1. Electric 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. - +
In which direction does the current flow?
from the Negative terminal to the Positive terminal of a cell.

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. Electrical Circuit Symbols
Electrical circuits often contain one or more resistors grouped together and attached to an energy source, such as a battery.
The following symbols are often used:
Ground
Battery - +
Resistor

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;
switch
Cell Battery
Bulbs
ammeter
voltmeter
motor
resistors

8. Laboratory Circuit Symbols
Rheostat
Emf - + V
Source of EMF
Voltmeter
Ammeter
Rheostat

9. Electromotive Force
A source of electromotive force (emf) is a device that uses chemical, mechanical or other energy to provide the potential difference necessary for electric current.
Power lines
Battery
Wind generator

10. Water Analogy to EMF
Low pressure
Pump
Water
High pressure
Valve
Water Flow
Constriction
Source of EMF
Resistor
High potential
Low potential
Switch R I + -
The source of emf (pump) provides the voltage (pressure) to force electrons (water) through electric resistance (narrow constriction).

11. Electric Current
Electric current [I] is the rate of the flow of charge Q through a cross-section A in a unit of time t. A + -
Wire +Q t
One ampere A is charge flowing at the rate of one coulomb per second.

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

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

14. Electric Resistance
Suppose we apply a constant potential difference of 4 V to the ends of geometrically similar rods of, say: steel, copper, and glass.
4 V
Steel
Copper
Glass Is Ic Ig
The current in glass is much less than for steel or iron, suggesting a property of materials called electrical resistance R.

15. Factors Affecting Resistance
1. The length L of the material. Longer materials have greater resistance.
1 W L
2 W 2L
2. The cross-sectional area A of the material. Larger areas offer LESS resistance.
1 W 2A
2 W A

16. Factors Affecting R (Cont.)
3. The temperature T of the material. The higher temperatures usually result in higher resistances.
R > Ro Ro
4. The kind of material. Iron has more electrical resistance than a geometrically similar copper conductor.
Ri > Rc
Copper
Iron

17. I = V / R Ohm’s Law Georg Simon Ohm (1787-1854)
I = Current (Amperes) (amps)
V = Voltage (Volts)
R = Resistance (ohms)
Georg Simon Ohm ( )

18. Ohm’s Law
Ohm’s law states that the current I through a given conductor is directly proportional to the potential difference V between its end points.
Ohm’s law allows us to define resistance R and to write the following forms of the law:

19. Ohm’s Law Review R=V or V = IR or I = V I R
R= resistance (ohms ) ( Ω )
V = Voltage (volts ) ( V )
I = Current (amps) ( A )
R=V or V = IR or I = V
I R

20. Example 1
The lights of a lantern are powered by a 24 V battery. What is the resistance if they draw 0.80 A of current when turned on?
24 V =
.80A
R= V = I
30 Ω

21. Example 2 When a 3-V battery is connected to a light, a current of
Example 2 When a 3-V battery is connected to a light, a current of .006 A is observed. What is the resistance of the light filament?
R = 500 W
Source of EMF
R=? I= + -
V = 3 V
.006 A
The SI unit for electrical resistance is the ohm, W: