国际课程IGCSE剑桥物理 chap18
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CHAPTER 18:ELECTRICAL QUANTITIES
INTRODUCTION
When electric charges start moving we say that there is a flow of electric current. Of course, electric current can not flow through everything. Materials that conduct electric current well are called conductors while materials that conduct badly are called insulators. In this and the following chapters, we will be concentrating on investigating the flow of electric current in an electric circuit. This is a closed loop network that contains a number of electrical components. In this chapter you will learn: •To explain how electric circuits work using notions of current, resistance, and potential difference. •How to measure and calculate electrical resistance. •How to calculate energy and power in electric circuits.
HOW IS ELECTRIC CURRENT CREATED?
A battery is needed to provide the necessary electro-motive force (e.m.f) to push the charges around the circuit. •A battery has a + terminal and a – terminal, and it will cause (negative charged) electrons to flow from – to +. However, as we think of current as a flow of positive charge, we will define the current as flowing from + to – . This is also called conventional current. •Note here that in the book they often speak of “cell” instead of battery. A cell is simply a electrochemical cell, and batteries are just a collection of electrochemical cells. They can both provide an e.m.f., and do not cause a difference in the physics of the circuit. Nonetheless, separate symbols have been invented to distinguish between a single cell and a battery:
HOW TO MEASURE RESISTANCE?
IS RESISTANCE CONSTANT?
ELECTRIC ENERGY AND POWER
ຫໍສະໝຸດ Baidu
HOW ARE EMF AND CURRENT QUANTIFIED?
RESISTANCE
HOW TO MEASURE CURRENT AND VOLTAGE
How do we measure current? •At any point in the circuit, it is possible to measure the current that goes through that point with an ammeter. You simply connect it in series into the circuit. •Note that for a simple one-loop circuit, the current is the same everywhere. How do we measure voltage? •As current flows through the circuit, it loses energy due to resistance. As such, the current between two different points in a circuit will have a energy difference between them. This difference is called potential difference or voltage between the two points. •We can measure this potential difference in a circuit using a voltmeter. The meter should be placed in parallel with the circuit and connecting the two points.
INTRODUCTION
When electric charges start moving we say that there is a flow of electric current. Of course, electric current can not flow through everything. Materials that conduct electric current well are called conductors while materials that conduct badly are called insulators. In this and the following chapters, we will be concentrating on investigating the flow of electric current in an electric circuit. This is a closed loop network that contains a number of electrical components. In this chapter you will learn: •To explain how electric circuits work using notions of current, resistance, and potential difference. •How to measure and calculate electrical resistance. •How to calculate energy and power in electric circuits.
HOW IS ELECTRIC CURRENT CREATED?
A battery is needed to provide the necessary electro-motive force (e.m.f) to push the charges around the circuit. •A battery has a + terminal and a – terminal, and it will cause (negative charged) electrons to flow from – to +. However, as we think of current as a flow of positive charge, we will define the current as flowing from + to – . This is also called conventional current. •Note here that in the book they often speak of “cell” instead of battery. A cell is simply a electrochemical cell, and batteries are just a collection of electrochemical cells. They can both provide an e.m.f., and do not cause a difference in the physics of the circuit. Nonetheless, separate symbols have been invented to distinguish between a single cell and a battery:
HOW TO MEASURE RESISTANCE?
IS RESISTANCE CONSTANT?
ELECTRIC ENERGY AND POWER
ຫໍສະໝຸດ Baidu
HOW ARE EMF AND CURRENT QUANTIFIED?
RESISTANCE
HOW TO MEASURE CURRENT AND VOLTAGE
How do we measure current? •At any point in the circuit, it is possible to measure the current that goes through that point with an ammeter. You simply connect it in series into the circuit. •Note that for a simple one-loop circuit, the current is the same everywhere. How do we measure voltage? •As current flows through the circuit, it loses energy due to resistance. As such, the current between two different points in a circuit will have a energy difference between them. This difference is called potential difference or voltage between the two points. •We can measure this potential difference in a circuit using a voltmeter. The meter should be placed in parallel with the circuit and connecting the two points.