Electricity and Circuits

Resistance Resistance is friction that electricity experiences while flowing thru something. Resistance (or the lack of resistance) is used in circuits to control the flow of current. Symbol :

Short Circuit

Short circuit Very low resistance of conductors. This example circuit connects the poles of the voltage source together using a simple piece of wire. High current ! DANGER : High conversion of energy into heat ! Burns, fire ! (don’t mess with it!).

Building fire (example of short circuit consequence)‏

Short circuit (conclusion)‏ In a short circuit the magnitude of the current can be very large. The release of energy is dramatic, usually in the form of heat : melts the metal, burns insulators, can start a fire, and injuries (burns) to the man.

Simple Circuit

Simple circuit Normally circuits are constructed to make practical use of the released energy in a manner as safe as possible. One typical and practical use of a circuit is for the production of electric lighting.

Light Bulb

Light bulb (incandescent lighting)‏

RESISTANCE Electrons encounter opposition when flowing thry the filament, due to friction. This phenomenon is known as RESISTANCE. Resistance depends on material, cross section, length.

Simple electric light circuit Bulb

Inside the light bulb Conductors have low resistance. Insulators have high resistance. Resistance limits the amount of current flowing thru the circuit for a given voltage of the power supply. In the light bulb, a relatively high amount of energy is dissipated due to the resistance of the filament, and manifests as heat – so high that light is emited (filament glows).

Open circuit

Open Circuit Breaker (switch)‏

Closed Circuit Current flows through the switch. Continuity is restablished. Light bulb glows.

KEYWORDS CIRCUIT RESISTANCE LOAD BREAKER SWITCH INCANDESCENSE CURRENT VOLTAGE SHORT CIRCUIT CONTINUITY

Static Electricity + + - - + - + + + - - - + + - + - - 08/12/2017 Static electricity is when charge “builds up” on an object and then stays “static”. How the charge builds up depends on what materials are used: + + - - + - + + + - - - + + - + - -

Static Electricity 08/12/2017 + - - -

Circuit Symbols Switch Bulb A V Ammeter Voltmeter Cell Fuse Battery 08/12/2017 Switch Bulb A V Ammeter Voltmeter Cell Fuse Battery

By definition, current is “the rate of flow of charge” Electric Current 08/12/2017 Electric current is a flow of negatively charged particles (i.e. electrons). Note that electrons go from negative to positive - + e- By definition, current is “the rate of flow of charge” e-

More basic ideas… 08/12/2017 If a battery is added the current will ________ because there is a greater _____ on the electrons If a bulb is added the current will _______ because there is greater ________ in the circuit

Current in a series circuit 08/12/2017 If the current here is 2 amps… The current here will be… The current here will be… And the current here will be… In other words, the current in a series circuit is THE SAME at any point

Current in a parallel circuit 08/12/2017 A PARALLEL circuit is one where the current has a “choice of routes” Here comes the current… Half of the current will go down here (assuming the bulbs are the same)… And the rest will go down here…

Current in a parallel circuit 08/12/2017 If the current here is 6 amps And the current here will be… The current here will be… The current here will be… The current here will be…

Some example questions… 08/12/2017 3A 6A

Voltage in a series circuit 08/12/2017 If the voltage across the battery is 6V… V …and these bulbs are all identical… V V …what will the voltage across each bulb be? 2V

Voltage in a series circuit 08/12/2017 If the voltage across the battery is 6V… V V …what will the voltage across two bulbs be? 4V

Voltage in a parallel circuit 08/12/2017 If the voltage across the batteries is 4V… What is the voltage here? V 4V And here? V 4V

Summary Current is THE SAME at any point 08/12/2017 In a SERIES circuit: Current is THE SAME at any point Voltage SPLITS UP over each component In a PARALLEL circuit: Current SPLITS UP down each “strand” Voltage is THE SAME across each”strand”

