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

Yüklə 439 b.
 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) • ## The E-field points from left to right • ## An electron between these two plates would move towards the positive plate (left) • ## An electron has the highest potential energy when it's near the negative plate • ## 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)
• ## If a point charge, q, has an electric potential energy at some point a, then the electric potential is

• V= PE/q • ## ΔPE = PEb-PEa=qVba • ## 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!

• ## V=4.2 x 107 m/s • ## V=9.8 x 105 m/s • ## 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 • ## How much work is needed to move a proton from a point with a potential of +100 V to a point where it is -50 V? • ## We're moving the proton from +100 V to -50 V

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

• ## Wba= -2.4 x 10-17 J • ## E=Vba/d

• d is the distance between the plates • ## V= 1.96 x 107 m/s • ## Equipotential lines are always perpendicular to electric field lines • ## Equipotential lines (green) are perpendicular to the electric field lines (red) • ## The electric potential at a distance r from a single point charge q is : V=kQ/r

• Potential is zero at infinity
• ## The potential near a positive charge is large and decreases toward zero at large distances • ## The potential near a negative charge is negative and increases toward zero at large distances • ## What minimum work is required by an external force to bring a charge q = 3.00 microC from a great distance away to a point 0.500 m from a charge Q= 20.0 microC? • ## Use our trusty equation:Vab=Va-Vb= -Wba/q • ## What is Vb?

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

• ## W= 1.08 J • ## When determining the electric potential at a point you can just add the electric potential from each charge, just be sure to include the correct sign of the charge when calculating potential • ## Therefore E is 43200 N/C right • ## V= -18000 N/C • ## Total V= -46800 V

• This is much easier! No directions...just make sure you include the sign! • ## A capacitor stores electric charge and consists of two conducting objects that are placed next to each other but not touching • ## 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

• ## (This is the permitivity of free space) • ## A charged capacitor stores electric energy • ## A 7.7 µF capacitor is charged by a 125 V battery and then is disconnected from the battery. When this capacitor (C1) is connected to a second, uncharged capacitor (C2), the voltage on the first drops to 15 V. What is the value of C2? (Charge is conserved) • ## When the capacitors are connected, the voltage on the first one is 15 V. That means the new charge on C1 is: • ## 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 • ## The voltage across each capacitor is the same  • ## If the capacitors are connected in series, the equivalent capacitance is given by the following expression • ## The charge on each capacitor is the same as the charge on the equivalent capacitor for capacitors in series • ## C2 and C3 are connected in parallel

• Combine them into one capacitor
• ## C23=C2 + C3 = 35 µF • ## C23 and C1 are connected in series • ## How much charge is stored on each capacitor?

• Q=CV
• V1= 50 V (this is the voltage across C1) • ## C1 and C23 are connected in series, therefore the charge on C23 is the same as the charge on C1 • ## C2 and C3 are connected in parallel, therefore: • ## The charge on C3 is: Yüklə 439 b.

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