Chapter 18

Electric Currents

Objectives

After studying the material of this chapter, the student should be able to:

Use the equations for electric power to determine the power and energy dissipated in a resistor and calculate the cost of this energy to the consumer.

Remember:

P= W / t (Power=Work/time)

• Remember: W= VQ and I = Q/t

So: P= I V

18.5 Electric Power

Power, as in kinematics, is the energy transformed by a device per unit time:

(18-5)

Hewitt – Electric Power

Electric Power, P= I V

Known as Joule’s Law

P: is the power consumed by a resistor, R.

Unit: Joule/s= Watt

Derive the following using P=IV and Ohm’s Law:

P=V2/R

P=I2R

18.5 Electric Power

The unit of power is the watt, W.

For ohmic devices, we can make the substitutions:

(18-6a)

(18-6b)

Problem 1

• The current through a car motor is 150A. The battery used is 12V. How much energy is supplied by the battery in 5s?

Problem 1

• The current through a car motor is 150A. The battery used is 12V. How much energy is supplied by the battery in 5s?

Answer: 9,000J

kWh

kiloWatt hour

What does the kWh measure, a) Energy or b) Power ?

18.5 Electric Power

What you pay for on your electric bill is not power, but energy – the power consumption multiplied by the time.

We have been measuring energy in joules, but the electric company measures it in kilowatt-hours, kWh.

Problem 2A small desktop radio has a resistance of 8,000 Ω. The voltage is 120V.

a)How much current does it draw?

b)How much power does it use?

c)How much does it cost to run the radio for 12 hours, if 1kWh costs $0.15?

Problem 2A small desktop radio has a resistance of 8,000 Ω. The voltage is 120V.

a)How much current does it draw?

b)How much power does it use?

c)How much does it cost to run the radio for 12 hours, if 1kWh costs $0.15?

Answers:

a)0.015A, b)1.8W, c) $0.00324

18.6 Power in Household Circuits

The wires used in homes to carry electricity have very low resistance. However, if the current is high enough, the power will increase and the wires can become hot enough to start a fire.

To avoid this, we use fuses or circuit breakers, which disconnect when the current goes above a predetermined value.

Household CircuitsHousehold circuits are:

parallel circuits Typically designed to carry 15 Amperes of

current - maximum.

Overloading CircuitsSince adding resistance to a parallel

circuit causes current to increase, adding appliances to a household circuit causes current in the house wiring to increase.

This means wires must dissipate more heat energy (P = I2R).

Short CircuitsA short circuit is a very low resistance

path in a circuit.A short circuit will also cause large

amounts of current to flow in a circuit and overload the circuit.

Fuses & Circuit BreakersFuses and circuit breakers act as

automatic switches in a circuit to prevent dangerous currents from flowing.

18.6 Power in Household CircuitsFuses are one-use items – if they blow, the fuse is destroyed and must be replaced.

When the currentexceeds a certainvalue, the metallicribbon melts and the circuit opens.then the fuse mustbe replaced.

18.6 Power in Household Circuits

Circuit breakers, which are now much more common in homes than they once were, are switches that will open if the current is too high; they can then be reset.

18-6 Power in Household Circuits

Example 18-11: Will a fuse blow?

Determine the total current drawn by all the devices in the circuit shown.

Example 18-11: Will a fuse blow?

Determine the total current drawn by all the devices in the circuit shown.

20-A fuse or 30-A fuseIf the circuit is designed for a 20-A fuse, what willhappen?Hopefully the fuse will blow. What if it does not?It can overload the wires, which will get hot and canstart a fire.

What do you do to prevent this from happening?Move one of the devices to another circuit.

If the circuit is designed for a 30-A fuse, what will happen?

If it has a heavier wire and a 30-A fuse, the fuse should not blow.If it does, a short could be the problem, which could be in cord.

18-6 Power in Household Circuits

Conceptual Example E: A dangerous extension cord.

Your 1800-W portable electric heater is too far from your desk to warm your feet. Its cord is too short, so you plug it into an extension cord rated at 11 A. Why is this dangerous?

18-6 Power in Household Circuits

Conceptual Example E: A dangerous extension cord.

Your 1800-W portable electric heater is too far from your desk to warm your feet. Its cord is too short, so you plug it into an extension cord rated at 11 A. Why is this dangerous?

Solution: An 1800-W heater operating at 120 V draws 15 A of current, which exceeds the rating of the extension cord. This creates a risk of overheating and fire.

Homework

Chapter 18 Problems: 26, 28, 29, 31, 33

Inside a Flashlight (optional)

Closure

Kahoot 18-5 and 18-6