As the electrons in an electric current move around a circuit, they bump into the atoms in the wires through which they pass. Atoms of different elements slow down the electrons by different amounts. For example, electrons pass easily through copper wire but much difficultly through tungsten or nichrome wires. We say that copper has a lower resistance than nichrome or tungsten. This is why copper is used for the connecting wires and cables in electrical circuits. Electrons collide with atoms in the wire that vibrate more quickly.
This causes the wire to warm up.
If the resistance is high and the current is large, the wire may get red hot. Conductors like this, which provide a high resistance, are called resistors. Resistors vary the current in a circuit. The resistor that I will use in my investigation is called a rheostat. Some resistors obey the Ohm’s law. A German scientist, Georg Ohm, investigated the resistance of various metal conductors.
The unit that we use for resistance is called the Ohm in honour of Georg Ohm. The symbol for the ohm is ? , so five ohms is written as 5 ?. The equation to measure the resistance (Ohm’s law) is: R = V Resistance = Voltage (Volts).
I Current (Amps) The more resistance there is the more electrical energy is converted to other forms of energy, such as heat and light energy. Hence, raising the temperature because the electrons in the substance are colliding, causing kinetic energy. Current is proportional to the voltage, providing that the temperature stays constant. When temperature is constant the current is proportional to the electric field. Conductors that obey the Ohm’s law are called ohmic or linear conductors because they produce straight-line graphs. Resistivity is mainly a fundamental characteristic of the material itself.
Some substances are bad conductors and some are excellent conductors. There are two types of circuits: Series circuit Parallel circuit In a series circuit, the current is shared and the voltage stays the same. In a parallel circuit, the voltage is shared and the current stays the same. The difference in a series and parallel circuit is that in a series circuit the current has no choice of route whereas in a parallel circuit there can be many choices of routes. If one of the bulbs, in a series circuit, breaks (or goes out) then so will all the remaining bulbs.
However, in a parallel circuit if one of the bulbs breaks none of the remaining bulbs are affected. A short circuit is when the current avoids a path with resistance, possibly one that contains a bulb, and takes the easiest route, providing that there is another wire at two points. In a series circuit, the resistance is calculated using the formula: R = R1 + R2 + R3 … In a parallel circuit, the resistance is calculated using the formula: R = 1 + 1 + 1 R1 R2 R3 The formula to calculate resistance is R = s l a ‘R’ is the resistance; ‘s’ is the resistivity; ‘l’ is the length; and ‘a’ is the cross-sectional area.
Another formula to calculate the resistance is Rt = R0 ( l + ? t ) ‘Rt’ means the resistance at ti?? C and ‘R0’ means the resistance at 0i?? C. Ohm’s law does not always apply. The current and voltage may not be proportional. The material obviously gets hotter and hotter. Since we can measure the resistance by the gradient of the graph, we have here an example where the resistance is increasing. A heat-dependant resistor or thermistor gives the opposite pattern. A thermistor is a device with a resistance that varies with its temperature is called a thermistor.
Its resistance decreases as the temperature increases. This is because the number of electrons carrying the current remains constant while the temperature rises. Several factors affect the resistance of a wire. They include: Length Cross – sectional area (thickness) Type of material I will be investigating all three factors. I am going to use two types of wires in my investigation. They are: > Constantan > Nickel Chrome (OR Nichrome) Hypothesis and Prediction In my introduction I stated that if the resistance of the wire were high the current would also be high.
The graph below shows my secondary source of data from ‘Letts Science Single and Double Award’. Ohm used his results to plot a graph of voltage (V) against current (I). The results gave a straight line through the origin (O). This shows that the current is directly proportional to the voltage because the temperature is constant. Such conductors are called ohmic or linear conductors. The resistance of an ohmic conductor therefore does not change when the voltage does. This led to Ohm’s law: ‘The voltage across a metal conductor is proportional to the current through it, provided that the temperature stays constant’.
Ohm’s results show that doubling the voltage doubles the current. The larger the resistance the greater the voltage needed to push each ampere of current through it. This means that the voltage is proportional to the current. This led to a definition of one ohm: ‘A resistor has a resistance of 1 ? , if a voltage of 1 V will drive a current of 1A through it’. The resistance of a resistor is the voltage per unit of current. However, thermistors do not obey the Ohm’s law. This is because their temperature starts to increase at a certain temperature. So, a material may or may not obey the Ohm’s law.
Below shows my secondary source of information from ‘GCSE Key Stage 4 Longman Study Guides Physics’ of a typical thermistor graph. As we can see it is a curved graph. It shows the resistance increasing then decreasing rapidly or coming to a halt. This change in resistance is due to the temperature increase. A graph of a thermistor Ohm’s law applies to all electric circuits for both direct current (DC) and alternating current (AC), but other principals must be raised for the analysis of complex circuits and for AC circuits also involving inductances and capacitances.
The resistance of an object is determined by the nature of the substance of which it is composed, known as the resistivity. Resistivity is expressed in terms of the ohms resistance per cubic centimetre of the substance at 20i?? C (68i?? F). As the current flows, it comes upon a certain amount of resistance from the conductor and any resistors in the circuit. Each material has a characteristic resistance. For example, wood is a bad conductor because it offers high resistance to the current; copper is a better conductor because it offers less resistance.
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The Ohm’s law. (2017, Oct 17). Retrieved from https://studymoose.com/the-ohms-law-essay