Electricity - Shawon Notes

Physics > Section 2: Electricity

a) Units

2.1 use the following units: ampere (A), coulomb (C), joule (J), ohm (Ω), second (s), volt (V), watt (W).

Unit of current: ampere (A)
Unit of charge: coulomb (C)
Unit of energy: Joule (J)
Unit of resistance: ohm (Ω)
Unit of time: second (s)
Unit of voltage or potential difference: volt (V)
Unit of Power: watt (W)

b) Mains ELectricity

2.2 understand and identify the hazards of electricity including frayed cables, long cables, damaged plugs, water around sockets, and pushing metal objects into sockets

Electricity is very useful, but it can be dangerous if it is not used safely. Broken plugs and frayed wires can expose the metal wires or parts of the plug that are carrying the electricity. Anyone touching these would get an electric shock, so they should be replaced as soon as the damage occurs. Anyone poking a metal object into a socket will also get an electric shock. Cables to electrical appliances should be kept as short as possible to prevent those causing spills. Water can conduct electricity at high voltages, so spilling water onto electrical equipment can be dangerous. Water should also be kept away from sockets and you must never use electrical equipment with wet hands.

2.3 understand the uses of insulation, double insulation, earthing, fuses and circuit breakers in a range of domestic appliances

Insulation: Some appliances are cased with insulators like plastic rather than metal to prevent user from receiving shock. This casing is called insulation.

Double Insulation: If all the pars of an appliance are insulated in such a way , so that electric current cannot be touched by the user, the appliance is said to have double insulation.

Earthing: Many appliances have a metal casing. This should be connected to earth wire so that if the live wire becomes frayed or breaks and comes into contact with the casing, the current will pass through the earth wire rather than the user. The current in the earth wire is always large enough to blow the fuse and turning off the circuit. So the user is safe from electric shock.

Fuses: Fuse is a safety device usually in the form of a cylinder or cartridge which contains a thing piece of wire made from a metal that has low melting point. If too large a current flows in the circuit the fuse wire becomes very hot and blows, shutting the circuit off. This prevents you getting a shock and reduces the possibility of an electrical fire. One the fault in the current is corrected, it should be replaced again.

Circuit Breakers: Circuit Breaker is similar to fuses. If too large a current flows in a current a switch opens making the circuit incomplete. Once the fault in the circuit is corrected, the switch is reset, usually by pressing a reset button.

2.4 understand that a current in a resistor results in the electrical transfer of energy and an increase in temperature, and how this can be used in a variety of domestic contexts

Normal wiring in the house are said to have low resistance and the current pass through them easily. Heating elements like nichrome wire have high resistance. When current flows through them current cannot pass, and the energy is transferred to heat energy and the element heats up. We use the heating effect of current in electric kettle, iron, filament lamps etc.

2.5 know and use the relationship:

power = current × voltage
P = I × V
and apply the relationship to the selection of appropriate fuses

Power is amount that represents how much voltage or energy is converted every second. It is calculated using this equation:
Power, P (in watts) = current, I (in amps) x voltage, V (in volts)
P= I x V

2.6 use the relationship between energy transferred, current, voltage and time:

energy transferred = current × voltage × time
E = I × V × t
The power of an appliance (P) tells you how much energy it converts each second. This means that the total energy (E) converted by an appliances is equal to its power multiplied by the length of time the appliance is being used.

Total energy, E(in joules) = power, P (in watts) x time, t (in seconds)
E= P x t
Since, P = I x V
E= I x V x t

2.7 understand the difference between mains electricity being alternating current (a.c.) and direct current (d.c.) being supplied by a cell or battery.

The mains electricity supply provides alternating current (a.c.). Alternating current constantly changes their direction, which is useful in electricity generator and transformers. Battery cell provide direct current (d.c.) where the current is always in the same direction.

c) Energy and potential difference in circuits

2.8 explain why a series or parallel circuit is more appropriate for particular applications, including domestic lighting

Series Circuit:

  • one switch can turn off the components on and off together
  • if one bulb ( or other component) breaks, it causes a gap in the circuit and all of the other bulbs will go off
  • the voltage supplied by the cell or mains supply is “shared” between all the components, so the more bulbs you add to a series circuit the dimmer they all become. The larger the resistance of the component, the bigger its “share of voltage”

Parallel Circuit:

  • switches can be placed in different parts of the circuit to switch each bulb on and off individually or all together
  • if one bulb (or other components) breaks, only the bulbs on the same branch of the circuit will be affected
  • each branch of the circuit receives the same voltage, so if more bulbs are added to a circuit in the parallel they all stay bright.

Decorative lights are usually wired in series. Each bulb only needs a low voltage, so even when the voltage from the mains supply is shared between them, each bulb still gets enough energy to produce light. The lights in our house are wired in parallel. Each bulb can be switched on and off separately and the brightness of the bulbs does not change.

2.9 understand that the current in a series circuit depends on the applied voltage and the number and nature of other components

In a series circuit the current is the same in all parts. Current is not used up as it passes around a circuit.

