alternating current

Current flowing in alternating direction. It is usually used to represent current varying with time in a sinusoidal way.


A device used to measure current. Ammeter is usually connected in series with the electric components to be measured. The resistance of ammeter is very small so that it will not affect the current to be measured.

cathode ray

Cathode ray is a beam of electrons emitted by the cathode of a vacuum tube.


Charge is a property of materials. Charged objects have electrostatic interaction between one and others. There are two types of charges: positive and negative charges. Opposite charges attract and similar charges repel each other. The unit for charge is coulomb (C). The charge of one electron is . The charge of one proton is .


There are freely movable charges inside these materials, allowing electric current to pass through them. Metals (like copper, iron and gold etc.) usually have many free electrons and are good conductors. Besides, ions can move freely in solutions and therefore they can also conduct electricity.

Coulomb's law

If two particles, with charges and respectively, are separated by a distance , then the electrostatic force between them is , where is permittivity in vacuum. represents repulsive force and represents attractive force.


Current is a measure of charge flow. It is defined as the amount of charges passing through a particular point in unit time. In convention, the direction of current is defined as the direction of positive charge flow. However, current is usually produced by the flow of negative charges (electrons). The moving direction of these negative charges is opposite to the direction of current. The unit for current is ampere (A).


Diode is a device that can let current flows through it in one direction only.

direct current

Current with fixed direction. The term is usually used to represent steady current, i.e. the magnitude of current does not vary with time.

Earth magnetic field

The Earth carries magnetic field just like a huge magnet. The two poles are very close to the South Pole and North Pole of the Earth. Therefore the magnetic pin in compass lies along the direction of the Earth's magnetic field and can indicate direction.

eddy current

In a changing magnetic field, induced electromotive force will drive a current inside a large conductor. This current is called eddy current. It will heat up conductors and leads to energy dissipation. For example, the soft iron cores in transformers are often laminated, so as to reduce the energy dissipation by eddy current.

electric field

It is the effect of charged object on the space around it. If we put a test charge , the electric field strengthat the position is. The SI unit of electric field is Newton/Coulomb () or Volt/metre ().


A device used to produce magnetic field by passing current through a solenoid. An iron core is often placed inside the solenoid to produce a larger field.

electromagnetic induction

When a conducting wire, carrying a current, is moving in a magnetic field, or when a magnetic field is changing with time, an e.m.f. will be induced across the wire. For example, when a magnet passes through a solenoid, an e.m.f. will be induced across the two ends of the solenoid.

electromotive force

When the output current of an electric source is zero, the potential difference across the two poles of the source is called the electromotive force of the source. The short form of electromotive force is e.m.f. The unit for e.m.f. is the same as potential difference, i.e. volt (V).

equivalent resistance

When some resistors are connected together, the resistance of the whole system can be represented by a single resistance. This resistance is called the equivalent resistance of the system. For example, there are resistors,,, etc. When they are connected in series, the equivalent resistance is ; when they are connected in parallel, then


Fuse is a device protecting electric appliances from large current. Fuses are made of materials with low melting points. When a large current flows through it, it will be heated up and melt. The circuit is therefore broken.


A device that can change mechanical energy into electrical energy. A coil is set to turn in a magnetic field. According to the law of electromagnetic induction, an e.m.f. is induced across the coil.

induced charge

Consider a neutral conductor under the influence of electrostatic force generated by another charged object. The positive charges inside the conductor will be separated from the negative charges. Therefore the conductor is now locally charged.

For example, if we put a positively charged plastic rod near a neutral metal ball. The region nearer to the plastic rod will become negatively charged, while the farther part is left positive.


Since the charges inside these materials cannot move freely, no current can flow inside the materials. Plastic, wood and rock are examples of good insulators.

Kirchhoff's laws

Kirchhoff's first law is about the conservation of current. For every point in space, the current flowing into this point must be equal to the current flowing out.

Kirchhoff's second law states that for all closed paths, the sum of the potential differences must be equal to the sum of the e.m.f. of the same path.

