Electrical Quantities Lecture

IGCSE Question: 4.2 Electrical Quantities Dangers of Static Electricity Markscheme: A 4.2.2 Electric Current An electric current consists of moving electric charges. The van de Graaff machine is working, it produces a continuous supply of charge which produces an electric field between the metal plates to which it is connected. How the Van De Graaff engine works: 1. The table-tennis ball shuttles rapidly backwards and forwards between the plates and the very sensitive meter records a small current. 2. As the ball touches each plate it becomes charged and is repelled to the other plate. In this way charge is carried across the gap. 3. This also shows that static charges, produced by friction in the van de Graaff machine, cause a deflection on a meter just as current electricity produced by a battery does. Effects of a Current The Ampere and the Coulomb An electric current has three effects that reveal its existence 1. Heating and lighting: The lamp lights because the small wire inside (the filament) is made white hot by the current. 2. Magnetic: The plotting compass is deflected when it is placed near the wire because a magnetic field is produced around any wire carrying a current. 3. Chemical: Bubbles of gas are given off at the wires in the acid because of the chemical action of the current. An electric current is defined as the charge passing a point per unit time and can be written in symbols as Current (I) with unit is Ampere (A). Charge (Q) with the unit is Coulomb (C). Conventional Current Before the electron was discovered scientists agreed to think of current as positive charges moving round a circuit in the direction from positive to negative of a battery. This agreement still stands. Arrows on circuit diagrams show the direction of what we call the conventional current, i.e. the direction in which positive charges would flow. Electrons flow in the opposite direction to the Worked Example Ammeter An ammeter is used to measure currents. It should always be placed in series in a circuit with the positive terminal on the ammeter connected to the positive terminal of the supply. When making a measurement on either type of ammeter a suitable range must be chosen. Direct and Alternating Current In a direct current (d.c.) the electrons flow in one direction only. In an alternating current (a.c.) the direction of flow reverses regularly. • The pointer of an ammeter for measuring d.c. is deflected one way by the direct current. • Alternating current makes the pointer move back and forth about the zero if the changes are slow enough; otherwise no deflection can be seen. • Batteries give d.c.; generators can produce either d.c. or a.c Frequency of a.c Frequency is the number of complete alternations or cycles in 1 second. The unit of frequency is the hertz (Hz). In Figure 4.2.15: The frequency of the a.c. is 2 Hz, which means there are two cycles per second, or one cycle lasts 1/2 = 0.5 s. Youtube : Alternating and Direct Current 4.2.3 Electromotive force and potential difference 1. The chemical action inside a battery produces a surplus of electrons at one of its terminals (the negative) and creates a shortage at the other (the positive). It is then able to maintain a flow of electrons, i.e. an electric current, in any circuit connected across its terminals for as long as the chemical action lasts. 2. Work is done by the battery in moving charge around the circuit. 3. Electromotive force (e.m.f.) is defined as the electrical work done by a source in moving a unit charge around a complete circuit. Electromotive force is measured in volts (V). 4. Potential difference (p.d.) is defined as the work done by a unit of charge passing through a component. Like e.m.f., potential difference between two points is measured in volts (V). The term voltage is sometimes used instead of p.d.