Drift velocity and power

Question 1

Describe the motion of the electron in a metal when there is no current flowing.

Answer 1

When there is no current flowing in a metal, the electrons are moving about 1. rapidly 2. with a range of speeds, in 3. random directions.

Question 2

Describe the motion of the electrons when a potential difference is applied across a metal.

Answer 2

When a potential difference is applied across a metal 1. an electric field is set up and 2. the free electrons begin moving under the influence.
3. As the free electrons accelerate they collide continuously with metal ions.

Question 3

Define drift velocity.

Answer 3

Drift velocity is the mean value of the velocity of the electrons in a conductor when an electric field is applied. It is made up of the velocity due to the influence of the electric field as well as the random motion when no electric field is applied.

Question 4

What is the relationship between I - current flowing in the conductor/A, n- the number of free electrons per unit volume/m^3, e - the charge on each electron, C and v - the mean drift velocity of the electrons/ms^-1 in a metallic conductor of length, L and cross-sectional area, A.

Answer 4

Volume of section = AL

number of electrons in section = nAL

Total amount of charge flowing = number of electrons x e = nALe

Time take for electrons to travel from one end of the section to the next = L/v

Electric current, I = Q/t = nALe/(L/v)

I = nevA

Question 5

What is the relationship between W - the energy dissipated in a time t, Q = the charge that flowed during a time t and V = the potential difference across a resistor.

Answer 5

V = W/Q

Question 6

Write an equation for W, the energy dissipated as the current, I flows through a resistor in t, time/s and V, potential difference across the resistor.

Answer 6

V = W/Q

Q = It

W = QV

W = ItV

Question 7

Define power.

Answer 7

Power is defined as the rate at which energy is converted.

P = W/t

Question 8

What is the SI unit of power.

Answer 8

The SI unit of power is the watt (W).

Question 9

Show and explain how power is the product of current and potential difference between the terminals of a resistor..

Answer 9

Since P = W/t

and W = ItV

P = ItV/t

so P = IV

Question 10

Show and explain another equation for power using current and resistance.

Answer 10

since V = IR

and P = IV
P = I(IR)
P = I^2 R

Question 11

Show another equation for power using potential difference and resistance.

Answer 11

V = IR and I = V/R

P = IV
P = (V/R)V
P = V^2/R