SME
Grade 8 Science · Physical Science
Electric charge, current and simple circuits
Use this for “A 6 V battery runs a 2 Ω and a 4 Ω resistor in series. What is the current?”
Diagram
I = V ÷ (R₁ + R₂)
= 6 V ÷ (2 Ω + 4 Ω) = 1 A
This is an example. Type your own values to start a new problem.
I (A)
=
V (V)
÷
(
R₁ (Ω)
+
R₂ (Ω)
)
s
Switch
example
Formulas
I
=
V
÷
(
R₁
+
R₂
)
(current
=
battery
voltage
÷
(first
bulb’s
resistance
+
second
bulb’s
resistance))
1 = 6 ÷ (2 + 4)
Assumptions
•
A circuit is a closed loop of conductor from one end of the battery to the other. Open it anywhere and the current stops.
•
Voltage (V) is the battery’s push. Current (A) is how much charge flows each second. Resistance (Ω) is how hard a part makes it to flow.
•
In series the bulbs are in one loop, so the same current goes through each and their resistances add: I = V ÷ (R₁ + R₂).
•
The switch is closed or open: tap it in the picture. Open, no current flows.
Step-by-step
WE KNOW
V = 6 V (battery voltage), R₁ = 2 Ω (first bulb’s resistance), R₂ = 4 Ω (second bulb’s resistance), s: closed
FIND
current (I)
Step 1 · Find current (I)
Use I = V ÷ (R₁ + R₂) (current = battery voltage ÷ (first bulb’s resistance + second bulb’s resistance))
Add the resistances, then divide the voltage by the total.
the switch is closed: the loop is complete
I
=
6
2 + 4
I
=
6
6
I
=
1
A
Check
Put the numbers back in. Both sides should match.
1 = 6 ÷ (2 + 4) ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
Refresh
More problem types
Bulbs in parallel
Use this for “Two bulbs are wired in parallel across a 6 V battery. What current does each get, and what does the meter read?”
More bulbs in series
Use this for “A third identical bulb is added in series. What happens to the current and the brightness?”
Conductor or insulator?
Use this for “A gap in a circuit is bridged with one object at a time. Which ones make the bulb glow?”
Charges that pull together or push apart
Use this for “Two rubbed balloons are brought near each other. What happens?”