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## Today’s lesson

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**Today’s lesson**• Understand the term resistance • Resistance = voltage / current • Recall and use: V = IR • The gradient of a Voltage current graph is known as the conductance**Electric current flow**Electric current flows from the POSITIVE terminal of a power supply around a circuit to the NEGATIVE terminal. The longer thinner line of the symbol for a cell is the positive terminal. In the circuit above the diode is aligned so that it allows current to flow through the radio.**a resistor**circuit symbol for a resistor Resistance Resistance is the opposition that an electrical device has to the flow of electrical current. All devices have some resistance. A resistor is a device that has a particular resistance. Now fill in the resistance sheet**Circuit used for measuring the resistance of an indicator**lamp Measuring Resistance The resistance of a component can be found by measuring the current through, and voltage across, the component.**Current-voltage graphs**These are used to show how the current through a component varies with the applied voltage. The circuit opposite could be used to obtain a current-voltage graph of a variable resistor. Typical results: Now draw a graph with the data given**I**V The current-voltage graph of a wire or a fixed resistor at a constant temperature The graph is a straight line through the origin. The wire or resistor obeys Ohm’s law which states that that the current is proportional to the voltage at a constant temperature.**Copy and complete:**An electric ________ will only flow around a circuit if there are no ______ in the circuit. All components have __________. The greater the resistance the ________ is the current for the same applied voltage. Resistance is measured in ______. A current – voltage graph for a ________ is a straight line through the _______. This shows that the current through the resistor is ___________ to the applied voltage. current gaps resistance smaller ohms resistor origin proportional WORD SELECTION: smaller resistor proportional gaps origin resistance ohms current**The irresistible Georg Ohm**Resistance is a measure of how hard it is for electrons to move in an electrical circuit. The connection between current, voltage and resistance was discovered in 1827 by Georg Ohm, a German physics and maths teacher. The formula R = V/I is known as Ohm’s Law. It was such an important discovery in electricity that the unit of resistance is called the ohm. This unit is represented by the symbol W.**voltage**current V I voltage = current x resistance V = I x R resistance = R = What is the formula/equation for Ohm’s law? Ohm’s law is usually written as: This formula can also be written as: What are the units of voltage, current and resistance? • Voltage is measured in volts (V). • Current is measured in amps (A). • Resistance is measured in ohms ().**V**I R X Ohm’s Law • V = IR**V = I x R**R = V / I I = V / R What does Ohm’s Law show? What do the different arrangements of Ohm’s law show about the links between current, voltage and resistance? This version of Ohm’s Law shows that as the voltage increases, the current increases. The voltage and current are proportional, while the resistance remains constant. The voltage and current are proportional, so the resistance of a material is constant, as long as the temperature does not change. For a low resistance material, more current is allowed to flow for a given voltage. For a high resistance material, less current will flow at the same voltage.**VR**I = Resistance formula triangle A formula triangle helps you to rearrange a formula. The formula triangle for V = IR is shown below. Cover up the quantity that you have to work out and this gives the formula needed. So to find current (I), cover up I… …which gives the formula… x**= 5V**0.2A Calculating the resistance of a bulb A filament bulb has a current of 0.2A running through it, with a potential difference of 5V across it. What is the resistance of the filament in the bulb? V = IR R = V I = 25 **Question 1**Calculate the resistance of a lamp if a voltage of 12V causes a current of 3A to flow through the lamp. resistance = voltage current = 12V / 3A resistance = 4 ohms (4Ω)**Question 2**Calculate the resistance of a heater if a voltage of 230V causes a current of 200mA to flow through the heater. resistance = voltage current = 230V / 200mA = 230V / 0.200A resistance = 1150 Ω**Question 3**Calculate the voltage across a resistance of 40Ω when a current of 5A is flowing. V = I x R = 5A x 40Ω voltage = 200V**Question 4**Calculate the current flowing through a wire of resistance of 8Ω when a voltage of 12V is connected to the wire. current = voltage resistance = 12V / 8Ω current = 1.5A Now complete the Ohm’s Law questions**What affects resistance?