SME
Grade 11 Science · Physics
Circuits: Ohm's law, series and parallel
Use this for “A 9 V battery drives a 20 Ω resistor. What current flows?” with Ohm’s law.
Diagram
I
=
V
R
=
9
V
20
Ω
=
0.45
A
Drag the source or a resistor up or down to change it.
This is an example. Type your own values to start a new problem.
V (V)
=
I (A)
×
R (Ω)
Formulas
V
=
I
×
R
9 = 0.45 × 20
Assumptions
•
The resistor keeps the same resistance whatever the current (it obeys Ohm’s law).
•
V is the voltage across the resistor and I the current through it.
•
Volts, amps and ohms go together: 1 V = 1 A × 1 Ω.
Step-by-step
WE KNOW
V = 9 V, R = 20 Ω
FIND
current (I)
Step 1 · Find current (I)
Use V = I × R
Divide the voltage by the resistance: more resistance, less current.
I
=
V
R
I
=
9
20
I
=
0.45
A
Check
Put the numbers back in. Both sides should match.
9 = 0.45 × 20 ✓
Topic check
See what you know: 5 questions on this topic
New questions each time. Pass to mark it complete.
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More problem types
Resistors in series
Use this for “A 12 V battery drives 10 Ω and 20 Ω in series. What does the ammeter read, and what is the voltage across each?”
Resistors in parallel
Use this for “40 Ω, 60 Ω and 120 Ω are in parallel on 24 V. Find each branch current, the total current and the equivalent resistance.”
A series-parallel circuit
Use this for “24 V drives R₁ = 4 Ω in series with 6 Ω and 12 Ω in parallel. Find the total current, the voltage across R₁ and each branch current.”
Electric power and energy
Use this for “A lamp on 120 V draws 0.5 A. What is its power and resistance, and how much energy does it use in 2 hours?”