Current Electricity

Question 1

What is the symbol for time, the unit and the symbol for the unit?

Answer 1

Symbol - t
Unit - second
Symbol for seconds - s

Question 2

What is the symbol for current, the unit and the symbol for the unit?

Answer 2

Symbol - I
Unit - Amp
Symbol for Amps - A

Question 3

What is the symbol for energy, the unit and the symbol for the unit?

Answer 3

Symbol - E
Unit - joule
Symbol for joules - J

Question 4

What is the symbol for voltage, the unit and the symbol for the unit?

Answer 4

Symbol - V
Unit - volt
Symbol for volts - v

Question 5

What is the symbol for resistance, the unit and the symbol for the unit?

Answer 5

Symbol - R
Unit - omh
Symbol for ohms - Ω

Question 6

What is the symbol for power, the unit and the symbol for the unit?

Answer 6

Symbol - P
Unit - joules per second/watt
Symbol for units - J/s / W

Question 7

What equation links current, charge and time?

Answer 7

I = Q/t

Question 8

What equation links energy, voltage and charge?

Answer 8

V = E/Q

Question 9

What equation links voltage, current and resistance?

Answer 9

R = V/I

Question 10

What equation links power, energy transferred and time?

Answer 10

t = e.t./P

Question 11

What equation links power, voltage and current?

Answer 11

V = P/I

Question 12

Voltage is low ->

Answer 12

current is low -> temperature is low -> resistance of the filament is low
• Gradient is high

Question 13

Voltage is high ->

Answer 13

current it high -> temperature is high -> resistance of the filament is high
• Gradient is low

Question 14

What is the symbol for charge, the unit and the symbol for the unit?

Answer 14

Symbol - Q
Unit - coulombs
Symbol for coulombs - C

Question 15

How to find out resistance when voltage is proportional to current and the gradient is known?

Answer 15

Resistance = 1/gradient

Question 16

Rules for current and voltage in a series circuit

Answer 16

• 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.

Question 17

What is charge measured in?

Answer 17

Charge = Q

Measured in Coloumbs (C)

Question 18

What is a thermistor and how does its resistance change?

Answer 18

• Thermistors can detect and sense temperatures.

* As the temperature increases, the resistance decreases.

Question 19

What is an LDR and how does its resistance change?

Answer 19

• Light dependent resistors can detect light levels.

* As light intensity increases, their resistance decreases.

Question 20

Where do you put an ammeter in a circuit?

Answer 20

• Anywhere in the main circuit in series, but not in parallel.

Question 21

Where do you put the voltmeter?

Answer 21

• In parallel, across the component.

* Not around the variable resistor or battery!!!

Question 22

What are alternating (ac) and direct (dc) currents?

Answer 22

• 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.

Question 23

Recipe for a safe plug:

Answer 23

• 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).

Question 24

How does the earth wire work?

Answer 24

• 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.

Question 25

What does it mean to double insulate an appliance?

Answer 25

* 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.

Question 26

How do fuses work?

Answer 26

* 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.

Question 27

How do circuit breakers work?

Answer 27

* 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).

Question 28

Why is it bad to have water near electrical objects?

Answer 28

Water is a very good conductor, so the current would pass through it and cause an electric shock.

Question 29

What are the different types of fuses?

Answer 29

• 3A, 5A and 13A.

Question 30

Why do some appliances have thick cables?

Answer 30

* Thick cables have a lower resistance. * Thick cables are easy to insulate. * Thick cables produce less heat.

Question 31

What is voltage?

Answer 31

The amount of energy transferred per unit of charge (Coulomb). 1 Volt = 1 Joule/Coulomb.

Question 32

What is current?

Answer 32

The rate of flow of charge.

Question 33

Why might a series circuit be more appropriate for a particular application?

Answer 33

* 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.

Question 34

Why might a parallel circuit be more appropriate for a particular application?

Answer 34

* 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.

Question 35

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?

Answer 35

* 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.

Question 36

What does a variable resistor do?

Answer 36

It enables someone to control the current in a circuit.

Question 37

Why would you want to keep the temperature of a wire constant when working out the difference in resistance by changing voltage?

Answer 37

* 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.