1. Introduction to Ohm’s Law
Voltage, Current and Resistance

2. Voltage and Current review
· Electrical potential difference: A quantity that provides a measure of the electrical potential energy a unit of charge (coulomb)* gains when passing through a source such as a battery.
* One coulomb equals 6.24×1018 electrons.
· We remember from last lesson that electric potential difference is also known as voltage and is measured in volts.

3. Voltage and Current review
Why is it called electrical potential difference?
· It measures the difference in electrical potential energy per unit of charge (coulomb) between the positive terminal and the negative terminal in an electrochemical cell.
· When we measure the voltage (potential difference) in a circuit, we always measure across two points.

4. POTENTIAL DIFFERENCE
Just like the water at the top of the waterfall has more potential energy so to do the charges at the negative end of the battery.
MORE POTENTIAL ENERGY

5. Voltage and Current review
· Electric current: is the flow of electric charge and is measured in amperes.
· 1 A is equal to one coulomb of charge passing a point on a circuit every second.

6. The figure here shows how current flows through a simple, closed circuit. Use your own words to explain what is happening at A, B, and C in the figure. Complete this in your notes.

7. Part A
•Negative terminal repels the negative charges already in the conductor
•Positive terminal attracts the negative charges already in the conductor
•Electrons move along the conducting wires; electrons from the electrochemical cell move into the conductor

8. Part B
•As the electrons pass through the load, they transfer some of their energy to the load
•The electrons then leave the load and return to the electrochemical cell

9. Part C
•Electrons enter the electrochemical cell; combine with positive ions to become neutral.
•Over time: fewer electrons at negative terminal; fewer positive ions at positive terminal.
•The worker (chemical energy) can carry more electrons up the ladder, keeping the number of separated charges equal.

10. Resistance
· Recall that a load is a device that converts electrical energy into another form of energy. (eg. heater, lightbulb, toaster etc.)
· A load resists the flow of electrons.
· As electrons pass through a load, they lose energy as electrical energy is transformed to another type of energy (heat, sound, mechanical etc.).
· Resistance: Describes the amount that current is hindered by a load.
· Resistance is represented by the symbol R and measured in ohms (___).

11. Example of Resistance: Filament in a lightbulb
How does it appear that a lightbulb filament resists the flow of electrons?
Charges move from a large wire (electrical cord) into a very thin wire (filament)
Since the charges have less room in the filament (the filament resists the movement of charges), they collide with atoms so hard that the filament gets very hot
The heat makes the filament glow (“light up”)

12. Resistors
· In some electrical circuits special metal wire or carbon components called resistors are made for the purpose of creating a precise quantity of resistance for insertion into a circuit.
· Although they produce heat, the purpose of a resistor is to simply to provide a precise quantity of electrical resistance.

13. Resistors on circuit boards

14. Resistors Resistors use colour coding to tell us their value in ohms.
For a four band resistor, we use the format below with the chart:

15. 10 x 102 𝛀 +/- 5% or 1000 𝛀 +/- 5% Resistors
For example, the resistor in the picture below has bands: brown (1),, black (0),, red (2) and gold as the “tolerance” or “percentage error”
Therefore the resistance value is:
10 x 102 𝛀 +/- 5%
or 1000 𝛀 +/- 5%
For “percentage error” - red is 2%, gold is 5%, silver is 10% and no band is 20%
More info here:

16. Complete these problems on your sheet:
Calculate the size of a resistor. (don’t forget units!)
a) yellow, red, brown, gold
Answer: 420 Ohms 𝛀 +/- 5%
b) violet, green, red, silver
Answer: 7500 𝛀 +/- 10%
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17. Ohm’s Law
Ohm’s Law states that the relationship of voltage (V), current (I), and resistance (R) is given by the equation: V=IR.

18. Example problem #1
An source of 6.0V is connected to a purely resistive lamp and a current of 2.0 A flows. What is the resistance of the lamp?
R=V/I
R= 6.0V/2.0A
R = 3.0 Ω

19. Example problem #2
A nine volt battery supplies power to a cordless curling iron with a resistance of 18 Ω. How much current is flowing through the curling iron?
I = V/R
I = 9V/18V
I = 0.5 A