero for the average voltage. The rms value, however, is obtained in this way: To find the rms average, you square everything to get 1, 1, 9, and Take care to avoid short-circuiting the coil by using a wooden lolly stick or spatula as the stirrer.
This is not a straight line showing constant resistance, but rather a curve showing that the resistance of the wire increases with temperature. Pure metals do obey Ohm's law when their temperature remains constant.
Therefore, they do not need to be placed in a constant temperature bath in order to show ohmic behaviour. This experiment can be used to teach about the idea of validity of scientific results. Results may be rendered invalid if an uncontrolled factor affects the results. In this case, the temperature of the wire is a factor which affects measurements of its resistance. Discuss how this can be taken account of. One approach is as discussed above to keep the wire at a constant temperature by immersing it in a water bath.
An alternative would be to use the p.