"Thermal Effect of Electric Current" Electric Work and Electric Power PPT Courseware 5

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"Thermal Effect of Electric Current" Electric Work and Electric Power PPT Courseware 5

teaching objectives

1. Use experiments to explore the factors related to the conversion of electrical energy into the internal energy of the conductor when current passes through a conductor. Focus on studying the relationship between the heat generated when the conductor is energized and the electrons in the conductor through experimental research methods.

2. Understand what factors are related to the heating effect of current, and understand and be able to use Joule's law to solve some practical problems.

3. Understand examples of applying Joule’s law in life and learn some methods of saving electricity.

Use your brain

There are three lights in one room, and there are three switches that control the lights in another room. You can only enter the switch room once, and then enter the light room once. How to determine which light each of the three switches controls?

1. Thermal effect of electric current

1. Concept: When electric current flows through a conductor, the conductor will generate heat. This phenomenon is called the thermal effect of electric current.

2. Electric heater: A device that uses electric current to do work to convert electrical energy into internal energy.

3. Principle of electric heater:

Utilize the heating effect of electric current.

Generally, electric heaters are mainly composed of resistors.

Explore: What factors are related to the heat generated by electric current passing through an electric heater?

① Resistor R

②Current I

③Power-on time t

2. Expression of heat generated by electric current

1. Formula: Q=I2Rt (universally applicable) Unit: I—ampere R—ohm t—second Q—joule

2. Joule’s law

Through a large number of experiments, the British physicist Joule accurately determined the relationship between the heat generated by current and current, resistance and energization time in 1840, which is Joule's law.

Content: The heat generated when current flows through a conductor is proportional to the square of the current, proportional to the resistance of the conductor, and proportional to the time of electricity.

3. Other expressions for heat generated by electric current

(1). Q=UIt

(2). Q=U²/R t

It is only applicable to circuits with only resistors (all electrical energy is converted into internal energy).

4. The relationship between electric power and electric heat (the relationship between W and Q)

①In the electric heater circuit (a circuit with only resistance): (almost all electrical energy is converted into internal energy)

W = Q

(W=UIt)=(Q=I2Rt)

②In other circuits: (for example, when current passes through a motor to do work, most of the electrical energy is converted into mechanical energy)

W > Q

(W=UIt)>(Q=I2Rt)

[Example 1]: The resistance of the resistance wire of a certain electric heater is 2 ohms. When a current of 1 A passes through, how many joules of heat are generated in 1 minute?

Solution: Q= I2Rt=(1A)2 × 2Ω × 60S =120J

[Example 2]: When a motor works normally, the voltage across the coil is 380 volts, the resistance of the coil is 2 ohms, and the current in the coil is 10 amps. If the motor works normally for 1 second,

Find: 1) The electrical energy consumed, W.

2) The heat Q generated.

Known: Motor U=380V I=10A R=2Ω t=1s

Find: (1) W (2) Q

Solution: 1) W＝UIt＝380×10×1J＝3800J

2) Q＝I2Rt＝10 ×10 ×2 ×1J＝200J

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Update Time: 2024-09-04

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