DC Circuits

Current and Resistance

A circuit is a closed loop that electrons travel through. If a circuit is open, that means that the wires are cut off or there is a gap so that electrons cannot make a full trip around the circuit. If a circuit is closed, there is a complete path that electrons can take.

DC Circuits stand for  a Direct Current (DC) circuit. This is opposed to an Alternating Current (AC) circuit which is not covered in AP Physics 1 as it is much more complicated.

Current (I) - How much charge passes through an area over time. Measured in amperes (A).

Resistance (R) - How much an object resists current flow. Measured in ohms ( Ω ).

Voltage (V) (potential difference) - the difference in potential energy between two locations. This is sometimes referred to as potential difference. Measured in volts (V).


DC Circuit Graphs

DC circuits in real life often look like a tangled ball of yarn. In order to have a clearer visualization of circuits in physics, we use DC circuit graphs as a simpler representation of a circuit.

Parts of a graph



A battery moves electrons from a low potential state to a high potential state.



A resistor resists current. Lightbulbs and motors can also be treated as resistors because they convert electrical energy to light or kinetic energy and therefore resist the current.



A switch switches the current. It turns something on and off by opening and closing the circuit.



An ohmmeter measures the resistance of a resistor (ohms -> ohmmeter)



An ammeter measures the current (amperes -> ammeter)



A voltmeter measures the voltage (volts -> voltmeter)

Important Equations

Formula Sheet + Explanations

V = IR: Voltage is Current times Resistance. 

I = V/R

R = V/I

P = IV: Power is Current times Voltage. Remember Poison IVy!


Calculating voltage can be thought of as adding the voltages to get form point A to point B.

Series circuit voltage: Vtotal = V1 + V2 + V3 + V4... 

Total voltage is the sum of all voltage drops in series.

Parallel circuit voltage: Vtotal = V1 = V2 = V3 = V4

Total voltage is the same as just one of the voltage drops. All voltage drops are equal.


Series circuit curren: Vtotal = V1 + V2 + V3 + V4...