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Electricity

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### Transcript of Electricity. Topics 1.What is Electricity? 2.Electrical Circuits 3.Practical Electricity 4.Effects...

Electricity

Topics

1. What is Electricity?2. Electrical Circuits3. Practical Electricity4. Effects of Electricity

Recall

• When we introduced protons and electrons in chemistry, we said that protons are positively charged and electrons are negatively charged

• We also noted that when an atom has unbalanced protons and electrons, it becomes an ion. Ions can either be positively or negatively charged.

• We also noted that equal amounts of positive and negative charges cancel each other out, resulting in no net charge

What is electricity?

• Electricity is the study of what happens when charges move

• When charges move, it is said to form an electric current.

Electric Current

• Symbol for Current: I• S.I. Units for Current: A (Ampere or Amps)• Current depends on two things– How many charges are moving– How fast the charges are moving

• Current has direction; the direction of current depends on how the charges are moving

Direction of Current

• When positive charges move, the direction of the current is in the same direction as the positive charge

• When negative charges move, the direction of the current is in the opposite direction as the negative charge

Test Yourself!

• What is the direction of current in each of the examples below?

• 1) Sodium ions moving to the left• 2) electrons moving upwards• 3) a neutral helium atom moving downwards

• Ans: 1) left, 2) downwards, 3) no current

Electric Current• 99% of the time, we are interested in electric

current in wires (i.e. metallic conductors)• In these situations, it is electrons which are

moving in the metals• Current flows in the OPPOSITE direction of

electron flow• We distinguish between this by calling direction

of conventional current or electron flow• (why so confusing? how this came about was a

result of an unlucky guess)

Measuring Current

• We measure current using an instrument called an ammeter

• If the ammeter is connected wrongly, the needle will attempt to go left (i.e. the negative direction)

• Just reverse the connections of the ammeter (or the battery) to get an ammeter reading

Why is there a current?• We have established that when charges move,

there is a current• But WHY do charges move in the first place? Is

there something which pushes them to move?• Think about it: if I connect a circuit without a

battery is there a current? Therefore, somehow the battery is essential in “pushing” the charges to move!

• The more batteries we use, the harder the charges are being pushed!

Voltage

• We might have said before that “the voltage of a battery is 1.5 volts”

• “If we use two batteries together, the voltage increases to 3 volts”

• How do I understand what this voltage is?• 2 types of “voltage”:– Electromotive Force (e.m.f.)– Potential Difference (p.d.)

Analogy: Water Slide

• Height water gets pumped: how high the voltage

• Water pump pushing water upwards – electromotive force

• Water going downwards (and doing work) – potential difference

Measuring Voltage

• Voltage is measured using a voltmeter• Note that voltmeter must measure at two

difference points• Just like height difference needs 2 different

points to measure• Both e.m.f. as well as p.d. can use voltmeter to

measure• Usually the symbol “V” is used to represent

voltage (note: units also symbol “V”!!)

Resistance

• Recall that current is the flow of electrons• Imagine you are an electron trying to move in

one direction. You enter a large street with very few people inside. Can you move easily down this street?

• Now you reach a narrow alleyway, which is crowded with people. You have to move through this alleyway. How is your movement affected?

Resistance• Similarly, when an electric current moves through a

circuit, it may pass through different circuit components

• Some components are like the wide street, allowing the current to pass through easily. These components are said to have low resistance

• Some components are like the narrow alley. These components are said to have high resistance

• Note: in theory we assume connecting wires to have zero resistance

Resistance

• Symbol for resistance is R• Units for resistance is Ohm (symbol Ω )• In Physics, there is a mathematical definition

for resistance of a component, it is:• R = V/I– V is potential difference across the component– I is current

Test Yourself

• A lightbulb has a resistance of 5.0 ohms. What is the potential difference across the lightbulb when a 1.5 A current is passing through it?

High Resistance Wire

• We assume connecting wires (i.e. copper wires) have zero resistance

• But you may come across high resistance wire (e.g. nichrome)

• The thicker the wire, the lower the resistance (imagine the street being wide)

• The longer the wire, the higher the resistance (imagine the electron has a longer path to travel)

Rheostat

• A rheostat is a device which can change it’s resistance

• It’s made of two parts: a copper bar (which has zero resistance) and a high resistance coil, and a sliding contact (which connects the two)

SERIES AND PARALLEL

Recap

• Important Terms you’ve learnt so far:• Current (symbol: I, units: A)• Voltage (symbol: V, units: V)– can be either electromotive force (e.m.f.) or

potential difference (p.d.)• Resistance (symbol: R, units: Ω)• Definition of resistance: R = V/I

Resistors in Series

• If there are two or more resistors in series, the total resistance is given by:

• Rtotal = R1 + R2 + R3 + …..

Example

• What is the total resistance of this arrangement of resistors?

• Rtotal = 1+2+3 = 6.00 Ω

1 Ω 2 Ω 3 Ω

Resistors in Parallel

• When there are two or more resistors in parallel, the total resistance is given by:

• 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + …..

Example

• What is the total resistance of this arrangement of resistors?

• 1/Rtotal = ½ + ¼ = ¾

• Rtotal = 4/3 = 1.33 Ω (3 sf)

2 Ω

4 Ω

Voltmeters and Ammeters

• When measuring the current, an Ammeter is always connected in series

• When measuring voltage across a particular component, a voltmeter is always connected in parallel (across that component)

Measuring Resistance• Simple Circuit Diagram:

• R = V/I

V

A

Finding Effective Resistance

• You may be asked to find a mixture of resistors in series and parallel

• The method to use is to replace a cluster of resistors with an effective resistor of the same resistance

Example

• What is the total resistance of this arrangement of resistors?

