More than 300 years ago. The question is not the most difficult, but understanding it will require a little attention and patience.
A special section of mechanics is devoted to the study of friction force - the so-called mechanics of frictional interaction (or tribology).
Friction force is the force with which bodies in contact and moving relative to each other interact with each other. It is the friction force that prevents the free movement of contacting bodies.
Types of friction and frictional forces
Where does the static friction force come from?
If we examine the surface of the floor and cabinet legs through a microscope, we will find multiple microscopic tubercles of unimaginable shapes.
When the bodies rest on each other, the tubercles engage with each other, which is why the bodies remain in an immobilized state.
Impact on one of the bodies or on both bodies at once to move them relative to each other will lead to deformation of the tubercles, which will cause electromagnetic repulsion of molecules, which underlies the static friction force.
If physical efforts are applied smoothly, until a certain critical moment the static friction force will be equal in magnitude to the force with which we are trying to move the cabinet from its place.
Sliding friction force
At the moment when the cabinet does move from its place, the static friction force will reach its maximum value.
At this moment, the destruction of the tubercles occurs and, as a result, the cabinet begins to slide.
Photo 1. Wheels and other devices are used to reduce sliding friction
A new type of friction force arises - sliding friction force. This force arises from the interaction of surfaces sliding over each other.
This force is manifested at the moment of physical movement (sliding) of the cabinet legs along the floor or when the skate of a hockey player or figure skater slides along the surface.
If we translate what is happening into “tubercles”, when sliding there is a breaking of bonds between molecules concentrated in different tubercles.
When objects are stationary - that is, when the static friction force is acting - such discontinuities do not occur.
The “tubercle model” is conditional. It was invented to present complex things in simple language.
The same processes can be explained in deeper scientific terms, the understanding of which will require special training from the reader.
The simplest physical laws related to friction force
The answer to the question of what friction force is can be obtained not only by studying theoretical principles, but also by solving practical problems.
To solve problems related to calculating the values of the friction force, you will need some scientific facts characterizing the friction force.
For example, the vector of the sliding friction force applied to a body from the sliding surface is always directed in the opposite direction from the direction of the velocity vector of the object.
If the direction of speed changes, the direction of the sliding friction force will also change. The dependence of the friction force on speed is an important distinctive feature inherent in this force (which, for example, does not exist in the force of gravity or the force of elasticity).
The simplest model of dry friction is characterized by the following laws:
. The sliding friction force is equal to the maximum value of the static friction force.
. The friction coefficient does not depend either on the area of interacting surfaces or on the speed of movement of interacting objects relative to each other.
. There is a directly proportional relationship between the support reaction force and the absolute value of the sliding friction force, calculated by the formula: f = µN.
The proportionality coefficient µ is called friction coefficient.
Physicists have calculated friction coefficients for tens of thousands of pairs of materials.
For example, static friction coefficient for the pair “rubber – dry asphalt” is 0.95, and sliding friction coefficient for the same pair varies from 0.5 to 0.8.
By changing the properties of interacting objects, you can influence the magnitude of the friction force that arises during their interaction.
For example, improving the external shape of racing cars or the tread pattern of the tires used makes it possible to increase their speed by reducing the sliding friction force.
Electromotive force electromotive force
(emf), a quantity characterizing the source of energy of a non-electrostatic nature in an electrical circuit, necessary to maintain an electric current in it. Emf is numerically equal to the work of moving a unit positive charge along a closed circuit. The total emf in a direct current circuit is equal to the potential difference at the ends of the open circuit. The induction emf is created by a vortex electric field generated by an alternating magnetic field. In SI it is measured in volts.
ELECTROMOTIVE FORCEELECTROMOTIVE FORCE (emf; e) - a quantity characterizing the source of energy of a non-electrostatic nature in an electrical circuit, necessary to maintain an electric current in it (cm. ELECTRICITY). The potential forces of an electrostatic (or stationary) field cannot maintain a constant current in a circuit. To maintain continuous current in the circuit, a current source is required ( cm.), or generator (cm. GENERATOR) electric current, providing the action of external forces (cm. THIRD PARTY FORCES). Third-party forces are of non-electrostatic origin and act inside current sources (generators, galvanic cells, batteries, etc.), creating a potential difference between the ends of the rest of the circuit and setting charged particles inside the current sources in motion.
