Electricity and Magnetism Done 4.2

DONE 4.2 Next subtopic 4.3 4.3 Electric Circuits 4.3.1 Circuit Diagrams & Components Some of the symbols used for the various parts of an electric circuit are shown in Figure 4.3.2 Series & Parallel Circuits Series Circuit Parallel Circuit • The current at every point in a series circuit is • The current splits: some goes through one the same lamp and the rest through the other Electric current in a circuit cannot be stored. This means that when circuits join or divide, the total current going into a junction must be equal to the total current leaving the junction. • The total p.d. across the components in a • The p.d. across devices in parallel in a circuit series circuit is equal to the sum of the are equal. individual p.d.s across each component. 4.3.2 Series & Parallel Circuits Series Circuit Parallel Circuit • A battery consists of two or more electric cells. • If two 1.5V cells are connected in parallel, as in Greater e.m.f.s are obtained when cells are Figure, the e.m.f. at terminals P, Q is still 1.5V joined in series, i.e. + of one to − of next. but the arrangement behaves like a larger cell and will last longer. 𝜀 𝑇𝑜𝑡𝑎𝑙=1 ,5 𝑉 +1 ,5 𝑉 =3 𝑉 𝜀 𝑇𝑜𝑡𝑎𝑙=1 ,5 𝑉 − 1 , 5 𝑉 =0 𝑉 4.3.2 Series & Parallel Circuits Series Circuit Parallel Circuit • Resistance in series circuit. The same current I • Resistance in parallel circuit. The voltage V flows through each and the total voltage V between the ends of each is the same and the across all three is the sum of the separate total current I equals the sum of the currents voltages across them, i.e. in the separate branches, i.e. Dividing both sides by I, Dividing both sides by V, Properties of parallel circuits (i) the current from the source is larger than the current in each branch (ii) the combined resistance of two resistors in parallel is less than that of either resistor by itself. Lamps are connected in parallel (Figure 4.3.5) rather than in series in a lighting circuit. The advantages are as follows: • The p.d. across each lamp is fixed (at the supply p.d.), so the lamp shines with the same brightness irrespective of how many other lamps are switched on. • Each lamp can be turned on and off independently; if one lamp fails, the others Resistor Colour Code Resistors have colour-coded bands as shown in Figure 4.3.12. In the orientation shown • the first two bands on the left give digits 2 and 7; • the third band gives the number of noughts (3) • the fourth band gives the resistor’s ‘tolerance’ (or accuracy, here ±10%) So the resistor has a value of 27 000 Ω (±10%). 4.3.3 Action and Use of Circuit Components Increase in resistance of a conductor In a metal the current in a circuit is carried by free electrons. When the temperature of the metal increases, the atoms vibrate faster and it becomes more difficult for the electrons to move through the material. This means that the resistance of the metal increases. From Ohm’s law V = IR, so that if R increases then if a constant current I is to be maintained, the p.d. V across the conductor also increases. Variable Potential Divider The resistance of materials other than metals does not necessarily rise when their temperature increases. For example, in a semiconductor thermistor. The voltage across is Characteristics of thermistor: • If a thermistor is part of a potential divider circuit then its resistance decreases when the external temperature rises. • The combined resistance of the two resistors then decreases, • so if the supply voltage remains constant, the current in the circuit will increase. The voltage across is Also the ratio of the voltages across the two resistors is Relays • Small to Big Power: The relay lets a small current (from a weak signal) control a much bigger current (for a powerful device). • Switching Appliances: Relays can also be used to turn on household appliances. • Safety: The relay also keeps the small, safe circuit separate from the dangerous highvoltage power. In short, relays are like a bridge that lets a small signal control big power safely. Light-dependent resistor (LDR) How the LDR work? The action of an LDR depends on the fact that the resistance of the semiconductor cadmium sulfide decreases as the intensity of the light falling on it increases (and the current increases). LDRs are used in photographic exposure meters and in series with a resistor to provide an input signal in switching circuits such as a light-operated intruder alarm. 1. Light on the LDR: When light shines on the LDR, its resistance decreases, making it easier for electricity to pass through. This causes less voltage across the LDR and more voltage across the other components, like the relay. 2. Relay Activation: As the voltage across the relay increases, it reaches a level where the relay turns on, acting like a switch. This switch allows electricity to flow to the bell, making it ring. 3. When Light is Gone: If the light is removed, the LDR's resistance increases, causing the voltage across the relay to drop. If the voltage Thermistor A negative temperature coefficient (NTC) thermistor contains semiconducting metallic oxides whose resistance decreases markedly when the temperature rises. The temperature may rise either because the thermistor is directly heated or because a current is in it. Figure 4.3.15b shows the symbol for a thermistor in a circuit to demonstrate how the thermistor works. When the thermistor is heated with a match, the lamp lights. 1. When the temperature rises, the resistance of the thermistor falls, and so does the p.d. across it. 2. The voltage across resistor R and the relay increases. When the voltage across the relay reaches its operating p.d. the normally open contacts close, so that the circuit to the bell is completed and it rings. 3. If a variable resistor is used in the circuit, the temperature at which the alarm Semiconductor diode • A diode is a device that lets current pass in one direction only. • The wire nearest the band is the cathode and the one at the other end is the anode. • The diode conducts when the anode goes to the + terminal of the voltage supply and the cathode to the − terminal (Figure 4.3.20a). It is then forward-biased; its resistance is small and conventional current passes in the direction of the arrow on its symbol. • If the connections are the other way around, it does not conduct; its resistance is large Light-emitting diode (LED) • An LED, is a diode made from the semiconductor gallium arsenide phosphide. • When forward biased the current in it makes it emit red, yellow or green light. • No light is emitted on reverse bias. • If the reverse bias voltage exceeds 5 V, it may cause damage. • LEDs are used as indicator lamps on computers, radios and other electronic equipment. • Each segment is an LED and, depending on which have a voltage across them, the display lights up the numbers 0 to 9. • LEDs are small, reliable and have a long life; their operating speed is high and their current requirements are very low.