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### Course: MCAT>Unit 9

Lesson 10: Electrostatics

# Voltage

Explore the distinction between electric potential energy and electric potential, often confused terms in electrostatics. Understand how electric potential, or voltage, is associated with position, while electric potential energy is tied to a charge. Reinforce your understanding of these concepts through examples involving charged plates and electric fields. Created by Sal Khan.

## Want to join the conversation?

• at you say the charge closer to the plate has a higher PE. Why is this if the E field is uniform? if F= qE then wouldnt the force be the same at both pts?
• To relate this to gravity, the gravitational force=mass*gravity at the Earth's surface. If you raise the height of an object, you are increasing the potential energy, but you are not changing the force. The force is always mg near the surface of the Earth, but the equation PE=mgh is changing only because h (height) is changing.
• I had a question. My physics teacher actually tried to explain but I still didn't understand. Suppose, my car's battery is dead and I need to start the car. My car's battery voltage is 12V, so if I were to use a regular 9V battery used in, let's say, in a flashlight (suppose, I put two 9V batteries in series to make them 18V); why wouldn't I be able to start the car?

I understand that the voltage depends on electrical field's strength in batteries, which is proportional to the charge density. So, if I have more charge I should have more Voltage.
Since the voltages are approximately the same in the scenario I mentioned, can I safely assume that they have the same charge?
Please, explain in detail what I am really missing here.
• Batteries have limitations on the amount of current they can supply. A car battery can supply a large current whereas a 9V flashlight battery can only supply a small current, even though the voltage ratings are similar. For the car to start, it requires a certain amount of power which is current times voltage. If the voltage is there, but the current is too low, the car will not start.
• Won't you have to exert more than 6N to push the charge inwards, since 6N will just counteract the Electric Force and hence it will stay stationary?
• At first, there would actually be more than 6N to push the charge inwards (thus creating acceleration and therefore causing the charge to move). However, the charge needs to stop and the slowing down requires negative acceleration, one that is caused by decrease in force exerted and creates a negative net force. Because we need more than 6N in beginning and less than 6N at the end, the average force applied for the total work is 6N.
I hope this helped.
• at , since we divide by the charge, can we define electric potential as electric field times distance?
• Yes! V=E.d where V is potential , E is field and d is distance.
• so what does it mean when we say like "this is a 1.5 voltage cell" or "thats a 220 volt battery"?
• 1.5 volt battery just means that if you connect the battery to a circuit it will create a potential difference of 1.5 V across the conductor.It simply means that it will give you a energy of 1.5 J per every coulomb of charge.
• I have quite a few questions because I have always had trouble with voltage, I have watched the circuit videos and most of my questions relate to circuits but they would be more fitting here. How is voltage used up in a circuit? Why is voltage the same everywhere you measure in just a normal circuit (one wire connected to a battery)? How does a voltmeter work and how does it measure the voltage used by an appliance such as a light bulb? How does voltage split equally for appliances in a series circuit but is equal in parallel?
If someone can answer just a few of these questions I will be extremely grateful! Voltage really gets me confused
• Voltage is not used up in a circuit. Voltage is an electric potential difference, just like you have a gravitational potential difference between the top and bottom of a hill. A ball rolling down the hill doesn't use up gravity and a charge moving along a electric potential doesn't use up that potential. The difference in potential means that a charge at the higher potential has more energy than it does at the lower potential, it is this energy that is used by the circuit.

To have an electrical potential difference (voltage) you need to have a separation of charges. A conductor with no resistance carries the same potential throughout the entire conductor because any difference in potential will be equalized because there is no resistance to the flow of charge.

A volt meter is just a sensitive current meter in series with a resister a very high resistance. When you put a voltage meter across a potential you are basically measuring the current through a known resistance, since you know the resistance in the meter you know the voltage needed to cause the current. The resister in the voltage meter needs to be very high to minimize its effect on what you are measuring.

In a parallel circuit you have the same conductor connected to the Voltage source and one end of each element of the circuit. Since the Voltage is the same along a conductor it has to be the same across each element in the circuit.

WIth a series circuit lets look at the current instead of the voltage. Since there is one path through all of the elements the current has to be the same through all of them. Since voltage is proportional to current and resistance and we know the current is the same the voltage across each element is directly proportional to each elements resistance.
• What is the exact definition of electric potential?
• The work done per unit positive charge in moving a small positive test charge from infinity to a point in an electric field.
• Sal said at that electric potential tells us how much work is required to move per unit of charge from one place to another. So if I increase the charge of the charged particles to a hundred coloumbs, will the electric potential remains the same?
• Yes electric potential will remain the same.By definition, it is the word done per unit charge to bring to a position with respect to a charge, from infinity, or in other words, the word done to bring a UNIT POSITIVE CHARGE from infinity to that position.Thus, electric potential is a property of a POINT(or position) in an electric field.-So it depends only on the electric field at the point
• Is there any difference between Potential Difference and Voltage ?
• Voltage is a somewhat vague term that we don't really use in physics, but when electrical engineers use it, yes, they are referring to a potential difference.