Potential Energy and Potential Difference
What is the potential difference?
Work required to move a charge between two points in an electric field divided by the charge?
What is the purpose of a capacitor? Name 3 devices that use capacitors.
A capacitor is used to store electric potential energy by separating the positive and negative charges on two conducting parallel plates.
Cameras, TAZOR or cattle prod, Radio, Microwave, X-Ray machine, MRI Machine, Electric motor.
What is an electric current? What causes an electric current?
Electric current is the rate at which charge flows through a conductor.
A potential difference (electric field) causes charges to flow.
What is meant by resistance?
opposition to electric current by a material or a device.
What is electric power? What are its units? What are two formulas used to find it?
Electric Power is the rate at which electric carriers do work.
Unit is Watts (W)
P = IV and P = V2/R
What is the point of using potential difference instead of potential energy?
It is possible for charge to change as it moves through an electric field (I.E. circuits discharge) so Potential difference is usually a better indication of work required to move charges since it disregards the magnitude of the charge.
What is required for a capacitor to be discharged?
A capacitor is discharged when a conductor connects the charged plates.
In which direction does conventional current flow?
Conventional current is defined as the direction that positive charges flow. Conventional current flows opposite the direction of electrons in a conductor. (I.E. from positive to negative terminal in a battery)
How are potential difference and current related in a circuit if the resistance is constant?
For a constant resistance, current is directly proportional to potential difference.
Name five different types of sources of potential difference. Name the type of energy that is converted to electrical energy in each case.
Piezoelectric Cell - energy from pressure on crystal
Photovoltaic Cell - Light energy
Thermal Electric Cell - Thermal energy
Electro-chemical Cell - Chemical energy
Electromagnetic Induction - electromagnetic energy
The distance a charge needs to travel doubles, and the electric field triples. How does the new potential difference compare to the original?
The new potential difference is 6x greater than the original.
What is a dielectric and what is its purpose in a capacitor?
Dielectric is an insulator placed between the two parallel plates in a capacitor.
By using an insulator (dielectric) between the plates a larger charge can be placed on the plates.
What is meant by the drift velocity of a current?
Drift velocity is the average velocity of an electron in a conductor. It is only a few millimeters per second.
Name four factors that the resistance of a conductor depends upon.
Length of conductor, Cross-sectional area of conductor, material, temperature of conductor.
A TV operates at a potential difference of 120 V with a resistance of 75 Ω. What is the power output of the TV? What is the current through the TV
P = 190 W
I = 1.6 A
A particle with a charge of 7.5 x 10-12 C moves 7.0 cm along an electric field of strength of 75 N/C. How much does its potential energy change? What is the potential difference?
ΔPE = -3.9 x 10-11 J
V = -5.2 V
A capacitor is made of two plates with an area of .0062 m2. The plates are separated by a vacuum at a distance of 3.7 mm. Find the capacitance.
C = 1.5 x 10-11 F
The current passing through a light bulb is .90 A. How long would it take a total charge of 2.5 C to pass through the bulb?
t = 2.8 s
What is the purpose of an ohmic resistor? What is the purpose of a nonohmic resistor?
Ohmic: Constant Resistance; controls magnitude of current.
Nonohmic: Varying resistance; controls direction of resistance.
A 120.0 W light bulb draws a current of .7456 A. Find the resistance of the light bulb and the required potential difference through the light bulb.
R = 215.9 Ω
V = 160.9 V
A particle with a charge of 7.5 nC moves 1.5 m along an electric field. What is the potential difference between the two positions?
V = 45 V
A parallel-plate capacitor has a charge of 7.0 nC when charged to a potential difference of 9.0 V. Find its capacitance. How much electric potential energy is stored?
C = 7.8 x 10-10 F
PE = 3.2 x 10-8 J
The current in a wire is .45 A. How long would it take 0.75 C of charge to pass through the wire? How many electrons pass through the wire in 5.0 seconds?
t = 1.7 s
1.4 x 1019 electrons
A 9.0 V battery is connected to a circuit and the current of .95 A flows. What is the resistance of the circuit?
R = 9.5 Ω
How much money does it cost to run a desktop computer 10.0 hours per day for a 4.0 day work week with a 550.0 W power supply if the electricity costs the business $0.15 per kilowatt hour?
$3.30