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| Question | Answer |
|---|---|
| What is direct current (DC)? | it is a current that flows in one direction only (DC) |
| Name a source of DC? | a battery |
| What is alternating current (AC)? | it is a current that repeatedly reverses direction |
| Name a source of AC? | mains supply in the home |
| What does the potential difference do when there is a direct current? | the potential difference in a direct current circuit does not change direction |
| What does the potential difference do when there is an alternating current? | the polarity of the potential difference repeatedly reverses its direction |
| What type of current does mains supply? | alternating current (AC) |
| What is the frequency of alternating current (AC)? | it is number of cycles it passes through each second |
| What is a cycle? | the current flows in one direction then in the other completing a cycle before it repeats itself again |
| What is the mains frequency in the UK? | 50 cycles per second or 50 Hz |
| How long does a mains cycle take in the UK? | time for a cycle = 1 ÷ frequency = 1 ÷ 50 = 0.02s |
| What is the maximum voltage of mains in the UK? | around 325 volts |
| Name the wires in a mains circuit? | a live and a neutral wire |
| What is the potential of the neutral wire? | zero volts |
| What is the potential of the live wire? | it alternates between positive and negative each cycle |
| Why is the live wire dangerous? | because its potential repeatedly changes from positive to negative |
| What is the National Grid? | a nationwide network of cables and transformers to distribute electricity across the country |
| What is the potential difference of a typical power station? | an alternating potential of 25 000 V or 25 kV |
| Where are step-up transformers used in the National Grid? | at power stations to transfer electricity to the National Grid |
| What does a step-up transformer do? | it makes the size of the alternating potential much bigger typically either 25 000V or 132 000V |
| Where are step-down transformers used in the National Grid? | they are used to supply electricity to consumers |
| What does a step-down transformer do? | it makes the size of the alternating potential much smaller to 230 V for homes and 100kV or 33kV for businesses |
| Why is potential difference increased to distribute electricity around the country? | it reduces the power loss because increasing the potential difference means only a small current is needed |
| Name the equipment used to show how an alternating potential difference varies with time. | an oscilloscope |
| Question | Answer |
|---|---|
| What does the earth wire do? | it provides a path for a fault current to flow to earth to prevent users from a shock |
| Why must a metal heater be earthed?? | In case, the electric wire touches the case, it stops the metal case becoming live and potentially shocking even electrocuting a user |
| Name a group of materials that make good insulators? | plastics |
| What does it mean when an electrical device is double insulated? | the appliance is designed in such a way that the electrical parts can never come into contact with the outer casing of the device |
| What are the outer casings of plugs, sockets, and cables made of? | hard-wearing electrical insulators |
| Why are the outher casings of plugs, sockets, and casings made of hard-wearing electrical insulators/ | because they all contain live wires |
| How are mains appliances connected to the mains? | by a wall socket using a cable and a three-pin plug |
| What materials are sockets made of? | a stiff plastic |
| What wires does a wall socket have? | a live wire, a neutral wire, and an earth wire |
| What is the earth wire connected to? | to the ground in the home of the socket |
| What is the voltage of the earth wire? | zero volts, 0 V |
| What current does the earth wire carry? | it only carries a current if there is a fault |
| Which pin of a three-pin plug is connected to the earth wire? | the longest pin |
| Why does a plug have three pins? | to connect to the live wire, neutral wire, and earth wire |
| What metal are pins in a plug made of? | brass |
| Why are the pins in a plug made of brass? | because brass is a good conductor and does not oxidise or rust |
| What metal conducts better than brass? | copper |
| Why is copper not used instead of brass? | because brass is harder than copper |
| What type of material is used for the plug case? | an insulator |
| What is the layout of the inside of a plug designed to do? | to ensure that the wires and the pins can't touch each other when the plug is sealed |
| How is the live pin connected to the live wire? | via a fuse |
| Why is the live pin connected to the live wire via a fuse? | if too much current passes into the live wire the fuse will melt and cut the live wire off |
| What is the colour of the live wire? | brown |
| What is the colour of the neutral wire? | blue |
| What is the colour of the neutral wire? | green and yellow striped |
| What metal is used for cables? | copper |
| How are the wires in cables insulated? | the copper wire is surrounded by an outer layer of rubber or flexible plastic |
| When is a two core cable? | a cable that contains two copper wires each insulated with plastic |
| When are two core cables used? | for appliances with plastic cases like radios, hairdryers, and mobile phone charges |
| Which cables are thicker out of the wires that join the sockets in a house compared to the cables that join together light fittings? | the cables joining house sockets are much thicker than those joining light fittings |
| Why are the cables joining house sockets much thicker than those joining light fittings? | because the cables joining house sockets carry more current than those joining light fittings |
| Why are the cables that carry more current thicker? | so they have less resistance which reduces the heating effect |
| What is a short circuit? | when a live wire touches a neutral wire |
| What happens when there is a short circuit? | a large current flows very quickly which can damage or destroy electrical devices, or even start electrical fires |
| How does a fuse protect appliances against a short circuit? | the large current that results from a short circuit melts the wire in the fuse which then cuts the current off |
| Why is it dangerous to touch a live wire? |