Since when an electric charge moves along a closed circuit, the work done by electrostatic forces is zero, the charge moves only under the influence of external forces. Therefore, the electromotive force of the current source will be numerically equal to the work of external forces A in direct or alternating current sources to move a single positive charge Q along a closed circuit. The EMF acting in a circuit is defined as the circulation of the tension vector of external forces.
The origin of external forces may vary. The potential difference created at the terminals of an open generator is taken as a measure of the electromotive force acting in the generator. The same current source, depending on the strength of the current taken, may have different voltages at the electrodes. Current sources - batteries, thermoelements, electric generators - simultaneously close the electrical circuit. Current flows through the outer part of the circuit - the conductor - and through the inner part - the current source. The current source has two poles: positive (higher potential) and negative (lower potential). Third-party forces, the nature of which can be different (chemical, mechanical, thermal), separate the charges in the current source. The total emf in a direct current circuit (the maximum of these voltages that exists when the circuit is open) is equal to the potential difference at the ends of the open circuit and shows the emf of the source.
EMF determines the current strength in a circuit at a given resistance (Ohm's law (cm. OMA LAW)). EMF is measured, like voltage, in volts (cm. VOLT). To maintain a continuous electric current, generators are used as a source of electromotive force. In generators, external forces are forces from the vortex electric field that arises when the magnetic field changes over time, or the Lorentz force (cm. LORENZIAN FORCE), acting from the magnetic field on electrons in a moving conductor; in galvanic cells (cm. GALVANIC CELL) and batteries are chemical forces.
encyclopedic Dictionary. 2009 .
See what “electromotive force” is in other dictionaries:
Eds, Phys. a quantity characterizing the action of third-party (non-potential) forces in DC sources. or alternating current; in a closed conducting loop is equal to the work of these forces to move a unit position. charge along the entire circuit. If through Esgr... ... Physical encyclopedia
electromotive force- A scalar quantity characterizing the ability of an external field and an induced electric field to cause an electric current. Note - The electromotive force is equal to the linear integral of the external field strength and the induced... ... Technical Translator's Guide
- (EMF), the sum of POTENTIAL DIFFERENCES throughout the ELECTRIC CIRCUIT as a whole. When the circuit is open and no current flows, this force is equal to the potential difference between the terminals of the current source. When there is current in the circuit, the external potential difference decreases.... ... Scientific and technical encyclopedic dictionary Big Encyclopedic Dictionary
- (emf), the reason that causes electric current in a closed circuit. current. E. s. is created by a current source that converts into electricity. energy is any other type of energy (mechanical in electric generators, chemical in elements, etc.). If the circuit is a current source... ... Technical railway dictionary
Electromotive force- a scalar quantity characterizing the ability of an external field and an induced electric field to cause an electric current...
To maintain an electric current in a conductor for a long time, it is necessary that the charges delivered by the current are constantly removed from the end of the conductor, which has a lower potential (consider that current carriers are assumed to be positive charges), while charges are constantly supplied to the end with a higher potential. That is, it is necessary to ensure the circulation of charges. In this cycle, charges must move along a closed path. The movement of current carriers is realized using forces of non-electrostatic origin. Such forces are called third parties. It turns out that to maintain the current, external forces are needed that act along the entire length of the circuit or in individual sections of the circuit.
Definition and formula of EMF
Definition
A scalar physical quantity that is equal to the work of external forces to move a unit positive charge is called electromotive force (EMF), acting in a circuit or section of a circuit. EMF is indicated. Mathematically, we write the definition of EMF as:
where A is the work done by external forces, q is the charge on which the work is performed.
The electromotive force of the source is numerically equal to the potential difference at the ends of the element if it is open, which makes it possible to measure the EMF by voltage.
The EMF that acts in a closed circuit can be defined as the circulation of the tension vector of external forces:
where is the field strength of external forces. If the field strength of external forces is not zero only in part of the circuit, for example, on segment 1-2, then integration in expression (2) can be carried out only over this section. Accordingly, the EMF acting on circuit section 1-2 is defined as:
Formula (2) gives the most general definition of EMF, which can be used for any cases.
