Flashcards in Chapter 21 Deck (25):
What is the Basic Energy Model?
Every system in nature has a quantity we call its TOTAL ENERGY (E)
- The total energy (E) is the sum of the energies present in the system
What are the different forms of energy?
1. kinetic energy
2. gravitational potential energy
3. elastic potential energy
what is kinetic energy?
energy of an object of mass (m) moving with speed (v)
K = 1/2mv^2
what is gravitational potential energy?
energy of an object of mass (m) at height (y) (assuming Ug = 0 when the object is at y = 0)
Ug = mgy
what is elastic potential energy?
energy of a spring displaced (x) from equilibrium (assuming Us = 0 when the end of the spring is at x = 0)
Us = 1/2kx^2
Energy can be transformed into energy of another kind within a system
Energy Transferred and work?
energy can be transferred between a system and its environment through WORK and HEAT
what is the work-energy equation?
- work is the energy transferred in or out of a system mechanically, by pushing or pulling
- The total energy of a system changes by the amount of work done on it
- work can increase or decrease the energy of a system
ΔE = ΔK + ΔUg + ΔUs + ΔEth + ΔEchem +... = w
what is work?
- work is done on a system by external forces
- work is scalar
- SI unit is the Joule; 1 joule = 1J = 1Nm
How are work an electric potential energy related?
The electric potential energy of a charge when it is at a particular position is HOW MUCH WORK IT TOOK to move the charge to that position
How are energy and charge related?
Think of a mass in free-fall:
- a positive charge "falls" in the direction of the electric field
- a negative charge "falls" in the opposite direction of the electric field
When do you do positive work (gain energy/speed)?
- When a positive charge moves against the electric field
- when a negative charge moves in the direction of the
when do you do negative work (lose energy/speed)?
- when a positive charge moves in the direction of the electric field
- when a negative charge moves against the electric field
What is the potential energy of a system with multiple point charges?
For a system of many point charges, the total potential energy is equal to the work that must be done by an external agent to assemble the system from infinity
what is the potential energy for a system with two point charges?
- For a system of two point charges, the total potential energy is
U = Kq1q2/r1*2
- if q1 and q2 are LIKE charges, POSITIVE work must be done by an external agent to bring the charges together
- if q1 and q2 are OPPOSITE charges, NEGATIVE work must be done by an external agent to bring the charges together
what is electric potential?
Just as the electric field is the force per unit charge, electric potential is the electric potential energy per unit charge:
ΔV = ΔU/q
- Potential is NOT the sane as potential energy
- BOTH potential energy and potential are scalars
what is the electron volt?
The electron volt (eV) is the energy that an electron gains when accelerated through a potential difference of 1V
1 eV = 1.6x10^-19J
What is the electric potential (potential) due to a point charge?
The potential created by a point charge (q) at any distance (r) from the charge is
V = Kq/r
- the point charge creates a potential exist everywhere in space around it
- for systems of many point charges, the total potential is just the sum of potentials due to every point charge
what are equipotential lines?
- contour lines are lines of constant elevation
- lines close together indicate steep changes in elevation
- If we think of previous 3D plots of potential as mountains, and slice them across horizontally, we would get contour lines
- we call these equipotential lines
Equipotential lines have?
have physical meaning as well being helpful for visualizing
Mechanical energy is conserved in the presence of which of the following types of force?
What is the formula that relates Potential Difference (V) to Electric Field (capacitance)?
Q = C*V or E = Q/(Eo*A) or V = E*d
A = area
Eo = 8.85*10^-12 C^2/N*m^2
What are some formulas to solve for capacitance?
C =Q/V or C =Eo*k*A/d or C =Eo*A/d
A = area
Eo = 8.85*10^-12 C^2/N*m^2
C = Capacitance
V = Potential Difference
k = dielectric Constant
What is the formula for Potential Energy that links Capacitance and Potential Difference (Capacitance Potential Energy)?
Uc = 1/2CV^2