Current, Resistance and Electromotive Force

Question 1

What is the current?

Answer 1

The rate of charge flow through a specific area
I=dQ/dt

Question 2

What is the direction of the current?

Answer 2

Direction is given by the flow of positive charges

Question 3

What is the current density vector?

Answer 3

A measure of the current per unit cross-sectional area
J=I/A=nqv_d

Question 4

What is the direction of the current density vector?

Answer 4

In the same direction as the electric field

Question 5

What is the speed of an electron in a wire?

Answer 5

v_d=I/(abs(q)nA

Question 6

What is the equation for electrical resisitivty?

Answer 6

ρ =E/J

Question 7

What is the resistivity of a good conductor?

Answer 7

Low resistivity

Question 8

What is the resistivity of an insulator?

Answer 8

High resistivity

Question 9

What is the electrical conductitvy?

Answer 9

The reciprocal of the resistivity
σ =1/ρ

Question 10

What is the conductivity of a good conductor?

Answer 10

High conductivity

Question 11

What is the conductivity of an insulator?

Answer 11

Low conductivity

Question 12

What is the equation for electrical resiatnce?

Answer 12

R=ρL/A

Question 13

What is the equation of Ohm’s Law?

Answer 13

V=IR

Question 14

How do resistors add in series?

Answer 14

R_eq=R_1+R_2

Question 15

How do resistors add in parallel?

Answer 15

1/R_eq=1/R_1+1/R_2

Question 16

What is the power for a general circuit element?

Answer 16

P=IV_ab
P=VI=RI^2=V^2/R

Question 17

What is Ohmic heating?

Answer 17

The power dissipated by a resistor
P=Ri^2

Question 18

What is the power per unit volume?

Answer 18

P/V=ρJ^2

Question 19

What is the Force inside the battery?

Answer 19

F=-qE

Question 20

What is the electromotive force (emf)?

Answer 20

The work done by the force per unit charge

Question 21

What is the output voltage of a battery?

Answer 21

V= ϵ − rI
where r is the internal resistance