Ohm's law for an arbitrary section of a circuit
The section of the chain on which external forces act is called heterogeneous. It satisfies the following equality:
where U 12 =IR 21 – voltage drop (or voltage) in circuit section 1-2 (I-current); – potential difference between the ends of the section; – electromotive force contained in a section of the circuit. equal to the algebraic sum of the emf of all sources that are located in a given area.
It should be taken into account that EMF can be positive and negative. The EMF is called positive if it increases the potential in the direction of the current (the current flows from the minus to the plus of the source).
Units
The dimension of the EMF coincides with the dimension of the potential. The basic unit of measurement of EMF in the SI system is: =V
Examples of problem solving
Example
Exercise. The electromotive force of the element is 10 V. It creates a current in the circuit equal to 0.4 A. What is the work done by external forces in 1 minute?
Solution. As a basis for solving the problem, we use the formula for calculating the EMF:
The charge that passes through the circuit in question in 1 minute. can be found as:
We express the work from (1.1), use (1.2) to calculate the charge, we get:
Let's convert the time given in the conditions of the problem into seconds (min=60 s), and carry out the calculations:
Answer. A=240 J
Example
Exercise. A metal disk with a radius a rotates with an angular velocity and is connected to an electrical circuit using sliding contacts that touch the axis of the disk and its circumference (Fig. 1). What will be the emf that appears between the axis of the disk and its outer edge?
Electromotive force, popularly known as EMF, as well as voltage, is measured in volts, but is of a completely different nature.
EMF from a hydraulic point of view
I think you are already familiar with the water tower from the previous article about
Let's assume that the tower is completely filled with water. We drilled a hole at the bottom of the tower and inserted a pipe through which water runs to your home.
The neighbor wanted to water the cucumbers, you decided to wash the car, your mother started doing laundry and voila! The flow of water became smaller and smaller, and soon completely dried up... What happened? The water in the tower has run out...
The time it takes to empty the tower depends on the capacity of the tower itself, as well as how many customers will be using the water.
The same can be said about the radio element capacitor:
Let's say we charged it from a 1.5 volt battery and it accepted the charge. Let's draw a charged capacitor like this:
But as soon as we attach a load to it (let the load be an LED) by closing the S key, in the first fractions of seconds the LED will glow brightly, and then quietly fade away... and until it goes out completely. The decay time of the LED will depend on the capacitance of the capacitor, as well as on what load we connect to the charged capacitor.
As I said, this is equivalent to a simple filled tower and consumers who use the water.
But why then does our towers never run out of water? Yes because it works water supply pump! Where does this pump get water from? From a well that was drilled to extract groundwater. Sometimes it is also called artesian.
As soon as the tower is completely filled with water, the pump turns off. In our water towers, the pump always maintains the maximum water level.
So, let's remember what voltage is? By analogy with hydraulics, this is the water level in the water tower. A full tower means maximum water level, which means maximum voltage. There is no water in the tower - the voltage is zero.
EMF of electric current
As you remember from previous articles, water molecules are “electrons”. For electric current to occur, electrons must move in one direction. But for them to move in one direction, there must be tension and some kind of load. That is, the water in the tower is tension, and people who waste water for their needs are a load, since they create a flow of water from a pipe that is located at the foot of the water tower. And flow is nothing more than current strength.
The condition must also be met that the water must always be at its maximum level, no matter how many people use it for their needs at the same time, otherwise the tower will become empty. For a water tower, this lifesaver is a water pump. What about electric current?
For electric current to flow, there must be some force that pushes the electrons in one direction for an extended period of time. That is, this force must move electrons! Electromotive force! Yes exactly! ELECTROMOTIVE FORCE! We can call it abbreviated EMF - E lectro D seeing WITH silt. It is measured in volts, like voltage, and is generally designated by the letter E.
