Who is ohms law

If voltage drives current, what impedes it? The electric property that impedes current crudely similar to friction and air resistance is called resistance R. Collisions of moving charges with atoms and molecules in a substance transfer energy to the substance and limit current. Resistance is defined as inversely proportional to current, or. Thus, for example, current is cut in half if resistance doubles.

Combining the relationships of current to voltage and current to resistance gives. These include good conductors like copper and aluminum, and some poor conductors under certain circumstances. Ohmic materials have a resistance R that is independent of voltage V and current I. An object that has simple resistance is called a resistor , even if its resistance is small. Figure 1 shows the schematic for a simple circuit. A simple circuit has a single voltage source and a single resistor.

The wires connecting the voltage source to the resistor can be assumed to have negligible resistance, or their resistance can be included in R. Figure 1. A simple electric circuit in which a closed path for current to flow is supplied by conductors usually metal wires connecting a load to the terminals of a battery, represented by the red parallel lines. The zigzag symbol represents the single resistor and includes any resistance in the connections to the voltage source. What is the resistance of an automobile headlight through which 2.

This is a relatively small resistance, but it is larger than the cold resistance of the headlight. As we shall see in Resistance and Resistivity , resistance usually increases with temperature, and so the bulb has a lower resistance when it is first switched on and will draw considerably more current during its brief warm-up period.

Resistances range over many orders of magnitude. Resistance is related to the shape of an object and the material of which it is composed, as will be seen in Resistance and Resistivity.

This expression for V can be interpreted as the voltage drop across a resistor produced by the flow of current I. The phrase IR drop is often used for this voltage. For instance, the headlight in Example 1 above has an IR drop of If voltage is measured at various points in a circuit, it will be seen to increase at the voltage source and decrease at the resistor.

Voltage is similar to fluid pressure. The voltage source is like a pump, creating a pressure difference, causing current—the flow of charge. The resistor is like a pipe that reduces pressure and limits flow because of its resistance. Conservation of energy has important consequences here. The voltage source supplies energy causing an electric field and a current , and the resistor converts it to another form such as thermal energy.

Thus the energy supplied by the voltage source and the energy converted by the resistor are equal. See Figure 2. Figure 2. The voltage drop across a resistor in a simple circuit equals the voltage output of the battery. It can be imagined that the wider the pipe, the more water will flow. The reason for this is that it is easier for more water to flow through a wider pipe than a narrower one - the narrower one presents more resistance to the water flow. Also if there is more pressure into e pipe, then for the same pipe, more water will flow.

Ohm determined that for normal materials, doubling the voltage doubled the current flow for a given component. Different materials or the same materials with different shapes will present different levels of resistance to the flow of current.

Ohm's Law states that the current flowing in a circuit is directly proportional to the applied potential difference and inversely proportional to the resistance in the circuit. In other words by doubling the voltage across a circuit the current will also double. However if the resistance is doubled the current will fall by half. In this mathematical relationship the unit of resistance is measured in Ohms. Ohm's Law formula The Ohm's Law formula or equation is very straightforward.

The formula can be manipulated so that if any two quantities are known the third can be calculated. To help remember the formula it is possible to use a triangle with one side horizontal and the peak at the top like a pyramid. This is sometimes known as the Ohm's law triangle. In the top corner of the Ohms law triangle is the letter V, in the left hand corner, the letter I, and in the right hand bottom corner, R. To use the triangle cover up the unknown quantity and then and then calculate it from the other two.

If they are in line they are multiplied, but if one is on top of the other then they should be divided. In other words if current has to be calculated the voltage is divided by the resistance i.

If the voltage has to be calculated then it is found by multiplying the current by the resistance i. If a voltage of 10 volts is placed across a ohm resistor determine the amount of current that will flow. Looking at the Ohms Law triangle the current is the unknown leaving the voltage and resistance as the known values. Example 2 In a similar way it is possible to use Ohm's Law to find the resistance if the current and voltage are known.

Take, for example, a voltage of 10 volts, and a current 0. Using the Ohm's Law triangle, it can be seen that:. Example 3 Finally, the other combination is that is the resistance and current are known, then it is possible to calculate the voltage expected across the resistance.

Using Ohm's law it can be seen that if the voltage and current were plotted for a fixed resistor or a length of wire, etc, there would be a straight line. It can be seen that doubling the voltage doubles the current that passes through the particular circuit element. On the graph, there are two lines, one for a higher resistance - this one requires more voltage to be applied for a given current flowing.

Accordingly this must have a higher resistance. Conversely the curve for the lower resistance shows a component that requires a lower voltage to be applied for a given current.