GCSE · Physics · Edexcel · Spec 1PH0

Charge equation Q = I x t

A current of 2 A means 2 coulombs go past a point every second. How many have gone past after 10 seconds? Count them and you've got the equation.

Physics · Try it first

How much charge has gone past?
0 s1 s2 s3 s4 s5 s6 s7 s8 s9 s10 sdrag the time bar →

Time: 3/10. Time passed 3 s. Charge passed 6 C

Time passed3 sCharge passed6 C

A steady current of 2 A is flowing. Drag the time bar and read the coulombs that have passed a point in the circuit.

Exam line: Charge passed = coulombs each second × number of seconds. For 2 A that is 2 C × the seconds.
Watch out: The 2 A means 2 coulombs every second, so the time on the bar is counted in seconds.

Now do it with numbers

Problem

A current of 4 A flows past a point in a circuit for 15 s. How much charge flows past the point?

Which way round?

Cover the quantity you want to find. What is left shows what to do.

Tap the quantity you want to find. The triangle shows you the formula.

÷

Cover charge, current or time to reveal its rearranged formula, then plug in numbers to solve.

Your turn, with help

Find the current

72 C of charge flows past a point in a circuit in 24 s. What is the current?

  1. We know the charge and the time, and we want the current. Current is charge each second.
  2. missing step
Which line is step 2?

Catch the slip

Where did this answer go wrong?

Find the charge that flows when a current of 5 A runs for 3 minutes.

A student's answer — which line goes wrong?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

Count the coulombs, second by second, and the equation builds itself.

What you need to know

  • Electric charge is measured in coulombs, written C.
  • Electric current is how much charge flows past a point in a circuit each second.
  • A current of 1 ampere (1 A) means 1 coulomb flows past that point every second.
  • So 3 A means 3 C every second, and after 2 seconds 6 C have gone past.
  • Have a goA 5 A current is flowing. How many coulombs go past a point in 4 seconds?

    20 C

    5 A is 5 C each second, and that happens for 4 seconds, so 5 × 4 = 20 C.

  • Charge = current × time, which is Q = I × t.
  • Use Q in coulombs (C), I in amperes (A) and t in seconds (s).
  • That equation comes from I = Q ÷ t: multiply both sides by t.
  • Rearrange to find current, I = Q ÷ t, or to find time, t = Q ÷ I.
  • Have a go24 C goes past a point in 8 s. Which rearrangement finds the current, and what is it?

    I = Q ÷ t = 24 ÷ 8 = 3 A

    Current is charge each second, so you divide the charge by the time, not multiply.

  • Time must be in seconds before you substitute, and one minute is 60 seconds.
  • Have a goYour mate says: '5 A for 2 minutes? Easy, Q = 5 × 2.' Confident. Wrong. What must they fix first?

    Change 2 minutes into 120 seconds

    The 5 A is 5 C each second, so the time has to be counted in seconds before it goes into the equation.

The big picture

Electric current is the amount of charge flowing past a point in a circuit each second, so 1 A means 1 C every second. Multiply the current by the time in seconds and you get the charge: Q = I × t. Rearrange it to find current (I = Q ÷ t) or time (t = Q ÷ I), and always turn minutes into seconds first.

Key points

1Current is charge flowing past a point in a circuit each second, and 1 A is 1 C each second.
2Q = I × t, with charge in C, current in A and time in s.
3Find current with I = Q ÷ t and time with t = Q ÷ I.
4Convert time to seconds before substituting: 1 minute = 60 s.

Worked example

Problem

A charge of 120 C flows past a point while a current of 4 A is flowing. For how long does the current flow?

⚠ Watch out

Putting minutes straight into Q = I × t. The equation is built from coulombs each second, so the time has to be in seconds first. One minute is 60 seconds, not 1.

🧠

Memory hook

An amp is coulombs per SECOND, so the time in Q = I × t is always in SECONDS.

✓

Check yourself

Explain in one sentence what 1 A means. Then try it: a 7 A current flows for 6 s. How much charge goes past, and in what unit? (Check: 42 C.)

Flashcards

(10)
What is electric charge measured in?
Coulombs, written C.
What is electric current?
The amount of charge flowing past a point in a circuit each second.
What does a current of 1 ampere mean?
1 coulomb of charge flows past a point each second.
A current of 6 A flows. How much charge passes a point each second?
6 C.
Write the charge equation in symbols.
Q = I × t
What are the units in Q = I × t?
Charge in coulombs (C), current in amperes (A), time in seconds (s).
How does Q = I × t follow from I = Q ÷ t?
Multiply both sides by t.
Which equation finds current from charge and time?
I = Q ÷ t
Which equation finds time from charge and current?
t = Q ÷ I
How many seconds are in one minute?
60 seconds, so minutes must be converted before you substitute.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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