Lenz's law

The direction of induced e.m.f. is always against the change that causes the induction. For example, when a magnet is getting nearer to a coil, an induced e.m.f. will drive a current to flow inside the coil. The current will flow in such a way that the magnetic force of the coil will repel the magnet.

magnetic effect of electric current

Current can produce magnetic field. The magnetic field at any point in space due to a long and straight wire with current is , where is the distance between the point concerned and the wire, is the permeability in vacuum. The direction of magnetic field is determined by the right hand screw rule.

magnetic force experienced by charged particles

When a particle with chargeis moving in a magnetic fieldat a speed, it will experience a magnetic force , where is the angle between the direction of motion and that of the magnetic field. The direction of the magnetic force is perpendicular to the direction of motion and the magnetic field, as determined by the right hand screw rule.


The process of inducing magnetic property in an object.

A magnetic field produced by another object can force the magnetic domains inside the object concerned to rearrange, so that the object will produce its own magnetic field.


A device used to change electrical energy into mechanical energy. A coil that can turn freely is placed in a magnetic field. When we supply current to the coil, the coil will experience a magnetic force in the magnetic field. The coil will start turning under the action of force couple and can be used to move other objects.


Objects without charges or carrying equal amount of positive and negative charges are said to be neutral.

Ohm's law

At fixed temperature and pressure, the current flowing through an object is directly proportional to the applied voltagefor some materials. The proportionality constant is the resistanceof the object. In SI units, Ohm's law can be expressed as.

parallel circuit

A common method of connecting electric components. When current flows through parallel circuit, there are two or more paths for current to flow. At this time, the current will be divided into many parts, which will pass different paths. Then the current will join together at the other end. When current flows through electric components in parallel, all the components will have the same potential difference.

permanent magnet

Under the condition of no external magnetic force, permanent magnets can preserve its own magnetic property for a long time. Permanent magnets are made of ferromagnetic materials, such as iron and nickel.

potential difference

The following is the definition of potential difference between two points (point A and point B) in space. If we move one unit of positive charges from point B to point A (assuming that the initial and final kinetic energy of the charges are zero), then the work required is equal to the potential difference across points A and B. For example, in a uniform electric field, the potential difference across two points separated by a distancealong the direction of electric field is. The unit for potential difference is Volt (V). Volt = Joule/Coulomb ()


Resistance is a measure of reluctance to current flow in an electric component. It depends on the conductivity and the shape of material. The unit for resistance is Ohm. 1 Ohm = 1 Volt/Ampere ()


Semiconductor is a type of materials with resistivity between conductors and insulators. Electrons gain energy from thermal motion of atoms and transit from the valence band to the conduction band. Then the moving electrons or holes can conduct electricity. Silicon and germanium are the most important semiconductors, they are widely used in manufacturing electronic components and integrated circuits.

series circuit

A common way of connecting electric components. When electric components are connected in series, current must flow through all the components in the circuit.


A coil with length longer than its diameter. Consider a solenoid with its length much greater than its diameter and carrying a current. The magnetic field inside the solenoid is about, whereis the number of turns of wire per unit length, and is the permeability in vacuum.

test charge

Charged particles experience electric force with different magnitude and direction in different regions. In studying these problems, we use imaginary objects with very small charges to measure electric force at different location. These small charges are called test charges.


Transformer is a device that can change the voltage of an alternating current. It consists of two coils, coupled by ferromagnetic materials (soft iron core). An alternating current is input to the primary coil. It will produce a changing magnetic field and induce an e.m.f. in the secondary coil. The secondary coil is connected to the output. By varying the turns ratio of the primary coil to the secondary coil, we can get an output voltage greater or smaller than the input. If the output voltage is greater than the input, the transformer is called a step-up transformer. It is called a step-down transformer vice versa.


Electromotive force or potential difference expressed in volt is called voltage.


A device used to measure voltage. Voltmeter is usually connected in parallel with the electric components to be measured. The resistance of voltage is very large, so that the voltage to be measured is not affected.