**The resistance of a wire depends on several factors: • material • thickness • temperature • length For example, a copper wire has a lower resistance than a nichrome wire of the same size. Copper’s excellent ability to conduct electricity means it is often used in wiring. What experiments could be used to test how length and thickness affect resistance?**length (cm)**voltage (V) current (A) resistance (W) 5 2.6 5.0 0.52 10 2.9 4.3 0.67 20 3.0 2.1 1.48 How does length affect resistance? The table shows the resistance of different lengths of wire. What general pattern do these results show? When the length of the wire increases, resistance increases. Why would increasing the length of the wire result in a increase in resistance?**Why does length affect resistance?**The effect of length of a wire on resistance can be understood by looking at the atomic structure. Resistance is caused by electrons colliding with metal ions. When the length of the wire is increased, the electrons have to travel further. So the chance of collisions will increase, causing the resistance to increase.**thickness (mm)**voltage (V) current (A) resistance (W) 1 3.0 2.1 1.48 2 2.9 4.3 0.67 4 2.6 5.0 0.52 How does thickness affect resistance? The table shows the resistance of different thicknesses of wire. What general pattern do these results show? When the thickness of the wire increases, resistance decreases. Why would increasing the thickness of the wire result in a decrease in resistance?**Why does thickness affect resistance?**Increasing the thickness of a wire increases the surface area that the electrons can flow through. This decreases the chance of collisions with metal ions. In thick materials the charge carrying particles are able to move through the conductor more easily, reducing resistance.**How does temperature affect resistance?**If a component obeys Ohm’s law, then decreasing the temperature decreases the resistance of the material. This is because the positively charged ions in the metal do not vibrate as fast, and the electrons can flow more easily. A superconductor is a material that conducts electricity with very little resistance at very low temperatures. Superconductors can be used to make very fast circuits and to make magnets levitate.**Do Now**• Question paper 3 November 2006 • Question paper 6 June 2004 • Stick in notes on factors affecting resistance.**Today’s lesson**• Understand the term resistance • Know how the four factors affect resistance and their mathematical relationships needed for paper 6 questions – question November 2004 • How to calculate resistance in series • How to calculate resistance in parallel**2**4 total resistance in series = R1 + R2 How is resistance affected in a series circuit? When two (or more) resistors are connected in series, the combined resistance is higher than the individual resistors. There is only one path for the current to travel, which means that it flows through the resistors one after the other. Total resistance = R1 + R2 This has the same effect as using a longer piece of wire. = 4 + 2 = 6**2**4 total resistance in series = R1 + R2 How is resistance affected in a series circuit? When two (or more) resistors are connected in series, the combined resistance is higher than the individual resistors. There is only one path for the current to travel, which means that it flows through the resistors one after the other. Total resistance = R1 + R2 This has the same effect as using a longer piece of wire. = 4 + 2 = 6**Calculating resistance in series**What is the total resistance for each of these circuits? Total resistance = R1 + R2 = 6 + 34 34 6 = 40 Total resistance = R1 + R2 = 15 + 5 = 20 5 15**= R1 x R2**R1 + R2 4 = 4 x 2 4 + 2 1 1 1 = + 2 total resistance in parallel R1 R2 How is resistance affected in a parallel circuit? When two (or more) resistors are connected in parallel, the current splits at the branches and does not go through each resistor. This means the total combined resistance is less than any of the individual resistors. Total resistance This has the same effect as using a thicker piece of wire. = 1.33**Total resistance = R1 x R2**R1 + R2 = 8 x6 8 + 6 Total resistance = R1 x R2 R1 + R2 = 5 x5 5 + 5 Calculating resistance in parallel What is the total resistance for each of these circuits? 8 = 3.4 6 5 = 2.5 5**A**Complete use page 181 ammeter heater resistor diode filament bulb thermistor cell LDR**Resistance of a lamp**• As the current in a lamp increases, it gets hotter, and its resistanceincreases. Why?**Resistance of a lamp**• As the current in a lamp increases, it gets hotter, and its resistanceincreases. Copy please.