Charge and Current Density

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Electric charge and electric current

Electric charge and electric current

In general, the current I can also be described as the amount of charge Q that flows through the conductor in a given time:

I = Δ Q
. Δ t

The unit of the electric charge is C (coulomb). In other words: 1 C Δ 1 As (ampere-second).

Exercise electric charge:

According to the manufacturer's specifications, the battery of a smartphone has a capacity of 3000 mAh. The following consumption values were determined by testing:

Flight mode: 2.5 mA
LTE wireless connection: 5.7 mA
WLAN wireless connection: 4.5 mA

Express the electric charge of 3000 mAh in coulombs.
How many days will the battery last if the phone remains in airplane mode?
How many days will it take for the battery to run out if the LTE and Wi-Fi connections are enabled?
Since the wireless connection is not constant, power consumption also fluctuates during calls. From the recording shown below, determine the power consumption between 0 and 300 seconds.

Exercise electric charge

Show solution

What do you mean by current density?

Current density J is a measure of how much electric current flows through a given cross-sectional area.

. J = I I is current in ampere
. A A is area in m²

Current Density explained

Current density exercise

The cross-section of a supply cable to a three-phase consumer is 2.5 mm². A constant current with a current density of 400 mA/mm² flows through the supply cable.

What is the current I?
What is the charge Q that flows through the supply cable in one second?
What is the number of electrons that flow through the supply cable in one second?

Solution:

I = J ⋅ A = 400 mA / mm² ⋅ 2,5 mm² = 1 A

Q = I ⋅ t = 1 A ⋅ 1 s = 1 C

n = Q / e = 1 C / 1,602 176 634 · 10^-19C = 6,241509... ⋅ 10¹⁸