The size of the current is a series circuit depends on the voltage supplied to it, and the number and nature of the other components in the circuit. In a circuit if more cell is attached, the current will increase. If more resistance is attached to the circuit the current will get less. But current is same at all points in a series circuit.

2.10 describe how current varies with voltage in wires, resistors, metal filament lamps and diodes, and how this can be investigated experimentally

In parallel circuit, current varies with the resistance and voltage. Voltage are same at all branches.

This circuit shows a 10 Ω and 20 Ω resistor connected in parallel to a 6V cell of negligible internal resistance. The p.d. across 10 Ω and 20 Ω resistors is 6V.
I1 = 0.6A
I2 = 0.3 A
As the resistance in I2 is higher, the current is small.
I3 = I1 + I2 (The current in a parallel circuit is shared between the branches depending on the resistance.)

2.11 describe the qualitative effect of changing resistance on the current in a circuit

Resistance is inversely proportional to current. Higher resistance means lower current and higher current means lower resistance. In other words resistance is the opposite of current. Resistance blocks charge flow.

2.12 describe the qualitative variation of resistance of LDRs with illumination and of thermistors with temperature

An LDR is a light dependant resistor. Its resistance changes with the intensity of light. It has a high resistance in the dark but low in the light. A thermistor is a temperature dependant resistor. In hot conditions there will be less resistance but in cold conditions there will be high resistance.

2.13 know that lamps and LEDs can be used to indicate the presence of a current in a circuit

All lamp and LEDs emit light when current passes through them. If an LED or a lamp lights up when connected to a circuit, this shows that a current is present in the circuit.

2.14 know and use the relationship between voltage, current and resistance:

voltage = current × resistance
V = I × R

2.15 understand that current is the rate of flow of charge

The size of an electric current indicates the rate at which charge flows. Charge(Q) is measured in coulombs (C). Current is measured in amperes (A). If 1 C of charge flows along a wire every second the current passing the wire is 1A.

2.16 know and use the relationship between charge, current and time:

charge = current × time
Q = I × t

2.17 know that electric current in solid metallic conductors is a flow of negatively charged electrons

Current is the flow of charge. One coulomb of charge is equivalent of the charge carried by approximately six million, million, million (6 x 1018) negative electrons.

2.18 understand that:

  • voltage is the energy transferred per unit charge passed
  • the volt is a joule per coulomb.

d) Electric charge

2.19 identify common materials which are electrical conductors or insulators, including metals and plastics

Conductors: Electrical conductors are materials that allow current to pass through them. Conductors have free electron diffusion to pass current. Metals like copper, silver, aluminium have free electrons and can conduct electricity.

Insulators: Insulators do not conduct electricity because they don’t have free electrons. Example of insulator are plastics, rubber, wood etc.

2.20 describe experiments to investigate how insulating materials can be charged by friction

Experiment: To investigate how insulating materials can be charged by friction

Apparatus: Glass rod, silk cloth, electroscope

Procedure:

  1. Take a glass rod and silk cloth.
  2. Rub the rod with the cloth.
  3. Now, take any of the two materials near the metal plate of an electroscope.

Observation:

  1. You will notice that the leaf below will deflect.
  2. This will prove that charge can be produced by friction.

2.21 explain that positive and negative electrostatic charges are produced on materials by the loss and gain of electrons

If two material are rubbed together electrons will be transferred. The one that gains electrons will be negatively charged and the one that losses electrons will be positively charged.

2.22 understand that there are forces of attraction between unlike charges and forces of repulsion between like charges

Similar charges repel each other and unlike charges attract each other. The attraction and repulsion occurs because of electrostatic force.

2.23 explain electrostatic phenomena in terms of the movement of electrons

An electrostatic phenomenon is an event where electricity has a special effect, for example a static shock. Electrons move from one material to another. Materials with a negative charge will look for some way to earth like clouds through lightning.

2.24 explain the potential dangers of electrostatic charges, eg when fuelling aircraft and tankers

In some situations the presence of static electricity can be a disadvantage.

  • As aircraft fly through the air, they can become charged with static electricity. As the charge on an aircraft increases so too does the potential difference between it and earth. With high potential differences her is the possibility of charges escaping to the earth as a spark during refueling, which could cause an explosion. The solution to this problem is to earth the plane with a conductor as soon as it lands and before refueling commences. Fuel tankers that transport fuel on roads must also be earthed before any fuels is transferred to prevent sparks causing a fire or explosion.
  • Television screens and computer monitors become charged with static electricity as they are used. The charges attract light uncharged particles-that is dust.
  • Our clothing can, under certain circumstances become charged with static electricity. When we remove the clothes there is the possibility of receiving a small electric shock as the charges escape to the earth.

2.25 explain some uses of electrostatic charges, eg in photocopiers and inkjet printers.

Electrostatic charges can be used in electrostatic paint spraying, inkjet printers, photocopiers, electrostatic precipitators etc. In inject printers inks are given negative charges so they can drop exactly where the ink needs to be.

In photocopiers, the paper is shone in bright light which reflects to a rotating drum. The dark writings and pictures do not reflect. As a result the light removes the charges in the drum. Carbon powder attaches to the charges in the drum and the pictures and writings are pasted into a sheet of paper.

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