Physics unit 1 electricity aqa

Electricity Notes

Current: The rate of flow of charge. (Amperes A)

Potential Difference: Work done per unit charge. (Voltage V)

Ammeters are placed in series and have a very low resistance. Voltmeters are placed in parallel and have a very high resistance. We can investigate current and potential difference by using:

·         A variable resistor – changes current to a minimum.

·         A potential divider – varies pd from zero (advantage) to a maximum.

We can replace the ammeter and voltmeter with current and voltage sensors, allowing us to use data loggers to capture data on a computer.

Power: a measure of the rate at which energy is used. (Watts W) 1 watt of power means 1 joule of energy is used every second.

Resistance:

Resistance: opposition to the flow of current.

Resistance is caused by repeated collisions between the charge carriers in the materials with each other and with the fixed positive ions in the material. The higher the temperature, the faster the metal atoms vibrate, and the more likely they are to impede electron flow, increasing the number of collisions and hence increasing resistance.

Ohms law: the pd across a component is directly proportional to the current through it, provided the temperature I constant. (Meaning an Ohmic conductor has constant resistance).

In Series:

In Parallel:  (If 2 identical resistors, eg 2 x 10Ω resistors = 5Ω) (the overall resistance will always be less than the lowest resistor value in a parallel circuit).

Insulators: Very high resistance. No free electrons exist to move in the material when a pd is across it. At mains voltage (230V) a small current will flow, but at a high pd.

Conductor: Metals contain free electrons within the metal lattice which flow towards the positive of the battery.

Semi-Conductors: the number of charge carriers increases as the temperature increases, so they conduct better at a high temperature (thermistor), or as light increases (LDR). This reduces their resistance.

Thermistor: (negative temperature coefficient NTC) decreasing resistance as temp increases because electrons in the metal oxide of the thermistor gain energy, increasing the number of charge carriers, decreasing resistance.

LDR: (light dependant resistor) decreasing resistance as light intensity increases because photons of light hit a cadmium sulphide track, they give bound electrons enough energy to jump into the conduction band.

Rules:

Series:

·         Current is the same all the way around the circuit.

·         Voltage is shared between the components in the circuit in ration of their resistances. Higher resistance components will get a bigger share of the voltage.

Parallel:

·         The current split up to…