• Step 1: find the subtotal of the parallel resistors first• Step 2: add this subtotal to the other resistor in

series• Ans: 3.71 Ω (3sf)

2 Ω3 Ω

4 Ω

Current and Voltage in Parallel

• When a circuit breaks into two (or more) branches, it is said to be a parallel circuit

• You are required to know how to determine current and voltage (potential difference) in parallel circuits

Current in Parallel

• In a parallel circuit, there must be branches• Current follows the “what goes in must come

out” rule

Example

• What is the value of I?

I0.3 A

0.2 A

Example

• What is the value and direction of current in wire X?

0.2 A

0.3 A

0.2 A

X

Potential Difference in Parallel

• p.d. is the same across parallel branches

Example

• What is the reading of voltmeter X?

V

V

Voltmeter X

4.0 V

PRACTICAL ELECTRICITY

Power Rating

• If you look a your own electrical appliances at home, they will come with a power rating, e.g. “240 V, 60 W”

• What does this rating mean?• This rating tells you two things:1. This device was designed to be run on 240 V2. When the voltage used to power the appliance

is 240 V, then it will produce 60 W of power

Power and Energy

• Power is how much energy used per unit time• Symbol: P• Units: Watts (W)• Power = Energy / t• P = E/t• 1 Watt is 1 Joule of Energy per Second• E.g. a 60 W light bulb uses up 60 Joules of

energy per second

Power, Current and Voltage

• Current, voltage and power is related by the following equation:

• P = IV– Power (P) in Watts (W)– Current (I) in Ampere (A)– Voltage (V) in Volts (V)

Example

• A light bulb has a current of 0.1 A and a p.d. of 1.5 V.

• (i) Determine the Power of the light bulb.• (ii) Determine the energy consumed by the

bulb if it was left on for one minute.

Kilo-Watt Hour

• In real life, we have to pay money for our electricity usage in our utility bills

• We pay for the amount of energy we use per month

• However, the amount of energy we use is so large, we do not use Joules as units, instead we use the units of KiloWatt Hour (kWh)

• 1 kWh = 1000 Watts x 1 hour• Price of electricity is usually in cents per kWh

Example

• The price of electricity is 27 cents per kWh. Determine how much it costs in total to use a 3kW kettle for 20 minutes and a 100 W bulb for 5 hours.

• Tip: convert all units of power to kW, and all units of time to hours

ELECTRIC SAFETY

Electrical Mains

• Recall: in order for current to flow through a component, you need TWO connections

• A light bulb will not work if only one side is connected to a battery – that’s still an open circuit

• Your electrical mains has 3 connections, the live, neutral & Earth

Electrical Mains

• 1) Live – this wire is at high potential (“high voltage”). The wire is brown in colour. The Fuse is also attached to the Live Wire

• 2) Neutral – this wire is maintained zero potential. The wire is blue in colour.

• 3) Earth – this wire is connected to the Earth. It is yellow/green in colour.

Electrical Mains• Should you touch the live wire and your feet

are not insulated, current will flow through you to/from the Earth (through your feet), this may result in electric shocks/electrocution

• It is safe to touch the Neutral or Earth wires, no current will flow

• This is why the fuse is attached to the live wire, should a short circuit happen a large current will flow, and the fuse will blow, disconnecting the live wire.

Electrical Mains

• This also explains why the switch is attached to the live wire – so that the live wire is disconnected when the appliance is not in use.

• Every household also has a circuit breaker, which is designed to cut the circuit when a large current flows (works using electromagnetic means

Electrical Mains

• When an appliance is connected to the mains, it is connected to the live and neutral connections.

• If the appliance is has a metal exterior, the metal exterior is connected to the Earth.

Electrical Mains

• What happens when the live wire accidentally makes contact with the metal surface of an appliance?

• If it there is no fuse & no Earth wire, a human touching the appliance may get electrocuted (current flows through the human to/from the ground)

• In reality, a large current will momentarily flow from live wire to Earth, blowing the fuse in the process

Electrical Mains

• Some appliances have non-metallic exteriors (e.g. made of plastic). This is called double insulation. These appliances do not need an Earth wire, and they may use only 2 pin plugs.

Summary of Safety FeaturesSafety Feature How it WorksSwitch on Live Wire Live wire is disconnected when appliance

is not in useFuse on Live Wire Blows if current exceeds fuse rating,

preventing large current from flowingEarth Wire connected to outer metal surface of appliance

Prevents humans from being exposed to high voltage should live wire touch casing by accident

Double Insulation Humans not exposed to high voltage, even if live wire touches outer casing

Circuit Breaker Box Cuts current off should current flow be too large

3 Pin Plug• Each pin of the three pin plug is connected to

one of the following:• Brown Wire – to live• Blue Wire – to neutral• Yellow/Green Wire – to Earth• The brown wire is also connected to a fuse.

This fuse is meant to protect the appliance (not humans) should current flow be too large.

3 Pin Plug

EFFECTS OF ELECTRICITY

Heating Effect

• Some electrical devices convert electrical energy to thermal energy (e.g. iron, electric heater, kettle, light bulb, etc.)

• Usually this is done using a heating element• IMPORTANT: your notes and textbook are

incorrect. A heating element does NOT need to have high resistance. In fact, the lower the resistance, the greater the heating effect (but for safety, we don’t want resistance to be too low)

Chemical Effect

• Electrolysis – Passing an electric current through a compound can break it apart into its constituents atoms

• Pass current through water can result in oxygen and hydrogen gas

• Pass current through molten sodium chloride in molten sodium metal and chlorine gas

Magnetic Effect

• Coil a wire around a piece of iron / steel• Passing current through the wire will result in

the formation of an electromagnet• Iron – Temporary Magnet (will no longer be

magnetic when current is off)• Steel – Permanent Magnet (will still be

magnetic when current is off)