Rtotal = 100 Ω R1 = ? R2 = 200 Ω 1/Rtotal 1/100 1/R1 1/R1 1/R1 R1 = 1/R1 + 1/R2 = 1/R1 + 1/200 = 1/100 - 1/200 = 2/200 - 1/200 = 1/200 = 200 Ω Measuring Current, Voltage and Resistance Like all components, meters (which are used to measure values) have component symbols. Component Name Circuit Symbol Function Voltmeter Measures potential difference. Must be placed in parallel to measure the difference in electrical potential between two points. Ammeter Measures current. Must be placed in series to measure the current flowing in a circuit. Ohmmeter Measures resistance. Must be placed in parallel with the component(s) which are to be measured. They can be used in simple, or more complex circuits in order to investigate the size of any of these quantities. This circuit a battery and a resistor. The diagram also shows how the voltmeter and ammeter must be connected. The voltmeter must be connected in parallel because it is measuring the difference in electrical potential between two points. The ammeter must be connected in series because it is measuring the flow of current THROUGH a component or circuit. The two main are: To find the potential difference across a component, or components, the voltmeter must be placed in parallel with that component or those components. To find the current through a component or components, the ammeter must be placed in series with that component or those components.
The previous diagram does not show an ohmmeter because, in most cases, resistance should only be measured when a circuit is not ‘on’ or fully connected. To use an ohmmeter, you place it ‘across’ a component – in parallel, just like a voltmeter. Components There are lots of different components which can be used to make circuits, the ones which will often feature in National 5 Physics are shown in the table below. Component Name Circuit Symbol Function Cell Supplies electrical energy to a circuit, the longer line shows the positive side. Battery A battery of cells means 2 or more cells. DC Supply Supplies electrical energy to a circuit in the form of a direct current. AC Supply Supplies electrical energy to a circuit in the form of an alternating current. Lamp A lamp lights when current flows through it, converting electrical energy to light energy. Switch A switch allows you to complete or a circuit. Resistor A resistor restricts the flow of current, this may be to protect other components. Variable Resistor A resistor, the resistance of which can be varied in the circuit, could be used for a dimmer switch. LDR Dependent Resistor) Can be used to control a circuit. The resistance goes down as the increases.
A circuit is described as a series circuit if the components appear one after the other in the circuit. An example is shown below. To investigate the current in the circuit, we must place multiple ammeters in the circuit. If we an ammeter at each of these positions, we discover that the readings will be identical. In equation form, we can write this as The current in a series circuit is the same at all points. We can also investigate potential difference in a series circuit. A potential difference is the difference in electrical potential between two points, so in a series circuit, the potential difference across each of the components must add up to the total potential difference across the supply. If we call the supply p.d. Vs we write this as The p.d. across each of the components in a series circuit adds up to the supply p.d. Parallel Circuits A circuit is described as a parallel circuit if the components are laid out next to each other, with common connections. An example is shown below. To investigate the current in the circuit, we must place multiple ammeters in the circuit.
What is the maximum resistance of the circuit? What is the minimum resistance of the circuit? What is the maximum current in the circuit? What is the maximum p.d. across the lamp? A student sets up the circuit shown below. Calculate the current supplied to the circuit. What is the potential difference across the parallel section of this circuit? What is the current in the 100 Ω resistor? Power, Energy and Time A light bulb has a power rating of 60 W. How much electrical energy is transformed by the bulb in 1 s? State the energy changes involved when the lamp is switched on. The electric on a ceiling fan transforms 207 kJ of electrical energy in 30 minutes?. Calculate the power rating of the motor in the fan? State the energy changes involved when the ceiling fan is switched on. How much electrical energy is transformed by the following appliances? A 400 W used for 45 s. A 300 W food processor used for 20 s. An 800 W iron used for 40 minutes. A 2.4 kW kettle that takes 5 minutes to boil the water inside it. What is the power rating of an appliance that transforms: 500 J in 5 s?
Current and p.d. In Series And Parallel Circuits a. Series b. 0.5 A c. 8 V a. ACE b. BDF a. Parallel b. X = 0.8 A ,Y = 0.5 A c. 6 V I1 = 0.4 A I2 = 1.5 A I3 = 0.8 A I4 = 0.8 A I5 = 1.5 A I6 = 1.0 A VS = 13 V V1 = 9 V V2 = 9 V V3 = 4 V Ohm’s Law 3 Ω 3 A 12 V 6 kΩ 24 mV 1.5 A 60 mV 25 Ω 2 A mV Potential Dividers a. 8 V b. 1 V c. 2.1 V a. 19.2 kΩ b. 116 kΩ c. 40 Ω a. 108 Ω b. 9 Ω c. 1.1 A d. 8.9 V a. 25 mA b. 2 V c. 20 mA Power, Energy and Time a. 60 J b. - a. 115 W b. - a. 18 kJ b. 6 kJ c. 1.92 MJ d. 720 kJ a. 100 W b. 60 W c. 3 kW 50 s Power, Current, P.D. And Resistance a. - b. 3.6 W c. 13 kJ a. 1.3 A b. 8.64 MJ 30 Ω 1920 W a. 24 W b. 48 W a. 8.7 A b. 26.5 Ω 0.2 A 60 Ω – W 7.2 J b. 2160 J c. - d. - e. 13 A f. - - b. -