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Year 9 Electrical Circuits summary sheet Electricity is a flow of electrons. Electricity can flow through conductors but not through insulators. Metals are good conductors of electricity. Circuits A complete circuit is needed for electricity to flow. We use symbols when we draw circuits: Component Symbol Component cell variable resistor battery of cells ammeter bulb voltmeter switch motor Symbol resistor The current is a measure of how quickly the electrons flow around a circuit. The units for current are amps (A). Current is measured using an ammeter. An ammeter is placed in series with the other components in the circuit. The potential difference (voltage) is a measure of the difference in energy between two parts of a circuit. The units for potential difference are volts (V). Potential difference is measured using a voltmeter. A voltmeter is placed in parallel with the component being measured. Circuits can be series or parallel circuits. Series circuit • If one bulb breaks, all the others go off. • The current is the same everywhere. • If you put more bulbs in they will be dimmer, because it is harder for the electricity to get through. The resistance of the circuit is higher. • The voltage from the cell or power pack is divided between the components. Parallel circuit • If one bulb breaks, the bulbs in the other branches stay on. • The current splits up when it comes to a branch. The current in all the branches adds up to the current in the main part of a circuit. • If you add more bulbs they stay bright. It is easier for the current to flow with more branches, because there are more ways for the electrons to go. • The voltage is the same across all the branches of the circuit. Electrical current Current in a series circuit: The reading at A1 is the same as A2 E.g. if A1 = 2A then A2 = 2A Current in a parallel circuit: The reading at A1 is the sum of the readings at A2 and A3. A1 = A2 + A3 E.g. if A1 = 3A then A2 and A3 both = 1.5A each (assuming that the bulbs are identical Potential difference Potential difference in a series circuit: The sum of the readings at V1 and V2 is the total potential difference from the cell (or battery) E.g. If the cell supplies a potential difference of 3V then V1 = 1.5V and V2 = 1.5V (assuming the bulbs are identical) Potential difference in a parallel circuit: The reading at V1 is the same as the readings at V2 and V3. V1 = V2 = V3 E.g. If the battery supplies a potential difference of 4.5V (read at V1), then the readings at V2 and V3 will both be 4.5V also. Resistance The wires and the other components in a circuit reduces the flow of charge through them. This is called resistance. The unit of resistance is the ohm, and it has the symbol Ω (an uppercase Greek letter omega). For example, a 2 Ω component has a greater resistance than a 1 Ω component, and will reduce the flow of charge through it more effectively. The resistance increases when you add more components in series. For example, the resistance of two lamps is greater than the resistance of one lamp, so less current will flow through them. The resistance of a circuit is a way of saying how easy or difficult it is for electricity to flow. • high resistance = hard for electricity to flow = small current • low resistance = easy for electricity to flow = large current The units for resistance are ohms (Ω) and can be calculated using the following equation: resistance (Ω) = potential difference (V) current (A) For example, 3 A flows through a 12 V bulb. What is the resistance of the bulb? Answer: resistance = 12 ÷ 3 = 4 Ω