Tr Astella Physics Tr Astella Physics ▪ ammeter Tr Astella ▪ Since current is the same in series circuit, the current flowing through the buzzer is 2 A. ▪ The lamp will no longer be lit. ▪ The buzzer will stop making sounds. Physics Tr Astella Physics Tr Astella What is charge? There are two types of electric charge, positive (+) charge and negative (-) charge. ▪ A positive charge will repel another positive charge. ▪ A negative charge will repel another negative charge. ▪ Positive charges attract negative charges. * Like charges repel with repulsive force. Unlike charges attract with attractive force. Physics Tr Astella Physics Where does the charge come from? ▪ All materials are made of tiny particles called atoms. ▪ Every atom contains three types of particle: protons, neutrons, and electrons. ▪ Protons are positively charged. ▪ Neutrons have no charge. ▪ Electrons are negatively charged. ▪ There are protons and neutrons in the centre of the atom, called the nucleus. The electrons orbit the nucleus. Tr Astella Physics ▪ In an atom there is always the same number of electrons and protons, so an atom has no charge overall. ▪ It is neutral. ▪ There is no net charge. ▪ If you remove one or more electrons from the atom it becomes positively charged, because there is now more positive than negative charge. Tr Astella Physics Electric circuits ▪ Circuit symbols show which components are in a circuit. ▪ A cell has a positive and a negative terminal. ▪ On circuit symbols the long line represents the positive terminal, and a short line represents the negative terminal. ▪ If you connect two positive terminals together (or two negative terminals together) it will not work. Tr Astella Circuit diagrams ▪ You can join circuit symbols in a circuit diagram to show how the components in your circuit are connected. ▪ In the circuit diagram, the switch is open and the lamp is not lit because the switch is a break in the circuit. ▪ You need a complete circuit for the lamp to work. ▪ If you press the switch the circuit will be complete and the lamp will light up. Physics Tr Astella Physics Tr Astella Physics dimmer brighter dimmer Tr Astella Physics dimmer brighter Tr Astella Physics brighter dimmer Tr Astella Physics What is electric current? ▪ An electric current is the flow of charge. ▪ In a circuit made of metal wires the flowing charge is caused by the movement of electrons. Electrons are charged particles. ▪ Inside a copper wire there are atoms of copper. These atoms are arranged in a regular pattern. Some of the electrons on the outside of the copper atoms are not strongly bound to the atom and can move around. Tr Astella Physics ▪ When a battery is connected to a circuit, these electrons are moving from the negative terminal to the positive terminal in the circuit. ▪ The moving electrons make an electric current. ▪ The current is the amount of charge flowing per second. ▪ Current is measured in amperes (A) or amps for short. ▪ Small currents are measured in milliamps (mA). [1 mA = 0.001 A] ▪ The instruments used to measure the current is called an ammeter and circuit symbol for ammeter is: Tr Astella Physics ▪ There are two instruments that can measure the current. ▪ One type shows the current using a needle on a scale. (ammeter) ▪ The other type shows the current on a digital scale. (multimeter) ammeter multimeter Tr Astella Physics The drawing of the circuit with ammeter. The circuit diagram is with ammeter. Tr Astella Physics Series circuit ▪ A circuit with a single loop is called a series circuit. ▪ The current will only flow if it is a complete circuit. ▪ In series circuit with several lamps, you cannot turn the lamp on and off separately. They are either all on and all off. ▪ If one lamp breaks, or ‘blows’, then all the lamps go out. ▪ In the diagram, an ammeter is used to measure the current at different points in a series circuit. The current at P, Q and R is the same, so the reading on each of the ammeters is the same. ▪ Current is not used up in an electric circuit. It is the same in every part of the circuit. Tr Astella Physics Series means all the components are connected end-to-end or one after the other. ▪ In a series circuit, there is only one path for the current to flow. ▪ Current is the same all the way around a series circuit. I = Ilamp = Ibuzzer ▪ If the switch is opened in series circuit, both the lamp and the buzzer will stop operating. ▪ The current in the whole circuit stops flowing when the switch is opened. Tr Astella Physics Conventional current and