P4 electrical circuits

Seite 1

CURRENT AND CHARGE

Inside the atom- Protons and neutrons make up the nucleus of an atom. Electrons move about in the space around the nucleus.

A proton has a positive charge

An electron has an equal negative charge

A neutron is uncharged- this means it has an equal number of electrons and protons.

A charged atom is called an ion.

Adding electrons to an uncharged atom makes it negative. More electrons than protons.

Removing electrons makes it positive. More Protons.

Charging by friction

Rubbing 2 insulators together can create a charge as electrons are transferred from one material to the other. Eg perspex rod and cloth

The force between 2 charges objects

Two charged objects exert a non contact force on each other because of their charge. Because a charged object creates an electric field around itself.

second charged object in the field experiences a force because of the field. As distance between the objects decreases the force becomes stronger.

Same type of charge repel each other.

Different types of charge attract each other.

A cell- necessary to push electrons around a complete circuit diagram. Battery = 2 or more cells. There is a positive and negative end of the cell.

Fixed resistor- limits the current going around in a circuit.

A switch- enables the current in a circuit to be switched on and off.

Variable resistor- Allows the current to be varied.

bulb- Designed to emit light as a signal passes through it.

Fuse- Designed to melt and break the circuit if the current through it is greater than a certain amount.

Diode- allows current through it in one direction only.

Heater- Designed to transfer the energy from an electric current to heat the surroundings.

Ammeter- used to measure electric current

Voltmeter- used to measure potential difference.

Seite 2

Current = flow of electric charge

The size of the current is the rate of flow of charge per second - more electrons = more current

In a single loop circuit the current is the same at any point in the circuit

Current will only flow around a complete (closed) circuit if there is a potential difference (source of power)

Charge is measure in COULOUMBS C

Current is measure in AMPERES Amps

Charge flow Q (Columbs C) = Current (amps) I x time taken T (seconds) Q= I x t or I = Q/T

Charge = current x time

Q=I x T

POTENTIAL DIFFERENCE AND RESISTANCE

Potential difference = energy transferred - driving force that pushes charge around - measured in VOLTS

In a series circuit if you add more batteries the potential difference will increase. Cell potential difference add up and the total potential difference is shared.

Potential difference (V) = Energy transferred E (Joules) / Charge Q (Coloumbs)

Resistance = caused by the atoms resisting the passage of the electrons through the circuit - Anything in a circuit that slows the flow down is resistance - measured on OHMS

In a series circuit the total resistance is the sum of the component resistances

Resistance R (Ohms) = Potential Difference V / Current I (Amps)

or

Potential difference = Current x Resistance V=IR

Ohms Law

Current through a resistor at a constant temp is directly propotional to the potential difference across the resistor.

Ohmic conductors have a constant resistance eg wire or resistor

Seite 3

COMPONENT CHARACRTERISTICS

Filament lamp

when electrical charge flows through a filament lamp it transfers some of the energy to the thermal energy store of the filament which is designed to heat up. Resistance increases with temperature so as the current increases the filament heats up more and resistance increases.

Current can flow in either direction

Resistance of metal increases as its temp increases.

Fail when switched on as resistance is low when bulb off and large current causes filament to burn out.

Diode

Current flows in one direction only.

Resistance changes as current changes - non ohmic conductor

In reverse current is virtually zero so diodes resistance is much higher in reverse direction

LED - light emitting diode - emits light when current passes through in forward direction.

Thermistors and light-dependent resistors

Thermistor :

used in sensor circuits

temperature dependent resistor

resistance decreases as temperature increases

eg temperature dectors in car engine, thermostats

Light-dependent resistor - (LDR)

Resistance decreases if light intensity increases

used for automatic night lights / burglar dectors

FACTORS AFFECTING RESISTANCE

Temperature - higher temp higher resistance

Longer the wire greater the resistance

NB Graphs of Ohmic conductor, filament lamp and diode to show resistance with increased current.

Seite 4

SERIES CIRCUITS

Series circuits are all or nothing - components are connected in a line between the +ve and -ve of the power supply. (NB Volt meters are always conncected in parallel but they don't count as part of the circuit). If you remove or disconnect one component the circuit is broken.

Cell Potential Differences add up

Total potential difference is shared - potential differences of components in a series circuit always add up to the source potential difference.

Current is the same everywhere in the circuit

Resistance adds up

Adding more resistors to a series circuit increases the total reistance therefore the current is reduced but the total potential difference is the same just shared across the resistors. The bigger a components resistance the bigger its share of potential difference.

PARALLEL CIRCUITS

In a parallel circuit each compenent is separately connected to the +ve and -Ve of the power supply (except ammeters which are always connected in series)

If you remove a component it ardly affects the others

Good for everyday life where you need to switch things on and off separately

Independence and Isolation

Potential difference is the same across all components (not shared)

Current is shared between branches

Adding a resistor in parallel REDUCES the total resistance

Sensing circuits

Sensing circuits can be used to turn on or incraese the power to components depending on the conditions they are in.

eg operate a fan in a bathroom