SITEMAP   School-college Physics Notes: Electricity 3.5 Q = It calculations

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Electricity Section 3: 3.5 Movement of charge - calculation of the charge passing through a point in a circuit,  Q = It calculations

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ANSWERS to ALL the QUESTIONS at the end of the page

3.5 Movement of electrical charge

How do you calculate the amount of electric charge moving in a circuit?

Calculation of the charge passing through a point in a circuit Q = It Current (I in amps) is the rate of flow of electrical charge around a circuit - coulombs (C) per second (s) past a given point in a circuit.

The greater the flow of charge in a given time the greater the current.

The rate of flow of electric charge is measured in coulombs/second.

You can calculate the charge passing a point in a circuit in a given time from the formula

Q = It

where Q = electric charge transferred in coulombs (C) unit of electric charge

I = current flow in amperes (A) and t = time (s)

rearrangements from Q = It,  I = Q/t  and  t = Q/I

A current flow of 1 A equals a rate of flow of charge of 1 C/s.

Examples of calculation questions involving the equation Q = It

Q1 If a current of 3.0 A passes through an appliance for 1 hour and 30 minutes, how much electrical charge is transferred in the process?

Q2 If 9000 C of charge passes a point in an electrical circuit in 12.0 minutes, what is the current flow?

Q3 How long will it take, in minutes and seconds, for an electrical circuit current of 20.0 A to transfer 5000 C of charge?

Q4 A laptop computer battery charger passes a current of 1.20 A for 30 minutes with an output p.d. of 15.0 V.

(a) Calculate how much charge is transferred to the computer battery.

(b) What is the resistance of the battery charger?

(c) When the laptop battery is fully charged it stores 3000 C.

How long will it take to fully charge a flat battery?

Keywords, phrases and learning objectives for the movement of electrical charge

Be able to do calculations of movement of charge in circuits.

Know that current equals the rate of charge passing through a point in a circuit.

Know how to rearrange and use the equations Q = It I = Q/t t = Q/I in calculations, relating charge in charge in coulombs, time in seconds and current in amperes.

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ANSWERS to examples of calculation questions involving the equation Q = It

Q1 If a current of 3.0 A passes through an appliance for 1 hour and 30 minutes, how much electrical charge is transferred in the process?

Q = It,  Q = 3.0 x 1.5 x 60 x 60 = 16 200 C = 1.62 x 104 C

Q2 If 9000 C of charge passes a point in an electrical circuit in 12.0 minutes, what is the current flow?

I = Q/t = 9000/(12 x 60) = 9000/720 = 12.5 A

Q3 How long will it take, in minutes and seconds, for an electrical circuit current of 20.0 A to transfer 5000 C of charge?

t = Q/I = 5000/20 = 250 seconds = 4 mins and 10 seconds

Q4 A laptop computer battery charger passes a current of 1.20 A for 30 minutes with an output p.d. of 15.0 V.

(a) Calculate how much charge is transferred to the computer battery.

Q = It = 1.2 x 30 x 60 = 2160 C

(b) What is the resistance of the battery charger?

V = IR,  R = V/I = 15 / 1.2 = 12.5 Ω

(c) When the laptop battery is fully charged it stores 3000 C.

How long will it take to fully charge a flat battery?

Q = It, t = Q / I  = 3000 / 1.2 = 2500 s (41 min 40 secs)

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