"Thermal Effect of Electric Current" Electric Work and Electric Power PPT Courseware 6

"Thermal Effect of Electric Current" Electric Work and Electric Power PPT Courseware 6

Review the past and learn the new

1. The essence of work done by electric current is to convert ( ) energy into ( ) energy. The common units of electric work are ( ) and ( ).

2. The work done when current flows through a conductor is affected by three factors: ( ), ( ) and ( ).

3. The physical meaning of electric power is ( ). For example, there is a light bulb marked with "220V, 60W", where 220V means ( ) and 60W means ( ).

What is the heating effect of electric current?

When a current passes through a conductor, the conductor will generate heat. When this current passes through the conductor, the phenomenon of converting electrical energy into internal energy is called the thermal effect of the current.

think?

What factors are related to the heat generated by electric current?

After the light bulb glows for a while, how does it feel when you touch the light bulb with your hand? Does it feel the same when you touch the wire with your hand?

making plans

The purpose of the experiment is to study the factors related to the heat generated by current passing through a conductor.

Experimental principle: When current passes through the resistance wire, the heat generated by the current will increase the temperature of the kerosene in the bottle, and the liquid level of the thermometer will gradually rise. The more heat generated by the current, the higher the liquid level will rise. We can Compare the heat generated by the current through the height of the thermometer liquid level rise. (Make sure the quality, initial temperature, container, etc. of the kerosene are the same.)

Experimental method Control variable method

experiment analysis

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

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

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

get conclusion

The British physicist Joule conducted a large number of experiments and in 1840 was the first to accurately determine the quantitative relationship between the heat generated by current and current, resistance and energization time, namely 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 resistance of the conductor, and proportional to the energization time.

2. Formula: Q = I2Rt

3. Unit: I-ampere, R-ohm, t-second, Q-joule.

Note: 1. Heat is proportional to the square of the current. When the resistance and power-on time remain unchanged, if the current passing through an electrical appliance increases to 3 times, the heat generated by it increases to 9 times.

2. If the resistance of a conductor is zero, will heat be generated when current flows through the conductor?

The relationship between electric heat and electric power

①If all electrical energy is converted into internal energy: For a pure resistance circuit

All electrical power is used to generate heat, such as a rice cooker, then: Q = W (I2Rt=UIt)

② If the electrical energy is partially converted into internal energy: for non-pure resistance circuits

The heat generated by an electric current is only a part of the work done by the electric current. like:

When the current passes through the motor, then: Q < W (W = W + Q) (I2Rt < UIt)

Notice:

Joule's law applies to any electrical appliance.

Show off your skills

1. There are three electrical appliances, one of which is a "220V 60W" electric fan, one is a "220V 60W" incandescent lamp, and one is a "220V 60W" electric heater. They all work at the rated voltage for the same time. Compare these three. The heat generated by personal appliances is ( )

A. Electric fans are the most common B. Incandescent lamps are the most common

C. Most electric heaters D. All the same

What about the power consumed? What about the work done by the current?

2. As shown in the figure, the same containers a and b are filled with the same mass of water. The resistance values ​​of the resistance wires in the water are R1 and R2 respectively, and R1 < R2. After closing the branch switch S, containers a and b Which water temperature rises faster ( )

A. water in a container

B. Water in container b.

C. The water temperature in containers a and b rises at the same rate.

D.Unable to judge

3. Among the following household appliances, which one is an electric heater ( )

A. Electric iron B. electric fan

C. Telephone D. washing machine

4. Among the following examples, the ones that do not use electric heat are ( )

A. Use a rice cooker to cook rice

B. Hatching chicks in an electric incubator

C. Iron clothes with an electric iron

D. TV with heat dissipation window

Class summary

1. Thermal effect of current:

When electric current passes through a conductor, electrical energy is converted into internal energy. This is the thermal effect of electric current.

2. Joule’s theorem:

The heat generated by an energized conductor is proportional to the square of the current, the resistance of the conductor, and the time it is energized. This is Joule's law

Formula: Q = I2Rt

3. The advantages and disadvantages of the thermal effect of electric current:

After-class homework: If all the electrical energy consumed by electrical appliances is converted into internal energy, please use the electrical power calculation formula and Ohm's law to deduce Joule's law.

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