• people's bodies are at zero volts • touching a live wire will result in a large potential difference across the body • the large potential difference will cause a current to flow through the person • the person will suffer an electric shock, possibly a lethal shock |
| Question | Answer |
|---|---|
| What does the current through an electrical appliance do? | it transfers energy from the power source to the appliance |
| What is the power of an appliance? | the energy it transfers per second |
| What are the units of power? | Watts (W) |
| State the equation for power. | \( power\; (P)\; (watts,\; W)= \frac {energy\; transferred,\; E\; (joules,\; J)}{time,\; t\; (seconds,\; s)} \) |
| Rearrange the equation for energy. | E = Pt |
| Describe the energy in an artificial heart. |
• millions of electrons pass through the circuit of an artificial heart every second • work is done by a battery in the artificial heart to force the electrons around the circuit • each electron transfers a small amount of energy to the heart from the battery • the total energy transferred to the artificial heart each second is large enough to allow the appliance to function |
| What is the current through an electrical appliance? | the charge which flows through it each second |
| What is the potential difference across an electrical appliance? | the energy transferred to the appliance by each coulomb of charge that passes through it |
| What is the power supplied to an electrical appliance? | it is the energy supplied to the appliance by the current each second |
| State the equation for power in an electrical appliance. | \( power,\; (P) = {current,\; I\; } \times {potential\; difference,\; V} \) |
| How do you determine the right fuse for an appliance? | the power rating of the appliance divided by its potential difference |
| What is the effect of a current passing through a resistor? | the power supplied to it by the current heats the resistor |
| What happens to the heat in the resistor when a current passes through it? | the heat is dissipated to the surroundings |
| State the formula for the potential difference V across a resistor of resistance R with a current I through it. | V = I x R |
| State the formula for the power P dissipated by a resistor R when current I passes through it. | P = V x I |
| State the other version of the equation for power in a resistor. | P = I2R |
| What happens to the power dissipated by a resistor if the current is doubled? | the power is quadrupled |
| Question | Answer |
|---|---|
| What is a current? | the flow of charge |
| Name two charged particles. | electrons and protons |
| What makes a charge move to create a current? | potential difference (also known as voltage) makes charge move to create a current |
| What are charges make up the current in wires attached to electrical appliances? | electrons |
| State the equation for calculating the charge flow in a current | charge flow, Q = current, I x time, t |
| State the units for each quantity in the above equation. |
• charge: Coloumbs, C • current: Amperes, A • time: seconds, s |
| What is the role of the battery in making a current flow through a resistor? | the battery does work to make electrons pass through the resistor |
| Why does a resistor become hotter when a current passes through it? |
• the electrons moving through the resistor collide with the ions and molecules of the resistor • the ions and molecules of the resistor gain kinetic energy as a result of each collission • the ions and molecules thus vibrate more and the resitor becomes hotter |
| Decribe the changes in energy from battery to a warm resistor. |
• the chemical energy of the battery is transferred to the kinetic energy of the charged particles in the current • the kinetic energy of the charged particles in the current is transferred to the ions and molecules of the resistor • the increased kinetic energy of ions and molecules is manifested as heat energy |
| State the equation to calculate the energy transferred to a resistor. | energy, E = charge flow, Q x potential difference, V energy, E = power, P x time, t = potential difference, V x current, I x time, t |
| Describe the energy transfer in a series circuit with a resistor, battery and light bulb. |
• the charge leaves the battery with energy • the charge transfers some of its energy to the light bulb • the charge transfers some of its energy to the resister • the energy transferred to the light bulb is converted to light and heat • the energy transferred to the resistor is converted to heat • the heat energy of the light bulb and the resistor is transferred to the surroundings • the electrical work done by the battery is equal to the energy transferred to the light bulb and the resistor |
| Write an equation for the energy in the above circuit. | the energy from the battery = the energy transferred to the light bulb + the energy transferred to the resistor |
| Question | Answer | |
|---|---|---|
| How can one work out how much energy is supplied to an appliance by the mains? | the time the appliance has been switched on and the power of the appliance | |
| State the equation for the energy transferred to an appliance in a given time. | energy transferred from the mains, E = power, P × time, t | |
| State the units for energy, power and time |
• energy Joules J • power Watts, W • time seconds, s |
|
| State the equation to calculate the power supplied to an appliance. | power, P = current, I × potential difference, V | |
| State the units for power, current and potential difference |
• power Watts, W • current Amperes, A • potential difference Volts V |
|
| State the formula for efficiency for an electrical appliance. | efficiency = (output power) ÷ (input power) × 100 | |
| Rearrange the efficiency equation for output power. | output power = efficiency × (input power) | x |
| What are the units of efficiency? | efficiency is expressed either as a ratio or a percentage | |
| What is the maximum efficiency of an appliance as a percentage? | 100 % | |
| What is the equation to calcuate the wasted energy percentage of an appliance. | wasted energy = 100 % − percentage efficiency | |
| List the ways an electrical appliance wastes energy. |
• the heating effect of the current due to the resistance in the wires • the heating effect of the current due to the components in the appliance • the heating effect due to friction of moving parts like motors |
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