"Joule's Law" Electric Power PPT

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"Joule's Law" Electric Power PPT

Part One: Guidance Design

Learning point 1: Thermal effect of electric current

Experiment 1: After the light bulb glows for a period of time, what does it feel like when you touch the light bulb with your hand? Think about why.

Experiment 2: After using the electric fan for a period of time, touch it with your hand

What do you feel about the motor part? Think about why.

Question: The above experiments show that when current passes through electrical appliances, generally electrical appliances will ________. When the current passes through the conductor, the electrical energy is converted into ______ energy. This phenomenon is called the ______ effect of current.

Learning point 2: Explore: What factors are related to the amount of heat generated when electric current passes through a conductor?

Hypothesis: The amount of heat generated when an electric current passes through a conductor may be related to _________, _________, and ____________.

Experiment 1: As shown in Figure 18-4-3, explore the relationship between electric heating and resistance.

Question 1: When equal amounts of air are heated in the experiment, the temperature of the air in the container rises and the volume expands. By comparing the height difference of the liquid columns on both sides of the U-shaped tube, the amount of heat generated when the current passes through the conductor can be compared. What research method was used here?

[Answer] Conversion method.

Question 2: What quantity should be controlled constant in this experiment? How?

[Answer] The current and power-on time must be controlled to be the same. Connect two pieces of resistor wire with different negative values ​​in series, because the current in the series circuit is equal everywhere.

Question 3: What conclusion can you draw?

[Answer] When the current and power-on time are the same, the greater the resistance, the more heat generated by the current.

Question 4: What quantity should be controlled constant in this experiment? How?

[Answer] The resistance and power-on time must be controlled to be the same. The same resistance wire is connected to the two containers, and a resistor shunt is connected in parallel to the outside of one container.

Question 5: What conclusion can you draw?

[Answer] When the resistance and energization time are the same, the greater the current, the more heat the current generates.

Experiment 3: Explore the relationship between electric heat and energization time.

Question 6: What quantity should be controlled constant in this experiment? How?

[Answer] To control the current and resistance to be the same, connect the same resistance wire to the two containers and heat them for different times.

Question 7: What conclusion can you draw?

[Answer] When the current and resistance are the same, the longer the current is energized, the more heat the current generates.

Joule's Law PPT, Part 2: Application Examples

Type 1: Thermal effect of electric current

Example 1 2017·Rizhao Under the guidance of the teacher, a study group explored the factors related to the amount of heat generated when electric current passes through a conductor. The experimental equipment they used is shown in Figure 18-4-2A. Two transparent containers are sealed with equal amounts of air. The liquid levels in the U-shaped tube are initially level. There is a section of resistance wire in both sealed containers.

(1) Please use pen lines instead of wires to connect the resistance wires in the two containers in Figure A to the circuit.

(2) In the experiment, the amount of heat generated by the current passing through the conductor is compared by observing the change of the liquid level in the U-shaped tube. This method is called ________.

(3) Connect the circuit, close the switch, and after powering on for a period of time, the liquid level in the U-shaped tube on the ________ (optional "left" or "right") side changes greatly. This experimental phenomenon shows that when the current and power-on time are the same In this case, the larger the ________, the more heat is generated by the current passing through the conductor.

(4) Make the resistances in the two sealed containers the same, and connect a resistor with the same resistance outside one of the containers in parallel with the resistance in the container (as shown in Figure B). Remove the resistor in Figure A, replace it with the resistor in Figure B, and repeat the experiment. At this time, the currents passing through the resistors in the two containers are different. When the energization time is the same, observe the changes in the liquid level in the U-shaped tube. The conclusion drawn is ______________________________________________________.

(5) If heat is represented by Q, current is represented by I, resistance is represented by R, and time is represented by t, then Q = ________.

[ Analysis As shown in FIG. (2) The heat generated by the current passing through the conductor is not easy to observe and measure directly with the eyes. It is reflected by the change of the liquid level in the U-shaped tube. This method is a commonly used conversion method in junior high school physics. (3) Since the currents everywhere in the series circuit are equal, the currents passing through the two resistance wires when they are connected in series are equal; from Q = I2Rt, it can be seen that when the current and energization time are the same, the resistance on the right side is larger, and the heat generated by it is larger. After the sealed air absorbs heat, the volume expands. The gas in the container on the right expands even more, so the liquid level in the U-shaped tube on the right changes significantly.

Type 2 Joule’s Law – Multi-gear problem

Example 2 Figure 18-4-5 shows the internal simplified circuit of a two-speed electric water heater. S is the temperature control switch. When S is connected to a, the current in the circuit is 5 A. When S is connected to b, the current in the circuit is 5 A. The electrical power is 22 W. Find:

(1) Resistance of R1.

(2) The electrical power of the circuit at high temperature.

(3) Work at low temperature for 10 minutes,

The heat generated by current passing through R2.

Type 3: Distinguish between electric power and electric heat in practical problems

Example 3 The resistance of a DC motor coil is 1.5 Ω. Connect it to a regulated power supply and close the switch. The motor works normally. The current passing through the coil is 2 A. The mechanical work done by the motor in 20 s is 1080 J. ask:

(1) How much heat is generated by the current passing through the coil during this period?

(2) How much work is done by the current passing through the motor during this period of time? What is the supply voltage?

(3) If the motor is stuck and unable to rotate due to a fault during operation, what is the current in the motor at this time?

Joule's Law PPT, Part 3: Class Feedback

1. As shown in Figure 4-1, among the following common electrical appliances, the ones that do not use the heating effect of current are ()

2. The resistance of a certain electric furnace wire is 100 Ω, and the heat generated by energizing it for 10 s is 4×103 J. Then the current passing through the electric furnace wire is ()

A． 2 A B． 4A C. 20A D． 40A

3. An electric heater is a device that uses ________ of electric current to heat. An electric furnace with a rated voltage of 220 V and a resistance of the electric furnace wire is 44 Ω. If it works normally for 10 minutes, the heat generated by the electric furnace wire is _________J.

4. Shown in Figure 4-2 is an experimental device to explore "what factors are related to the heat generated when electric current passes through a conductor." Please answer:

(1) The heat generated by the current is reflected by the ____________ of the liquid level in the U-shaped tube. This method of studying physical problems is called the ________ method.

(2) In order to easily compare the amount of heat generated by two resistance wires after passing current, two identical transparent containers are sealed with air of equal mass and _________.

(3) The resistance wires in the two containers shown in Figure A are connected in series and connected to both ends of the power supply. The purpose of this is to control _________ and ____________ to be the same, and to study the relationship between the heat generated when the current passes through the conductor and _________ .

(4) In Figure B, a resistance wire with the same resistance is connected in parallel outside the container on the right side. The purpose is to study the relationship between the heat generated when the current passes through the conductor and ________.

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