Current Electricity Flashcards
What is the symbol for time, the unit and the symbol for the unit?
Symbol - t
Unit - second
Symbol for seconds - s
What is the symbol for current, the unit and the symbol for the unit?
Symbol - I
Unit - Amp
Symbol for Amps - A
What is the symbol for energy, the unit and the symbol for the unit?
Symbol - E
Unit - joule
Symbol for joules - J
What is the symbol for voltage, the unit and the symbol for the unit?
Symbol - V
Unit - volt
Symbol for volts - v
What is the symbol for resistance, the unit and the symbol for the unit?
Symbol - R
Unit - omh
Symbol for ohms - Ω
What is the symbol for power, the unit and the symbol for the unit?
Symbol - P
Unit - joules per second/watt
Symbol for units - J/s / W
What equation links current, charge and time?
I = Q/t
What equation links energy, voltage and charge?
V = E/Q
What equation links voltage, current and resistance?
R = V/I
What equation links power, energy transferred and time?
t = e.t./P
What equation links power, voltage and current?
V = P/I
Voltage is low ->
current is low -> temperature is low -> resistance of the filament is low
• Gradient is high
Voltage is high ->
current it high -> temperature is high -> resistance of the filament is high
• Gradient is low
What is the symbol for charge, the unit and the symbol for the unit?
Symbol - Q
Unit - coulombs
Symbol for coulombs - C
How to find out resistance when voltage is proportional to current and the gradient is known?
Resistance = 1/gradient
Rules for current and voltage in a series circuit
- Current doesn’t change in a series circuit.
- In series, voltage is shared between all components.
- In parallel, voltage is the same with each branch of the circuit.
What is charge measured in?
Charge = Q
Measured in Coloumbs (C)
What is a thermistor and how does its resistance change?
- Thermistors can detect and sense temperatures.
* As the temperature increases, the resistance decreases.
What is an LDR and how does its resistance change?
- Light dependent resistors can detect light levels.
* As light intensity increases, their resistance decreases.
Where do you put an ammeter in a circuit?
• Anywhere in the main circuit in series, but not in parallel.
Where do you put the voltmeter?
- In parallel, across the component.
* Not around the variable resistor or battery!!!
What are alternating (ac) and direct (dc) currents?
- Alternating currents are constantly changing direction - mains electricity is an ac supply.
- Direct current are always the same direction - cells and batteries supply direct current.
Recipe for a safe plug:
- Firmly screw in the rightly coloured wire to its corresponding pin.
- No bare wires inside the plug.
- Fasten the cable grip tightly over the cable outer layer.
- Metal parts are made of copper or brass (good conductors).
- Case, cable grip and cable insulation are made of flexible rubber or plastic (good insulators).
How does the earth wire work?
- Live wire manages to touch the metal case.
- The case is earthed, so a large current flows through the live.
- The current then flows through the case and out down the earth wire.
- The surge of current causes the fuse to melt (or the circuit breaker to be tripped) and cuts off the live supply.
- The appliance cannot get electricity - no chance of an electric shock or fire from overheating.
What does it mean to double insulate an appliance?
- If an appliance has a casing made out of plastic (instead of metal - which conducts electricity making electric shocks possibles) and has no metal parts showing, it is double insulated.
- If an appliance is double insulated, it doesn’t require an earth wire.
How do fuses work?
- Fuses contain a thin piece of wire which has a low melting point.
- If there is a current which is too large flowing in the circuit, the fuse wire will become very hot and melt.
- If the fuse has melted (‘blown’), the circuit is shut off and you cannot get an electric shock or have an electrical fire.
How do circuit breakers work?
- Very similar to fuses.
- If a circuit breaker detects a surge in the current, it will break the circuit by opening a switch.
- To reset them, one flicks a switch on the device (more convenient than fuses, as once a fuse melts it has to be replaced).
Why is it bad to have water near electrical objects?
Water is a very good conductor, so the current would pass through it and cause an electric shock.
What are the different types of fuses?
• 3A, 5A and 13A.
Why do some appliances have thick cables?
- Thick cables have a lower resistance.
- Thick cables are easy to insulate.
- Thick cables produce less heat.
What is voltage?
The amount of energy transferred per unit of charge (Coulomb). 1 Volt = 1 Joule/Coulomb.
What is current?
The rate of flow of charge.
Why might a series circuit be more appropriate for a particular application?
- In a series circuit the voltage is shared out between all the components and therefore might be useful to supply low power things, such as fairy lights.
- In a series there are fewer wires used.
- In a series the resistance values are lower.
Why might a parallel circuit be more appropriate for a particular application?
- In a parallel circuit, each component receives full voltage, so it is useful for supplying power to high energy things.
- It might be more practical because if component in the circuit breaks, the rest of the circuit will still work because the different components are connected separately to the supply.
A kitchen has a water supply, an electricity supply and electric lighting. There is a toaster, a kettle, a clothes iron, a microwave oven and a dishwasher.
What are the hazards of electricity in this kitchen?
- Electrical devices with heating elements (e.g. the kettle) can reach high temperatures, which can cause (skin) burns and fires (if the insulation melts).
- Electricity coming into contact with water is hazardous (e.g. dishwasher) because water conducts electricity and can therefore cause an electric shock.
- Damaged equipment can cause many hazards, such as a fault microwave leaking cancer-causing microwaves. Live parts must not be exposed which could lead to electric shocks.
- With many electrical appliances, there is a danger of the sockets being overloaded (which can cause a fire). Circuit must have correct fuses and there must be sufficient sockets.
- Having many electrical appliances means there is a trip hazard of all the cables. Short leads could be used to reduce the hazard.
- There is a danger of sticking metal objects into sockets/heating elements which can cause an electric shock. Sockets could be covered to stop people being able to put things in them and tools must be insulated properly.
What does a variable resistor do?
It enables someone to control the current in a circuit.
Why would you want to keep the temperature of a wire constant when working out the difference in resistance by changing voltage?
- Resistance changes with temperature.
- A hot wire would be dangerous as it might melt or catch fire.
- Voltage is only proportional to current at a constant temperature.
- Ohm’s law is only true if the temperature is constant.
