“Zip Zap”

Electrical Safety

Charges

What is a Circuit?• An electric circuit consists of:

– A source of electrical energy (battery or cell)– A user of electrical energy (eg lamp)– Connecting wires

Component Symbol Function

Cell An energy supply: converts chemical potential energy to electrical energy

Battery Cells connected together

Switch Allows user to switch current on/off

Lamp Converts electrical energy to light + heat

Resistor Transforms electrical energy into heat energy (by slowing down the passage of electrons)

Variable

Resistor

Allows user to control the amount of electrical energy moving through the cricuit

Ammeter Measures current

Voltmeter Measures voltage

Wire Conductor connecting up parts of the circuit

Crossing wires

(connected, not connected)

Fuse Breaks / melts when current is too high

Diode Allows current to flow in one direction only

Name Symbol FunctionSwitch

Two way

switch

*Push button switch (normally off)

*Push button switch (normally on)

*Motor

*Buzzer

Rules for Drawing Circuit Diagrams

3D Drawing Circuit diagram

• Use a ruler

• No gaps / overlaps in wires

• Neat

• Organised

Eg:

ConductorsElectric current is the flow of electrons

Conductors• These are materials with electrons held loosely in

atoms• When connected to a cell these electrons can be

made to move (electric current)• Examples: all metals, graphite, salt water, acids• Uses: where you want current to flow (power lines,

circuits)• Static: conductors can’t get a build up of static

charges because the charges move (producing current) as soon as they are separated.

InsulatorsInsulators• Are materials with electrons held tightly in atoms• Connecting to a cell can’t make the electrons move

(no current)• Examples: plastic, glass, rubber, wood• Uses: where you don’t want current to flow, to protect

users against electric shock (sheath of wires, handles of electrician’s tools)

• Static: insulators can get a build up of static charges because when the charges are separated by friction they stay there

This means:Conductors are also poor insulatorsInsulators are also poor conductors

Examples

Conductors Insulators

Series Circuits

• In a series circuit the components are connected one after another in a row.

• In series:– lamps are dimmer, – but the battery lasts

longer (less current is drawn from it)

– if one bulb blows, the others won’t work.

Parallel Circuits• In a parallel circuit the

components are connect in parallel branches of the circuit

• In parallel: – lamps are brighter, – but the battery goes flat

more quickly (more current is drawn from it).

– if one bulb blows, the others will still go.

Problem Solving• You have a stairway with a light in the middle. You want

to be able to control the light from the bottom and top of the stairs

Design a circuit that will achieve this.

• Flipping either of the two way switches closes the circuit. So you can install one switch at the bottom and one at the top of a flight of stairs and both will turn on the light.

Problem Solving

• Design a circuit that will operate a doorbell buzzer

Problem Solving

• Design a circuit that will turn off a fridge light when the door closes

Voltage

• “A measure of how much energy can be given to moving electrons in a circuit”

• When electrons go through a cell they gain energy

• When electrons go through a user (eg lamp) they lose energy.

• Voltage is measured in volts (V) using a voltmeter.

• The voltmeter measures gain/loss of energy in volts (V). * This is also called “potential difference”

• 1V = 1000 millivolts (mV)

*Energy Gains & Losses

• A 9V battery gives an electron a parcel of energy of 9 V

• When you measure voltage across a user you are measuring the electrical energy needed to get through that component; eg 5V across a lamp means each electron gives up 5V of energy to get through that lamp

Using a Voltmeter

• Connect in parallel around a component: this is so that it can measure the amount of energy the electrons use (eg in a lamp) or gain (eg when moving through a cell).

• Connect positive terminal to positive terminal of battery / cell (and negative to negative)

+ -

Voltage and circuits

• An electron leaves the battery with a package of energy and returns to the battery with nothing.

Voltage in Series

• In series, the voltage is shared amongst the users and it adds up to the total energy that left the battery.

Voltage in Parallel

• In parallel branches the voltage is the same in each branch because each electron only goes through one branch and must drop off all energy in that branch and return with 0 volts

Current

• “A measure of the rate of flow of electrons passing a point in a circuit each second”

• Current is measured in Amperes or Amps (A) using an ammeter. 1A = 1000 mA.

• *1A = 6.25x1018 electrons per second

(6,250,000,000,000,000,000 per second!)

Using an Ammeter

• Connect in series with a component: this is so that it can measure the number of electrons going through the component (eg lamp or cell) per second

• Connect positive terminal to positive terminal of battery / cell (and negative to negative)

+

-

Current and Circuits

• The amount of current that leaves the cell also returns to the cell because this is the number of electrons moving through the circuit.

Current in Series

• Current stays the same everywhere in a series circuit

Current in Parallel

• Current in the parallel parts of a circuit adds up to the current in the main branch.

Finding Lamp PowerAim: To find out the power used by a lamp.Method:

1 Set up the circuit (below)2 Use P = VI to find the power used by the lamp.

Results:VoltageCurrentPower used = V x I

==

Finding Lamp ResistanceAim: To find the resistance of a lampMethod:

1 Set up the circuit (below)2 Use R = V / I to find the power used by the lamp.

Results:VoltageCurrentPower used = V / I

==

Resistances in Series and Parallel - *Extension

The total resistance increases when resistors are added in series.

Rtotal = R1 + R2 + …

The total resistance decreases when resistors are added in parallel.

1/Rtotal = 1/R1 + 1/R2 + …

Rtotal = total resistance of the arrangement of resistors, ΩR1 = resistance of the first resistor, Ω.

Fuses

• Make a steel wool fuse

PowerPower is a measure of electrical energy transferred (gained or used) per second. For example, a 60 Watt light bulb uses 60 Joules of energy every second.

Power has the symbol P and is measured in Watts, W.

P = V x I

P = power measure in watts, symbol: WV = voltage across a component, measured in volts, symbol: VI = current through that component, measured in amps, symbol: A

*Power could also be found using this equation: P = E / t (E is energy used or supplied in joules, t is time the energy is used or supplied for in seconds). So, if a lamp uses 60J in 1s it would have a wattage of 60W (same as above)

Energy Conservation & Changes

The law of the conservation of energy states that “energy cannot be created or destroyed it can only be transferred or transformed from one form into another”

Example: a lamp – converts the electrical energy it receives into heat and light (energy in = energy out)

Lamp: electrical energy heat + light

Billing Electricity• You are billed in “units” what are they?• 1 unit:

= 1kwh (one kilowatt hour)

= 1000 watts for an hour

= 1000 joules per second for an hour

= 1000 joules per second for 3600 s

= 3,600,000 joules

• Prices vary but a unit may cost between $0.21 and $0.25

• Why do they bill you in units, not joules?• Power Company is a misnomer – what should they be called

instead?

Energy

The total energy used by a component is the power of the component multiplied by the time it was used for (in seconds).

E = P x t

E = total energy used, JP = power rating of the component, Wt = time the component was used for, s.

At What Cost?

10Mi are notorious for charging their laptops at school. Is the cost significant to the school? If so, should the school add an extra charge to accounts to cover this?

Cost = power (in kW) x hours used x unit cost

10mi (2013) est of $763 for all laptops (based on 21.41c (actually probably 10c). Total bill hostel + school inc heating $200,000