1. Electricity and Magnetism
Physics 208
Dr. Tatiana Erukhimova
Lectures 21-23

2. Current, Ohm’s Law, Etc.

3. The Continuity Equation for Steady State Currents
Currents and current densities are constant in time – steady state. The flux of out of any closed surface must be zero.

4. For steady state situation
Kirchhoff’s junction rule: The algebraic sum of the currents into any junction is zero.
Kirchhoff’s loop rule: The algebraic sum of the potential differences in any loop must be zero.

5. Resistors in series:
Resistors in parallel:

6. Circuit with capacitors

7. Joule’s Law

8. Drift velocity

9. Why does a light bulb burn out when you switch the light on and never when you turn it off?

10. The amount of current drawn by a given device is determined by its power input
The current in a 100-W light bulb is

11. A kitchen circuit
An 1800-W toaster, a 1.3-kW electric frying pan, and a 100-W lamp are plugged into the same 20-A, 120-V circuit.
What current is drawn by each device, and what is the resistance of each device?
Will this combination blow the fuse?

12. To prevent the fuse from blowing, you replace the fuse with one rated at 40 A.
Is this a reasonable thing to do?