"Scientific Exploration: Thermal Effect of Current" PPT Courseware on Current Work and Electric Power

"Scientific Exploration: Thermal Effect of Current" PPT Courseware on Current Work and Electric Power

learning target

1. Know the thermal effect of current and the working principle of electric heaters.

2. Understand the factors related to the heat generated by current.

3. Master the content, formula and units of Joule's law.

4. Ability to use formulas for calculations.

A device that uses the heating effect of electric current.

Common points: When electric current passes through a conductor, it generates heat.

When electric current passes through a conductor, it generates heat, converting electrical energy directly into internal energy. This phenomenon is called the heating effect of electric current.

Common structural features of electric heaters

⑴The main component of the electric heater is the heating element made of resistance wire.

⑵The heating elements are made of materials with high resistivity and high melting point.

life phenomenon

1. The light bulb and the wire are connected in series in the circuit. The current is the same. The light bulb is so hot that it glows, but the wire is not very hot. Why is this?

2. If the light bulb is replaced by a high-power electric stove with the same wire, the wire will heat up significantly and even burn the wire. Why is this?

3. Electric iron. If it is powered on for too long, it will generate more heat. If you are not careful, it will damage the clothes. Why is this?

How to observe electric heat:

method one

The heating wire generates heat and lights the match

advantage:

⑴The phenomenon is intuitive and obvious;

⑵The equipment is simple and easy to prepare;

⑶The experiment time is short and the operation is simple.

shortcoming:

The phenomenon is easily disturbed by factors such as matches, placement location, environment (such as air flow);

Method Two

The heating wire heats up the kerosene and expands it.

advantage:

⑴The phenomenon is intuitive and precise;

⑵ Less interference from the environment.

shortcoming:

⑴The phenomenon is subtle and difficult to see from a distance;

⑵The equipment is complex and difficult to prepare;

⑶The experimental time is slightly longer.

3. Design experiments and verify hypotheses

(1) Study the effect of the resistance of the electric heater on the heat generated

To study the effect of resistance on the amount of heat generated, ________ and ________ should be controlled unchanged. Then two resistors with different resistances can be connected to the circuit ________ (series/parallel).

(2) Study the effect of current size on heat generation

To study the effect of current on heat generation, the resistance of the resistor should be ______(change/unchanged), and the amount of current flowing through the resistor should be ______(change/unchanged). As long as the slider of the sliding rheostat in the circuit is adjusted, the current in the circuit can be changed.

(3) Study the effect of power-on time on heat generation

To study the effect of current on heat generation, _______ and _______ should be kept constant and _______ should be changed.

Experimental results

Conclusion 1: Under the same conditions of energizing current and energizing time, the greater the resistance, the more heat generated by the current.

Conclusion 2: Under the same conditions of resistance and energization time, the greater the current, the more heat the current generates.

Conclusion 3: Under the same conditions of energizing current and resistance, the longer the energizing time, the more heat generated by the current.

Information Express

The British physicist Joule spent nearly forty years exploring the "quantitative relationship between electric heat and current, resistance, and time." He was not afraid of difficulties and worked hard. He devoted almost his entire life to promoting the progress of human society and to science. His story is very touching. In order to commemorate this great scientist, "Joule" is used as the unit of work or energy in physics. This law is called Joule's law

Joule's law

1. Content: The heat generated by current passing through a conductor is proportional to the square of the current, proportional to the conductor resistance, and proportional to the energization time.

2. Formula: Q heat = I²Rt

3. Unit:

Current I--Ampere (A) Resistance R--Ohm (Ω)

Time t--second (s) Heat Q--joule (J)

3. 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

That is UIt = 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

That is UIt > I2Rt

Conclusion: Electric power W ≥ Electric heat Q

illustrate:

Joule's law is an experimental law. Joule's law is suitable for calculating the portion of energy that is converted into internal energy due to the presence of resistance, that is, electric heat. Without resistance, there would be no electric heat!

4. Application and prevention of electric heating phenomena

What hazards can be caused by electric heating when electrical appliances are used improperly?

When the current in the wire is too large,

⑴Electric heating will cause insulation materials to age rapidly and even burn and catch fire;

⑵Some electrical appliances will be burned out if heat dissipation measures are not taken.

What measures are taken to prevent the hazards of electric heat when using electrical appliances?

⑴Some motors are equipped with fans;

⑵ There are heat dissipation windows on the TV and radio casings;

Summary of this lesson

1. Know the thermal effect of electric current; know the heating principle of general electric heaters; and be able to perform simple calculations on some household electric heaters.

The thermal effect of current, the heating principle of electric heaters, and the calculation of simple household electric heaters.

2. Explore the factors that determine electric heating, understand and correctly apply Joule’s law

What factors are related to electric heat, Joule's law, formulas, units;

Applicable conditions of Joule's law and calculation of electric heat

Operation:

1. Review the content of this lesson

2. Class workbook

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