Exam 1 Chapters 19-22 Flashcards
What is the difference between a conductor and an insulator?
Conductor- Usually metal, has 0 electric field, has free charges
Insulator- Usually non metal, has an electric field, has NO free charges
What does Coulomb’s law state?
Where
F= ma (mass x acceleration)
Q and q= Charges
r= Distance between the two charges
k= 8.99 x 10^9 (N x m^2/C^2)
The attractive force between the point charges 8.44E-6C and Q has a magnitude of 0.975N when the separation between the charges is 1.31m Find the sign and magnitude of the charge Q.
Q= 2.2E-5
Given that Q1= +12uC and d= 0.16m, find the direction and magnitude of the net electrostatic force exerted on the point charge q1
F1= F(1,2) + F(1,3)
F1= [k(+q)(2q) / (d^2)] + [k(+q)(3q) / 2(d^2)]
F1= [8.99E9(1 x 12E-6)(-2 x 12E-6) / (0.16^2)] + [8.99E9(1x 12E-6)(3x 12E-6) / (2 x0.16^2)]
F1= -101.13 + 37.89
F1= 63.24 (Would be negative to specify direction)
Suppose that a charge q2 can be moved left or right along the line connecting the charges q1 and q3. Given that q= +12uC, find the distance from q1 where q2 experiences a net electrostatic force of 0. The charges q1 and q3 are separated by a fixed distance of 0.32m.
Distance between 1 and 2= ? (X)
Distance between 1 and 3= (.32m-X)
F2= F(2,1) + F(2,3) BUT where F2=0
0= [k(-2q)(1q) / x^2] + [k(-2q)(3q) / (0.32m-x)^2]
- [k(-2q)(1q) / x^2] = [k(-2q)(3q) / (0.32m-x)^2]
(k’s and -2q’s cancel eachother out)
- [(1q)(0.32m-x)^2] = [(3q)(x^2)] > Do 3q/q= 3 and take square root
- (0.32m-x) = sqrt3(x)
x= (0.32m)/ 1 +- sqrt 3
x= 0.12 and -0.45
Find the orbital radius for which the kinetic energy of the electron in example 19-1 is 1.51 ev.
*1ev = 1 electron volt = 1.6E-19
* F electrical = F centripetal
[k(q1)(q2)/r^2] = ma (centripetal) is equivalent to…
[k(e)^2/r^2] = m (v^2/r) > e is for electron
V^2 = k(e^2)/ (r)(m)
Kinetic energy= (1/2)(m)(v^2)
V^2= (K)(2)/(m) > Set v^2’s equal to each other
k(e^2)/ (r)(m) = (K)(2)/(m)
Where r= k(e^2)/ 2K
r= (8.99E9)(1.6E-19C^2)/ 2(1.51ev)(1.60E-19)
r= 4.76E-10
Four point charges are located at the corners of a square with sides of length a. Two of the charges are +q and two are -q. Find the magnitude and direction of the net electric force exerted on a charge +Q, located t the center of the square, for each of the following 2 arrangements of charge; the charges alternate in sign +q,-q,+q,-q as you go around the square; the two positive charges are on the top corners, and the two negative charges are on the bottom corners.
- The first box has a F(net) of zero because the charges cancel out
- F(net)= 2F(x) + 2F(y) where a= Length
F(net)= 2[KqQ/ (a/sqrt2)^2] x sin(45) + 2[KqQ/ (a/sqrt2)^2] x sin(45)
F(net)= [-4(sqrt2)kqQ/ a^2]
What is the equation when charge Q is distributed over a sphere?
What is the magnitude of the electric field produced by a charge of magnitude 7.5uC at a distance of 1m and 2m?
Equation for electric field: E= kq/r^2
- E= [(8.99E9)(7.5)/(1)^2]= 6.74E10
- E= [(8.99E9)(7.5)/(2)^2]= 1.68E10
What is the equation for electric field?
E= k(q)/(r)^2
For point charges, each of magnitude q are located at the corners of a square with sides of length a. Two of the chargess are +q and two are -q. The charges are alternate in sign as you go around the square. The top 2 corners of the square have positive charges (-q,+q). In which case will the electric field at the center of the square have the greatest magnitude? Calculate the electric field at the center of the square for each of these 2 cases.
E= [kq/(1/2)(a^2)]
E1x + E2x + E3x + E4x= [kq/(1/2r^2) 1/sqrt2 +-1/sqrt2 + 1/sqrt2 +-1/sqrt2 = 0
E1y + E2y + E3xy+ E4y= -1/sqrt2 - 1/sqrt2 - 1/sqrt2 - 1/sqrt2 = [-4sqrt2(k)(q)/(r)^2]
The electric field lines surrounding 3 choices are shown. The center charge is q2= -10uC. What are the signs of q1 and q3? Find q3.
- Charges q1 and q3 must be positive
- The charge q1 has 8 lines leaving it, but q2 has 16 lines entering it. Because 8 is half of 16, and since the number of lines entering or leaving a charge is proportional to the magnitude of the charge, the magnitude of q1, is one-half of q2, or 5uC.
- By the reasoning of part b, the magnitude of q3 is 5uC
A surface encloses the charges q1= 3.2uC, q2= 6.9uC, and q3= -4.1uC. Find the electric flux through this surface.
6.8E-12 Nm^2/C
What is the equation for electric flux?
What is the eqaution of electric field?