So, our batteries also have such a “pump”? There is, and it would be more correct to call it “electron supply pump”). But, of course, no one says that. They say simply - EMF. I wonder where this pump is hidden in the battery? This is simply an electrochemical reaction, due to which the “water level” in the battery is maintained, but then, nevertheless, this pump wears out and the voltage in the battery begins to sag, because the “pump” does not have time to pump water. Eventually it breaks down completely and the voltage on the battery drops to almost zero.
Real EMF source
The source of electrical energy is a source of EMF with internal resistance R int. These can be any chemical batteries, such as batteries and accumulators
Their internal structure from the point of view of EMF looks something like this:
Where E is the EMF, and R int is the internal resistance of the battery
So, what conclusions can be drawn from this?
If no load is attached to the battery, such as an incandescent lamp, etc., then as a result the current in such a circuit will be zero. A simplified diagram would be like this:
But if we nevertheless connect an incandescent light bulb to our battery, then our circuit will become closed and current will flow in the circuit:
If you draw a graph of the dependence of the strength in the current circuit on the voltage on the battery, it will look like this:
What is the conclusion? In order to measure the EMF of a battery, we just need to take a good multimeter with a high input resistance and measure the voltage at the battery terminals.
Ideal EMF source
Let's say that our battery has zero internal resistance, then it turns out that R in = 0.
It is not difficult to guess that in this case the voltage drop across zero resistance will also be zero. As a result, our graph will look like this:
As a result, we simply got an EMF source. Therefore, an EMF source is an ideal power source in which the voltage at the terminals does not depend on the current in the circuit. That is, no matter what load we attach to such an EMF source, it will still produce the required voltage without drawdown. The EMF source itself is designated like this:
In practice, there is no ideal source of EMF.
Types of EMF
– electrochemical(EMF of batteries and accumulators)
– photoelectric effect(receiving electric current from solar energy)
– induction(generators using the principle of electromagnetic induction)
– Seebeck effect or thermoEMF(the occurrence of electric current in a closed circuit consisting of series-connected dissimilar conductors, the contacts between which are at different temperatures)
– piezoEMF(receiving EMF from)
Summary
EMF is a force of NON-electric origin that causes electric current to flow in a circuit.
Real the EMF source has internal resistance inside it, ideal EMF source internal resistance is zero.
An ideal EMF source always has a constant voltage value at its terminals, regardless of the load in the circuit.
Electromotive force (emf; ε) is a quantity characterizing a source of energy of a non-electrostatic nature in an electrical circuit, necessary to maintain an electric current in it. The potential forces of an electrostatic (or stationary) field cannot maintain a constant current in a circuit. To maintain a continuous current in the circuit, it is necessary, or an electric current generator, to ensure the action of external forces. Third-party forces are of non-electrostatic origin and act inside current sources (generators, galvanic cells, batteries, etc.), creating a potential difference between the ends of the rest of the circuit and setting charged particles inside the current sources in motion.
Since when an electric charge moves along a closed circuit, the work done by electrostatic forces is zero, the charge moves only under the influence of external forces. Therefore, the electromotive force of the current source will be numerically equal to the work of external forces A in direct or alternating current sources to move a single positive charge Q along a closed circuit. The EMF acting in a circuit is defined as the circulation of the tension vector of external forces.
The origin of external forces may vary. The potential difference created at the terminals of an open generator is taken as a measure of the electromotive force acting in the generator. The same current source, depending on the strength of the current taken, may have different voltages at the electrodes. Current sources - batteries, thermoelements, electric generators - simultaneously close the electrical circuit. Current flows through the outer part of the circuit - the conductor - and through the inner part - the current source. The current source has two poles: positive (higher potential) and negative (lower potential). Third-party forces, the nature of which can be different (chemical, mechanical, thermal), separate the charges in the current source. The total emf in a direct current circuit (the maximum of these voltages that exists when the circuit is open) is equal to the potential difference at the ends of the open circuit and shows the emf of the source.
EMF determines the current strength in a circuit at a given resistance (Ohm's law). EMF, like voltage, is measured in volts. To maintain a continuous electric current, generators are used as a source of electromotive force. In generators, external forces are forces from the vortex electric field that arises when the magnetic field changes over time, or the Lorentz force acting from the magnetic field on electrons in a moving conductor; in galvanic cells and batteries these are chemical forces.