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1 of 46 © Boardworks Ltd 2007 2 of 46 © Boardworks Ltd 2007 What obstacles affect the flow of current? How do different components affect the flow of current? 3 of 46 © Boardworks Ltd 2007 What affects the size of the current? Current is a measure of the rate of flow of electric charge through a circuit. A large current means that the rate of flow is more rapid. Current can be changed by increasing or decreasing the voltage of the circuit. Components in a circuit may reduce the size of the current. This is called resistance. All wires and components resist current. Resistance is a measure of how hard it is for electrons to move in an electrical circuit. 4 of 46 © Boardworks Ltd 2007 What causes resistance? Resistance is a measure of how much a material tries to stop electricity passing through it. Electricity is the flow of electrons along a wire. As the electrons move along the wire they collide with the metal ions in the wire. These collisions make the atoms vibrate more, which makes the metal hotter. All wires and components have some resistance, so electrical appliances always waste some energy as heat. 5 of 46 © Boardworks Ltd 2007 Resistance simulation 6 of 46 © Boardworks Ltd 2007 What is a resistor? A resistor is a component designed to reduce the current. A variable resistor has a resistance that can be changed. A fixed resistor has a resistance that remains the same. Many domestic appliances use resistance to transfer electrical energy to heat and light energy. The heating element in this kettle is a coil of metal that resists electricity. As current is passed through the metal, resistance causes the metal to get hot and so boils the water. 7 of 46 © Boardworks Ltd 2007 8 of 46 © Boardworks Ltd 2007 How can resistance be investigated? Set up this circuit with a resistor and a variable resistor. voltage current (V) (A) 0.0 0.0 0.5 0.6 1.0 1.1 1.5 1.8 2.0 2.5 2.5 3.0 3.0 3.5 3.5 Plot a current-voltage graph of the results. 4.2 A V Slowly move the variable resistor so that the voltage increases by 0.5 V and record the current for each setting. 9 of 46 © Boardworks Ltd 2007 Investigating current and voltage 10 of 46 © Boardworks Ltd 2007 Current voltage graphs 11 of 46 © Boardworks Ltd 2007 Plotting current–voltage results for nichrome and copper wires on the same graph gives straight lines with different gradients. At the same voltage, a copper wire allows a larger current than a nichrome wire of the same size. current (A) Is resistance the same in all wires? copper nichrome voltage (V) This means that the copper wire has a lower resistance than the nichrome wire. What does the gradient tell you about resistance? The steeper the gradient of a current-voltage graph, the lower the resistance of the wire. 12 of 46 © Boardworks Ltd 2007 Current-voltage graph for a bulb 13 of 46 © Boardworks Ltd 2007 Current voltage graph for a diode 14 of 46 © Boardworks Ltd 2007 Current – voltage graphs 15 of 46 © Boardworks Ltd 2007 16 of 46 © Boardworks Ltd 2007 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. 17 of 46 © Boardworks Ltd 2007 What is the formula/equation for Ohm’s law? Ohm’s law is usually written as: voltage = current x resistance V = IxR This formula can also be written as: resistance = R = voltage current V I 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 (W). 18 of 46 © Boardworks Ltd 2007 What does Ohm’s Law show? What do the different arrangements of Ohm’s law show about the links between current, voltage and resistance? V = IxR 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. R =V/I The voltage and current are proportional, so the resistance of a material is constant, as long as the temperature does not change. I = V/R 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. 19 of 46 © Boardworks Ltd 2007 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. …which gives the formula… So to find current (I), cover up I… I = V R x 20 of 46 © Boardworks Ltd 2007 Calculating the resistance of a bulb A filament bulb has a current of 0.2 A running through it, with a potential difference of 5 V across it. What is the resistance of the filament in the bulb? V = IR R = V I = 5V 0.2 A = 25 W 21 of 46 © Boardworks Ltd 2007 Resistance calculations 22 of 46 © Boardworks Ltd 2007 23 of 46 © Boardworks Ltd 2007 What affects resistance? The resistance of a wire depends on several factors: material thickness length temperature 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? 24 of 46 © Boardworks Ltd 2007 Investigating resistance and length 25 of 46 © Boardworks Ltd 2007 How does length affect resistance? The table shows the resistance of different lengths of wire. What general pattern do these results show? 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 When the length of the wire increases, resistance increases. Why would increasing the length of the wire result in a increase in resistance? 26 of 46 © Boardworks Ltd 2007 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. 27 of 46 © Boardworks Ltd 2007 Investigating resistance and thickness 28 of 46 © Boardworks Ltd 2007 How does thickness affect resistance? The table shows the resistance of different thicknesses of wire. What general pattern do these results show? 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 When the thickness of the wire increases, resistance decreases. Why would increasing the thickness of the wire result in a decrease in resistance? 29 of 46 © Boardworks Ltd 2007 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. 30 of 46 © Boardworks Ltd 2007 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. 31 of 46 © Boardworks Ltd 2007 Resistance – true or false? 32 of 46 © Boardworks Ltd 2007 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. This has the same effect as using a longer piece of wire. total resistance in series = R1 + R2 33 of 46 4W 2W Total resistance = R1 + R2 = 4W + 2W = 6W © Boardworks Ltd 2007 Calculating resistance in series What is the total resistance for each of these circuits? Total resistance = R1 + R2 6W 34 W = 6 W + 34 W = 40 W Total resistance = R1 + R2 = 15 W + 5 W 15 W 34 of 46 5W = 20 W © Boardworks Ltd 2007 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. This has the same effect as using a thicker piece of wire. 1 1 1 = + total resistance R1 R2 in parallel 35 of 46 4W 2W Total resistance = R1 x R2 R1 + R2 = 4W x 2W 4W + 2W = 1.33W © Boardworks Ltd 2007 Calculating resistance in parallel What is the total resistance for each of these circuits? 8W 6W 5W 5W 36 of 46 Total resistance = R1 x R2 R1 + R2 = 8 W x 6W 8W + 6W = 3.4 W Total resistance = R1 x R2 R1 + R2 = 5W x 5W 5W + 5W = 2.5 W © Boardworks Ltd 2007 Resistors in a circuit 37 of 46 © Boardworks Ltd 2007 38 of 46 © Boardworks Ltd 2007 How can resistance be used? When a material resists current, some of the electrical energy is transferred to heat or light. This property of resistance is used in many electrical appliances. Hairdryers and electric heaters contain coils of nichrome wire. Nichrome, an alloy of nickel and chromium, resists current and gets very hot. Filament light bulbs contain tungsten. Tungsten resists current and gets so hot that it glows. This is why bulbs waste energy as heat. 39 of 46 © Boardworks Ltd 2007 Uses of resistors 40 of 46 © Boardworks Ltd 2007 Resistor circuit diagram symbols 41 of 46 © Boardworks Ltd 2007 Resistor circuit diagram symbols 42 of 46 © Boardworks Ltd 2007 43 of 46 © Boardworks Ltd 2007 Glossary diode – A component that allows current to flow in one direction only. light dependent resistor – A component whose resistance changes in response to light intensity. ohm – The unit of electrical resistance, named after Georg Ohm. Ohm’s law – Formula used to relate current, voltage and resistance, if the temperature remains constant. R = V/I. resistance – The opposition to the flow of charge. resistor – A component that opposes the flow of charge. thermistor – A component whose resistance changes in response to temperature. variable resistor – A component whose resistance can be adjusted to vary the amount of current. 44 of 46 © Boardworks Ltd 2007 Anagrams 45 of 46 © Boardworks Ltd 2007 Multiple-choice quiz 46 of 46 © Boardworks Ltd 2007