# Let's go back to junior physics for a second

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 tarix 18.04.2018 ölçüsü 439 b. #39192

• ## What is gravitational potential energy?

• Energy that depends on an object's mass and its position relative to some point
• i.e. To calculate someone's potential energy relative to the surface of the Earth you'd need mass, g and height above the surface

• ## The idea of electric potential energy is similar to that of gravitational potential energy

• Electric potential energy for a charge is calculated based on the magnitude of the charge and its position relative to some point

• ## If you place a proton in that field, what is the magnitude and direction of the force acting on that proton?

• F=qE=7.2 x 10-16 N
• Right

• ## What will happen to the proton's kinetic energy and electric potential energy?

• Kinetic Energy will increase
• EPE will decrease (conservation of energy)

• ## By convention, the positive plate is at a higher potential than the negative plate

• Positively charged objects move from higher potential to lower potential (i.e. towards negative plate)
• Negatively charged objects move from lower potential to higher potential (i.e. towards positive plate)

• ## Electric Potential, V, is the potential energy per unit charge

• Unit is Volts (1 V= 1J/1 C)

• V= PE/q

• ## An electron in a television set is accelerated from rest through a potential difference Vba=+5000 V

• What is the change in PE of the electron?
• What is the speed of the electron as a result of the acceleration?
• Repeat for a proton that accelerates through a potential difference of -5000 V

• ## ΔPE = -8 x 10-16 J

• Potential Energy was lost!

• ## Conservation of Energy!

• The amount of PE lost, must be equal to the amount of KE gained!

• ## Since potential energy is always measured relative to some other point, only differences in potential energy are measurable

• Potential Difference is also known as voltage

• ## Vab=Va-Vb= -Wba/q

• The potential difference between two points a and b is equal to the negative of the work done by the electric force to move the charge from point b to point a, divided by the charge

• ## We're moving the proton from +100 V to -50 V

• Therefore point A is +100 V, point B is -50 V

• ## E=Vba/d

• d is the distance between the plates

• ## The electric potential at a distance r from a single point charge q is : V=kQ/r

• Potential is zero at infinity

• ## What is Vb?

• Vb=KQ/r=(9x109 Nm2/C2)(20x10-6C)/0.500m
• Vb= 360,000 V

• ## Total V= -46800 V

• This is much easier! No directions...just make sure you include the sign!

• ## Amount of charge for each plate:

• C= Capacitance of the capacitor (different for each capacitor)
• Unit for C is farad (F)

• ## A= Area of plates

• If A increases, C increases

• ## What happens to the rest of the charge?

• It must be on capacitor 2 because charge is conserved
• Since the two capacitors are connected, the voltage for the second one must also be 15 V

• ## C2 and C3 are connected in parallel

• Combine them into one capacitor

• ## How much charge is stored on each capacitor?

• Q=CV
• V1= 50 V (this is the voltage across C1)

• ## The charge on C3 is:

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