Electric Circuits

I. Circuits

A. Definition1. Any path along which electrons can

flow2. There MUST be a continuous path

(loop) for electrons to flowa. open switches prevent electron flowb. closed switches close the loop and allow

current to flow

B. Types of Circuits

1. Most circuits have more than one device that receives electrical energy, the way they are arranged defines the type of circuit

2. Series Circuits – devices are connected in a single pathway for electron flow between the terminals of the battery

3. Parallel Circuits – the devices are connected in separate branches which provide different paths for electrons to flow

II. Series Circuits

A. Current1. Electrons flow as soon as

the pathway is closed, from the negative terminal through the circuit

2. The current passing through each device is the same

3. Formula:I1 = I2 = I3

- Ohm’s Law can also be usedI = Vt / Rt

II. Series Circuits (continued)

B. Resistance1. The current is resisted by the

resistance of the 1st, 2nd, and 3rd device2. the total resistance to the current in

the circuit is the sum of the individual resistances along the circuit path

3. Formula: Rt = R1 + R2 + R3 + …

II. Series Circuits (continued)

C. Voltage1. the total voltage impressed across a

series circuit divides among the individual electrical devices in the circuit

2. Equation:Vt = V1 + V2 + V3 + ….

II. Series Circuits (continued)

D. Ohm’s Law for each device1. the potential difference (voltage drop)

across each device depends directly on the resistance of the device Reason: more energy is needed to move a unit of charge through a large resistance than through a small resistance

2. Formula: V1 = I R1

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II. Series Circuits (continued)

E. Disadvantage1. If one device fails, the current in the

whole circuit stops and none of the devices work

III. Parallel Circuits

A. Connections1. Each device

connects to the same 2 points on the circuit

2. Each device receives the same voltage

Vt = V1 = V2 = V3

III. Parallel Circuits (continued)

B. Current1. Total current in the circuit divides

among the parallel branches. The total current in the circuit equals the sum of all the currents in its parallel branchesIt = I1 + I2 + I3 + …

2. If there is a break in one path does not interrupt the flow of charge in other branches

III. Parallel Circuits (continued)

B. Current (continued)4. Current passes more readily through

devices that offer less resistance5. The amount of current in each branch

is inversely proportional to the resistance in each branch (Ohm’s law applies separately to each branch).Remember: I = V/R

III. Parallel Circuits (continued)

C. Resistance (Equivalent, Effective, Total)

1. As the number of parallel branches is increased, the overall resistance of the circuit is DECREASED

2. Overall resistance is lowered with each added path between any 2 points of the circuit.

3. Overall resistance of the circuit is less than the resistance of any one branch1/Req = 1/R1 + 1/R2 + 1/R3 + …

Circuit:A 200 W (R1) and a 350 W (R2) resistor are connected in parallel to a 12.0 V battery.

1. Sketch a schematic diagram to represent the circuit.

2. What is the equivalent resistance in the circuit?

3. What is the total current in the circuit?

4. What is the current that flows through R1?

5. What is the current that flows through R2?

• What is the current measured through ammeter A?

• What is the equivalent resistance of the circuit?

• How much power is dissipated in the 36-ohm resistor?

• What is the reading of ammeter A?