IBDP Physics: Electricity and Magnetism

Table of Contents

• Charge
• Coulomb’s Law
• Current, resistance and voltage/potential difference
• Ohm’s Law – The Three Properties in Series and Parallel Circuits
• Right-hand rule

Welcome! This is Nguyen Hoang Minh, and today we will be looking into a very important chapter in IBDP Physics: Electricity and Magnetism.

This blog will be a “run-through”, so to speak, of important concepts and formulas in electricity and magnetism.

Charge

An electric charge is a manifestation of energy that exerts forces in the form of a magnetic field or a transition of electrons. The SI unit for charges is coulomb (symbol: C). Charges can be negative (-) or positive (+). Objects can be negatively or positively charged; oppositely charged items attract, and vice versa. An object with no charge, or with equal negative and positive charge, is considered neutral.

An electron has a charge of around -1.6×10⁻¹⁹ C.

Similarly to the law of energy conservation, electric charge can only be transitioned from one object to another, and cannot be reduced or increased.

Coulomb’s Law

Coulomb’s law states that for charges with magnitudes q₁ and q₂ and distance r, the force created is equal to F. One thing to note is that if the charges are of the same sign, the force will make them repel, and the opposite is also true. Coulomb’s law is an “inverse-square law”, that is, a variable is inversely proportional to the distance.

Current, resistance and voltage/potential difference

Electric current is the flow of electrons through an object (usually a wire). Electrical current is usually denoted by I, and is measured using the SI unit ampere (symbol: A). The strength of a current can be calculated using the current formula:

Electrical resistance is the force acting upon an electric current that slows it down. Electrical resistance (or resistivity) is inversely proportional to an object’s cross-section surface area (in the case of wires, the diameter of the wire) and is proportional to an object’s length. Resistance is measured in ohms (symbol: Ω). It can be calculated using the following formula:

Electrical potential difference is the energy needed to move a current from one point to another. Potential difference is usually used interchangeably with another term: voltage. Voltage is measured in volts (symbol: V)

Along with these 3 properties of electricity, there are apparatuses related to them. A voltmeter can be used to measure voltage, an ammeter can be used to measure current, and a resistor can be used to change the resistance of a circuit.

Ohm’s Law – The Three Properties in Series and Parallel Circuits

Ohm’s law is essential for a lot of calculations and circuit exercises.

Ohm’s law says that the current of a circuit is proportional to the voltage of the circuit and inversely proportional to the resistance of the circuit. Put simply:

Series and parallel are two types of circuits. More specifically, they are two ways in which components can be attached to a circuit.

A series circuit has components all attached between each other, on one “route” of wire. On the other hand, a parallel circuits have multiple “routes”, and different components are attached to different routes. There are certain rules for calculating voltage, current and resistance for series and parallel circuits:

Remember the rules for each type of circuits. Remember: circuits can also have both mixed series and parallel.

Right-hand rule

Fleming’s right-hand rule is a well-known, popular rule used to determine forces of a wire with a magnetic field. You can see a visualisation here: