SP10: Electricity And Circuits Flashcards
SP10a
1) Describe the basic structure of an atom (positions, relative masses and relative charges of protons, neutrons and electrons).
2) What are the circuit symbols for voltmeters, ammeters, lamps, resistor, variable resistor, switches, cells, batteries, diode, fuse, light dependent resistors, light emitting diode, thermistor, motor?
3) What are the conventions for positive and negative terminals?
1) The atom has a central nucleus consisting of positive protons and neutral neutrons, with electrons around the nucleus on shells. Protons have a charge of +1 and a mass of 1. Neutrons have a charge of 0 and a mass of 1. Electrons have a charge of -1 and a mass of almost 0. The number of electrons is equal to the number of protons and so overall an atom is uncharged.
2) Voltmeter: a circle with a capital V inside, and horizontal 2 wires coming off either side of the circle.
Ammeter: a circle with a capital A inside, and 2 horizontal wires coming off either side of the circle.
Lamp: a circle with a capital V inside, and 2 horizontal wires coming off either side of the circle horizontally.
Resistor: A rectangle with 2 horizontal wires coming off either side.
Variable resistor: a rectangle with a diagonal arrow going through it from the bottom left to the top right.
Thermistor: a rectangle with a diagonal line going through it from the bottom left to the top right. There is a short horizontal line attached just before the bottom left of the horizontal line.
Cell: two parallel vertical lines, with one slightly taller than the other
Battery: looks like a cell, with a dashed line between each cell which makes it a battery.
Switch: a straight horizontal line with small circles on each end.
Diode: a circle with a triangle and a vertical line on the pointy end of the triangle.
Light emitting diode: a diode with two small arrows pointing away from it above the top right corner of the circle.
Light dependant resistor: a rectangle with a circle round it and two small or rows pointing towards it from above the top left corner of the circle.
Fuse: rectangle with horizontal line through it.
Motor: circle with the capital letter M.
3) In the circuit diagram of a cell: The long line is the positive (+) terminal. The short line is the negative(-) terminal. Electrons flow from the negative terminal to the positive terminal. However, the conventional current goes from the positive terminal to the negative terminal.
SP10a
1) Describe and explain the difference between the brightness of identical lamps in series and parallel circuits.
2) Describe and explain the effects of different numbers of identical lamps, switches and components in series and parallel circuits.
1) In a series circuit, bulbs become dimmer as the potential difference is shared equally across the bulbs. The current reads the same for each component. In parallel, each branch shows the same potential difference, so the bulbs on one branch will have the same relative brightness.
2) Adding more components (eg. cells) in series increases the potential difference. Adding more components in parallel increases the current.
Adding more switches in series increases the total resistance of the circuit. Adding more switches in parallel decreases the total resistance of the circuit.
In a series circuit, as more bulbs are added, the brightness of each bulb decreases.
In a parallel circuit, as more bulbs are added, the brightness of each bulb remains unchanged.
SP10b
1) Describe how to measure voltage.
2) Define the term ‘potential difference’
3) Describe how to measure current.
1) Voltage (or potential difference) is measured in volts, V, using a voltmeter. A voltmeter is always connected in parallel with a resistor to measure the potential difference across a component or circuit.
2) Potential difference (or voltage) is a measure of energy, per unit of charge, transferred between two points in a circuit.
3) Electric current is measured in units called amperes (often shortened to amps, A), using an ammeter. An ammeter is connected in series to measure the current passing through a component or circuit.
SP10b
1) Describe the conditions needed to produce an electric current
2) Describe the behaviour of current at a junction
1) For a current to flow, the circuit must be closed and contain a source of potential difference (such as a cell or battery). The electrons all move together when a current flows.
2) The total amount of current stays the same on its journey around the circuit. This is because current is conserved. In a parallel circuit, current splits at a junction to travel along different branches, but the total amount entering the junction is the same as the total amount leaving.
SP10c
1) Explain the link between the potential difference (voltage) across a battery or a component, the charge passing through it and the amount of energy transferred.
2) What is the unit of potential difference and explain it in terms of units of energy and charge?
3) Recall and use the equation to calculate the charge that flows, the current or the time the current flows.
1) When a charge moves through a potential difference, electrical work is done and energy is transferred. Energy transferred = charge moved × potential difference
2) Potential difference is measured in volts. There is a potential difference of 1 volt when there is a transfer of 1 joule of energy to each coulomb of charge (1 volt = 1 joule per coulomb)
3) The charge that flows in a set time can be calculated: charge (C) = current (A) x time (s)
SP10c
1) Explain the link between electric current and electric charge.
2) Explain electric current in metals in terms of electrons.
3) What is the equation that allows you to calculate the charge that flows in a set time?
1) Electrical current is the rate of flow of electric charge. Moving charged particles form an electric current. Electric charge is measured in coulombs (C).
2) When a voltage is applied to a metal, the delocalised electrons travel through the lattice structure. The movement of these charged particles forms an electric current.
3) charge (C) = current (A) x time (s)
The equation can also be written as: Q = l x t
Where Q represents charge, l represents current, and t represents time
SP10d
1) Explain the link between potential difference, resistance and current in a circuit.
2) Define the resistance of a component or circuit
3) Recall and use the equation to calculate the potential difference, the current or the resistance
1) Wires and components with a large electrical resistance required a large potential difference to produce a current.
2) Resistance in a circuit slows down the flow of charge (the current). Resistance is measured in ohms (Ω).
3) The resistance is measured: potential difference (V) = current (A) x resistance (Ω)
SP10g
1) Define power and the units used to measure it.
2) Recall and use the equation to calculate the power, the energy transferred or the time taken.
3) Explain how power transfer depends on the potential difference across a device and the current through it.
