# 500-600 Word Electricity And Magnetism

Categories: ElectricityPhysics

Resistors are often made to have a specific value of resistance so that the characteristics of the circuit can be accurately calculated. We can explain the flow of current through a resistor by comparing it to water flowing through a pipe. A pressure difference maintained across two ends of the pipe by a pump is like the potential difference, or voltage, across a wire maintained by a battery. The rate of flow of water, analogous to the rate of current, depends on the type of pipe used.

A long and thin pipe offers more resistance than a short and thick one or a pipe that has obstructions.

Similarly, the resistance of a wire is dependent upon several factors, including its length, cross-sectional area, temperature, type of material (resistivity). Resistivity is a fundamental characteristic of the material itself defined by the voltage divided by the density of current (current per unit cross-sectional area) flowing across the material. A material of high resistivity will require a higher electrical field to cause a given current density.

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If the resistivity of a material is known, as well as its dimensions, it can be used to calculate the resistance of a particular piece of material.

The resistivity of a material is also dependant upon temperature. When a material resists the flow of current, it converts the electrical energy into other kinds of energy such as heat and light. This energy causes resistors to heat up and glow when enough current flows through them. Resistors are designed to have a specific value of resistance.

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Most resistors used in electric circuits are cylindrical items a few millimetres long with wires at both ends to connect them to the circuit. Resistors are often colour coded by three or four colour bands that indicate the specific value of resistance.

Some resistors obey the Ohm’s law, which states that the current density is directly proportional to the electrical field when the temperature is constant. The resistance of a material that follows Ohm’s law is constant, or independent of voltage or current, and the relationship between current and voltage is linear. In diodes, the current does not increase linearly with voltage and is different for two directions of current. Variable resistors are used in electronics (and are then called potentiometers) as volume and other controls. Larger current versions are useful in laboratory experiments.

Constantan wire is an alloy of 60% of copper and 40% of nickel. In the laboratory the constantan wire is wound on a tube with a sliding contact on a metal bar above the tube. The variable resistor can also act as a potential divider for changing the p. d. applied to a device, all three connections being used. So, it can be used also as a rheostat. From my background information I can now predict that the thinner and longer the wire the high resistance and lower the current. The thicker and shorter the wire the lower the resistance and higher the current. The results in turn should produce a straight-line graph.

Fair Testing To ensure that my experiment is carried out fairly I should do the following:  I should not keep all the variables changing at one time. I should keep one variable constant and the other varying each time. E. g. When varying the length I should keep everything else the same apart from the length of the wire itself. When varying the thickness everything else should be the same apart from the thickness of the wire itself.  When cutting the length of the wire I should make sure that it is exactly the same length; not any more and not any less, as this would affect my results.

I should have a good range of readings. The more readings I have the more accurate the answer will be.  After one reading I should disconnect the whole circuit and then connect it again instead of just turning the power off. This will also help in giving me more accurate answers.  I should keep the voltage at 4 volts the maximum at all times, as above this voltage the wire will burn.  I should not give priorities to one variable only. Preliminary work I have done only one previous experiment of which I did wrong. I kept varying the resistance on the rheostat for each reading I made.

This was incorrect, as the resistance should have been kept the same. I also turned the power off instead of disconnecting the circuit, which I was not supposed to do. However, everything else turned out right. In my preliminary work I did the above. Therefore, I am not going to do these mistakes. I will not vary the resistance at all. Neither will I turn the power off. I will disconnect the circuit. I will also take three readings each time. Safety Procedures  Handle all substances with care.  When you increase the resistance decrease the voltage.  Do not set the voltage on the power pack over 4V, as this will burn the wire.

Do not keep the resistance very low, as this can also burn the wire and is dangerous. Keeping the resistance low can also result in the apparatus being damaged.  Remove loose clothing and make sure the wires are secure and do not touch them.  You should not eat during any part of the experiment and keep water well away from the apparatus.  You should not leave experiment unattended.  To prevent electrocution ensure that your hands are dry during all parts of the experiment.  Do not run around in the laboratory and keep all bags underneath tables, as people can trip over them. Variables (Varying length) Factor.

Independent variable Dependant variable Changing variable Fixed variable Length 1. Voltage is specific and not dependant. 2. The type of material does not depend on anything. 1. Resistance depends on the length and thickness of the material. 2. Current depends on the voltage. The length changes: 10 cm – 70 cm. 1. The thickness stays constant. 2. Voltage stays same. 3. Type of material – constantan or nickel chrome stays constant. (Varying thickness) Factor Independent variable Dependant variable Changing variable Fixed variable Thickness 1. Voltage is specific and is not dependant. 2. The type of material does not depend on anything.

1. Resistance depends on the length and thickness of the material. 2. Current depends on the voltage. The thickness of the material changes: 122g 125g 1. The length stays constant. 2. Voltage stays same. 3. Type of material – constantan or nickel chrome stays constant. Apparatus Power pack  Connecting wires Variable resistor – rheostat Types of material – constantan and nickel chrome  Ammeter  Ruler Clip The following shows a diagram of how the apparatus will be connected: Method Varying length Step 1: Set the power pack to 4V. Step 2: Connect a wire from the power pack to the rheostat.

Step 3: Connect another wire from the rheostat to the Step 4: Connect a wire from the clip to the ammeter. Step 5: Connect another wire from the ammeter back to the power pack. Step 6: Cut a piece of wire at 10 cm using a ruler and put it in the Step 7: Make sure all the apparatus is connected properly and the resistance is not very low. Step 8: Turn the power pack on and take a reading. Step 9: Disconnect the circuit and connect it again and take another reading. Step 10: Repeat step 9 again. Step 11: Repeat steps 1-10 again for a piece of wire at 20 cm. Keep increasing the length by 10 cm each time. Only keep the length varying.

All the other variables should be kept constant. Varying thickness Step 1: Set the power pack to 4V. Step 2: Connect a wire from the power pack to the rheostat. Step 3: Connect another wire from the rheostat to the Step 4: Connect a wire from the clip to the ammeter. Step 5: Connect another wire from the ammeter back to the power pack. Step 6: Cut a piece of wire of 20 cm using a ruler and put it in the Step 7: Make sure all the apparatus is connected properly and the resistance is not very low. Step 8: Turn the power pack on and take a reading. Step 9: Disconnect the circuit and connect it again and take another reading.

Step 10: Repeat step 9 again Step 11: Repeat steps 1-10 again 3 times for another piece of wire of the same length. Keep the length constant but vary the thickness (cross-sectional area). Observations My observation has given me the following results that I have obtained: Table (Varying length) – SWG 22 Material SWG Thickness (Diameter in g) Length (cm) Reading No Current (A) Average (A) Resistance (? ) Thickness (Diameter in g) Length (cm) Reading No Current (A) Average (A) Resistance (? ) Constantan.

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