Chapter 9: Current and Resistance

Question 1

What is electricity?

Answer 1

The movement of electrons.

Question 2

What is the definition of current (I)?

Answer 2

The current is the amount of positive charge passing some location each second.

Question 3

What are the units of current (I)?

Answer 3

Amps or Amperes (A), which is the same thing as Joules per second (J/s).

Question 4

Which direction does the current move relative to the motion of the electrons?

Answer 4

The current points in the direction opposite the motion of the electrons.

Question 5

What is the definition of the drift velocity?

Answer 5

The drift velocity is the net motion of electrons over time through a conductor.

Question 6

Why is the drift velocity so much smaller than the instantaneous electron velocity?

Answer 6

Electrons bounce around very quickly (~1,000,000 m/s!) but constantly collide with atoms in a conductor, so their net overall motion in some direction is actually very small in comparison (~0.001 m/s), like balls in a ping pong machine.

Question 7

What does resistivity (rho) mean?

Answer 7

The resistivity (rho) is the ability of some specific material (like gold or copper) to resist the flow of electrons.

Question 8

What does resistance (R) mean?

Answer 8

The resistance (R) is the ability of a specific component (such as a resistor made up of multiple materials with their own resistivities) to resist the flow of electrons.

Question 9

What are the units of resistivity (rho)?

Answer 9

Ohms per meter (Omega/m).

Question 10

What are the units of resistance (R)?

Answer 10

Ohms (Omega).

Question 11

Does resistivity (rho) change with temperature?

Answer 11

Yes, resistivity (rho) increases with temperature according to an equation on the equation sheet.

Question 12

Does resistance (R) change with temperature?

Answer 12

Yes, resistance (R) increases with temperature. To find how, use the change of resistivity (rho) with temperature, where R is proportional to rho.

Question 13

What is Ohm’s Law?

Answer 13

V = I R

V = voltage difference between two points (such as the terminals on a battery) in units of Volts (V)

I = current in units of Amps (A)

R = resistance in units of Ohms (Omega)

Question 14

What is the definition of power (P)?

Answer 14

Power = energy / time

Question 15

What are the units of power (P)?

Answer 15

Watts (W) where a Watt = Joules / second (W = J/s)

Question 16

Is a kilowatt hour (kW hr) a unit of energy or a unit of power?

Answer 16

A unit of energy

Power = energy / time,
so energy = power * time

1 kW hr = 1,000 W * 3600 seconds = 3.6 x 10^6 J