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Basic Electricity

Basic Electricity. All the Electricity You Need to Know for Physics. Current is measured with an ammeter. It is measured in amps. It is a flow of charge. 1 A = 1 coulomb per second. 1 C = 6.2 × 10 18 electrons. Potential difference is measured with a voltmeter. It is measured in volts.

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Basic Electricity

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  1. Basic Electricity All the Electricity You Need to Know for Physics

  2. Current is measured with an ammeter. It is measured in amps. It is a flow of charge. 1 A = 1 coulomb per second. 1 C = 6.2 × 1018 electrons. Potential difference is measured with a voltmeter. It is measured in volts. It is the energy contained per unit charge. 1 V = 1 J/C. Measured Quantities

  3. Power is the rate of changing electrical energy into other forms. Power is measured in watts (W). Power = voltage × current (P = VI). Resistance is the opposition of a conductor to the flow of electricity. Resistance is measured in Ohms (W). Resistance = voltage ÷ current (R = V/I). Calculated Quantities

  4. Heating Effect of a Current • If P = VI and V = IR, we can see that P = I2R. • This means that if the current doubles, the power through a component goes up four times. • Therefore heavy currents need thick wires, otherwise the wires get hot. • It is important to check the power rating of a resistor; a ¼ watt resistor will burn out if it has to dissipate 5 watts.

  5. 3 1 2 The heating effect can be dramatic! This was a 33 W resistor connected to a 20 V supply. The current would be 20 V ÷ 33 W= 0.61 A The power would be 0.61 × 20 V = 12 watts. Plenty enough to fry a 1 watt resistor. It is important that we ensure that any current limiting resistors can dissipate the power through them. The above situation could be highly dangerous.

  6. Short Circuits can be Dramatic You can see the effects of the intense heating effect on a circuit breaker, and the bogie of an electric locomotive. Pick-up shoe for third rail

  7. Short Circuit in a Transformer

  8. Wire wound resistors can dissipate more heat

  9. Voltage – Current Characteristics • These are graphs plotted from measurements of voltage and current. • We usually put current on the horizontal axis and voltage on the vertical axis. • This allows us to work out the resistance by working out the gradient. • For the resistor and the lamp, the graph is symmetrical about the origin. If the voltage is positive, the current is positive.

  10. A V Basic Circuit We then put a lamp in instead of a resistor.

  11. Characteristic Graph for a Resistor It is a straight line as Ohm’s Law is obeyed

  12. Voltage (V) Current (A) Characteristic Graph for a Lamp It is a curve as the resistance increase as the filament gets hotter

  13. 150 W 6 V 5 k V mA Characteristic for a Diode This circuit is more complex, allowing us to control the voltage more effectively.

  14. Voltage Current Graph for Diode

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