Edexcel Physics Unit 2 Notes

 Physics Edexcel notes for Uni 2: Electricity

Notes can be downloaded from the link given below:

Physics Edexcel Unit 2 notes download here

  Notes excerpt:

UNIT 2: ELECTRICITY

Current: is defined as flow of charges through the closed circuit. When current is more, the flow of charges is more. Unit of measurement is Ampere (A-amp).

Voltage: is defined as the electrical pressure which drives the electrical charges around the closed circuit. Higher voltages have high amount of current to be driven and hence they are more hazardous.

In diagram Circuit A, has one cells and so the current is driven at slow rate and light is dull whereas in circuit B, there ae two cells and so current is driven more quickly and so the lamp is much brighter.

• Electric shocks can occur as a result of high voltage delivered. This is very painful, and

can even cause death.

Main’s supply: is where electricity is provided for home and voltage of mains supply can be wither 120 V or 230 V, both of them are very high voltages and can result in serious accidents.

Hazards of electricity include the following:

·        Damaged plugs or bare wires.

·        Electric toasters and fryers have high voltage and using metal knife can lead to accidents.

·        Connecting many plugs to socket and overloading it which may melt wire due to high voltage.

·         Clear overhead cables and railway lines.

Alternating CURRENT (AC) and direct current (DC):

Direct current: Current flowing in one direction.

Alternating current: Voltage switches direction each second ad so current also switches direction.

In circuit diagrams: a cell is represented by 2 vertical lines: one smaller than the other.

The bigger line is the positive terminal and small is the negative terminal.

There are two types of current flow:

Conventional Current: Current flows from the positive to the negative terminal.

New theory suggests that the electrons flow from negative terminal to the positive terminal.

For the Edexcel IGCSE exams it is assumed that current flows conventionally from positive to negative terminal unless stated in exams.

Current flow is denoted in arrow.

 

Mains supply:

Most home supply use a.c. supply for home with standardised voltage. Described using the frequency which is mostly 50 Hz (cycles per second) or 60 Hz (cycle per second).

 

Electrical heating:

Resistors: appliances which provide heat to the current, marked with symbol ‘R’. The current passing through resistor provides with electrical transfer of energy into resistor’s thermal store. This causes the resistors turn red due to heat.

 

Fuses:

Used with electrical devices for safety purposes mainly to prevent fire. It contains a wire that heats up and melts down if there is too much of current.

Most common fuses used are 3 A, 5 A or 13 A. The fuse is always connected in the circuit before any appliances to the live wire to prevent damage as shown in the diagram. The supply of current is done through the ‘live’ or ‘hot’ wire and a ‘neutral’ wire is used to complete the circuit.

 

Circuit breakers:

Electromagnetic switch that opens or closes when current is more than the required amount.

Pros: quickly reset once fault has been corrected.

 

Earthing:

Installing a third wire named as ‘Earth’ wire, it is attached to the earth.

The live wire is brown in colour, the neutral is blue and earth is green or yellow. Metal are conductors of electricity and allow current to pass and plastics are insulator and do not conduct electricity.

Double insulation:

Many appliances are made up of plastics which do not conduct electricity and so can be connected only using two wires. In such cases, earth wire is not needed and this method of protection is known as double insulation, shown by symbol’.

 

Residual-current circuit breaker (RCCB):

Is a device that detects leaks of current from a circuit when current in live wire and neutral wire is not same. The leakage could cause shock to anybody getting in contact with the circuit.

 

Electrical power:

Power: defined as energy supplied per second (J/s) or watts (W) or Kilowatt (kW-1000 W).

Power can be calculated using the equation below:

P=VI

power, P.      voltage, V.       current, I

How to choose a fuse:

·        Calculate the amount of current using the above equation rearranged as I=P/V

·        Substitute your values and find out the amount of current flowing and then chose from 3 A, 5 A or 13 A fuse.

 

 

 

Energy calculations:

Energy calculations are needed while paying out bills and it can be calculated using the formulas (available in download version)

Electric circuits:

Circuit diagram: Diagram represents a continuous circuit that is connected with wires and symbol of different elements. The current passes through positive to negative terminal. A diagram is given:

Ammeter: device connected in series and helps measure the amount of current flowing through a circuit. (All circuits symbols are there at the end of document).

Switch: A closed switch means that the circuit is closed and current is flowing through the circuit.

An Open switch indicates that the current is not flowing through the circuit.

The current in the circuit can be changed by:

·        Changing the size of the battery.

·        Changing components of the circuit.

The current in a series circuit doubles when voltage is doubled and when resistance is increased the voltage reduces.

Variable resistor: a resistor whose resistance can be changed

Fixed resistor: in which resistance is fixed.

Calculating the resistance:

Ammeters: are connected in series to the resistor. It measures the amount of current flowing. Measured in Amperes or milliamperes (1 mA is 0.001 A)

Voltmeter: are connected in parallel with the resistor. Measured in Volts (V).

Resistance: Measured in ohms (ohm (Ω)). 1 kΩ is 1000 Ω.

Calculated using the equation:

R=V *I

 

Ohm’s law:

States that when temperature is kept constant and current is proportional to the voltage. The resistor in such a case is said to be ohmic resistors. The voltage against current graph shows a linear graph (straight line passing through the origin-as shown in graph 1)

Resistance can be calculated using the formula: R=V*I.

When the resistor does not behave the same i.e., voltage is not proportional to the current, then it is said to be non-ohmic. In this case the voltage against current graph is a curve (as shown in graph 2). In non-ohmic resistors, when the current increases, resistance also increases and so the temperature increases.

 

The diode:

A device that regulates the flow of current in one direction and that is a forward voltage. When voltage is reversed there is no current flowing through the circuit.

LED: Light-Emitting Diode: A device that lights up when there is current through it. When there are is dim light, there is less resistance and when light intensity increases the resistance also increases.

Thermistor: A diode which reacts to surroundings temperature, the thermistors resistance decreases when temperature increases.     

Adding resistance:

When resistors are connected in series, the total resistance is calculated by adding up the resistance of each resistor.

Current, charge and voltage:

Current: measure of rate at which the electrical charge flows.

The flow is from the positive terminal of the battery to the negative terminal.

Charge: the amount of charge flowing in a circuit is measured in Coulombs (C).

Charge and Current are linked with equation shown below:

Q=IT

Where Q=charge,

I=current,

T= time.

Equation relating voltage, charge and energy shown below:

E=QV 

E=Energy, Joules, J.

Q=Charge, Coulombs, C

V=Voltage, Volts, V

Current and voltage rules:

When appliances are connected in a series connection the amount of current flowing through each one of the appliances is equal.

If the appliances are connected in parallel, then the current is split either equally or unequally between them. But the input current is always equal to output current, so missing currents could be found using simple maths.

Rules of Current:

Current is same at any given point of series circuit.

The current splits up in parallel connection

Rules of Voltage:

In parallel: voltage across all appliances remain same. Whereas in series the voltage adds up to battery voltage.

Uses of parallel circuits:

·        Domestic appliances are connected in parallel so that all appliances receive full mains voltage and are individually operated with switch.

·        Voltmeters are always placed in parallel to measure voltage across them.

Uses of series circuits:

·        Lights of decoration are used in series connection as they require low voltages.

·        A switch is placed in series connection

·        An ammeter is connected in series

·        A resistor is connected in series with a component such as diode or thermistor to prevent overheating.

·        A fuse is connected in series and before the appliance as a safety measure.

Batteries: consists of two or more cells.

Cell; an individual cell.

Batteries connected in series as shown in circuit add up to produce a larger voltage. If they face in opposite direction then the resultant voltage is zero.