Chapters 1718 Circuits Unit

1
Chapters 17/18 Circuits Unit

• Electrical Power
• Electrical Energy
• Kirchoffs Rules

2
Electrical Power

• Power is the rate at which work is done
• In mechanics
• For Electrical Power
• To get max power, would you make a series or
  parallel circuit? Why?

W
P

t
V2
P

VI

I2R

R
3
Power Example

• A potential drop of 50 volts is measured across a
  250 ? resistor. What is the power developed in
  the resistor?

4
Power Example 2

• Which circuit uses the greatest power?

Think about this
V2
P

R
5
Electrical Energy

• The amount of electrical energy used is equal to
  the work done on an electrical component
• For mechanics W Fd
• For electrical energy

V2t
w




Pt
VIt
I2Rt
R
6
Electrical Energy Example

• An Electrical heater draws a current of 8 amps
  and has a resistance of 15 ?. How much energy
  does the heater use in 1 minute?

7
Kirchhoff's First Law

• The algebraic sum of the currents flowing through
  a junction is zero.
• Generally speaking, it says, the total current
  entering a junction must equal the total current
  leaving the junction.

10A
20A
3A
??
8
Kirchhoff's Second Law

• The algebraic sum of the potential differences in
  a circuit loop must be zero.
• Generally speaking, it says, around any loop in a
  circuit, the voltage rises must equal the voltage
  drops

9
Junction Practice Problem
How much current must be flowing out the other
wire?
10 A
6 A