An example question: 08/12/2017 6V A3 3A A1 V1 A2 V2 V3

Another example question: 08/12/2017 10V A3 3A A1 V1 A2 V2 V3

Resistance V R I Resistance = Voltage (in V) (in ) Current (in A) Georg Simon Ohm 1789-1854 08/12/2017 Resistance is anything that will RESIST a current. It is measured in Ohms, a unit named after me. The resistance of a component can be calculated using Ohm’s Law: V R I Resistance = Voltage (in V) (in ) Current (in A)

An example question: Ammeter reads 2A A V 08/12/2017 Ammeter reads 2A V A What is the resistance across this bulb? Assuming all the bulbs are the same what is the total resistance in this circuit? Voltmeter reads 10V

What is the resistance of these bulbs? More examples… 08/12/2017 3A 6V 12V 4V 2A 1A 2V 3A What is the resistance of these bulbs?

Resistance Resistance is anything that opposes an electric current. 08/12/2017 Resistance is anything that opposes an electric current. Resistance (Ohms, ) = Potential Difference (volts, V) Current (amps, A) What is the resistance of the following: A bulb with a voltage of 3V and a current of 1A. A resistor with a voltage of 12V and a current of 3A A diode with a voltage of 240V and a current of 40A A thermistor with a current of 0.5A and a voltage of 10V

Power and fuses 08/12/2017 Power is “the rate of doing work”. The amount of power being used in an electrical circuit is given by: P I V Power = voltage x current in W in V in A Using this equation we can work out the fuse rating for any appliance. For example, a 3kW (3000W) fire plugged into a 240V supply would need a current of _______ A, so a _______ amp fuse would be used (fuse values are usually 3, 5 or 13A).

Power and fuses Copy and complete the following table: Appliance 08/12/2017 Copy and complete the following table: Appliance Power rating (W) Voltage (V) Current needed (A) Fuse needed (3, 5 or 13A) Toaster 960 240 Fire 2000 Hairdryer 300 Hoover 1000 Computer 100 Stereo 80

In other words, 1 Watt = 1 Joule per second Energy and Power 08/12/2017 The POWER RATING of an appliance is simply how much energy it uses every second. In other words, 1 Watt = 1 Joule per second E T P E = Energy (in joules) P = Power (in watts) T = Time (in seconds)

Some example questions 08/12/2017 What is the power rating of a light bulb that transfers 120 joules of energy in 2 seconds? What is the power of an electric fire that transfers 10,000J of energy in 5 seconds? Farhun runs up the stairs in 5 seconds. If he transfers 1,000,000J of energy in this time what is his power rating? How much energy does a 150W light bulb transfer in a) one second, b) one minute? Shaun’s brain needs energy supplied to it at a rate of 40W. How much energy does it need during a physics lesson? Damien’s brain, being more intelligent, only needs energy at a rate of about 20W. How much energy would his brain use in a normal day?

The Cost of Electricity 08/12/2017 Electricity is measured in units called “kilowatt hours” (kWh). For example… A 3kW fire left on for 1 hour uses 3kWh of energy A 1kW toaster left on for 2 hours uses 2kWh A 0.5kW hoover left on for 4 hours uses __kWh A 200W TV left on for 5 hours uses __kWh A 2kW kettle left on for 15 minutes uses __kWh

The Cost of Electricity 08/12/2017 To work out how much a device costs we do the following: Cost of electricity = Power (kW) x time (h) x cost per kWh (p) For example, if electricity costs 8p per unit calculate the cost of the following… A 2kW fire left on for 3 hours A 0.2kW TV left on for 5 hours A 0.1kW light bulb left on for 10 hours A 0.5kW hoover left on for 1 hour 48p 8p 8p 4p

Efficiency 08/12/2017 Efficiency is a measure of how much USEFUL energy you get out of an object from the energy you put INTO it. For example, consider a TV: Light (80J) Electrical Energy (200J) Sound (40J) Heat (?) Efficiency = Useful energy out Energy in x100%

Some examples of efficiency… 08/12/2017 5000J of electrical energy are put into a motor. The motor converts this into 100J of movement energy. How efficient is it? A laptop can convert 400J of electrical energy into 240J of light and sound. What is its efficiency? Where does the rest of the energy go? A steam engine is 50% efficient. If it delivers 20,000J of movement energy how much chemical energy was put into it?