electron flow ▪ In a circuit with a cell and a lamp, a current will flow. ▪ We normally draw the current flowing from the positive terminal of the battery to the negative terminal of the battery. This is conventional current (I). ▪ But the charges, electrons, flow from negative terminal to positive terminal. ▪ They travel in opposite directions. Tr Astella Physics In a parallel circuit, there is more than one path for the current to flow. The paths where the current can flow are called branches. ▪ Current from the cell flows to the branch in the circuit. ▪ At the branch, the current is divided. Tr Astella Physics ▪ If the two lamps are the same, the current will be divided equally between them. ▪ When the current comes to the other side where the branches join again, the current combines (adds together) again. Tr Astella Physics ▪ The parallel circuit in this diagram has ammeters to show how the current is shared between the branches. Tr Astella Physics The rule for parallel circuits is: ▪ the current through the cell is equal to the total of the current in all the branches. I = I1 + I2 + I3 I I1 I2 I3 Tr Astella Physics In this circuit, the readings on ammeters A1 and A4 are equal. ▪ The readings on ammeters A2 and A3 will add up to the reading on A1 (and A4). ▪ This rule still works even when the current in the branches is different. Tr Astella Physics ▪ The circuit has three parallel branches, each with a different current. ▪ The current through the cell is equal to the total of the currents through each of the branches. ▪ So, 1.0 A + 2.0 A + 0.5 A = 3.5 A. Tr Astella Physics Q. Calculate the missing current through the buzzer. each of the current = total current in the branches X + 3.0 A + 1.5 A = 5.5 A X + 4.5 A = 5.5 A X = 5.5 – 4.5 X = 1.0 A The missing current though the buzzer is 1.0 A. Tr Astella Physics In a parallel circuit, the current through a branch can keep flowing, even if the current stops flowing in the other branches. ▪ Components in the same circuit can be switched on and off independently ▪ If a component in one branch stops working, the other branches are not affected. Tr Astella Physics ▪ When switch S1 is closed, then lamp L1 will light. This will not affect lamp L2 because L2 is on a different branch. ▪ When switch S2 is closed, then lamp L2 will light. This will not affect lamp L1 because L1 is on a different branch. ▪ If both lamps are switched on and lamp L1 stops working, then lamp L2 will not be affected. • Switch on = Switch closed • Switch off = Switch open Tr Astella Physics ▪ Components in a parallel circuit can be switched on an off separately by having switches on each branch. ▪ The components can also be all switched on or off together if the switch is between the cell and the branches. Tr Astella Physics ▪ For any lamp to light in this circuit, switch S4 must be closed. ▪ Switches S1, S2 and S3 can then be used to control each lamp separately. ▪ If all the lamps are on, then opening S4 will cause all of the lamps to go off. ▪ If all the lamps are off, but switches S1, S2 and S3 are closed, then closing S4 will cause all lamps to light together. • Switch on = Switch closed • Switch off = Switch open Tr Astella Physics Tr Astella Physics Tr Astella (i) Physics (ii) Tr Astella Physics Any two from: ▪ if one lamp fails/ is removed the others will continue to work. ▪ the four lamps could be switched on and off separately. ▪ more (identical) lamps could be connected in parallel without affecting the brightness. Tr Astella Physics (c) Total current = 2.0 A 𝟐.𝟎 The current through each lamp = 𝟒 = 0.5 A Tr Astella Physics (a) X = 1 + 1 =2A (b) X = 2 + 2 =4A Tr Astella Physics (c) X = 1.5 + 1.0 + 0.5 =3A (d) X = 0 + 1 + 0.5 = 1.5 A Tr Astella Physics (A) 6 = X + 3 (B) 1.0 = 0.4 + X X=6-3 X = 1.0 – 0.4 =3A = 0.6 A Tr Astella Physics (C) 0.2 + X = 0.3 X = 0.3 – 0.2 = 0.1 A (D) 0.15 + 0 + X = 0.25 X = 0.25 – 0.15 = 0.1 A Tr Astella Physics 9.1 A Current flow in parallel circuits This is a parallel circuit because current can follow more than one path. C Tr Astella Physics Current in X = 3 + 3 =6A 6 Tr Astella Physics Total current = 4 A Total current = current in P + current in R Since the component used in the circuit is the same, Current in P & R = 𝟒 𝟐 =2A Tr Astella 9.1 B Facts about parallel circuits Physics Tr Astella Physics A4 A3 A2 or A3 A1 or A4 Tr Astella Physics Tr Astella Physics 9.1 C Understanding current in parallel circuits A1 and A2 are equal. A1 and A2 are greater than A3. A4 = A1 + A2 + A3 Tr Astella Physics no change decreases
