MYP Physics 3.4 Electric Circuits Notes

How Does Electric Current Flow Through Circuits?

1. An electric circuit is a complete, closed path that allows electric charge to move.
2. Understanding circuits means keeping track of three linked ideas:
   1. Charge: the "stuff" that moves (in metals, this is mainly electrons)
   2. Current: how fast charge passes a point
   3. Potential difference (voltage): the energy difference that pushes charge around the circuit
3. Electric current can only flow when the circuit forms a closed loop from the energy source, through the components, and back to the source.
4. If there is a break anywhere in the pathway, charges cannot complete the loop and the circuit stops working.
5. Circuits are used to transfer electrical energy from a source to devices such as lamps, motors, or heaters.

Open and closed circuits

1. A closed circuit provides a continuous loop, so charges can flow without interruption.
2. An open circuit has a break in the path, which prevents current from flowing.
3. Switches are commonly used to deliberately open or close a circuit.

What Parts Are Needed To Make A Circuit Work?

1. To transfer electrical energy, a circuit must contain certain essential components.
2. Each component has a specific role, and removing any one of them prevents the circuit from working properly.

Cells and Batteries

1. A cell provides energy to charges in the circuit.
2. The cell pushes charges around the circuit by providing voltage.
3. A battery is made of two or more cells connected together.
4. Increasing the number of cells increases the energy transferred to charges.

Conducting Wires

1. Wires connect all components to form a complete loop.
2. Wires are made of metals because metals allow charges to move easily.
3. Wires are designed to have very low resistance, so little energy is lost in them.

Switches

1. A switch controls whether a circuit is open or closed.
2. Opening a switch breaks the circuit and stops current.
3. Closing a switch completes the circuit and allows current to flow.

Electrical Devices (Loads)

1. Devices in a circuit are often called loads.
2. Loads transfer electrical energy into other forms:
   1. Lamps → light and heat
   2. Motors → movement
   3. Heaters → thermal energy
3. Loads usually have resistance, which affects how much current flows.

Circuit Diagrams Let You Represent Circuits Clearly

1. A circuit diagram uses standard symbols so a circuit can be communicated accurately.
2. Instead of drawing realistic pictures, scientists use diagrams to show how circuits are connected.
   1. Circuit diagrams focus on connections, not physical appearance.
   2. Standard symbols are used so circuits can be understood internationally.
   3. Diagrams make it easier to trace current paths and identify components.

Essential Circuit Symbols To Know

1. Cell and battery
2. Open and closed switch
3. Lamp
4. Resistor
5. Ammeter
6. Voltmeter
7. Connecting wire

Series Circuits Have One Path, So Current Is The Same Everywhere

1. A series circuit has only one loop, meaning there is only one path the current can take.
2. In a series circuit, there is only one path for charges to move around the circuit.
3. All charges must pass through every component in turn.
4. Because the same charges flow through all components, the current is the same everywhere in the circuit.
5. Series circuits are simple to build but have important limitations.

Calculating Voltage in Series Circuit

$$V_{\text{total}} = V_1 + V_2 + V_3 + \dots$$

How Is Energy Shared in a Series Circuit?

1. Although current stays the same, energy does not.
2. Electrical energy supplied by the cell is shared between the components.
3. Each component transfers some of the energy carried by the charges.
4. Adding more components means each one receives less energy.

Parallel Circuits Have Multiple Paths, So Current Splits At Junctions

1. In a parallel circuit, components are connected on separate branches.
2. Each branch provides an independent path for charges.
3. Charges can choose different paths through the circuit.

Current in a Parallel Circuit

1. The total current from the cell splits between the branches.
2. Some charges flow through one branch, and others through another.
3. The total current equals the sum of the currents in each branch.

$$I_{\text{total}} = I_1 + I_2 + I_3 + \dots$$

Voltage in a Parallel Circuit

1. The same voltage is supplied to each branch.
2. Each component receives the full energy per charge from the cell.
3. Adding more branches does not reduce the voltage across existing components.

Measuring Current with an Ammeter

1. An ammeter measures current.
2. It must be connected in series with the component.
3. This allows all charges to pass through the meter.

Measuring Voltage with a Voltmeter

1. A voltmeter measures voltage (energy per charge).
2. It must be connected in parallel with the component.
3. This allows it to compare energy before and after the component.

Resistance in Series and Parallel Circuits

The arrangement of components affects the total resistance in a circuit.

Resistance in Series Circuits

1. In a series circuit, charges must pass through every component one after another.
2. Each component adds to the total resistance of the circuit.
3. Adding more components increases total resistance.
4. Increased resistance causes the current in the circuit to decrease.
5. Lower current means less energy is transferred per second to each component.

$$R_{\text{total}} = R_1 + R_2 + R_3 + \dots$$

Resistance in Parallel Circuits

1. In a parallel circuit, charges have more than one path.
2. Adding more branches gives charges more paths to follow.
3. The total resistance of the circuit decreases.
4. A lower total resistance allows a larger total current from the cell.
5. Each branch still receives the same voltage.

$$\frac{1}{R_{\text{total}}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + \dots$$