1. ELECTRICAL CIRCUITS

2. 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.

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

4. 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.

5. 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.

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

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

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

9. 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.

10. 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.

11. 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.

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

13. 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

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

15. 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

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

17. 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

18. 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.

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

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

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

22. 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.

23. measuring current & voltage

24. measuring current & voltageb) 6V 4A A V A V A

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

26. 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.

27. 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

28. 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...

29. 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 

30. 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

31. 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 ?

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

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

34. 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 

35. 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 

36. 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 

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

38. 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 ?

39. 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 ?

40. 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

41. 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

42. 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