Electric Current (SQA National 5 Physics): Revision Note

When will 8 A of current pass through an electrical circuit?

A.     When 8 J of energy is used by 1 C of charge

B.     When a charge of 4 C passes in 0.5 s

C.     When a charge of 8 C passes in 0.1 s

D.     When a charge of 1 C passes in 8 s

E.     When a charge of 0.5 C passes in 4 s

ANSWER:  B

• The equation relating current, charge and time is:

$Q=I\times t$

• Rearrange to make current I the subject of the equation:

$I=\frac{Q}{t}$

• Consider option B, where Q = 4 C and t = 0.5 s:

$I=\frac{4}{0.5}=8\mathrm{A}$

• Therefore, the correct answer is B

A is incorrect as this is the definition of a voltage of 8 V between two points and does not describe current

C is incorrect as  $I=\frac{8}{0.1}=80\mathrm{A}$

D is incorrect as  $I=\frac{1}{8}=0.125\mathrm{A}$

E is incorrect as  $I=\frac{0.5}{4}=0.125\mathrm{A}$

Electric currents in everyday circuits tend to be quite small, so it's common for examiners to throw in a unit prefix like 'm' next to quantities of current, e.g. 10 mA (10 milliamperes). Make sure you can convert these into standard units, e.g. 10 mA = 10 × 10^-3 A.

Make sure to only use the triangle to help you rearrange the equation that links charge, current and time. Don't draw it if you are asked to write out the equation in full, such as Q = I × t, as you may lose marks for doing so.

Check out this revision note on speed, distance and time if you need a reminder on how to use relationship triangles.