Topic 10 - Electricity And Circuits

Question 1

What is a fixed resistor?

Answer 1

A resistor restricts or limits the flow of electrical current. A fixed resistor has a resistance that does not change.

Question 2

What is a variable resistor?

Answer 2

Adjusting this resistor changes its resistance. A variable resistor is used in some dimmer switches and volume controls.

Question 3

What is a thermistor?

Answer 3

The resistance of a thermistor depends on its temperature. At low temperatures, the thermistor has a high resistance. As the temperature increases, the resistance decreases. A thermistor can be used in thermostats or heat activated fire alarms.

Question 4

What is a Light-dependent resistor (LDR)?

Answer 4

The resistance of a LDR depends on light intensity. At low light levels, the LDR has a high resistance. As the light intensity increases, the resistance decreases. A LDR can be used as a sensor in cameras or automatic lights that come on when it gets dark.

Question 5

What is a semiconductor diode?

Answer 5

A semiconductor diode allows current to flow in one direction only. Current will not flow in the other direction. Diodes are used to convert an alternating current into a direct current.

Question 6

What is a light-emitting diode?

Answer 6

A LED, or light-emitting diode, is a light source based on a semiconductor – materials that are neither pure conductors nor insulators but have an electrical conductivity somewhere in between. When a current passes through the LED, electrons recombine with holes in the semiconductor, emitting light in the process.

Question 7

What is a motor?

Answer 7

Electric motors involve rotating coils of wire which are driven by the magnetic force exerted by a magnetic field on an electric current. They transform electrical energy into mechanical energy.

Question 8

What is a voltmeter?

Answer 8

A voltmeter is an instrument used for measuring the potential difference, or voltage, between two points in an electrical circuit. A voltmeter is connected around the device (in parallel with the device) but outside the main circuit.

Question 9

What is an ammeter?

Answer 9

An ammeter is a measuring device used to measure the electric current in a circuit. An ammeter is connected in the cirucit with a device (in series with a device) to measure its current.

Question 10

What is a series circuit?

Answer 10

In series circuits, the different components are connected in a line, end to end, between the +ve and-ve of the power supply (except for voltmeters, which are always connected in parallel, but they don’t count as part of the circuit).
If you remove or disconnect one component, the circuit is broken and they all stop working. This is generally not very handy, and in practice very few things are connected in series.

Question 11

What are the electrical properties of a series circuit? (4)

Answer 11

• There’s a bigger supply p.d, when more cells are in series (if they’e all connected in the same way) E.g. when two batteries with a p.d. of 1.5 V are connectod in serion they supply 3 V between them
• The current is the same everywhere. The size of the current depends on the total pd. and the total resistance of the circuit ( I = V ÷R).
• The tołal potential difference of the supply is shared between components. The p.d. for each component depends on its resistance.
• The total resistance of the circuit increases as you add resistors.

Question 12

What is a parallel circuit? (4)

Answer 12

• In parallel circuits, each component is separately connected to the +ve and -ve of the supply (except ammeters, which are alwaus connected in series).
• If you remove or disconnect one of them, it will hardly affect the others at all.
• This is obviously how most things must be connected, for example in cars and in household electrics. You have to be able to switch everything on and off separately.
• Everyday circuits often contain a mixture of series and parallel parts - when looking at components on the same branch the rules for series circuits apply.

Question 13

What are the electrical properties of parallel circuits? (4)

Answer 13

• The potential difference is the same across all components.
• Current is shared between branches. The total current flowing around the circuit is equal to the total of all the currents through the separate components.
• In a parallel circuit, there are junctions where the current either splits or rejoins. The total current going into a junction has to equal the total current leaving. (If two identical components are connected in parallel then the same current will flow through each component).
• The total resistance of the circuit decreases if you add a second resistor in parallel.

Question 14

What is the potential difference?

Answer 14

The energy transferred per unit (coulomb) of charge that passes through two points. It is also called voltage.

Question 15

How do you work out the energy transferred?

