Electricity - Voltage and Current
Voltage
Everyone uses this term, even though they might not understand it. You probably know that a single cell from a flashlight creates an electrical force of 1.5 Volts. You also know that a transistor radio battery (with the snaps on the top) creates an electrical force of 9 Volts. (There are six little cells of 1.5 Volts each inside of the 9 Volt battery.) You might also know that household electrical outlets are ~240 Volts in Australia (and different in other countries eg the US ~ 120 Volts), and you know that "High Voltage" signs represent danger.
What else should you know about voltage? Again, lets look at the atomic level.
Every atom has its own complement of electrons. In a conductor, some of those electrons can jump from atom to atom. But electrons don't move from atom to atom without a reason. When electrons are flowing there is always an electrical force pushing them along. We refer to this force as "Voltage".
Current
Again, everyone uses this term without really knowing what it means. In very simple terms, current is the flow rate of the electrons in the circuit. How is that different from voltage? Let's use a water tank and a pipe as an example.
In some neighborhoods where there might not be water piped into the house from a treatment works, you'll see a water tank raised high above the ground on strong legs. The water in this tank has been raised up there to create pressure in the system. A series of pipes carry the water down from the tank, under ground, into your house, and then to each sink, bathtub, and toilet. The water in your pipes is under pressure because the water in the tank is pushing down on it. This pressure is similar to Voltage. Voltage is the pressure pushing on the electrons in a circuit.
If all of the faucets in your house are closed, no water flows through the pipes. If you open one faucet, some water flows. If you open all of the faucets, a lot of water flows. This flow of water is similar to electrical current. Current is the flow rate of electrons through the circuit.
Are Voltage and Current Related?
Voltage and current are not the same thing, although they are closely related. In simple terms, Voltage causes Current. Given a Voltage and a path for the electrons, current will flow. Given the path, but no Voltage, or Voltage without the path, there will be no current.
A Simple Circuit
Voltage
Everyone uses this term, even though they might not understand it. You probably know that a single cell from a flashlight creates an electrical force of 1.5 Volts. You also know that a transistor radio battery (with the snaps on the top) creates an electrical force of 9 Volts. (There are six little cells of 1.5 Volts each inside of the 9 Volt battery.) You might also know that household electrical outlets are ~240 Volts in Australia (and different in other countries eg the US ~ 120 Volts), and you know that "High Voltage" signs represent danger.
What else should you know about voltage? Again, lets look at the atomic level.
Every atom has its own complement of electrons. In a conductor, some of those electrons can jump from atom to atom. But electrons don't move from atom to atom without a reason. When electrons are flowing there is always an electrical force pushing them along. We refer to this force as "Voltage".
Current
Again, everyone uses this term without really knowing what it means. In very simple terms, current is the flow rate of the electrons in the circuit. How is that different from voltage? Let's use a water tank and a pipe as an example.
In some neighborhoods where there might not be water piped into the house from a treatment works, you'll see a water tank raised high above the ground on strong legs. The water in this tank has been raised up there to create pressure in the system. A series of pipes carry the water down from the tank, under ground, into your house, and then to each sink, bathtub, and toilet. The water in your pipes is under pressure because the water in the tank is pushing down on it. This pressure is similar to Voltage. Voltage is the pressure pushing on the electrons in a circuit.
If all of the faucets in your house are closed, no water flows through the pipes. If you open one faucet, some water flows. If you open all of the faucets, a lot of water flows. This flow of water is similar to electrical current. Current is the flow rate of electrons through the circuit.
Are Voltage and Current Related?
Voltage and current are not the same thing, although they are closely related. In simple terms, Voltage causes Current. Given a Voltage and a path for the electrons, current will flow. Given the path, but no Voltage, or Voltage without the path, there will be no current.
A Simple Circuit
This picture illustrates a single cell pocket flashlight. The 1.5 Volt cell is pushing the electrons through the bulb and the wire. Without this push, the electrons would be happy to remain stationary. In this case, chemical action within the battery causes the push. When the battery gets old, its chemical reaction slows down and its internal push gets weaker and weaker. (That's why the bulb gets dim.)
Who Does the Work?
Current, not Voltage, does the work in electrical circuits. The flow of water through a turbine is what makes the turbine spin. The flow of current through an electrical circuit is what lights the bulb, heats the stove, runs the motor, etc. Routing and controlling the flow of current is the goal of every electrical circuit.
Resistance
Resistance is the friction in an electrical circuit that controls the flow of current.
As previously mentioned, voltage causes current. When a voltage is present and there is a path (circuit) for electron flow, then there will be a current.
