Electric circuits

Electrical charges and fields

Protons and neutrons make up the nucleus of an atom and electrons move about in the space around the nucleus. Electrons are negative, protons are positive, and neutrons are neutral.

Only electrons can be transferred to and from atom. A charged atom is called an ion. 

• Adding electrons to an uncharged atom makes it negative because it has more electrons than protons.
• Removing electrons from an uncharged atom makes it positive because there are more protons than electrons.

Some insulators become charged when you rub them with another insulator. Electrons are transferred from one of the materials to the other.

• Rubbing a polythene rod with a dry cloth transfers electrons to the surface atoms of the rod from the cloth; the polythene rod is negatively charged.
• Rubbing a perspex rod with a dry cloth transfers electrons from the surface atoms of the rod to the cloth so the perspex rod becomes positively charged.

Two charged objects exert a non-contact force on each other because of their charge, creating an electric field around itself. A second object in the field experiences a force because of the field. The field becomes stronger as the distance between the two decrease. Lines of force represent an electric field - a path positive charges would follow because of the field. Lines away from the centre mean that the force is directed away from the sphere. Negative charges point towards the sphere.

If two objects are oppositely charged, electrons in the air molecules between them experience a force towards the positive object, but if the field is too strong, sparking occurs, since electrons are pulled out of air molecules by the force of the field, hitting other air molecules and knocking electrons out of them, creating a sudden flow of electrons between the two objects.

Like charges repel; unlike charges attract.

Current and charge

Components in a circuit are connected and each has its own symbol.

• A cell has to push electrons around a complete circuit; a battery consists of two or more cells. The + symbol next to the long line of the cell indicates it is the positive terminal of the cell.
• A switch enables current in a circuit to be turned on or off.
• An indicator emits light as a signal when a current passes through it.
• A diode only allows current through in one direction only.
• Light-emitting diodes (LED) emits light when a current passes through it.
• An ammeter measures electric current. It is connected in series with the bulb so the current flowing through them is the same.
• A fixed resistor limits the current in a circuit.
• A variable resistor allows the current to be varied.
• A fuse melts and breaks the circuit if the current through it is greater than a certain amount.
• A heater transfers energy from an electric current to heat the surroundings.
• A voltmeter measures voltage. It is connected in parallel to measure voltage.

An electric current is a flow of charge…