Answer 15

Energy transferred (J) = charge moved (Coulomb) x potential difference (V)

Question 16

What is an electric current?

Answer 16

The rate of flow of charge. In metals, current is a flow of electrons.

Question 17

How do you work out charge?

Answer 17

Charge (coulombs) = current (A) x time (s)

Question 18

When does current flow through a component?

Answer 18

Current will only flow through a component if there is a potential difference across that component and if it s a closed circuit.

Question 19

What is resistance?

Answer 19

Anything that slows the flow down. Unit: Ohms

Question 20

When is current conserved?

Answer 20

At a junction in a circuit

Question 21

What happens when the potential difference is increased?

Answer 21

The current also increases.

Question 22

What happen when the resistance is increased?

Answer 22

The current flowing decreases.

Question 23

How do you work out potential difference using current and resistance?

Answer 23

Potential difference (V) = Current (A) x Resistance (ohms)

Question 24

Why is it that in series, the resistance of the network is equal to the sum of the two individual resistances whereas in parallel, the resistance of the network is less than either of the two individual resistances?

Answer 24

Placing the resistors in series causes the resistance to be double that of a single resistor because there is only one path for the electrons to follow - the supply must drive current through one resistor and then the other.

Placing the resistors in parallel causes the resistance to be half that of a single resistor.

Question 25

Construct electrical circuits to investigate the relationship between potential difference, current and resistance for a resistor and a filament lamp. (6)

Answer 25

1. Connect the circuit as in the form of a standard test circuit. The component and the ammeter are in series, which means they can be put in any order in the main circuit. (Remember the voltmeter must be in parallel around the component under test)
2. Change the output potential difference of the power supply. This alters the current flowing through the circuit and the potential difference across the component.
3. Take several pairs of readings from the ammeter and voltmeter to see how the current through the component varies as the potential difference across it is changed.
4. Plot the current against the potential difference to get I-V graphs.
5. You can use this data to work out the resistance for each measurement of I and V so you can see if the resistance of the component changes as I and V change.
6. Make sure the circuit doesn’t get too hot over the course of your experiment, as this will mess up your results. If the circuit starts to warm up, disconnect it for a while between readings so it can cool down. And, like any experiment, you should do repeats and calculate means.

Question 26

How does the current vary with the potential difference for resistors and wire?

Answer 26

Current is directly proportional to p.d.

Question 27

How does current vary with p.d for filament lamps?

Answer 27

The increasing current increase the temperature of the filament, which makes the resistance increase so their I-V graphs curve.

Question 28

How does current vary with p.d for Diodes?

Answer 28

Current will only flow through a diode in one direction, as shown. The diode has very high resistance in the opposite direction.

Question 29

How does the resistance of a LDR vary with light intensity?

Answer 29

In bright light, the resistance falls. In darkness, the resistance is highest. They have lots of applications such as automatic night lights, outdoor lighting and burgling detectors.

Question 30

How does the resistance of a thermistor vary with temperature?

Answer 30

In hot conditions, the resistance drops. In cold conditions, the resistance rises. Thermistors make very useful temperature detectors such as car engine temperature sensors and electronic thermostats.

Question 31

Compare the total resistance in series and parallel arrangements . (7)

Answer 31

1. Set up a parallel circuit with 2 branches with the resistor on the first branch and the voltmeter on the second, turn the power supply on and close the switch.
2. Record the voltmeter and ammeter readings and calculate the resistance of the resistor using R = V/I, where R is resistance, V is potential difference and I is current.
3. Change the resistor and repeat step two to find the resistance of a second resistor.
4. Arrange the two resistors in series and close the switch.
5. Record the voltmeter and ammeter readings once again and determine the total resistance of both resistors in series using R = V/I.
6. Arrange the two resistors in parallel, so both resistors have their own branch and close the switch.
7. Record the voltmeter and ammeter readings once again and calculate the total resistance of both resistors in parallel.

Question 32

What happens when an electric current is in a resistor?

