True or False 1. The larger the E field value inside the conductor is, the large

Question

True or False

1. The larger the E field value inside the conductor is, the larger the current density will be. The ratio of current density over E field is called resistivity of the conductor.

2. If both the radius and length of a cylindrical wire are doubled, its resistance will be halved?

3. The larger the E field value inside the conductor is, the larger the current density will be. The ratio of current density over E field is called conductivity of the conductor.

4. When the voltage from a wall plug produces a current in the coils of a toaster, when it is turned on, current (charge flow) exists in wires and coils. And those wires and coils are NOT conductors in equilibrium.

5. When the potential difference across different wires is fixed, the conductivity determines the current density, the smaller the conductivity, the larger the current density.

6. The current density J describes how much charge passes through a unit of cross-section area per unit time.

7. When the potential difference across different wires is fixed, the resistance determines the current, the smaller the resistance, the larger the current.

8. The current I describes how much charge passes through the conductor’s cross-sectional area per unit time. It has unit 1 A= 1 C/s.

9. The current density J describes how much charge passes through the wire’s cross-sectional area per unit time.

10. If both the radius and length of a cylindrical wire are doubled, its resistivity will _______?

11. The longer and narrow the wire, the smaller the resistance.

12. The resistance R is defined to be the ratio of potential difference V across a conductor to the resulted current I.

13. Wires with different length and shape made with the same material have the same conductivity and resistivity, but different resistance.

14. The current I describes how fast charges flow through a cross-sectional area of a wire. It has unit of m/s.

Explanation / Answer

1. The larger the E field value inside the conductor is, the larger the current density will be. The ratio of current density over E field is called resistivity of the conductor. False

J = sigma *E

J/E = sigma where su=igma is conductivity

2. If both the radius and length of a cylindrical wire are doubled, its resistance will be halved? TRUE

   R = rhoIl/A = rho*l /(pi*r^2) , R1/R2 = l1*r2^2/l2*r1^2

               l2 = 2l1, r2 = 2r1

                   R1/R2 = l1*(2r1^2)/(2l1*r1^2) ==> R1/R2 = 2 ==> R2 = R1/2

3.The larger the E field value inside the conductor is, the larger the current density will be. The ratio of current density over E field is called conductivity of the conductor. True

4.When the voltage from a wall plug produces a current in the coils of a toaster, when it is turned on, current (charge flow) exists in wires and coils. And those wires and coils are NOT conductors in equilibrium. False

5.When the potential difference across different wires is fixed, the conductivity determines the current density, the smaller the conductivity, the larger the current density. False

from the formula J = sigma *E

6.The current density J describes how much charge passes through a unit of cross-section area per unit time. True

   J = I/A

   J = q/(T*A)
7.When the potential difference across different wires is fixed, the resistance determines the current, the smaller the resistance, the larger the current. True , V = iR

8.The current I describes how much charge passes through the conductor’s cross-sectional area per unit time. It has unit 1 A= 1 C/s

   Q= i*t ==> i = Q/T C/s

9.The current density J describes how much charge passes through the wire’s cross-sectional area per unit time. True

       J = I/A
10.If both the radius and length of a cylindrical wire are doubled, its resistivity will halves

11.The longer and narrow the wire, the smaller the resistance. False

       R = rho*l/A

12.The resistance R is defined to be the ratio of potential difference V across a conductor to the resulted current I. True

   V = i*r ==> R = V/I

13. Wires with different length and shape made with the same material have the same conductivity and resistivity, but different resistance. True

14.The current I describes how fast charges flow through a cross-sectional area of a wire. It has unit of m/s. False

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