1) Power is the energy transferred per second, measured in watts (W). One watt is a transfer of one joule per second.
2) Power = energy transferred (J) / time taken (s)
P = E/t
3) The power in a component or appliance is proportional to the potential difference across it and the current through it.
SP10g
1) Recall and use the equation to calculate the electrical power, the current or the potential difference.
2) Recall and use the equation to calculate the electrical power, the current or the resistance.
1) Equation for electrical power:
Electrical power (W) = current (A) x potential difference (V)
P = I x V
2) Equation for electrical power: electrical power (W) = current² (A²)x resistance (Ω)
P = I² x R
SP10f
1) Describe the energy transfer that occurs when a current passes through a resistor.
2) Use the electron and ion model and the idea of electrical work to explain the energy transfer in a resistor and the resulting dissipation of energy in the surroundings.
3) Explain how unwanted energy transfers in wires can be avoided.
1) All circuits have some resistance, so they warm up when there is a current. When a current passes through a resistor, energy is transferred because electrical work is done against the resistance. The energy is transferred by heating and the resistor becomes warm.
2) As the electrons flow through the lattice of vibrating ions, they collide with the ions. The more collisions they make with the ions, the harder it is for them to pass through, so the higher the electrical resistance. When the electrons collide with the ions, they transfer energy to them.
3) Resistance in circuits can be avoided by using wires made from metals with a low resistance. Thicker wires also have lower resistance. Resistance can also be decreased by cooling the metals so that the lattice ions are not vibrating as much.
SP10f
1) Recall the advantages of the heating effect of an electric current.
2) Recall the disadvantages of the heating effect of an electric current.
3) What is the equation used to calculate the time, the current, the energy transferred or the potential difference? What assumption are you making when using this equation?
1) The heating effect is very useful in appliances which are intended to heat up. The heating effect is useful in an electric heater, electric oven, a toaster or a kettle.
2) The longer electricity is passing through the wire for, the more the wires (or motor) heat up. This increases the resistance of the wires, meaning less energy is transferred usefully.
It is not useful in a computer or in plugs and wires because it means that useful energy is being transferred from the circuit by heating, and spread out or dissipated. The surroundings gain thermal energy.
3) Energy transferred (J) = current (A) x potential difference (V) x time (s)
When using this equation, you are assuming that the object was 100% efficient, that all the energy transferred to the appliance was used for its intended purpose, and that no energy was transferred to heating the wires or surroundings.
SP10h
1) Describe energy transfers from d.c. batteries and the a.c. mains supply to motors and heaters.
2) Explain the difference between direct and alternating voltage.
3) Compare alternating and direct current (in terms of movement of charge).
1) Energy stored in a d.c. battery is transferred by electricity to the motor, where it is transferred to a store of kinetic energy in the moving parts (eg. A fan) or heaters. Energy stored in the battery is transferred by electricity to a high resistance wire where it is transferred by heating to a store of thermal energy in the wire. The energy then dissipates to the surroundings.
2) In cells and batteries, which have direct voltage, the electrons move round in a circle in the same direction: leaving one terminal and going towards the other. However, alternating voltage changes all of the time. It increases to a peak voltage before going down to zero and then peaks in the opposite direction before returning to zero. In the UK, there are 50 cycles per second (the frequency of the mains supply is 50Hz.
3) Direct current (d.c.) is when the direction of the movement of charge stays the same. Alternating current (a.c.) is earn the direction of the movement of charge keeps changing.
SP10h
1) Recall the frequency and voltage of the UK domestic supply.
2) Describe what the power ratings on domestic appliance mean
1) The frequency of the UK domestic supply is 50 Hz, and the voltage is equivalent to a d.c. voltage of 230 V.
2) A power rating of an appliance is measured in watts (W). A kettle with power rating of 3kW transfers 3000 joules of energy each second (from the mains electricity supply to a store of thermal energies the water).
SP10i
1) Explain the difference between the functions of the live and the neutral wires.
2) Explain how circuit breakers make circuits safer. State advantages and disadvantages of circuit breakers.
3) Explain how the earth wire and the fuse make circuits safer.
1) Live wire: connects the appliance to the generators at the power station. The voltage on this wire is 230V.
Neutral wire: the return path to the power station. If the circuit is correctly connected it is at a voltage of 0V.
The insulation that covers the live wire stops an electric current from flowing out of the Iive wire and potentially causing an electric shock (ie. for safety). The insulation also makes it easy to identify the live wire.
2) Circuit breakers are automatically operated electrical switches that protect electrical circuits from overloading or short circuiting. They detect faults and then stop the flow of electricity.
Advantages of circuits breakers include they break the circuit more quickly than fuses, and circuit breakers are easier to reset than fuses. A disadvantage of circuit breakers is that they are more expensive than fuses.
3) The earth wire is connected to a metal case. The metal case is a conductor. When the live wire touches the case, the resistance between the live wire and the earth wire is very low. A large current flows to the earth through the low resistance earth wire. The case is kept at the same potential as the earth, so a person cannot get a shock if they touch the metal case.
A fuse is made of thin wire. The fuse is connected between the live pin and wire to the kettle. When the current is very large, the temperature of the wire increases beyond the melting point of the wire. The fuse wire melts, which disconnects the mains supply to the kettle. This prevents damage to the house wiring.
SP10i
1) Explain why switches and fuses are connected to the live wire.
2) Recall the potential differences between the live, neutral and earth wires.
1) Switches are connected in the live wire of a circuit. When they are off, no current goes through the appliance. A fuse is a tube with a thin wire inside. The current passes through the wire and the wire gets hotter. If the current exceeds a certain value the wire melts. This breaks the circuit and stops the current.
2) The live wire has a potential difference of 230V, and the neutral and earth wires also both a potential difference of 0V.