Answer 32

There is an energy transfer which heats the resistor because of the collisions of electrons and ions in the lattice structure.

Question 33

What type of energy is dissipated when an electrical current works against electrical resistance?

Answer 33

Electrical energy is dissipated as thermal energy.

Question 34

How could you reduce unwanted energy transfers through low resistance wires?

Answer 34

Increase the voltage which would decrease the current

Question 35

What are the advantages of the heating effect from current? (3)

Answer 35

1. The heating effect of electric current is put to good use in room heaters and water heating coils where electricity is converted into heat energy.
2. An electric bulb also exploits the effect. The filament of the bulb is made of Tungsten metal, which glows when heated by the electric current flowing through it, thus giving out light.
3. An electric fuse also uses the heating effect of electric current. A fuse is rated for a specific amperage. When the current through it exceeds its rating, the heat produced melts the fuse wire thus opening the circuit.

Question 36

What are the disadvantages of the heating effect from current? (3)

Answer 36

1. Wastage of energy during power transmission in overhead cables is a major disadvantage of the heating effect. To counter the ohmic losses, electricity is transmitted in a high voltage.
2. The heating effect reduces the life of several components in a circuit.
3. Additional cooling systems and heat sinks need to be added to circuits to counter the effect of heating, which adds to the cost.

Question 37

How do you work out energy transferred?

Answer 37

Energy transferred (J) = current (A) x p.d (V) x time (s)

Question 38

What is power?

Answer 38

Energy transferred per second

Question 39

How do you work out power?

Answer 39

Power (W) = energy transferred (J) ÷ time taken (s)

Question 40

How does the power transfer in any circuit depend on p.d. and current?

Answer 40

The p.d. tells you how much energy each unit of charge transfers and how much energy you spend. Thus both will affect the rate that energy is transferred to an appliance, and the rate at which it transfers energy to other stores. The power of an appliance can be found with:

Question 41

How do you work out power with current and p.d?

Answer 41

Power (W) = current (A) x p.d (V)

Question 42

How do you work power with resistance and current?

Answer 42

Power (W) = resistance (ohm) x current squared (A)

Question 43

Describe alternating current

Answer 43

In a.c. supplies, the movement of the charges is constantly changing direction. Alternating currents are produced by alternating voltages (the positive and negative voltages ends of p.d. keep alternating). The Uk mains supply is an a.c. supply at around 230V. the frequency of the a.c. supply is 50 cycles per second (50Hz).

Question 44

Describe direct current

Answer 44

In d.c., the movement of the charges is only in one direction. It’s created by a direct voltage (a p.d. that is only positive or negative not both). Cells and batteries supply direct current

Question 45

What is the brown wire?

Answer 45

Live wire:

It carries the voltage (p.d.). It alternates between a high positive and a high negative of about 230V

Question 46

What is the blue wire?

Answer 46

Neutral wire:
It completes the circuit - electricity normally flows in through the live wire and out through the neutral wire. The neutral wire is always at 0V.

Question 47

What is the green and yellow wire?

Answer 47

Earth wire:
It is for safety and protecting the wiring. It carries the current away if something goes wrong and stops the appliance from becoming live. It is also at 0V.

Question 48

What is the function of switches on plug sockets?

Answer 48

These switches are connected in the live wire of the circuit. This is so that the circuit can be broken - the appliance becomes isolated and the risk of an electric shock is reduced.

Question 49

What is the p.d. between the live wire and the neutral wire?

Answer 49

It equals the supply p.d. (230V for the mains)

Question 50

What is the p.d. between the live wire and the earth wire?

Answer 50

230V for a mains-connected appliance

Question 51

What is the p.d. between the earth wire and the neutral wire?

Answer 51

They are both at 0V.

Question 52

Why shouldn’t you touch the live wire?

Answer 52

A large potential difference is produced across your body and a current will flow through you.

Question 53

What are the dangers of a connection between the live and neutral wire?

Answer 53

If this link creates a low resistance path to earth, a huge current will flow which could result in a fire.