Electric Circuits

Ohm’s Law Current, voltage, and resistance are related to one another. The relationship among resistance, voltage, and current is summed up in Ohm’s Law. Ohm’s Law states that the resistance of most conductors does not depend on the voltage across them. Changing the voltage in a circuit changes the current but will not change the resistance. Ohm concluded that conductors and most other devices have a constant resistance regardless of the applied voltage.

Ohm’s Law The equation for Ohm’s Law is: *current can also be abbreviated as “I” **draw your triangle!! Current (amps A) Resistance = (ohms Ω) Voltage (volts V)

Ohm’s Law If a circuit has a resistance of 30.5 ohms and a current of.05 amps, what is it’s voltage? The brake light on an automobile is connected to a 12-volt battery. If the resulting current is 0.40 amps, what is the resistance of the brake light?

Features of a Circuit Circuits have devices (resistors) that are run by electrical energy. A circuit has a source of electrical energy. Electric circuits are connected by conducting wires. These three things are mandatory components of an electric current.

Radios, appliances, and light bulbs are examples of electrical devices. All these devices resist the flow of electrical energy. As a result, electrical devices are known as resistors.

Features of a Circuit Sources of electrical energy in a circuit include batteries, generators, and electric power plants. When you plug a radio into a wall socket, the source of electrical energy is your local electric plant. Conducting wires complete the path of an electric circuit. Wires allow charges to flow from the energy source to the electric device and back to the energy source.

Often, a switch is placed in an electric circuit. With a switch, you can turn a device on or off by opening or closing the circuit.

Circuit Diagram

Series Circuits A series circuit has all the parts of the circuit connected on one path. An example of a series circuit is a circuit with a battery and two light bulbs connected by a single wire. Suppose a series circuit has two light bulbs. If one burns out, the second bulb goes out because the circuit is broken when the first bulb burns out.

Series Circuits If you add resistors to a series circuit, the resistance of the circuit increases. This can be shown as: R Total =R 1 +R 2 +R 3 In a series circuit, the current throughout the circuit is constant. This can be shown as: I Total = I 1 = I 2 = I 3 …

Series Circuits Also, in a series circuit, as bulbs are added in line, all of the bulbs become dimmer. Each bulb adds more resistance to the circuit, therefore, for a constant voltage, when resistance increases, current decreases.

Series Circuits An ammeter is an instrument used to measure current.

Parallel Circuits A parallel circuit has more than one path for current to take. There are separate branches in a parallel circuit. Each resistor may be on its own branch. Suppose a parallel circuit has two light bulbs, each on its own branch. If one light bulb burns out, the other light bulb will stay lit. The second bulb stays lit because it is on its own branch.

Parallel Circuits Because the voltage source (battery) provides constant voltage, the only factors that can change in a circuit are resistance and current. In a parallel circuit, when more branches are added, the overall resistance decreases, meaning that the current will increase.

Parallel Circuits The major disadvantage of a parallel circuit is that as you add more things in parallel, the current draw on the source goes up with each new branch. If the source cannot supply the current that is demanded by the multiple resistors of the circuit, the voltage will (must!) decrease. This could be bad, as some devices, notably motors, do not like to run at low voltage and can actually be damaged if voltage decreases too much.

Parallel Circuits A voltmeter is a device used to measure voltage, or electrical potential energy difference.

Parallel Circuits The circuits in your home are parallel. Electrical energy enters your home through heavy-duty, low resistance wires, and then branch out in parallel to your wall sockets, then to appliances and lights in each room. The voltage in most household circuits is 120 V.