E 1.1
CURRENT & CHARGE
An electric current is a flow of charge. Current can be defined as the rate of
flow of charge.
The unit of charge is the COULOMB
If the current is 1 Ampere, it means that 1 Coulomb of charge is passing every
second.
The amount of charge that flows in a circuit depends on the size of the current
and the time for which the current flows.
The total amount of charge can be calculated using the following equation:
CHARGE = CURRENT X TIME
This is sometimes written as
Q=Ixt
Q
I
t
Where:
UNITS:
stands for
stands for
stands for
CHARGE
CURRENT
TIME
Charge
Current
time
must be in
must be in
must be in
COULOMBS
AMPS
SECONDS
THE TRIANGLE
Q
I
x
t
1
PROBLEMS
WORKING OUT
ANSWER
(WITH UNITS)
CHARGE
CURRENT
TIME
1
?
4A
30 s
2
?
0.2A
5 min
3
600 C
?
300 s
4
10,000 C
3.5A
?
5
0.5 C
5A
?
6
?
300 mA
10 s
7
300 mC
?
5s
8.
A student is performing an experiment to measure the charge stored by a
capacitor. When he discharges it he finds the average current is 200 mA
and that it takes 40 s to discharge. What was the charge on the
capacitor?
9.
The dome of a Van de-Graaf generator holds a charge of 100 mC. When
a man touches the dome it discharges in 3 x 10 -2 s. What is the current
that flows through the man to earth?
10.
In an electroplating experiment a student connects up a circuit that
delivers a steady current of 0.2 A. If it requires 96500 C of charge to
deposit 64 g of copper, how long will it take to deposit 20 g?
2