 # ELECTRICAL CIRCUITS.

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ELECTRICAL CIRCUITS

The CELL The cell stores chemical energy and transfers it to electrical energy when a circuit is connected. When two or more cells are connected together we call this a Battery. The cells chemical energy is used up pushing a current round a circuit.

What is an electric current?
An electric current is a flow of microscopic particles called electrons flowing through wires and components. - +

simple circuits Here is a simple electric circuit. It has a cell, a lamp and a switch. wires cell lamp switch To make the circuit, these components are connected together with metal connecting wires.

simple circuits When the switch is closed, the lamp lights up. This is because there is a continuous path of metal for the electric current to flow around. If there were any breaks in the circuit, the current could not flow.

circuit diagram Scientists usually draw electric circuits using symbols; cell lamp switch wires

circuit diagrams In circuit diagrams components are represented by the following symbols; cell battery switch lamp buzzer ammeter voltmeter motor resistor variable resistor

types of circuit There are two types of electrical circuits;
SERIES CIRCUITS PARALLEL CIRCUITS

SERIES CIRCUITS The components are connected end-to-end, one after the other. They make a simple loop for the current to flow round. If one bulb ‘blows’ it breaks the whole circuit and all the bulbs go out.

Light Bulbs in Series Lights connected in a row, one after the other are IN SERIES REMOVE one bulb and the circuit is BROKEN If Circuit is BROKEN, they all go out.

PARALLEL CIRCUITS The components are connected side by side.
The current has a choice of routes. If one bulb ‘blows’ there will still be a complete circuit to the other bulb so it stays alight.

measuring current Electric current is measured in amps (A) using an ammeter connected in series in the circuit. A

Light Bulbs in Parallel
Lights connected on different wires that share the same current flow as the original wire provide multiple paths for electricity to flow. REMOVE one bulb and the circuit is STILL ACTIVE IN OTHER AREAS. Other remaining bulbs still have current and glow

measuring current A A This is how we draw an ammeter in a circuit.
PARALLEL CIRCUIT SERIES CIRCUIT

measuring current SERIES CIRCUIT current is the same
at all points in the circuit. 2A 2A PARALLEL CIRCUIT 2A 2A current is shared between the components 1A 1A

copy the following circuits and fill in the missing ammeter readings.
? 3A 3A 4A ? 1A ? ? 4A 4A 1A ? 1A

measuring voltage The ‘electrical push’ which the cell gives to the current is called the voltage. It is measured in volts (V) on a voltmeter V

measuring voltage Different cells produce different voltages. The bigger the voltage supplied by the cell, the bigger the current. Unlike an ammeter a voltmeter is connected across the components Scientist usually use the term Potential Difference (pd) when they talk about voltage.

measuring voltage V V This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT

series circuit voltage is shared between the components 3V 1.5V 1.5V

parallel circuit voltage is the same in all parts of the circuit. 3V

measuring current & voltage
copy the following circuits on the next two slides. complete the missing current and voltage readings. remember the rules for current and voltage in series and parallel circuits.

measuring current & voltage

measuring current & voltage
b) 6V 4A A V A V A

answers a) b) 6V 6V 4A 4A 6V 4A 4A 3V 3V 2A 4A 6V 2A

SERIES CIRCUITS - RESISTANCE
Resistance means to “resist” or slow down the flow of current. RESISTORS are devices, like light bulbs, that make use of this slowing down of current.

Total Resistance in Series
Resistance is measured with an Ohmmeter, named after the unit of Resistance, the Ohm . In a series circuit, you ADD all the resistance in that wire section together. 3  3  + 3  = 6  total

PARALLEL CIRCUITS - RESISTANCE
Resistance along a parallel circuit is reduced because you give the current another way to go, multiple paths!! BUT THINGS GET WEEEEEIRD when you add these up...

Total Resistance in Parallel
(1/R1 + 1/R2) = 1/Rtotal In a parallel circuit, you ADD the INVERSE of all the resistance in that wire section together and then take the inverse of the answer… You are making the pathway BIGGER for the current to flow through by putting resistors side by side – parallel. 3  3  1/3  + 1/3  = 2/3  now flip and divide 1/Rtotal = 2/3 (then press 1/x button) = 1.5 

Results When you added these two together inversely, the total resistance was LOWER than either of the resistors. That is because you are providing more area for the charge to flow, so you are reducing resistance. 3  3  1.5  Total

WHICH IS BRIGHTER? So… you know that resistance is different in these circuits. Which type of circuit would allow current to flow faster, making the brightest bulbs? SERIES CIRCUITS PARALLEL CIRCUITS ?

WHICH IS BRIGHTER? The bulbs in PARALLEL will be brighter because the circuit allows current to flow more easily SERIES CIRCUITS PARALLEL CIRCUITS

Combination Circuits Combo circuits contain both parallel and series portions in one single circuit.

Solving Combo Circuits – STEP 1
Solve for the Current going through the total circuit. STEP 1 Calculate total resistance of Parallel path first. (1/3 + 1/9) = 1/0.44 = 2.25 

Solving Combo Circuits – STEP 2
Solve for the Current going through the total circuit. STEP 2 Now it is as if the circuit is just a bunch of resistors in series, EASY! Add the resistance together to get the total: =  2.25 

Solving Combo Circuits – STEP 3
Solve for the Current going through the total circuit. STEP 3 Total R =  I = V / R = 120/20.25 I = 6 A 2.25 

Solving Combo Circuits CHALLENGE
Solve for the Current going over the 3  resistor ONLY

Solving Combo Circuits – STEP 1
Solve for the Current going over the 3  resistor STEP 1 Remember that current along a series circuit is always the same at all points. Current in a parallel circuit is shared according to I = V / R 6A ?

Solving Combo Circuits – STEP 2
Solve for the Current going over the 3  resistor STEP 2 Must find the voltage going across the parallel path!!! Voltage at parallel branch = the total I X total R of the branch Total I = 6 A Total R = 2.25  V = 13.5 V 6A 13.5 V ?

Solving Combo Circuits – STEP 3
Solve for the Current going over the 3  resistor STEP 3 V = 13.5 V Now just calculate current at 3  resistor I = V / R = 13.5 / 3 = 4.5 A And at the 9  resistor I = V / R = 13.5 / 9 = 1.5 A 6A 13.5 V 4.5 A 1.5 A

Power in Circuits Remember that power is just a measure of how fast you do work. In circuits Power is the same. Work is voltage and the rate is Current Unit of Power is a Watt P = VI

Calculate Current with Power
A 40 Watt light bulb is connected to a standard 120 V outlet. What is the current being drawn through the bulb? P = VI… I = P / V = 40 / 120 = 0.33 A A brighter 100 W bulb? 0.83 A More current = More power usually