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EDUF 1017 Physical Phenomena - L9 Dynamic charges - Current • • • Current The figure shows current carriers (considered positive) flowing through a current with cross-section area A . • We define the current through A as the net charge (in Coulombs) flowing through the area per unit time • The unit of current is the ampere • 1 A = 1 C/s • usually measured with an ammeter A current is a net flow of charge from one region to another - it is another way to move energy! In a conductor the charges free to move are electrons. They jiggle about very quickly, and randomly When a voltage (really an electric field) is applied, there is a superimposed systematic motion of the electrons moving at the ‘drift velocity’ - by convention, from + to - . • EDUF 1017 Physical Phenomena L9 EDUF 1017 Physical Phenomena L9 Potential difference - voltage when energy gain or loss occurs between two points in a circuit then there is a potential difference (a “voltage”) between the two points - measured in volts (symbol V) using a voltmeter. • A loss of electrical energy from the circuit occurs in devices such as a light bulb or a ‘resistor’. A gain in energy occurs in devices such as a battery • • Resistance • • • • Most devices ‘resist’ the flow of current - they have resistance (measured in Ohms - Ω). The relation between voltage (in V), current (in A) and resistance (in Ω) is : V = IR Power is the rate at which energy is used (e.g. in heating a resistor or lighting a globe) - i.e. energy/time P = VI or P = I 2 R or EDUF 1017 Physical Phenomena L9 • • • A battery provides electrical energy. something which provides electrical energy has an emf (electromotive force - not a good name) The emf of the battery originates from the chemical reaction that goes on inside the battery. In colloquial terms, a battery has a nominal “voltage” (actually an emf) - commonly 1.5 V EDUF 1017 Physical Phenomena L9 See on-line version for print that is too small V2 R in J/s = Watts EDUF 1017 Physical Phenomena L9 Models of current flow - which one? Batteries and emf • P= Lamps same brightness - current heads off in both directions Current Battery + Lamps progressively dimmer - current is “used up” Current Current + Battery - X Current Lamp Lamp Lamps same brightness - current is NOT “used up” Current Lamp X + Battery - Lamps same brightness - current is “used up” √ Current Current Lamp EDUF 1017 Physical Phenomena L9 Battery + X Current EDUF 1017 Physical Phenomena - L9 How does a torch work? Batteries in Series and Parallel • • • • • • Use the switch to complete the circuit • Batteries in series - the current must flow through all 3 voltages add, current is the same Why? To get higher voltage Batteries in parallel - the current has a choice currents add, voltages are the same Why? To get more current EDUF 1017 Physical Phenomena L9 EDUF 1017 Physical Phenomena L9 The Electric Eel each 0.15 V, 0.25 Ω Resistors in Series and Parallel 1250 Ω Resistors • in series Rtotal = R1 + R2 + .... • • in parallel 1 1 1 = + + .... Rtotal R1 R2 1250 Ω 8.93 Ω EDUF 1017 Physical Phenomena L9 EDUF 1017 Physical Phenomena L9 ~0.93 A Individual rows ~6 mA See on-line version for print that is too small ~800 Ω