How can ohms law correctly be expressed

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. A voltmeter is a device equipped with probes that can be touched to two locations on a circuit to determine the electric potential difference across those locations. By altering the number of cells in the battery pack, the electric potential difference across the external circuit can be varied.

This can be compared to the manufacturers stated value to determine the accuracy of the lab data and the validity of the Ohm's law equation. The tendency to give attention to units is an essential trait of any good physics student. Many of the difficulties associated with solving problems may be traced back to the failure to give attention to units. As more and more electrical quantities and their respective metric units are introduced in this unit of The Physics Classroom tutorial, it will become increasingly important to organize the information in your head.

The table below lists several of the quantities that have been introduced thus far. The symbol, the equation and the associated metric units are also listed for each quantity. It would be wise to refer to this list often or even to make your own copy and add to it as the unit progresses. Some students find it useful to make a fifth column in which the definition of each quantity is stated.

The next section of Lesson 3 will involve taking another look at the quantity power. A new equation for power will be introduced by combining two or more of the equations in the above table. Which of the following will cause the current through an electrical circuit to decrease? Choose all that apply. Answers: A and D. The current in a circuit is directly proportional to the electric potential difference impressed across the circuit and inversely proportional to the resistance of the circuit.

Reducing the current can be done by reducing the voltage choice A or by increasing the resistance choice D. A certain electrical circuit contains a battery with three cells, wires and a light bulb. Which of the following would cause the bulb to shine less brightly?

Answers: B and D. The bulb will shine less brightly if the current in it is reduced. Reducing the current can be done by reducing the electric potential difference impressed across the bulb choice B or by increasing the resistance of the bulb choice D. You have likely been warned to avoid contact with electrical appliances or even electrical outlets when your hands are wet. Such contact is more dangerous when your hands are wet vs. Wet hands have less resistance and thus less hindrance to the flow of charge; the current would thus be increased.

Touching an outlet with wet hands increases the risk of charge flowing through you and causing electrical shock or even electrocution. Current is inversely proportional to the resistance. A threefold increase in the resistance would cause a threefold decrease in the current. Current is directly proportional to the voltage. A fourfold increase in the voltage would cause a fourfold increase in the current. A circuit is wired with a power supply, a resistor and an ammeter for measuring current.

The ammeter reads a current of 24 mA milliAmps. Determine the new current if the voltage of the power supply was But current is also inversely proportional to the resistance; a doubling of the resistance will halve the current.

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.