1. Current and Resistance
An introduction to Ohm’s Law

2. Recall: Potential Difference

3. Potential Difference Provided by the battery
AKA voltage measured in volts
It is a “push” for the electrons in the wire that causes them to flow.
The battery DOES NOT supply the electrons, they are already in the wire.

4. Current: The rate at which electrons flow through a conductor
𝐼= 𝑞 𝑡
Current: I measured in C/s (amp)
The battery supplies the potential difference needed for the electrons to flow. THE BATTERY DOES NOT SUPPLY THE ELECTRONS THEMSELVES!
Current only flows if there is a complete circuit!

5. Conservation of charge
Total current into a junction must equal the total current out.
3 Amps
Magnitude and direction of current flow?
6 Amps
4 Amps In
out
6 amps
4 amps
3 amps
Total in: 9 amps
Total out: 4 amps
The unknown current is 5 amps out of the junction (to the right)

6. Simple Circuit Every Circuit must have
A source of potential difference (battery)
A conducting material (wires)
An electrical load (resistor/lightbulb)

7. Circuit Symbols
Draw a circuit with a battery, a lamp and a switch using the symbols above

8. Resistance A resistor is any electrical device that you plug in.
It impedes the flow of electrons
The resistance of an object is DIRECTLY related to temperature.
The hotter the wires, the more resistance they have.
The resistance of a wire depends on
Its length
The material its made of (resistivity)
The cross-sectional area of the wire

9. Calculating Resistance
Resistance is measured in Ohms (Ω)
Calculating Resistance

10. Circuits Measurements
Ammeter
Voltmeter
Measures current (in Amps)
Wired IN SERIES with the resistor
Has a very LOW resistance
Measures potential difference (in Volts)
Wired IN PARALLEL with the resistor
Has a very HIGH resistance

11. Types of Circuits
Series
One complete loop
Parallel
Multiple paths

12. Ohm’s Law
𝑽=𝑰𝑹
Slope = Resistance V I
Slope = 1/Resistance I V
Resistance and current are INVERSES of each other. As one increases the other decreases! R I

13. Electrical Power When resistance is constant “ohmic”
“Follows ohm’s law”
“constant temperature”
When Potential Difference is constant P I P I

14. Electrical Power When resistance is constant “ohmic”
“Follows ohm’s law”
“constant temperature”
When current is constant P V P V

15. When Voltage is constant When current is constant
Electrical Power
When Voltage is constant
When current is constant P R P R

16. Electrical Energy (AKA work)
Measured in Joules