E= kq/r^2
The cell membrane of a typical nerve cell consists of an inner and an outter wall separated by a distance of 0.10um. The electric fields within the cell membrane is 7.5E5. Approximating the cell membrane as a parallel-plate capacitator, determine the magnitude of the charge density on an inner and outer cell walls
A uniform electric field of magnitude 4.1E5 N/C points in the positive x-direction. Find the change in electric potential energy of a 4.5uC charge as it moves from the origin to the points (0,6m) (6m,0) (6m,6m)
What is the difference between electrical potential energy and electrical potential
An electrical plate capacitor has plates separated by 0.00075m. If the electric field between the plates has a magnitude of a.) 1.2E5 V/m or b.) 2.4E4 N/c, what is the potential difference between the plates?
A typical 12V car battery can deliver 7.5E5C of charge. If the energy supplied by the battery could be converted entirely to KE, what speed would give to a 1400kg car initially at rest?
The electrons in a TV picture tube are accelerated from rest through a potential difference of 25,000V. What is the speed of the electrons after they have been accelerated by this potential difference?
A proton has an initial speed of 4E5 m/s. a.) What potential difference is required to bring the proton to rest? b.) What potential difference is required to reduce the initial speed of the proton by a factor of 2? c.) What potential difference is required to reduce the initial kinetic energy of the proton by a factor of 2?
The hydrogen atom consists of one electron and one proton. In the Bohr model of the hydrogen atom, the electron orbits the proton in a circular orbit of radius 0.529E10 m. What is the electric potential due to the proton at the electrons orbit?
a.) Find the electric potential at point P in the figure. uppose the 3 charges shown are held in place. A fourth charge, with a charge of 6.11uC and a mass of 0.00471kg is released from rest at point p. b.) What is the speed of the fourth charge when it has moved infinitly far away from the other 3 charges?
A 0.40uF capacitor is connected to a 9v battery. How much charge is on each plate of the capacitor?
To operate a given flash lamp requires a charge of 32uC. What capacitance is needed to store this much charge in a capacitaor with a potential difference between its plates of 9V.
A parallel-plate capacitor has plates with an area of 0.012m^2 and a separation of 0.00088m. The space between the plates is filled with a dielectric constant is 2. What is the potential difference between the plates when the charge on the capacitor plates is 4.7uC? Will your answer to part a increase, decrease, or stay the same if the dielectric constant is increased? Explain. Calculate the potential difference for the case where the dielectric constant is 4.
In the Bohr model of the hydrogen atoms, what is the smallest amount of work that must be done on the electron to move it from its circular orbit, with a radius of 0.529E-10m, to an infinite distance from the proton? This value is referred to as the ionization energy of hydrogen.
How much work is required to bring 3 protons, initially infinitly far apart, to a configuration where each proton is 1.5E-15 m from the other two?
Many cells in the body have a cell membrane whose inner and outer surfaces carry opposite charges, just like the plates of a parallel-plate capacitor. Suppose a typical cell membranehas a thickness of 8.1E-9 m, and its inner and outer surfaces carry charge densities of 0.58E-3 C/m^2 and +5.8E-3 c/m^2, respectively. In addition, assume that the material in the cell has a di electric constant of 5.5. Find the direction and magnitude of the electric field within the cell membrane. Calculate the potential difference between the inner and outer walls of the membrane, and indicate which wall of the membrane has the higher potential
A flashlight bulb carries a current of 0.18A for 78 sec. How much charge flows through the bulb in this time? How many electrons?
A bird lands on a bore copper wire cayying a current of 32A. The wire is 8 gauge, which means that its cross-sectional area is 0.0013m^2. Find the difference in potential between the birds feet, assuming they are separated by a distance of 0.06m. Will your answer to part a.) increase or decrease if the separation between the birds feet increases? Explain.
A typical cel membrane is 8nm thick and has an electrical resitivity of 1.3E7 nxm. If the potential difference between the inner and outer surfaces of a cell membrane is 0.075v, how much current flows through a square area of membrane 1um on a side? Suppose the thickness of their membrane is doubled.
It costs 2.6 cents to charge a car batter of a voltage of 12V and a current of 15A for 120 minutes. What is the cost of electrical energy per kilawatt/hour of this location?
Suppose point A is grounded V=0. Find the potential at points B and C.
The tungsten filament of a lightbulb has a resisitence of 0.07 mu. If the filament is 0.27m long, what is its diameter?
Find the power dissapated in a 25mu electric heater connected to a 120-v outet.
Find the equivalent resistence between points A and B for the group of resistors shown.
What is the minimum number of 65mu resistors that must be connected in parallel to produce an equivalent resistance of 11mu or less?
Suppose A is grounded (V=0). Find the potential at points B and C.
An electron moves at right angles to a magnetic field of 0.18T. What is its speed if the force exerted on it is 8.9E-15 N?
Find the radius of an electrons orbit whenit moves perpendicular to a magnetic field of 0.66t with a speed of 6.27E5 m/s.
Charged particles pass through a velocity selector with electric and magnetic fields at right angles to each other, as shown in the figure. If the electricfield has a magnitudeof 450 N/C and the magnitude field has a magnitude of 0.18T, what speed must the particles have to pass through the selector undeflected?
What is the magnetic force exerted on a 2.15m length of wire carrying a current of 0.899A perpendicular to a magnetic field of 0.720T?
The maximum current in a superconducting solenoid can be as large as 3,750A. If the number of turns per meter in such a solenoid is 3,650, what is the magnitude of the magnetic field it produces?
Two parallel wires, each carrying a current of 2.2A in the same direction, are shown in the figure. Find the direction and magnitude of the net magnetic field at points A, B, and C.
