P2-Electricity

Current and Circuit Symbols

Current= the flow of electrical charge. Electrical charge will only flow round a complete circuit if there is a potential difference. The unit of current is ampere. In a single closed loop the current has the same value everywhere in the circuit.

Potential difference is the driving force that pushes the charge around units volt. Resistance is anything that slows the flow down units ohm. The current flowing through a component depends on its potential difference across it and the resistance of the component.

THE GREATER THE RESISTANCE ACROSS A COMPONENT, THE SMALLER THE CURRENT THAT FLOWS.

Total charge through a circuit depends on current and time. 

CHARGE FLOW= CURRENT X TIME

(Circuit symbols)

Resistance and V=IR

POTENTIAL DIFFERENCE= CURRENT X RESISTANCE

Practical:

You can investiaget the effect of wire length using the (circuit below)

Attatch a crocodile clip to the wire level with 0cm on the ruler. Attatch the second crocodile clip to the wire. Write down the length of the wire between the clips. Close the switch then record the current though the wire and the pd across it. Open the switch then move the second crocodile clip along the wire. Close the switch and record the new length, current and pd.Repeat and then calculate the resistance for each lengthof wire using the results.

The ammeter= measures the current flowing through the test wire. The ammeter must alwasy be placed in series with whatever your investigating.

The voltmeter= measures the potential difference across the test wire. The volt meter must alwasy be placed parallel around whatever your investigating.

Resistance and IV Characteristics

Ohmic conductors have a constant resistance. The resistance of ohmic conductors (a wire or resistor) does change with the current. At a constant temperature the current flowing through is directly proportional to the potential difference across it. The resistance of some resistors and components does change.

Three IV characteristics:

1. Ohmic conductor- eg a resistor at a constant temperature. The current through a ohmic conductor is directly proportional to pd so you get a straight line.

2. Filament lamp- As the current increases the temperature of the filament lamp increases. This means less current can flow per unit pd so the graph gets shallower so its a curve.

3. Diode- current will only flow through a diode in one direction. The diode has a very high resistance in the reverse direction.

Circuit Devices

Light dependant resistor= a resistor that is dependent on the intensity of light. In bright light, the resistance falls. In darkness the resistance is highest. They have lots of applications including automatic night lights, outdoor lighting and buglar detectors.

Thermistor= a tempertaure dependent resistor. In hot conditions the resistance drops. In cool conditions the resistance goes up. Thermistors make useful temperature detectors, eg- car engine temperature sensors and electronic thermostats.

Series Circuits

In series circuits, the different components are connected in a line, end to end, between the +ve and -ve of the power supply. 

If you remove or…