Electrical Quantities (Cambridge O Level Physics)

A student rubs a polythene rod with a dry cloth. The polythene rod becomes negatively charged.

Describe and explain how the rod becomes negatively charged.

The negatively charged polythene rod hangs from a nylon thread so that it is free to turn.

 The student charges a second polythene rod and brings it close to the first rod, as shown in Fig.11.1.

Describe and explain what happens when the negatively charged rods are close to each other.

A student experiments with electric charge.

The student uses a dry cloth to rub a plastic rod. The rod becomes positively charged.

Explain how the friction between the rod and the cloth causes the rod to become positively charged.

Use your ideas about the movement of charge.

The student suspends a balloon from an insulating thread, as shown in Fig. 9.1.

The balloon has an electric charge.

 
Explain how the student can use a positively charged rod to determine the charge on the balloon.

The lamp of a car headlight is rated at 12 V, 50 W.

 
Calculate the current in the lamp when operating normally.

current = ......................................................... 

A car is driven at night.

In a journey, the total charge that passes through the 12 V battery is 270 kC.

Fig. 9.1 shows the symbol for a 12V battery.

Two lamps are connected in parallel with the battery.

On Fig. 9.1, using the correct symbols, complete the circuit diagram.

One of these lamps has a resistance of 6.0 Ω.

 
Calculate, for this lamp:

The power of the other lamp is 36 W.

 
Calculate the total energy delivered to this lamp in 20 hours.

Fig. 9.1 shows current-potential difference graphs for a resistor and for a lamp.

The p.d. across the lamp is 6.0V. Calculate the resistance of the lamp.

resistance = ......................................................... 

The lamp and the resistor are connected in parallel to a 6.0 V supply.

Calculate the current from the supply.

current = .........................................................

The lamp and the resistor are connected in series to another power supply. The current in the circuit is 4.0 A.

Calculate the total p.d. across the lamp and the resistor.

p.d. = .........................................................

Fig. 10.1 shows a circuit used by a student to test a metal wire made of nichrome.

State the name of component Z.

The current reading on ammeter X is 0.8 A. State the reading on ammeter Y.

The current in the nichrome wire is 0.8 A. The potential difference (p.d.) across the nichrome wire is 4.5 V.

Calculate the resistance of the nichrome wire.

The student tests a different nichrome wire, which is thicker than the wire in (c), but of the same length. When testing this wire, the current in the wire is different from the value given in (c).

 
State and explain the difference in current.

Fig. 10.1 shows an electrical circuit.

An electrical heater has a resistance of 21.8 Ω when connected to a 240 V mains supply.

An uncharged conducting metal plate rests on insulating supports. Fig. 10.1 shows the plate and a positively charged insulating plastic sheet placed on top of the metal plate.

Fig. 10.2 shows two uncharged conducting spheres suspended on insulating threads.

State, in terms of their structure, why metals are good conductors of electricity.

A cylindrical metal wire W₁, of length  and cross-sectional area A, has a resistance of 16 Ω.A second cylindrical wire W₂ having length and cross-sectional area 2A, is made from the same metal.

Determine

The parallel pair of resistors in (b)(ii) is connected to a battery that is made from three cells in series, each of electromotive force (e.m.f.) E. There is a current in each resistor.

(i)

State the e.m.f. of the battery.

[1]

 

(ii)

The current in the battery is I_B, the current in W₁ is I₁ and the current in W₂ is I₂.

 

Place a tick in one box to indicate how these three currents are related.

 

  I₁ > I₂ > I_B

  I₁ > I_B > I₂

  I₂ > I₁ > I_B

  I₂ > I_B > I₁

  I_B > I₁ > I₂

  I_B > I₂ > I₁

  I₁ = I₂ = I_B

[1]

In terms of a simple electron model, describe the differences between conductors and insulators.

On Fig. 8.3, draw the electric field pattern around a single-point positive charge.

Fig. 8.1 shows a polystyrene ball covered with aluminium paint. The polystyrene ball is suspended between two charged metal plates by an insulated thread.

 

The ball oscillates between the two charged plates.
Explain why the ball oscillates.

There is a current of 0.29 A in an electrical circuit.

Calculate the time taken for a charge of 15 C to flow through the electrical circuit.

time = .........................................................

A light-emitting diode (LED) is a diode that emits light when there is a current in it.

Draw a circuit diagram showing an LED, connected so that it is lit, in series with a battery and a fixed resistor. Use standard electrical symbols.

The p.d. across the LED when lit is 3.1 V and the current in the LED is 0.030 A.

Calculate the value of the resistance of the LED when lit.

resistance = .........................................................

  
The p.d. across the lamp is 2.1 V and the current in the lamp is 1.5 A.

Calculate:

A light polystyrene ball is coated in conducting paint and suspended from a thread as shown in Fig. 1

The ball is given a positive charge

Fig. 1

Draw the field lines around the positively charged ball.

A second, identical ball is suspended close to the first one. The second ball is also given a positive charge.

Marion says that both balls will hang straight downward, due to the effect of gravity.

State whether Marion is correct, and explain your answer.

Sketch the arrangement of the two balls hanging next to each other.

Add field lines to your diagram, in support of your answer to part (b).