Most of our devices and homes are powered by electricity, and circuits allow this energy transfer to happen. We often think of charge as a particle, but circuits allow us to also think of charge as a fluid. If a container is full of water and has the ability to flow to a lower pressure system, the water flows until the pressure has equalized. If we include a pump into our system, water will continue to flow, continually transferring energy. This is very similar to how a circuit works. If we have the appropriate elements, charge will continue to flow. Circuits explain this ability for charge to flow and transfer energy.
Potential Difference/EMF/Voltage (V)
Previously we said potential difference tells us about the energy required for a positive charge to move from one point to another. It is also sometimes referred to as the electromotive force. In a circuit, this electromotive force is the pump which is pushing charge around the circuit. This constant potential difference is the source of all the energy in the circuit. We know energy cannot be created or destroyed, so when a circuit is used to power your lights, or your TV, that energy must be coming from the EMF which often times takes the form of a battery.
The time dependent rate at which charge flows is a measurable quantity called current. When a conductor is placed in an electric field, the free electrons move towards the higher potential. This electric field is created by the circuit's EMF elements. The greater the voltage, the greater the desire for the charge to flow, the greater the current.
When current flows in a circuit, we know that is is the negatively charged electrons that are moving through the wires. However, the convention is to describe the direction of current as the direction a positive charge would flow. While electrons would move in a circuit from the negative terminal to a positive terminal of a battery. So we would say that the current flows from the positive to negative.
For the scope of our AP Physics 1 class, we will be dealing with Direct Current (DC) Circuits. Direct Current Circuits (as opposed to Alternating Current Circuits) Are circuits in which the charged particles all flow in the same direction.
The goal of a circuit is to transfer energy from the potential difference created by a battery. The elements responsible for this transfer all have resistance. Resistance, as its name implies, is a measure of how difficult it is for charge to flow through a conductor. The physical properties of the conductor determine the measure of its resistance. The properties responsible for resistance are the length of the conductor, the cross-sectional area of the conductor and the conductor's resistivity. Use the simulation to the right to adjust those three properties and see how they affect the resistance.
We said before we can think of current in a circuit like a fluid. Think of a resistor as a straw. If you want to have air pass easily through (low resistance), which would be better a long straw or short straw? As you can see in the simulation, and if you have two straws, a longer straw is more challenging to blow through. Longer resistors increase resistance. Using that same straw analogy, would it be easier for air to pass through a narrow straw or a fat straw? A smaller cross-sectional area means a greater resistance. It is more challenging for charge to move when it has less space to make that happen. Resistivity is a property of the material that describes how easy charge can flow through that substance. Hopefully you notice materials with a high resistivity had a high resistance. Imagine trying to blow molasses through the same straw you were blowing air through. Some substance just have a greater resistance to flow.
- A battery is a constant source of potential difference. This means it will maintain a certain electromotive force that will push current and allow it to flow.
- A wire is a conductor and is assumed to have no resistance. This is a conductor that most likely represents copper in the real world.
- A light bulb is a specific type of resistor that radiates light when a charge is flowing through it. We have moved away from tungsten filaments that glowed red hot due to friction, but the logic remains the same: more current, brighter light.
- A resistor represents anything else utilizing current as a way to transfer energy
- Switches represent purposeful ways to break the circuit and prevent the flow of charge. They can be open (no flow) or closed (charge is free to flow.
Completing the Circuit
Now let's put it all together and take a look at a complete circuit. In order to have a complete circuit, we need conductors (wires), an electromotive force (battery), and a resistor to power (light bulb?). So let's start buy creating a circuit that lights a light bulb. The battery is what drives the current, but in order for charge to flow, it needs access to the positive and negative terminals of the battery. Try to make the circuit using the simulation below.
If you got something like this circuit, congrats! You just figured out how to complete a circuit! This is something that plenty of college graduates are unable to accomplish. If you figured it out, you realized you needed a complete loop. connected by wires.
You have successfully completed a circuit, now feel free to add more elements to your circuit above. Try to make it more complex, add different resistors ans start measuring current and potential difference at various stages. Try to develop a pattern for what is related and how.