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  • Define electric current, ampere, and drift velocity
  • Describe the direction of charge flow in conventional current.
  • Use drift velocity to calculate current and vice versa.

Electric current

Electric current is defined to be the rate at which charge flows. A large current, such as that used to start a truck engine, moves a large amount of charge in a small time, whereas a small current, such as that used to operate a hand-held calculator, moves a small amount of charge over a long period of time. In equation form, electric current     I size 12{I } {} is defined to be

I = Δ Q Δ t , size 12{I = { {ΔQ} over {Δt} } ","} {}

where Δ Q size 12{ΔQ} {} is the amount of charge passing through a given area in time Δ t size 12{Δt} {} . (As in previous chapters, initial time is often taken to be zero, in which case Δ t = t size 12{Δt=t} {} .) (See [link] .) The SI unit for current is the ampere    (A), named for the French physicist André-Marie Ampère (1775–1836). Since I = Δ Q / Δ t size 12{I = ΔQ/Δt} {} , we see that an ampere is one coulomb per second:

1 A = 1 C/s size 12{"1 A "=" 1 C/s"} {}

Not only are fuses and circuit breakers rated in amperes (or amps), so are many electrical appliances.

Charges are shown as small spheres moving through a section of a conducting wire. The direction of movement of charge is indicated by arrows along the length of the conductor toward the right. The cross-sectional area of the wire is labeled as A. The current is equal to the flow of charge.
The rate of flow of charge is current. An ampere is the flow of one coulomb through an area in one second.

Calculating currents: current in a truck battery and a handheld calculator

(a) What is the current involved when a truck battery sets in motion 720 C of charge in 4.00 s while starting an engine? (b) How long does it take 1.00 C of charge to flow through a handheld calculator if a 0.300-mA current is flowing?


We can use the definition of current in the equation I = Δ Q / Δ t size 12{I = ΔQ/Δt} {} to find the current in part (a), since charge and time are given. In part (b), we rearrange the definition of current and use the given values of charge and current to find the time required.

Solution for (a)

Entering the given values for charge and time into the definition of current gives

I = Δ Q Δ t = 720 C 4.00 s = 180 C/s = 180 A.

Discussion for (a)

This large value for current illustrates the fact that a large charge is moved in a small amount of time. The currents in these “starter motors” are fairly large because large frictional forces need to be overcome when setting something in motion.

Solution for (b)

Solving the relationship I = Δ Q / Δ t size 12{I = ΔQ/Δt} {} for time Δ t size 12{Δt} {} , and entering the known values for charge and current gives

Δ t = Δ Q I = 1.00 C 0.300 × 10 - 3 C/s = 3.33 × 10 3 s.

Discussion for (b)

This time is slightly less than an hour. The small current used by the hand-held calculator takes a much longer time to move a smaller charge than the large current of the truck starter. So why can we operate our calculators only seconds after turning them on? It’s because calculators require very little energy. Such small current and energy demands allow handheld calculators to operate from solar cells or to get many hours of use out of small batteries. Remember, calculators do not have moving parts in the same way that a truck engine has with cylinders and pistons, so the technology requires smaller currents.

Got questions? Get instant answers now!

[link] shows a simple circuit and the standard schematic representation of a battery, conducting path, and load (a resistor). Schematics are very useful in visualizing the main features of a circuit. A single schematic can represent a wide variety of situations. The schematic in [link] (b), for example, can represent anything from a truck battery connected to a headlight lighting the street in front of the truck to a small battery connected to a penlight lighting a keyhole in a door. Such schematics are useful because the analysis is the same for a wide variety of situations. We need to understand a few schematics to apply the concepts and analysis to many more situations.

Questions & Answers

a15kg powerexerted by the foresafter 3second
Firdos Reply
what is displacement
Xolani Reply
movement in a direction
Explain why magnetic damping might not be effective on an object made of several thin conducting layers separated by insulation? can someone please explain this i need it for my final exam
anas Reply
What is thê principle behind movement of thê taps control
Oluwakayode Reply
what is atomic mass
thomas Reply
this is the mass of an atom of an element in ratio with the mass of carbon-atom
show me how to get the accuracies of the values of the resistors for the two circuits i.e for series and parallel sides
Jesuovie Reply
Explain why it is difficult to have an ideal machine in real life situations.
Isaac Reply
tell me
what's the s . i unit for couple?
its s.i unit is Nm
Force×perpendicular distance N×m=Nm
İt iş diffucult to have idêal machine because of FRİCTİON definitely reduce thê efficiency
if the classica theory of specific heat is valid,what would be the thermal energy of one kmol of copper at the debye temperature (for copper is 340k)
Zaharadeen Reply
can i get all formulas of physics
BPH Reply
what affects fluid
Doreen Reply
Dimension for force MLT-2
Promise Reply
what is the dimensions of Force?
Osueke Reply
how do you calculate the 5% uncertainty of 4cm?
melia Reply
4cm/100×5= 0.2cm
how do you calculate the 5% absolute uncertainty of a 200g mass?
melia Reply
= 200g±(5%)10g
use the 10g as the uncertainty?
which topic u discussing about?
topic of question?
the relationship between the applied force and the deflection
sorry wrong question i meant the 5% uncertainty of 4cm?
its 0.2 cm or 2mm
thank you
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the meaning of phrase in physics
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is the meaning of phrase in physics
Practice Key Terms 3

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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