Question: How much current will flow? Answer: As much as can be produced by the pressure (Voltage) working against the friction (Resistance) of the circuit.
If you open all of the faucets in your house, a lot of water will flow, but not enough to instantly empty the water tank on the hill above your house. The faucet valves that the water must flow through tend to limit the flow rate of the water. In a sense, they have friction that works against the water pressure to limit the flow. You can adjust the flow by how far you open each faucet. In electrical terms, the faucet is an adjustable resistor.
Wires, like water pipes, have very low resistance. Other circuit elements are intentionally designed to have specific amounts of resistance in order to accomplish the operating goals of the circuit. Light bulb filaments, stove heating coils, and motor windings all have resistance that regulates the flow of current through them.
Why do we need Resistance?
Every real-world circuit must include an appropriate resistance so that there is an appropriate current flow. Recall the battery and bulb circuit on the voltage/current page. In that circuit the bulb is the primary circuit resistance. Here are three examples of voltage, resistance, and resulting current in that circuit:
1. Not Enough Resistance
If we use a 6 Volt battery and a bulb that is intended for 1.5 Volts, the pressure is too high and the filament resistance allows too much current to flow. We get a single bright flash as the bulb burns out!
2. Too Much Resistance
If we use a 1 .5 Volt battery and a bulb that is intended for 6 Volts, the pressure is too low and the filament resistance allows very little current to flow. The battery lasts for a very long time but the bulb is too dim to be of much use.
3. Exactly Enough Resistance
When we use a 1 .5 Volt battery and a bulb that is also intended for 1 .5 Volts, the current is just right. The bulb is bright, but not doesn't burn out, and the battery lasts for quite a while.
Ohm's Law
What is the exact relationship between Voltage, Current, and Resistance?
Here are two statements that describe the relationship:
1. Given a fixed Resistance, more Voltage causes more Current.
2. Given a fixed Voltage, more Resistance causes less Current.
(Before proceeding I need to remind you that we use Volts for electrical pressure, Amps for electrical current, and Ohms for resistance.)
Ohm's Law Interactive
Who Does the Work?
Current, not Voltage, does the work in electrical circuits. The flow of water through a turbine is what makes the turbine spin. The flow of current through an electrical circuit is what lights the bulb, heats the stove, runs the motor, etc. Routing and controlling the flow of current is the goal of every electrical circuit.
Resistance
Resistance is the friction in an electrical circuit that controls the flow of current.
As previously mentioned, voltage causes current. When a voltage is present and there is a path (circuit) for electron flow, then there will be a current.
Question: How much current will flow? Answer: As much as can be produced by the pressure (Voltage) working against the friction (Resistance) of the circuit.
If you open all of the faucets in your house, a lot of water will flow, but not enough to instantly empty the water tank on the hill above your house. The faucet valves that the water must flow through tend to limit the flow rate of the water. In a sense, they have friction that works against the water pressure to limit the flow. You can adjust the flow by how far you open each faucet. In electrical terms, the faucet is an adjustable resistor.
Wires, like water pipes, have very low resistance. Other circuit elements are intentionally designed to have specific amounts of resistance in order to accomplish the operating goals of the circuit. Light bulb filaments, stove heating coils, and motor windings all have resistance that regulates the flow of current through them.
Why do we need Resistance?
Every real-world circuit must include an appropriate resistance so that there is an appropriate current flow. Recall the battery and bulb circuit on the voltage/current page. In that circuit the bulb is the primary circuit resistance. Here are three examples of voltage, resistance, and resulting current in that circuit:
1. Not Enough Resistance
If we use a 6 Volt battery and a bulb that is intended for 1.5 Volts, the pressure is too high and the filament resistance allows too much current to flow. We get a single bright flash as the bulb burns out!
2. Too Much Resistance
If we use a 1 .5 Volt battery and a bulb that is intended for 6 Volts, the pressure is too low and the filament resistance allows very little current to flow. The battery lasts for a very long time but the bulb is too dim to be of much use.
3. Exactly Enough Resistance
When we use a 1 .5 Volt battery and a bulb that is also intended for 1 .5 Volts, the current is just right. The bulb is bright, but not doesn't burn out, and the battery lasts for quite a while.
Ohm's Law
What is the exact relationship between Voltage, Current, and Resistance?
Here are two statements that describe the relationship:
1. Given a fixed Resistance, more Voltage causes more Current.
2. Given a fixed Voltage, more Resistance causes less Current.
(Before proceeding I need to remind you that we use Volts for electrical pressure, Amps for electrical current, and Ohms for resistance.)
Ohm's Law Interactive
All material on this site originally accessed from http://www.reprise.com/host/electricity/voltage.asp