P2bL 1 Circuit Diagrams

Key Words

Ammeter - Instrument for measuring current

Battery - several cells connected together.

Cell - a single unit producing electrical energy from chemical energy (often called a battery).

Closed switch - switch turned on so the electrical current can flow through it.

Conductor - wire.

Diode - a semi-conductor component that allows electricity to flow one way only.

Fuse - weak link in an electrical circuit that melts to protect components if too big a current flows, breaking the circuit.

In parallel - components each have their own branch of the circuit.

In series - current goes through the components one after the other.

Lamp - bulb.

LDR - light dependent resistor.

Open switch - switch that is turned off.

Resistor - component that reduces the flow of current.

Thermistor - component whose resistance changes as the temperature changes.

Variable resistor - resistor whose resistance can be changed.

Voltmeter - instrument to measure potential difference

Grade E

Circuit symbols are used to show:

• what components are used in the circuit;
• how they are connected to each other.

It is important to use standard symbols, so that everyone knows what you are talking about:

Look at the key word list to learn what each does.

Grade C

In a series circuit, the components are in a line.  The electric current passes through each component in turn.

Note that the ammeter is placed in series with the components, and the voltmeter is placed in parallel with the component.  We can say two important things about a series circuit:

• The current is the same all the way round.
• The voltages add up to the battery voltage.

Note also that the connections between the wires (junction of conductors) are joined with black dots (a blob of solder).

In a parallel circuit, each component is in its own branch as below:

In this circuit,

• The voltage across each branch is the same;
• The currents add up to a total current supplied by the cell.

The circuit below shows a variable resistor that can make the lamp glow brightly or dimly, depending on the resistance.

The next circuit shows a fuse protecting the lamp.  The fuse is a thin wire that gets hot and melts if too big a current flows.  It then breaks and stops further damage to the circuit.  The fuse is easily replaced.  If the fuse isn't there, the circuit could overheat and catch fire.

Note that in some books the symbols for some components like resistors and lamps are different.  For the exam, these symbols will be used.  However, if you did use a different symbol correctly, you would get the mark.

Grade A

Remember that in electrical circuits, we always treat currents as conventional currents, which means that the current flows from positive to negative.  In reality the electrons flow from negative to positive.  This is confusing, but the early physicists got it wrong.  When they realised, it meant that they would have to rewrite all the laws of physics, so they stuck to the conventional current.

Electrons carry energy from the source (battery) to the components.  The components must convert all that energy into some other kind of energy.  The energy given out by the components must be the same as the energy supplied by the source, otherwise energy would be created or destroyed.

Electrons do not leak out of the circuit, so the number of electrons leaving the source is the same as the number of electrons getting back to the source.

Potential difference is formally defined as the energy transferred per unit charge, i.e. joules per coulomb.   1 V = 1J/C