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Transcript
P6A Rheostats A rheostat is a variable resistor( Type of resistor whose resistance can be varied to change the amount of current flowing through it.) It can be used in an electric circuit to vary the brightness of a lamp or the speed of a motor. It does this by controlling the current flowing through the circuit. The simulation below shows how this works. The resistance in ohms Ω is controlled by the slider. Wires are shown as straight lines in circuit diagrams. Rheostat A rheostat contains a long length of resistance wire (wire that has a high resistance to the flow of electric current). A slider changes the length of the wire that the current has to flow through. The resistance is low when a short length of wire is involved, and it is high when a long length is involved. Ohm’s Law Ohm’s Law relates to current and voltage (potential difference). It states that the current flowing through a conductor is directly proportional to the voltage across the conductor. In other words, the current increases as the voltage increases. Circuit symbols Circuit symbols are used in circuit diagrams. They show the different components involved and how they are connected. The ones you need to know are shown below. This equation shows the link between resistance, voltage and current: resistance = voltage ÷ current where: resistance is measured in ohms, Ω voltage is measured in volts, V current is measured in amps, A What causes resistance? Metals are good conductors of electricity. The structure of metals consists of positively charged metal ions surrounded by a ‘sea’ of negatively charged electrons. For example, what is the resistance if the voltage is 12 V and the current is 3 A? resistance = 12 ÷ 3 = 4 Ω Ohmic conductors An ohmic conductor is a component that obeys Ohm’s law. A graph of voltage on the vertical axis against current on the horizontal axis (a voltage-current graph) gives a straight line with a positive gradient. The steeper the gradient, the higher the resistance will be. The electric current in a metal conductor such as a wire is the flow of electrons through the metal. The electrons are the charge carriers in a metal. Electrical resistance is due to these charge carriers colliding with metal atoms as they flow through the metal. Effect of temperature In a metal, the charge carriers (the electrons) collide with metal atoms. This causes them to vibrate more, which in turn causes: an increase in collisions, which increases the resistance, and an increase in the temperature of the metal The higher the resistance, the larger the voltage needed to get a given current to flow. For example, no current flows at 0 V, but 5 A flows at 30 V: resistance=(30 – 0) ÷ (5 – 0) = 30 ÷ 5 = 6 Ω The resistance increases as a wire gets hotter. Non-ohmic conductors A bulb is a non-ohmic conductor. Its voltage-current graph does not follow a straight line. Instead, it gives a curve with an increasing gradient. It shows that the resistance increases as the current increases. As the voltage across a non-ohmic conductor such as a bulb increases, the electrons carry more energy. When they collide with metal atoms in the conductor, they transfer more energy. This makes the atoms vibrate more and more. This increases the resistance and the temperature, and causes the shape of the graph seen above.